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CLIMATE THROUGH THE AGES
By the same author
THE EVOLUTION OF CLIMATE
CLIMATE
BRITISH FLOODS AND DROUGHTS
THE WEATHER
CLIMATE
THROUGH THE AGES
A STUDY OF THE CLIMATIC FACTORS
AND THEIR VARIATIONS
By
C. E. P. BROOKS
I.S.O., D.Sc., F.R.Met.Soc.
ERNEST BENN LIMITED
LONDON
First published 1926
Revised edition 1949
Second impression 1950
PUBLISHED IN GREAT BRITAIN BY ERNEST BENN LIMITED, LONDON
PRINTED IN GREAT BRITAIN BY HHADLEY BROTHERS
IOQ KINGSWAY, LONDON, W.C.2J AND ASH FORD, KENT
PREFACE TO THE FIRST EDITION
SOME years ago I had the privilege of laying before the
public " The Evolution of Climate." As its title
implies, that book mainly consists of an account of the
variations of climate during geological times, and it deals
especially with the Quaternary Ice-Age and the Post-glacial
period. The causes of the events described were touched on
only very briefly, and chiefly for the purpose of emphasising
the climatic importance of the period of continental emergence
which intervened between the mild -climates of the Mesozoic
and Tertiary periods and the beginnings of the Ice-Age. It
was my intention then to write a companion volume dealing
with the mechanism of climatic changes and drawing on the
first volume for illustrations. But there were many problems
requiring further investigation and much reading to be done
before such a work could be approached with any confidence,
and so for several years desire outran performance. Then
came the work of Wegener on the theory of continental drift,
and of Koppen and Wegener on the interpretation of the
climatic record in terms of the travels of the continents across
the parallels of latitude, and this stimulated me to complete
the investigation in order to examine Wegener's theory from
the climatic side.
" Climate through the Ages " has not been altogether easy
writing ; the present is the key to the past, but there have
been many strange meteorological situations for which we
have no present parallel to guide us. Moreover, the
meteorology of the present has still to solve many problems
of its own, and I am even encouraged to hope that the
meteorology of the past may at times help in the study of the
present. The theory of the circulation of the earth's
atmosphere, for instance, is not yet complete, and it may be
that the modifications of the circulation during the varied
climatic history of the globe, as deduced from the distribution
of rainfall and temperature, will provide just the additional
material required for a solution.*
PREFACE
The point of view being meteorological, it has not been
considered necessary to keep to a strict chronological order
in discussing the climates of different geological periods, as
was done in " The Evolution of Climate." For convenience*
of reference, a list of the geological periods, with some idea
of their duration and a brief note on the general type of
climate, has been given in Appendix I. For the purposes of
the discussion, geological climates are classed as " warm "
or " glacial." We are living now in a " glacial " period,
though fortunately not at its maximum, and the meteorological
conditions during the Quaternary Ice-Age are not really
strange to us. The " warm " periods, in which a genial
climate extended almost or quite to the poles, are meteoro-
logically much more remote. In the best developed warm
periods it is probable that ice was unknown. This approach
to uniformity of temperature was inevitably associated with
very great changes in the winds, the ocean currents, and the
distribution of rainfall. The way in which this situation came
about was not simple, and I have thought it best to open the
book by setting out, in an Introduction, the geological evidence
as to the existence and nature of these warm periods, and
especially the extent to which climatic zones were developed.
This greatly simplifies the subsequent meteorological discussion
by allowing us to take the main facts for granted and to
consider the variation of the individual factors of climate one
by one, until the ground has been prepared for a more complete
and logical discussion in Chapter X.
" Warm " periods have been the rule in the history of
the earth, but from time to time the revolutionary forces
of the underworld have succeeded in overthrowing this
quiet and genial existence and have brought about crises in
the earth's history, and the greatest of these crises have been
marked by ice-ages. There have been at least four major
ice-ages, in the early Proterozoic, in the late Proterozoic or
Algonkian, in the Upper Carboniferous, and in the Pleistoce;ne-
Recent periods. Of the first two of these we know little, but
that little suggests that they were entirely analogous to the
Quaternary. The Upper Carboniferous glaciation, on the
other hand, was highly abnormal, in that the greatest ice-
sheets developed in regions which are now not far from the
equator. It seems probable that there was another great
PREFACE 7
period of mountain-building at the close of the Cretaceous,
which developed in such a way that the distribution of land
and aea, and of mountain ranges, was not favourable for
extensive glaciation.
The book* is divided into three sections. In the first, the
various factors of climate are discussed, and the scope which
they offer for the introduction of climatic changes is considered,
quantitatively when possible. This part of the work is
essentially a text-book of meteorology, in which, however,
some of the constants of the ordinary meteorological text-book
ar.e treated as variables. Various theories of climatic change
are discussed in successive chapters as they arise, but no
attempt has been made to include all the theories which have
been put forward from time to time. No useful purpose
would be served by solemnly refuting, for instance, the theory
that the mild polar climates of the Tertiary were due to
radiation from a warm moon, while the author of another
theory of the warm periods, who attributes them to the heat
set free by decomposing animal remains in the stratified
deposits, can only be described as a " Prophet of the Utterly
Absurd." A fairly full list of palaeoclimatic theories, possible
and impossible, is given in Appendix II.
Even when we leave out of consideration the " Patently
Impossible and Vain," however, there still remain theories
innumerable. Most of the earlier adventurers in this subject
had a single cause oceanic circulation, carbon dioxide,
eccentricity of the orbit, etc. and saw in it complete explana-
tion of the whole of climatic history. This spirit is not yet
entirely dead, but of recent years a broader outlook has become
manifest :
There are nine and sixty ways
Of constructing tribal lays,
And every single one of them is right.
.There are at least nine and sixty ways of constructing a
theory of climatic change, and there is probably some truth
in quite a number of them. The greatest extremes of climate
are not to be attributed to the abnormal development of any
one factor, but to the co-operation of a number of different
factors acting in the same direction. It seems probable that
the dominant factors have always been " geographical/* but
8 PREFACE
we must give this term a very wide meaning, and include not
only the distribution of land and sea and the systems of ocean
currents which result from it, but also the vertical circulation
of the sea, the general elevation of the land, and the amount
of explosive volcanic activity. It will be seen in ^Chapter XII.
that all these " geographical " factors are so closely inter-
related that no discussion which takes account of only one of
them can be complete.
But although the results of this work seem to indicate that
the " geographical " factors have dominated the climatic
history of the globe, this does not mean that other npn-
geographical factors are without effect. In Chapter XXII.
it is shown that during the past two thousand years, when
the distribution and elevation of the land remained practically
constant, a number of minor fluctuations of climate have been
associated with changes of solar activity. No doubt similar
fluctuations have occurred throughout geological time, but
they have been masked by the much greater effect of the
" geographical " factors, and it is only exceptionally that
solar influences can be recognised, as in the " varve " clays
formed during the melting of the Quaternary ice-sheets.
When the " varve " clays of earlier glacial periods, the rings of
the fossil trees found in various geological horizons, and other
remains which depend on seasonal changes of temperature
or rainfall come to be examined in detail, it is to be expected
that further evidence will be discovered of the existence
of minor climatic fluctuations which can be attributed
to small variations of solar radiation. Any such material
which can be analysed in the way that the tree-rings or the
Nile flood records have been analysed will be especially
welcome.
We know that the more or less regular variations in the
inclination of the earth's axis, the eccentricity of its orbit,
and the precession of the equinoxes must have influenced the
seasons in the past. So far no climatic variations have b^en
found of such a nature that they can be unhesitatingly
attributed entirely to these astronomical factors, though it
is suggested in Chapter V. that they may have been responsible
for the sequence of deposits in the coal measures and in the
Tertiary of Southern Europe, and there is good reason to
believe that the large annual range of temperature in Northern
PREFACE 9
Europe during the Boreal or Continental period was partly
due to a high inclination of the earth's axis. Even in the
last exVmple, however, the main cause of the extreme climate
was probably geographical, not astronomical, while the
historical period in which geographical causes fell into the
background has not been long enough for any of these
astronomical factors to change sufficiently to make their
presence felt.
Any change, geographical or astronomical, can only become
effective through its action on the atmosphere, but it will be
seen that this action is complex, and may at times produce
unexpected results. During the warm periods, for example,
there was a large amount of water vapour in the air, yet the
cloudiness, rainfall, and evaporation were less than at present,
because the extension of the sub-tropical anticyclones into
higher latitudes led to more stable conditions. The discussion
of the effect of cloudiness in Chapter VII. shows that Marsden
Manson was on the right lines when he emphasised the great
climatic importance of cloudiness, but he erred in attempting
to dissociate this element from the pressure distribution and
the atmospheric circulation in general, and in relating it
directly to earth heat. His neglect of meteorological principles
led him into some absurd conclusions, and the same is true
of many other theorists in the domain of palaeoclimatology.
The theory which attributed all the great climatic changes to
variations in the amount of carbon dioxide is a case in point ;
after a series of highly laborious and intricate discussions from
the geological side, it was laid gently to rest by the application
of simple physical experiments, and it is now generally
recognised that variations in the amount of carbon dioxide,
while not entirely without some climatic significance, are of
relatively slight importance. Chamberlin's ingenious theory
of the reversal of the oceanic circulation to give warm periods
in high latitudes, although it never reverberated so widely
thrQugh the world as did the original carbon dioxide theory,
really has a better physical basis. It is shown in Chapter III.
that a change in the type of oceanic circulation, although on
different lines to Chamberlin's, may have been of great
climatic importance, but only as an auxiliary factor in con-
junction with favourable geographical and meteorological
conditions, *
IO PREFACE
The second section of the book applies the principles laid
down in the first section to the various problems presented
by geological climates. It opens with a compari/on by
the method of correlation of the climatic history of the
north temperate and polar regions with the 9 corresponding
changes of elevation, land and sea distribution, oceanic
circulation, and volcanic activity. In this way numerical
measures are obtained of the influence of these various
geographical factors on geological climates, which are found
to be in good agreement with the theoretical values deduced
from recent meteorological work. The next two chapters
are mainly devoted to a consideration of the theory of
continental drift. In Chapter XIII. the theory is stated, with
some reference to its geological basis. Chapter XIV. considers
the extent to which the theory satisfies the requirements of
palaeoclimatology, and it is shown that even on Wegener's
reconstruction of the Upper Carboniferous geography, the
climate of that period still presents many difficulties. Next,
in Chapter XV., the distribution of land and sea, mountain
ranges and ocean currents during the Upper Carboniferous
is set out on the basis of the present positions of the continents,
and the probable climate to which such a distribution would
give rise is deduced from the principles developed in Part I.
It is found that these give at least as good an approximation
to the facts as does Wegener's theory, and it is accordingly
inferred that continental drift is not necessary to account for
the distribution of past climates.
I may, I hope, be excused for the length to which the
discussion of continental drift has run. The theory is at
present on its trial before the tribunal of the world's scientists,
and the verdict appears to be wavering in the balance.
Geology, naturally, will be the final arbiter, but the voices
of other sciences are not without some weight. Meteorologists,
thinking always in terms of the well-known distinction that
while weather changes from day to day climate goes op. all
the time, are naturally averse to considering the possibility
of changes in the atmosphere circulation of sufficient magnitude
to give climatic revolutions on the geological scale. To most
meteorologists, therefore, Wegener's theory of continental
drift presents an easy escape from the palaeothermal problem.
But it seems to me that this point of view is similar to that
PREFACE 1 1
of the captain of a vessel who, crossing the North Atlantic,
should attempt to determine his latitude with the aid of a
thermometer and isothermal chart.
* There seems also to be some confusion of thought as to
Wegener's theory itself. There is some definite geological
evidence in favour of that part of the theory which states that
the alternate stretching and compression of the crust, in
conjunction with tidal forces, has led to the gradual drifting
apart of the continents in an east-west direction. This being
granted, there is a tendency to assume that the remaining
part of Wegener's theory is also well grounded, namely, that
which attributes to the continents, in addition to the com-
paratively small east-west movements, enormous drifts from
north to south or vice versa. The only real evidence adduced
in support of this view is climatological, and, practically
speaking, the climate of the Upper Carboniferous ; the geo-
logical evidence is quite inadequate. It is therefore necessary
to examine the climatic evidence carefully from all points
of view, and especially to explore the possibility of alternative
explanations to it. Wegener's explanation, though not
probable, is possible ; in Chapter XV. I have attempted to
set out an alternative explanation which likewise, though
not probable, is possible. Which of us is on the right lines,
time will show. But even if the theory of continental drift
should ultimately be established in all its parts, the necessity
for a science of " palaeometeorology " will still remain. It
is now quite beyond doubt that the earth has passed through
periods of mountain formation alternating with long periods
of comparative rest, and that in consequence the average
elevation of the land has varied considerably from time to
time. Similarly, on any theory (and on Wegener's theory
more than on any other), the distribution of land and sea has
varied greatly from one period to another. These changes
must inevitably have produced corresponding changes in the
distribution of climate, which can only be arrived at by
applying the principles of meteorology . Hence it is hoped that
this book will be of service to geologists and palaeoclimatologists
no matter what basal theory of past geography they may
adopt.
The third section of the book deals in considerable detail
with the climates of different pal ts of the world during the
12 PREFACE
historical period, or from about 5000 B.C. to the present day.
In recent years a large amount of valuable work has been
published dealing with the climatic changes during this
period, and this part of the book is materially more complete*
and definite than would have been possible had it been written
even as little as ten years ago. I have been fortunate in being
able to make use of a number of detailed and entirely in-
dependent records for different parts of the world, such as the
annual rings of the big trees of America, the literary and
historical records of Europe and of China, the levels of the
Caspian, the racial movements of Asia, and the floods and
low-water stages of the Nile, and these have shown so good
an agreement with each other and with such records of solar
activity as we possess, that I cannot but feel that the climatic
fluctuations portrayed are definitely real and demonstrate the
solar control of climate in the absence of disturbing
geographical changes.
Apart from its meteorological interest, I hope that this part
of the work will prove of service to archaeologists and
historians. Mr Harold Peake, in his brilliant study of " The
Bronze Age and the Celtic World," found it necessary to call
in the aid of climatic changes in order to understand the
migrations of the " Aryans," which laid the foundations of
the present distribution of peoples in Europe and Western
Asia. Similarly, Ellsworth Huntington believes that the
rise and decline of the ancient Maya civilisation of Yucatan
can only be explained by changes in the climate, and there
are other examples. The literature of historic climates is,
however, so chaotic that the historian or archaeologists has little
inducement to trust himself to its mazes. A co-ordination
of the evidence was urgently needed, and this need I have
attempted to meet.
I wish to make acknowledgment to the Council of the
Royal Meteorological Society for permission to reprint parts
of several papers published in the Quarterly Journal, including
Figures i, 8, and 16 to 19, for several of which they have
kindly lent the blocks. Mr W. H. Dines, F.R.S., has kindly
accorded me permission to use his illustration of the heat
balance of the atmosphere (Fig. n). Messrs G. W. Bacon &
Co. have accorded permission for the use of their outline chart
of the globe on Mercator'^ projection as a basis for several of
PREFACE 13
the charts. Several friends have read parts of the manuscript
and have made valuable criticisms and suggestions, or have
assisted me with expert advice on different aspects of a subject
which is so wide that no one man can hope to master it.
C. E. P. B.
September 1926.
PREFACE TO THE SECOND EDITION
The twenty-two years which have elapsed since the pre-
pafation of the first edition have added to the literature,
among other works, Sir George Simpson's theory of variations
of solar radiation and Professor Zeuner's brilliant exposition
of the astronomical causes of the succession of glacial and
interglacial periods ; the explanation of the glacial succession
in the Quaternary now seems to rest with one or possibly both
of these theories. Neither of them accounts for the occurrence
of the Ice-Age as a whole or for the warm periods, and with
the piling up of objections to Wegener's hypothesis of con-
tinental drift, the case for the " geographical " theory has
been strengthened. There has been a general acceptance
of the idea of " glacial " and " non-glacial " climates which
forms the basis of that theory. In the post-glacial period the
principal changes of the past twenty years have been a new
conception of the climate of the Sub-boreal and the dating
of the beginning of the Sub-atlantic as 500 B.C. instead of
850 B.C., both of which make the post-glacial sequence more
intelligible. It is now possible to present a much more
complete interpretation of the changes of climate in the
historical period than could be given in 1926.
While the general plan of the book remains unchanged,
several chapters have been almost entirely rewritten. It
seemed unnecessary to reprint the Appendix describing the
mathematical theory of correlation, which is now widely
known and is available in many text-books on Statistics.
My thanks are due to the many authors who have sent me
copies of their publications dealing with climatic changes,
which have saved me much arduous search in libraries, to
Sir George Simpson and the Manchester Literary and
Philosophical Society for permission to reproduce Figures
9 and 10, to the publishers for bringing out a new edition in
the face of great difficulties, to Miss N. Garruthers for reading
the revision and making a number of valuable suggestions,
and especially to my wife for her great practical help and
encouragement during the course of the revision.
C. E. P. B.
February 1948.
13
Chapter
I.
Chapter
II.
Chapter
III.
Chapter
IV.
Chapter
V.
Chapter
VI.
Chapter
CONTENTS
PAGE
PREFACE TO THE FIRST EDITION . . . . . 5
PREFACE TO THE SECOND EDITION 15
INTRODUCTION. The Normal Climate of Geological Time . 2 1
PART I. THE CLIMATIC FACTORS AND THEIR VARIATION
" Glacial " and " Non-Glacial " Periods . 31
Pressure and Winds .... 46
The Circulation of the Oceans . . 68
Radiation from the Sun .... 89
Astronomical Factors of Climate . . 102
VI. The Absorption of Radiation by the Atmo-
sphere . . . . . .no
VII. The Effect of Cloudiness on Temperature 122
Chapter VIII. Continentality and Temperature . . 129
Chapter IX. Precipitation Rain, Snow, and Hail . 1 58
Chapter X. Mountain-building and Climate . . 177
Chapter XI. The Weather of the Warm Periods . . 192
PART II. GEOLOGICAL CLIMATES AND THEIR CAUSES
Chapter XII. The Geography of the Past . . .201
Chapter XIII. The Theory of Continental Drift _ . . 221
Chapter XIV. An Examination of the Climatic Evidence
for Continental Drift . . . .231
Chapter XV. The Climate of the Upper Carboniferous
Glacial Period 247
Chapter XVI. The Climate of the Quaternary . . 263
PART III. THE CLIMATES OF THE HISTORICAL PAST
Chapter XVII. The Nature of the Evidence . . .281
Chapter XVIII. Europe . .... 295
1 8 CONTENTS
PAGE
Chapter XIX. Asia 318
Chapter XX. Africa 329
Chapter XXI. America and Greenland .... 342
Chapter XXII. The Interpretation of Climatic Fluctuations
in the Historical Period . . . 359
APPENDIX I. The Geological Time-Scale . . . 379
APPENDIX II. Theories of Climatic Change . . . 384
INDEX ........ . 387
LIST OF ILLUSTRATIONS
PAGE
Fig. i. Cooling at edge of floating ice-cap . . . 37
Fig. 2. Temperature difference between " non-glacial "
and " glacial " climate ..... 44
Fig. 3. Mean pressure, January ..... 47
Fig. 4. Mean pressure, July ...... 48
Fig- 5- Winds over North Pacific, January . . . 57
Fig. 6. Winds over North Pacific, July .... 58
Fig. 7. Reconstruction of pressure and winds during a
warm period ....... 62
Fig. 8. Ocean currents in winter ..... 70
Fig- 9- Effect of increasing radiation on precipitation
and accumulation of snow . . . . 92
Fig. 10. Effect of two cycles of solar radiation on glaciation 93
Fig. 1 1 . Heat exchange of atmosphere . . . 1 1 1
Fig. 12. Average temperature distribution, January . . 130
Fig. 13. Average temperature distribution, July . . 131
Fig. 14. Isotherm of 32 F. in Tertiary and Quaternary . 135
Fig. 15. Change of temperature due to formation of an
island . . . . . . . .145
Fig. 1 6. Observed, and calculated temperature changes,
Litorina period, January . . . . .147
Fig. 17. Changes of level and land and sea distribution,
Quaternary . . . . . . .152
Fig. 1 8. Changes of temperature due to geographical
changes, January . . . . . 154
Fig. 19. Changes of temperature due to geographical
changes, July 155
Fig. 20. Tracks of depressions 160
Fig. 21. Variation of rainfall with latitude . . .165
Fig. 22. Relations of soil to temperature and rainfall 167
19
2O LIST OF ILLUSTRATIONS
PAGE
Fig. 23. Mountain-building and glaciation (schematic) . 178
Fig. 24. Geographical factors and temperature during
geological time ...... a 06.'
Fig. 25. The transition from a warm period to an 'ice-age . 218
Fig. 26. Carboniferous and Permian deserts . . . 237
Fig. 27. Mesozoic deserts . . . . . . .238
Fig. 28. Land and sea distribution, Cretaceous . . 240
Fig. 29. Geography of the Upper Carboniferous . . 248
Fig. 30. Ocean currents of Middle Permian . . . 252
Fig. 31. Variations of rainfall in Europe .... 299
Fig. 32. Variations of temperature in Europe . . . 311
Fig* 33- Variations of rainfall in Asia . . . .321
Fig. 34. Variations in the level of the Caspian . . . 323
Fig- 35- Levels of Nile. Flood stage and low-level stage . 330
Fig. 36. Variations of rainfall in Africa .... 339
Fig. 37. Variations of rainfall in U.S.A. .... 343
Fig. 38. Variations of rainfall, world .... 359
Fig. 39. Causes of climatic variations since 6000 B.C. . 361
INTRODUCTION
THE NORMAL CLIMATE OF GEOLOGICAL TIME
DURING some hundreds of millions of years with
which we are acquainted through the records of
the rocks, the surface of the earth has passed through
some strange climatic vicissitudes. At least four, probably
five or more times in its history great ice-sheets have spread
out from various centres, covering the plains and even filling
the shallow continental seas. There have been other periods
of somewhat less intense climatic stress, when perhaps only
small glaciers were able to develop among the high hills.
These strenuous episodes have been of great importance in the
development of the earth's living beings, but always they have
been brief, and always after them the earth has returned to
more genial conditions, which have endured for long periods.
Hence we may regard these genial conditions as the normal
state of affairs on this our earth, and the glacial periods as
episodes disturbing the normal climate for a brief time, as at
long intervals a passing cyclone disturbs the peaceful life of a
tropical island. Many references to these warm periods will
occur in the next few chapters, and it is necessary to have
some preliminary knowledge of the climatic conditions which
characterised them. I am therefore beginning this study,
not with the stir and strife of an ice-age, but with the everyday
life of the genial periods. We are not yet in a position to
discuss their meteorology, but we can see what the geologists
have to say about their general climate.
The first point to notice about these periods, however, is not
climatic, although we shall see later that it has a very great
beaming on their climatology ; it concerns the absence of
mountain ranges. Warm periods were without exception,
periods of low relief, while glacial periods occurred when the
earth's crust had been thrown into folds and ridges by great
internal convulsions. The processes of denudation wave
action on the coasts, running water on the slopes and in the
valleys, and blown sand in the %teppes and deserts go on
22 INTRODUCTION
all the time, and are constantly tending to level up the earth's
crust, and they are powerfully aided during the cold periods
by the action of frost and ice on the high ground. Against
them are set the internal mountain-building forces, whfch
act with great intensity for comparatively sh'ort periods at
great intervals. The forces of denudation are most powerful
when the land is highest ; following a period of mountain-
building the general level is reduced very rapidly at first, and
the normal stage of low relief is soon reached again.
During the periods of low relief also the sea generally
encroached on the land and the continental areas were greatly
reduced in size. This we shall see was another factor of
great climatic importance, first, because it did away with the
areas of intense winter cooling in the centres of the great
continents in middle to high latitudes ; and secondly, because
the extension of the seas allowed the warm ocean currents to
penetrate very readily into the polar regions through many
broad channels. This gives us the background for our
climatological study groups of broad low islands rather than
continents, with rounded hills instead of mountains, and wide
oceans extending through broad channels from pole to pole.
The next point is the comparatively small difference of
temperature between the tropical and the polar regions.
The marine fauna, and to a less extent even the land vegetation,
differ little whether we are in Greenland, in Yorkshire, or in
Central America. When the rich fossil faunas and floras
from high latitudes were discovered in the middle of the
nineteenth century and this similarity in the life from widely
different latitudes was first remarked, it was believed that
during the early geological periods there were in fact no
biological zones at all, and that life was really uniform over the
whole extent of the seas. Radio-activity had not been heard
of, and it was believed that the high temperatures of the earth's
interior indicated by volcanic eruptions were mainly a legacy
from the time, not long before the Cambrian, when the (jarth
was entirely molten. It was believed that for a long ime
after the formation of a solid crust, the condensation of water
as oceans, and the origin of life, this internal reservoir of heat
continued to make itself felt at the surface, and maintained
genial temperatures in all latitudes throughout the Palaeozoic,
the Mesozoic, and a large part of the Tertiary. The
INTRODUCTION 23
pre-Cambrian and Carboniferous glaciations were unknown, and
the apparently uniform, equable temperatures of these earlier
periods fitted excellently into this theory of a cooling earth.
It is now recognised, however, that the earth has owed its
surface warmth to the sun as far back as we can see into the
past, and that even in the warmest periods there must have been
zonal differentiation of climates ; the tropics were perhaps
a little warmer than they are to-day, while the polar oceans
and their shores had a temperature now found in temperate
latitudes. Since the deposits which have come down to us
were almost invariably formed in the sea or in the estuaries
of wide rivers, we know comparatively little of the plants which
grew in the interior of the continents in high latitudes ; what
we do know suggests that they were of hardier types than those
from the low coastal valleys.
Neumayr ( i ) was the first to bring forward definite evidence
of a zonal distribution of animals in earlier geological periods ;
his conclusions, as revised by V. Uhlig (2), distinguish five
faunal zones during the Jurassic : Boreal, Mediterranean-
Caucasian, Himalayan, Japanese, and South Andean but
the four latter all seem to be facies of a tropical zone which
is contrasted with the cooler boreal. The boundary between
these zones does not run strictly parallel with the lines of
latitude, but shows divagations which may reasonably be
attributed to ocean currents. Similarly, during other
geological periods, the faunas of different latitudes when
examined critically invariably show differences between
different latitudes which are best accounted for by a slow
decrease of temperature towards the poles ; for example,
during the Cambrian, the Archaocyathim of the Antarctic
show the same species as those of Australia, but in a dwarfed
and crippled condition. In the Lower Cretaceous we have
a clear division of the marine faunas into northern boreal,
Mediterranean-equatorial, and southern boreal, the latter
containing some species identical with those in the first, but
absent in the second. The principal difference between the
temperate and the polar faunas throughout the Mesozoic is
the absence or dwarfing of the corals in the latter.
Similarly, although a rich vegetation apparently extended
as near the present poles as the land surfaces permitted,
there are considerable differences* between the lower Tertiary
24 INTRODUCTION
floras of the sub- Arc tic regions and those of lower latitudes.
Thus E. W. Berry (3) records " the total dissimilarity between
the Canadian floras, which are a part of the [Upper Eocene]
Arctic flora of Alaska, Greenland, Iceland, Spitsbergen, etc.,
and the contemporaneous flora of the [Mexican] Gulf States."
The Arctic flora is found also in Northern and Eastern Asia,
so that its distribution covers an irregular area surrounding
the pole, the southern limit varying from 45 N. to the Arctic
Circle. The plants include poplar, willow, alder, birch,
hazel, beech, oak, plane, laurel, andromeda, ash, guelder
rose, cornel, magnolia, ivy, spindle-tree, buckthorn, sumach,
and hawthorn. All these are now found in some parts of the
cool temperate zone, and the assemblage, while presenting a
very different picture from the present life of the sub-Arctic
regions, cannot be described as sub-tropical. Berry considers
that the identifications of palms, etc., which have given rise
to the ideas as to very high polar temperatures in the early
Tertiary, are erroneous. The most northerly Eocene flora
at present known is from near Cape Murchison in Grinnell
Land, latitude 71 55' N. ; Berry reduces the list of Heer's
determinations of this flora to the following : horse-tail,
yew, pine, spruce, poplar, birch, hazel, sedge or grass, and
apparently water-lily but he remarks that even this flora is
sufficiently remarkable when we consider the scantiness of the
present vegetation of the region, and would require the
present isotherms to swing 15 or 20 northward.
Not only has the northern boundary of this cool temperate
flora moved southward since the Eocene, but so also has its
southern boundary, the shift being about 10 of latitude.
This suggests that the tropical as well as the polar regions
were warmer in the Eocene than they are at present, though
not to the same extent, and that definite climatic zones were
in existence at that time.
The third point to notice about the warm periods is their
general aridity. Deserts have apparently existed throughout
geological time, but during most of the warm periods, and
especially in the Mesozoic, they expanded greatly, extending
from the sub-tropical regions far into the present temperate
zones. Among the most remarkable of these " fossil deserts "
we have the " Old Red Sandstone " of the Devonian period,
consisting mainly of inlancf lake or lagoon deposits formed
INTRODUCTION 25
under arid or semi-arid conditions. This regime extended
from Ireland across Britain to Southern Norway, Poland,
Cgurland, and the White Sea, and similar deposits are found
in Nova Scoti^, New Brunswick, Canada, and the north-east
of the United States. This region was not altogether rainless,
but the rain came in occasional heavy showers which caused
brief floods in the water-courses, carrying large quantities of
coarse detritus into the lakes and depressions. The desert
character of the land was accentuated because the specialisation
of the plants for life on dry land had scarcely begun, and it is
quite likely that regions with a similar climate at the present
day would have a moderately rich vegetation. They have
therefore been described as biological rather than climatic
deserts.
Desert deposits are again well developed in the Permian,
in Britain, France, Germany, and the Tyrolese Alps. During
this period a great inland sea was formed over a large part of
Europe ; since evaporation exceeded rainfall this sea became
highly saline and deposited thick layers of salts. These show
in part regular annual layers, gypsum, which is more soluble
in cold than in warm water, being formed in summer, while
in winter its place was taken by rock salt. Kubierschky,
quoted by Koppen and Wegener, on the basis of similar
phenomena in the Sahara, estimates the annual range of
temperature as between 60 and 95 F. The number of annual
layers indicates that the salt lake existed for some 10,000 years,
after which the salt deposits were covered by a layer of desert
sand.
In the Triassic we have a very wide development of desert
formations, especially in Western and Central Europe and in
the east of the United States, Texas, Colorado, and Idaho.
Many of these may have been biological rather than climatic
deserts, but the occurrence of numerous salt deposits, especially
in Central Europe, shows that the rainfall was less than the
evaporation over wide areas. In the Jurassic and Cretaceous
there is less evidence of extensive deserts, partly because the
climate was not so arid as that of the Triassic, but partly
because the land vegetation had evolved more specialised
types which were able to exist without the constant presence
of water. Occasional salt deposits show, however, that the
climate of Europe was still rather* dry. During the Tertiary,
26 INTRODUCTION
the Eocene period shows a return of moister conditions, but
the Oligocene and, to a less extent, the Miocene, were again
dry over much of Europe. The Mediterranean regions had
for a time a true arid climate, while Egypt wjis for long an
absolute desert, but in Central Europe, although the summer
was hot and dry and was prolonged into autumn, there was
heavy rainfall of the thunderstorm type in spring, which tore
leaves and twigs from the trees and bushes and carried them
to the lakes, where they were buried in mud. The winter
was generally mild, and there were many evergreens, but some
of the Miocene leaves from Central Europe show traces of
frost action, indicating that there were occasional cold nights.
Generally speaking, the climatic zones in Europe lay 10 or
15 degrees north of their present position.
In Western U.S.A., where the succession of events has been
worked out in great detail, the Eocene and early Oligocene
enjoyed a sub-tropical climate with ample rainfall, becoming
temperate farther north in Alaska. In middle Tertiary the
climate became increasingly cooler and more arid, especially
to the east of the Rocky Mountains, this process culminating
in the cool semi-arid climate of the Pliocene.
It is probable that one of the most momentous steps in the
history of life was taken during a period of drought, namely,
the origin of air-breathing land vertebrates. A similar
development on a smaller scale was the evolution of the Lung-
fish (Ceratodus], adapted to breathe both air and water, which
to-day inhabits regions subject to alternating floods and
droughts. Ancestral forms appeared in the Devonian, and
the true lung-fish as early as the Permian ; it was widely
distributed in the Triassic of Central Europe.
Thus it will be seen that the predominant features of the
normal geological climate were warmth and dryness. Broadly
speaking, the polar regions had the climate of the present
temperate belts, while the latter had the climate of the sub-
tropics. In the latitude of the British Isles the rainfall oame
almost entirely in the form of brief, heavy showers, of the type
associated with thunderstorms, and the steady but gentle rains
which are characteristic of our winter months did not occur.
These rains are associated with the passage of extensive
barometric depressions or cyclones, and from their absence
we can infer that during ttc warm periods such depressions
INTRODUCTION 27
either did not occur, or were limited to high latitudes. Thus
the meteorology of these warm periods was very different
from that of to-day, and it is these differences which we have
to analyse and, if possible, account for, in the following pages.
REFERENCES
(1) NEUMAYR, M. " Uber klimatische Zonen wahrend der Jura- und Kreidc-
zeit." Wieri, Denkschr. K. Akad. Wiss., Math. nat. KL, 47, 1883, p. 277.
(2) UIILIG, V. " Die marinen Reiche des Jura und der Unterkreide." Wien,
Mitt. Geol. Ges., 4, 1911, p. 329.
(3) BERRY, EDWARD W. " A possible explanation of Upper Eocene climates. "
Proc. Amer. Phil. Soc. 9 61, 1922, p. i.
PART I
THE CLIMATIC FACTORS AND THEIR
VARIATION
CHAPTER I
" GLACIAL " AND " NON-GLACIAL " PERIODS
IN the preceding Introduction we saw that one of the
characteristics of the " warm " periods which have formed
the major part of geological time was the small temperature
difference between equatorial and polar regions, small, that
is, relative to the present difference. This at once introduces
a difficulty which the meteorologist feels very acutely in
attempting to account for geological climates, for the basis
on which the existence of the major climatic zones depends
is not to be found in any surface features of the earth, which
might have been different in some former geological period,
but in the fact that the earth is very nearly spherical. So
long as the axis of rotation remains in nearly its present position
relative to the plane of the earth's orbit round the sun, the
outer limit of the atmosphere in tropical regions must receive
more of the sun's heat than the middle latitudes, and the
middle latitudes more than the polar regions ; this is an
inviolable law. It is not difficult to think of causes, such as
a change in the heat of the sun, or the formation of a veil of
volcanic dust in the atmosphere, which will slightly raise or
lower the mean temperature of the earth as a whole, or change
the temperature of equatorial regions more than that of the
polar regions ; it is much more difficult to think of a cause
which will raise the temperature of polar regions by some
30 F. or more, while leaving that of equatorial regions almost
unchanged, and so bring about an approach to the distribution
of climatic zones during the warm periods.
Let us consider what is the essential difference between
polar and equatorial regions to-day. We can say that the
mean temperature of the polar regions is 20 F., and that
of the equatorial regions 80 F., and the difference between
20 and 80 is very striking. But the zero on the Fahrenheit
scale is purely arbitrary ; the true zero of temperature is at
459 F., or to adopt the units employed by physicists, 273
C., and if we express these temperatures on the absolute
32 CLIMATE THROUGH THE AGfci>
scale the figures become polar, 266 A, equatorial, 299 A,
and the difference does not look so alarming. The essential
difference is not between 20 and 80 F., but between the fact
that the former is below the freezing point of water and ihe
latter is above it. The real difference betweeh the centre of
Africa and the centre of Antarctica is that in the former,
water is water and in the latter, water is ice. Similarly, the
essential way in which the polar regions during the warm
periods differed from the polar regions at present was that
they had no great ice-sheets and no floating ice on the sea, that
water was water, and they were " non-glacial."
The winter cold of Siberia is well known, and is explained
in the geographical text-books by the statement that the
climate is " continental." With the coming of the short days,
in which the sun has little heating power, and the long nights,
the surface of the ground loses its heat very rapidly, and soon
falls below freezing point. At a depth of a foot or two the
soil may be much warmer, but heat passes very slowly through
the ground, and this underground store of heat does not help
appreciably to keep up the temperature of the surface. Soon
snow falls on the frozen ground ; snow, too, is a very poor
conductor of heat, and moreover its white surface reflects
four-fifths of the sun's heat which falls on it ; it absorbs heat
very slowly and loses heat very readily, hence the temperature
falls still more rapidly, until life becomes barely supportable.
Compare this with the conditions in the centre of the Pacific
Ocean in the same latitude. As the surface layer of water
cools, the winds and waves, aided by convection, mix it with
the underlying water, and in this way the cooling which was
practically limited to a few inches of soil or snow over the
land, becomes spread through many feet of water in the oceans.
This means that the sea surface cools very much more slowly
than the land, quite apart from the fact that water has a higher
heat capacity than soil or rock. But when water freezes and
loses its mobility, it takes on some of the properties of land,
and when snow falls on the ice, we have a surface which is
indistinguishable from the surface of the snow-covered land.
Thus an Arctic Ocean covered by a snow-encrusted layer of
floating ice acts almost as a northward extension of the con-
tinents of Asia and America. The temperature does not fall
quite so low as in Siberia, partly because, the surface being
" GLACIAL " AND " NON-GLACIAL " PERIODS 33
more level, the cold air can escape more readily, and partly
because a little more heat finds its way from below through
ice than through the subsoil, but a temperature of 62 F. was
recorded over the ice in March 1894 during Nansen's " Fram "
expedition.
But the cooling power of a snow-covered surface is not
limited to the surface itself; the winds spread its influence
over the surrounding land or ocean. The winds blowing
off the snow of Canada lower the mean temperature of the
whole of North America except the Pacific coast, and this
cooling allows the snow-cover to extend much farther south
than it would do if, for instance, these cold winds were held
off by a range of mountains extending from east to west.
Investigations to be described in detail later (Chapter VIII.)
have shown that if in the middle of a large open polar ocean
a circular cap of ice were formed, with its centre at the pole
and its southern edge in latitude 80 N. (that is, having a
radius often degrees of arc or about 690 miles), the temperature
would be lowered by about 45 F. at the pole and about
20 F. in latitude 80 on the edge of the ice, while the cooling
would still be quite appreciable at a distance of 500 miles
south of the ice-edge, across the open ocean.
Now let us suppose (after Brooks (i)) that the earth is
entirely covered by water for a distance of more than 2,000
miles from the North Pole, and further suppose that the surface
of the water at the pole itself is just warm enough to prevent
freezing, while the temperature rises outwards from the centre
at a uniform rate. Now suppose that a small uniform decrease
of temperature occurs over the whole ocean. This results in
the formation of a mass of floating ice in the centre, which
will extend to the limit of the area reduced below the freezing
point by the initial fall of temperature. This ice itself exerts
an additional cooling effect on the air, not only above the ice,
but for some distance round it, so that a further area of the
sea surface has its temperature reduced below freezing point
and becomes converted into ice. The ice-cap accordingly
extends beyond the limits of the area of freezing due directly
to the initial fall of temperature. Equilibrium is reached only
when the initial decrease of temperature, plus the cooling effect
exerted by the ice-cap at a point on its edge, is balanced by the
rise of temperature due to increasing distance from the pole.
34 CLIMATE THROUGH THE AGES
Now we come to the consideration that while the rise of
temperature due to the horizontal temperature gradient is
proportional to the distance from the pole, the area of the
ice-cap increases with the square of its radius. Consequently,
if the cooling effect of an ice-cap at a point on its edge varies
directly with its area, the cooling effect is small at first, but
increases more and more rapidly as the ice-cap grows in size,
while the counteracting effect of the normal horizontal
gradient increases at a uniform rate. Sooner or later a point
is reached at which a further growth of the ice-sheet causes a
greater lowering of temperature on its edge than can be
neutralised by the horizontal temperature gradient, and beyond
that point, in the conditions postulated, the ice-cap must
continue to grow indefinitely. The critical point at which
the ice-cap becomes unstable depends on the numerical values
assigned to the cooling power of ice and to the horizontal
gradient.
This conclusion is so important, underlying as it does the
whole theory of climatic changes as set out below, that I may
be excused for dwelling on it at some length. First, is the
cooling power of an ice-cap on its edge proportional to its
area ? The winter cooling by floating ice is quite comparable
to the winter cooling due to the presence of land in high
latitudes, since in winter the land is almost invariably snow
covered. It is shown in Chapter VIII. that with increasing
land area, the cooling effect exerted by each square kilometre
of land in winter actually increases at first with the increase
in total area, so that by doubling the area we more than
double the cooling. In the centre of a round island the
maximum cooling effect per square mile of total area occurs
when the island has a radius of about ten degrees of arc ;
beyond this the more distant parts cease to exert their full
effect, and while the total cooling effect continues to increase
with increasing area, the average effect per square mile begins
to decrease. On the edge of a large island the maximum
effect is exerted only by that portion which lies within a circle
of ten degrees radius centred at a point on the edge ; when the
radius of the island exceeds eight degrees, any further increase
of radius makes a comparatively small difference in the area
within this ten-degree circle. We shall not be far out if we
assume that the fall of teniperature due to the ice at a point
" GLACIAL " AND " NON-GLACIAL " PERIODS 35
on its edge is directly proportional to the area of ice included
in a circle of ten degrees radius, drawn round that point,
everything outside that circle being ignored. As the average
effect per square mile of land between ten and twenty degrees
away is only one-fifth of the average effect of land inside the
circle of ten degree radius, this simplification is justified.
The next point is to determine the critical temperature
given by definite values of horizontal gradient and cooling
power of ice. We still start with a simple numerical example.
Let us put the cooling power of a floating ice-cap as 0-5 F.
for each one per cent, of a circle of ten degrees radius which
is occupied by ice. Then on the edge of the ice-cap the
lowering of temperature due to the ice will be 0-5 F. when it
has a radius of one degree ; four times 0-5 or 2-0 F. when
it has a radius of two degrees; nine times 0*5 or 4-5 F.
when it has a radius of three degrees, and so on. We will
suppose that before the ice-cap developed the temperature
increased uniformly outwards from the pole at the rate of i F.
for each degree of latitude. Then, taking the freezing point of
sea water as 28 F. and supposing that initially the sea at the
pole was just on the point of freezing, we have the following
distribution of temperatures before the formation of the ice-
cap and at different stages of its growth :
Distance from pole, degrees
of latitude o i 23 45
Initial temperature, F. . 28 29 30 31 32 33
Cooling due to ice-cap, F. o 0*5 a 4-5 8 12-5
Temperature on edge of
ice-cap, F. . . . 28 28*5 28 26-5 24 20-5
Table i . Temperature at edge of floating polar ice-cap.
This table shows that the floating ice-cap will experience most
difficulty in establishing itself; once it has reached a certain
size the temperature on its edge will be below the freezing
point of sea water, and it will continue to expand owing to
the lowering of temperature which the ice itself introduces.
Suppose that, starting with a temperature of 28 F. at the pole,
we have a general fall of temperature by 0-5 F. An ice-cap
will form and will grow until it has a radius of one degree of
latitude. At this stage the temperature at its edge, at a distance
36 CLIMATE THROUGH THE AGES
of one degree from the pole, which was originally 29 F., will
have been lowered 0-5 F. by the initial cooling and a further
0-5 F. by the ice, a total decrease of i F. The water on
the edge of the ice will therefore be exactly at freezing point ;
lower the general temperature a little more, and the ice-cap
will continue to grow until it reaches very large dimensions ;
lower it a little less, and the ice will extend less than one
degree from the pole. This result is not dependent on the
special numerical values which have been taken. Suppose
for instance, that the cooling power of ice is only 0-25 F.
for each one per cent, of a ten-degree circle, instead of 0-5 F.
as before. Then the critical radius of the ice-cap is four
degrees of latitude instead of one degree, and for it to grow
to large dimensions the initial lowering of temperature must
exceed i F. instead of 0-5 F.
Let us now put the matter quite generally. Let the horizontal
gradient of temperature be h degrees per degree of latitude
and let the cooling power of ice be k degrees for each one per
cent, of a circle with a radius of ten degrees. An initial small
fall of temperature by / degrees will cause the formation of an
ice-cap with a final radius greater than t/h ; call this radius
R, where R is measured in degrees of latitude. The cooling
power of this ice-cap is A:R 2 , and in a condition of equilibrium
the total cooling t+kR 2 is balanced by the horizontal increase
of temperature AR. Thus we have the equation :
the solution of which is
When =o, R=o, so that only the negative value of the root
term is required here. The critical point occurs when
4^= h 2 , or th 2 l^k ; for values of t above this amount the
equation has no solution, and on the assumptions made the
ice-cap has no finite limit.
The average value of the horizontal gradient of air
temperature in January (K) which would prevail over an ice-
free ocean between 80 and 90 N. latitude is difficult to
estimate, but would probably be of the order of 0-5 C.
(0-9 F.) per degree of latitude. The cooling power of land
in winter in high latitudes is approximately 0-5 C. (0-9 F.)
GLACIAL AND NON-GLACIAL PERIODS
37
for one per cent, of a circle with a radius of ten degrees ; the
cooling power of a continuous surface of thick ice would be
nearly the same, but where the ice is interrupted by lanes of
open water caused by ocean currents the cooling power is less,
and we may take k as 0-25 C. (0-45 F.) for one per cent.
Hence the critical value of t for which the ice-cap becomes
unstable is
h 2 0-81
4# 4 x o 45
at which stage the radius R is one degree of latitude. We
can now calculate the total cooling on the assumption that
the initial fall of temperature is o 6 F. and that ice distant
from any point more than ten degrees of arc has no influence
on the temperature of that point.
The results are as follows :
Radius of ice-
cap R, in . o i 2 3 4 5 6 8 10 15 20 25
Total cooling
on edge, F. 0-6 1-05 2-4 4-6 7-8 n-8 14-5 17-2 18-2 20-4 21-3 22-2
Rise due to
normal hori-
zontal grad-
ient A, F. . o-o 0-9 1-8 2-7 3-6 4-5 5-4 7-2 9-0 13-5 i8'O 22-5
Table 2. Cooling at edge of ice-cap.
JS
^ 5
^ 10
20
25
'
5
i
10
Fig. i .
10 15 20 25
of Ice Cap in Degrees
-Cooling at edge of floating ice-cap.
25
These results are shown graphically in Fig. i, in which the
curve represents the cooling on the edge of the ice, while the
inclined straight line represents the normal horizontal gradient.
Over almost the whole of th^ diagram the curved line
38 CLIMATE THROUGH THE AGES
representing cooling is below the straight line representing the
normal rise of temperature with increasing distance from the
pole, and the two do not meet until the ice-cap attains a radius
of nearly twenty-five degrees of latitude. Hence an inifial
winter cooling to 0-6 F. below the freezing point will result
in the formation of a floating ice-cap with a radius of nearly
twenty-five degrees.
We have now to consider to what extent this ice-cap will
melt in summer. Let us suppose that the floating ice-cap has
been formed in winter owing to a depression of the temperature
to 24 F. or 4 F. below the freezing point of sea water. When
summer comes, there is a general warming up ; let us suppose
(in accordance with a calculation by F. Kerner (2)) that under
normal or " non-glacial " conditions the summer temperature
would be 13-5 F. above that of winter, or 9-5 F. above the
freezing point. The edge of the ice-cap, which was just at
freezing point in winter, will now rise above freezing point,
and the ice will begin to melt. But although the " non-
glacial " temperature, even at the pole, is now above the
freezing point of water, this does not mean that the ice-cap will
necessarily entirely vanish. As the ice melts back towards
the pole, its cooling effect at a point on its edge decreases
slowly very slowly at first and on the edge of an ice-cap of
ten degrees radius it is only 4 F. less than on the edge of an
ice-cap of 25 degrees radius. But the natural or " non-
glacial " temperature decreases towards the pole at the rate
of O'9 F. for each degree of latitude, and in a distance of
15 degrees or 1,035 m il es the temperature will have fallen
from this cause by 13-5 F. This is exactly equal to the
9-5 F. by which the summer temperature rises above the
freezing point, plus the 4 F. due to the decrease in size of the
ice-cap, and by the time the ice-cap has melted back to a
distance of ten degrees from the pole, the temperature at its
edge will again be at freezing point and it will be unable to
melt back any farther. Hence, so long as the amount <?f the
summer warming does not exceed the greatest difference
between the curved line and the straight line in Fig. i, the
ice-cap will not entirely disappear even in summer ; instead
a core of ice will remain from which the ice-cap will spread
out again in the following winter. It is evident that this
central core will in time cdme to consist of very thick old ice,
cc
GLACIAL " AND " NON-GLACIAL " PERIODS 39
while the outer region in which melting takes place in summer
will consist only of thinner ice of one season's growth.
In order that the ice-cap may entirely disappear in summer,
{Tie summer temperature must rise so far above the freezing
point that the curved line in Fig. i, representing the cooling
at the ice-edge, lies entirely above the straight line representing
the normal horizontal gradient of temperature. From Table
i, we see that these lines are 10 F. apart when the ice-cap
has a radius of eight degrees, and in order that the ice may
disappear in summer, the summer temperature must rise above
the freezing point by at least 10 F. Allowing an annual
range of i3*5F., this means that the winter temperature
must be above 24-5 F.
Fortunately this hypothesis as to the conditions under which
an extensive floating ice-cap develops does not rest entirely
on theoretical arguments. The assumptions made represent
very fairly a generalisation of the Arctic Ocean, and we can
confirm the conclusions reached by reference to the existing
ice-conditions in that ocean. In winter, with the exception
of an embayment west of Norway kept open by the Gulf
Stream Drift, the whole ocean north of the Arctic Circle is
covered by floating ice, much of which breaks up in summer.
Inside the outer or winter limit is another or summer limit
representing the area over which there exists a solid mass of
very thick ice (the " Palaeocrystic Ice ") which never melts,
winter or summer. The limit of this inner ice-cap has an
average latitude of about 78 degrees, or 12 degrees from the
pole, and this may be taken as representing the limit of the
region of maximum depression of temperature below freezing
point.
It may be remarked in parenthesis that this theory of a
critical temperature applies equally well to the case of an
ice-sheet over land, and affords a satisfactory explanation
of the rapid growth and decay of the Quaternary ice-sheets
at certain stages of their existence.
The critical point for an ice-sheet on a conical continent
with a surface slope of i in 1,000 (e.g., height 2,000 metres,
radius 20 degrees of latitude) occurs when the radius of the
ice-sheet is about one hundred miles. Once it extends beyond
this limit, it will grow rapidly until it attains a radius of more
than 500 miles, and then more slowly to a much greater radius.
4O CLIMATE THROUGH THE AGES
Conversely, during a period of amelioration of climate, a
large ice-sheet will decrease slowly in size until it attains a
radius of rather less than ten degrees (600 miles), after which
it will shrink rapidly, even without any further change *of
climate, until its radius is less than one hundred miles. This
is exactly what happened in Scandinavia during the closing
stages of the Quaternary Ice-Age. The ice-sheet withdrew
slowly at first, until the distance from the centre to the southern
edge was about nine degrees ; after this the retreat became
more and more rapid until the ice-sheet had sunk to in-
considerable dimensions. Here again the concordance of the
theory with the observations is as good as could be wished.
From this discussion we can see that if over an open polar
ocean the winter temperature at the pole falls only as much
as 0-6 F. below the freezing point of sea water, an ice-cap
will develop, which will extend rapidly until it reaches a
latitude of about 78. From this stage it will grow more
slowly to about 65, unless in the meantime its growth is
arrested by land, by the seasonal change of temperature, or
by a warm ocean current. The ultimate lowering of the
winter temperature brought about by the initial small fall of
temperature of 0-6 F. will amount to about 45 F.
This estimate of the cooling effect may appear large,
but consider the difference of conditions a little way below
and a little way above the critical point. Below the critical
point all the cooled water remains on the surface in the form
of ice, spreading freezing temperatures all round it, and even
when the ice melts, the cold thaw water, owing to its com-
parative freshness, is lighter than the warmer more saline
water and remains on the surface, freezing again very readily.
Fresh snow falls on the ice and increases its thickness, until the
surface of the central parts of the Arctic Ocean resembles that
of the Antarctic Ice Barrier, with correspondingly low tem-
perature. On the other hand, above the critical point, the
water as it cools becomes denser and sinks to the bottom,
where it drains away as a ground current, to be replaced by
warm surface currents from lower latitudes. Every inlet is
flooded with warm water, no ice can form, and the temperature
never sinks below the freezing point of sea water. The differ-
ence between these two pictures shows that they may easily
have a temperature difference of 45 F,
" GLACIAL " AND "NON-GLACIAL" PERIODS 4!
This good agreement between the conditions found in the
Arctic Ocean and the conditions deduced from theory suggests
that the " non-glacial " winter temperature of the Arctic
Oean may not be far below the freezing point of sea water,
and that if the Arctic ice could once be swept away, it might
find some difficulty in re-establishing itself. It therefore
becomes an interesting speculation to try to determine what
the " non-glacial " temperature of the Arctic would be. The
problem may be stated thus : to determine what would be
the distribution of temperature if sea water, with all its other
properties unchanged, reached its freezing point and point
of maximum density at a very much lower temperature than
28 F. This problem may be attacked in two ways. One
solution, which is due to F. Kerner (2), starts with an attempt
to determine the temperature distribution over an entirely
open ocean, and then superposes on this the effects of the
present land and sea distribution. Kerner considers that
there are only two regions in which the oceans are free from
the effects of floating ice, namely, the seas of the East Indies
and the centre of the North Pacific anticyclonic area, and
he takes the temperatures of the sea surface in these two regions
as the real oceanic temperatures which would obtain if there
were no transference of heat across the parallels of latitude by
ocean currents. From these temperatures, taking account
of the influence of solar and terrestrial radiation, he calculates
by various methods the " akryogenous " (i.e., " non-glacial ")
temperature of every tenth parallel of latitude, and obtains
for the North Pole an average annual temperature of 28 F.
He computes the annual range of temperature as i3'5F.,
and this gives him for the January temperature 2i'3F.,
which he regards as the lowest limit for the temperature of an
open ocean near the pole in the absence of ocean currents
crossing the lines of latitude. (This, of course, is well below the
freezing point of sea water, but we have guarded against this
contretemps by a suppositions change of the freezing point.)
Kerner next analyses the present distribution of temperature
in January along the 75th parallel of latitude, and obtains a
geographical formula which expresses the temperature of any
point in this latitude in terms of the land and sea distribution.
The geographical effects are twofold ; an elevation of tempera-
ture by the Gulf Stream Drift, and* a depression of temperature
42 CLIMATE THROUGH THE AGES
owing to the cooling effect in winter of the large land-masses
which nearly surround the Arctic Ocean in about 70 N.
From Kerner's discussion it appears probable that the Gulf
Stream effect is slightly the larger, and that the " non-glaci51 "
temperature near the pole in an Arctic ocean with the present
configuration would be a degree or two above the figure of
21 '3 F. calculated for the open ocean.
The second way in which a rough calculation of the " non-
glacial " temperature of the North Pole may be made depends
on my investigations of " continentality and temperature "
described in Chapter VIII. , and referred to earlier in this
chapter. By comparing the distribution of temperature with
the distribution of land and of ice, I obtained measures of the
thermal effects of land and of ice, in January and in July in
different latitudes. These enabled me to eliminate the land
and ice effects, but not the effect of ocean currents crossing
the parallels of latitude, and consequently gave me the
temperature of an open ocean under " non-glacial " conditions,
with a steady interchange of water between equator and pole.
The calculation was carried up to 70 N., but the figures
calculated for January are very erratic. The results for July
are more regular, since in that month the interference by ice
is much less than in January, and the July figures gave :
Latitude, N 40 50 60 70
Mean July temperature, F. 64-6 55 -o 45-9 43*3
By extrapolation the July water temperature of 90 N. would
be about 40 F., and since there is very little land within 20
of the North Pole, we may take the figure of 40 F. to represent
the " non-glacial " July temperature of the North Pole with
the present land and sea distribution. The annual range
over an open ice-free polar ocean being 13*5 F. according
to Kerner's calculation, the January " non-glacial " tempera-
ture would be about 26*5F. There are, however, many
objections to this method of calculation, and it cannot give
us more than a rough approximation.
Taking account of all three sources of information, the
ice-conditions in the Arctic which suggest a January " non-
glacial " temperature of not more than 24 F. (p. 39),
Kerner's calculation of not less than 21-3 F., and my own
rough calculation of 26'5*F., we find that the first of these
" GLACIAL " AND " NON-GLACIAL " PERIODS 43
figures, 24 F., is probably not far from the truth. This gives
us some very interesting information about the stability of
the present climatic regime. Quite a small rise in the general
temperature of the earth, say, 2 F., would suffice to make the
whole ice-sheet unstable in summer. This does not necessarily
mean that it would completely break up and disappear during
a single summer. The Palaeocrystic ice is so thick and
extensive that in order to melt it a very large amount of heat
would be required. Hence the most that could happen
during a single abnormally warm summer would be that,
after the fringe of thin ice of one winter's growth had been
broken up, a border a few miles wide round the Palaeocrystic
ice might be attacked before the winter cold came again to
reverse the process. But suppose that there occurred a real
and permanent change in the general climatic conditions,
so that the warm summers came year after year, alternating
with mild winters in which the outer fringe of ice did not form
again with quite its old thickness or extent. Under these
conditions, aided by the gradual drift of the whole ice-mass
across the Arctic Ocean, the Palaeocrystic ice would gradually
break up and be replaced by much thinner and looser ice.
After this stage had been reached it might be possible for the
ice to disappear completely during favourable summers,
forming again in the following winter.
Such a " semi-glacial " condition would require a nice
adjustment of temperatures, for it seems that it would be
possible only if the temperature in winter fell so little below
the critical figure of 27'5F. that the initial stages in the
formation of the ice-cap took place very slowly. If the worst
of the winter were over by the time the ice-cap had attained its
critical radius of one degree, the subsequent extension beyond
this radius would be small and weak, and the ice-cap would
be readily dissipated in summer. It will be shown in Part III.
that this condition in which ice formed in winter but dis-
appeared in summer, probably existed during the fifth to
seventh centuries of the Christian era.
If the general warming up went a little farther, so that the
" non-glacial " winter temperature at the pole rose above
the freezing point of sea water, the process would at first be
very similar, though presumably more rapid. The Palaeo-
crystic ice would melt back farther and farther each summer,
44
CLIMATE THROUGH THE AGES
but so long as it persisted with a radius exceeding one degree,
an extensive ice-cap would form again in the winter. Finally,
however, there would come a summer in which the ice-cap
completely disappeared, and in the following winter it wo&ld
not form again ; the climate would become definitely
" non-glacial."
Apart from the transitional or " semi-glacial " stage,
which can have occurred but rarely in geological times and
then only for short intervals, only two types of oceanic climate
70
60
50
40
X*
| 30
1*20
I-
-20
90 60 70 60
Latitude, Degrees
50
Fig. 2. Temperature difference between " non-glacial "
and " glacial " climate.
are possible, the " glacial " and the " non-glacial/ 3 and these
are very wide apart. The former is characterised by a very
rapid decrease of temperature in high latitudes, the latter
by a slow decrease. In order to show this, I have constructed
Fig. 2, which gives the variations of temperature between
latitudes 50 and 90. The straight line represents conditions
in winter over an open ocean in which the temperature falls
at the rate of o 9 F. for every degree of latitude, the water
at the pole being one degree above the freezing point. The
curve below it represents the temperatures over the same
" GLACIAL " AND " NON-GLACIAL " PERIODS 45
ocean in winter which would be the final result of a general
initial fall of temperature by 5 F., causing the formation of
an ice-cap in the way described above. The cooling power
of ice is taken as 0-45 F. for each one per cent, of a ten-degree
circle. The original fall of temperature by 5 F. has increased
to 50 F. in high latitudes as the result of the formation of
the ice-cap.
This result greatly simplifies the problem presented by
warm polar climates. Instead of having to account for
changes of temperature of the order of 50 F. we have only
to account for initial changes of 5 F. or so, since we can
safely leave the floating ice to make up the odd 45 F. Before
we are in a position to begin a serious search for the causes
of these greatly reduced changes, however, it will be necessary
to carry our preliminary studies a little further, and obtain
some information about the winds and ocean currents, and the
way they may have been modified by the change from a
" glacial " to a " non-glacial " climate.
REFERENCES
(1) BROOKS, C. E. P. " The problem of mild polar climates." London,
Q,. jf. /?. Meteor. Soc., 51, 1925, p. 83.
(2) KERNER, F. " Das akryogene Seeklima und seine Bedeutung fur geolo-
gischen Probleme der Arktis." Wien, Sitzungsber. Akad. Wiss., 131,
1922, p. 153.
CHAPTER II
PRESSURE AND WINDS
THE weather at any place on any day is mainly
governed by the winds which are blowing at the
time ; similarly, the climate is largely determined by
the winds which blow most frequently. The winds in one
place are closely related to the winds at other places, the
whole forming a more or less orderly system which ultimately
depends on the differences of temperature between different
latitudes. This system of winds over the earth as a whole is
known as the circulation of the earth's atmosphere. Since
the winds are bound up with the distribution of pressure, their
discussion must also include the distribution of pressure in
the various seasons.
Figs. 3 and 4 show the average pressure distribution, reduced
to mean sea-level, prevailing at present in January and July
respectively. In January there is a belt of relatively low
pressure (below 1,012 millibars) extending the whole length
of the equatorial regions, with local minima over the
continents south of the equator (South Africa, South America,
and Australasia). On either side of this low-pressure belt are
the sub-tropical high-pressure belts between 20 and 40
latitude. The high-pressure belt in the Northern Hemisphere
is especially well developed ; in addition to the maxima
over the oceans there are marked anticyclones near the
centres of the continents. In the Southern Hemisphere
the anticyclones lie entirely over the oceans, with their centres
rather nearer the eastern than the western shores.
On the poleward sides of these high-pressure belts lie
marked areas of low pressure. In the Northern Hemisphere
these are in the form of isolated depressions over the oceans.
One area, known as the Aleutian low, is centred near the
Aleutian Islands in latitude 52 N. ; the other, the Icelandic
low, lies west-south-west of Iceland in 60 N., and extends in
a long tongue towards the Arctic Circle between Norway and
46
PRESSURE AND WINDS
47
4 8
CLIMATE THROUGH THE AGES
PRESSURE AND WINDS 49
Spitsbergen. In its centre the average pressure is below
996 mb. In the Southern Hemisphere, where the Southern
Ocean is not broken up by land-masses, the area of minimum
pressure is remarkably deep, and extends in a continuous
belt completely round the globe in about 60 S. ; it is,
however, accentuated in the Ross Sea and Weddell Sea. On
the poleward sides of these minima, pressure rises again,
especially towards the Antarctic continent ; in the Arctic
Ocean the polar increase is masked by the Siberian and
American anticyclones.
In the chart for July (Fig. 4) the relative distribution over
the two hemispheres is partly reversed. The equatorial
low-pressure belt is still shown, but the lowest pressure is now
found north of the equator, near Jacobabad in North-western
India, about latitude 30 N. In the Northern Hemisphere
the sub-tropical high-pressure belt is limited to two areas over
the Atlantic and Pacific Oceans, but in the Southern Hemi-
sphere it is nearly continuous in about 25 S. The Icelandic
minimum still persists with greatly reduced intensity, but the
Aleutian low has disappeared ; on the other hand, the low-
pressure belt in the Southern Ocean is very sharply defined.
As in January, pressure rises again near the poles.
If the earth were not rotating about its axis, air would
simply flow directly from areas of high pressure to areas of
low pressure. Owing to the rotation the air is deflected, to
the right in the Northern Hemisphere and to the left in the
Southern Hemisphere, and in the free air it blows parallel
with the isobars. Close to the earth's surface, however, the
wind blows obliquely inwards towards the low pressure, and
in the Northern Hemisphere a system of straight isobars with
low pressure to the north and high pressure to the south gives
surface winds from between west-south-west and south-west.
Thus in accordance with the pressure distribution shown in
the figures, on the eastern and south-eastern sides of the
sub-tropical high-pressure centres there are great systems
of winds, blowing towards the equator, known as the Trade
Winds, north-east in the Northern and south-east in the
Southern Hemisphere. Near the equator these two wind
systems unite in a slow drift which on the whole is from east
to west, though it is diversified by frequent calms and variable
winds ; this region is known as the Doldrums. On the
50 CLIMATE THROUGH THE AGES
poleward side of the high-pressure belts occur westerly winds,
which blow round the globe in temperate latitudes and are
varied by frequent " cyclonic depressions." Finally, on the
edge of the polar high-pressure areas easterly winds agin
increase in frequency.
This generalised system is modified by the continents,
which tend to be occupied by high-pressure areas in winter
and by low-pressure areas in summer. These are associated
with continental systems of winds known as monsoons, blowing
outwards from the continents in winter and inwards towards
the continents in summer. The classic example of a monsoon
area is Asia ; the winter pressure over Siberia, when corrected
to sea-level, is the highest known on the earth, on the average
exceeding 1,040 mb. south of Lake Baikal, while the summer
low pressure in North-western India rivals in intensity the
great barometric minimum in the Southern Ocean. There is
a correspondingly great reversal of wind direction, for instance
on the China coast the winds blow from the north or north-east
with remarkable persistence for several months in winter, and
from the south with almost equal steadiness in summer. The
neighbouring continent of Australia south of the equator is
undergoing the same alternation, though not so marked, in
the reverse sense, so that when pressure is high in Asia it is low
in Australia and vice versa ; consequently there takes place
an immense ebb and flow of air between these two regions.
It is evident that we have to deal with two factors which
combine to form the main features of the distribution of
pressure and winds over the globe. The first is known as the
planetary circulation, the second is the influence of the
continents and oceans and also of the distribution of ice ; it
must have varied greatly from one geological epoch to another
in accordance with the varying land and sea distribution, and
may be termed the geographical circulation. The planetary
circulation also cannot be regarded as a constant ; it is
governed by the contrast of temperature between low and high
latitudes, and before we can estimate the part which the
atmospheric circulation has played in geological changes of
climate, it will be necessary to discuss the planetary circulation
somewhat more fully, especially in relation to the vertical
and horizontal distribution of temperatures.
Near the surface of the earth, temperature decreases upwards
PRESSURE AND WINDS 51
at the average rate of about 19 F. per mile, but this decrease
does not continue indefinitely. It is found that at a height of
several miles temperature ceases to fall, and above that height
ifremains constant or even rises a little. The lower part of
the atmosphere, in which temperature decreases upwards, is
termed the troposphere, the succeeding layer in which there
is no change with height is termed the stratosphere, the
junction between them being the " tropopause." Now the
tropopause is not always found at the same height ; it is
slightly higher in summer than in winter, it is higher over
anticyclones than over cyclones, and it is very much higher
near the equator than near the poles. Over the equator its
average height is about 18 kilometres (n miles), over the
British Isles about n km. (7 miles), near the North Pole
only about 7^ km. (less than 5 miles). This curious structure
of the atmosphere has some very important consequences.
In the troposphere the temperature at any given height is
roughly proportional to that at sea-level ; the air five miles
above the equator is warmer than the air five miles above the
pole. But at a height of ten miles the conditions are very
different. Let us work it out. At sea-level the temperatures
are, say, 80 F. at the equator and 20 F. near the North Pole.
Allowing a decrease of 19 F. for each mile of height, at five
miles we have: equator, 15 F., pole, 75 F. But as
we go on to a height of ten miles the temperature above the
equator goes on falling, while that above the pole stays at
75 F., and at ten miles we have: equator, noF.,
pole, 75 F., that is, the air ten miles above the pole is
35 F. warmer than the air at the same height above the
equator. In fact, the lowest temperatures naturally existing
anywhere on earth are found at a height of about 10^ miles
above the equator. Cold air is heavier than warm air, and
therefore the colder the air above any locality, the greater will
be the pressure there.
The barometric pressure on any part of the earth's surface
is the result of the whole column of air above it, but we may
follow Sir Napier Shaw (i) in dividing the atmosphere into
two parts, one above the level of 8 kilometres (5 miles) and the
other below that level, and we may further suppose that at a
height of about 20 kilometres (12^ miles) the pressure
differences between different parts of the earth are small. The
52 CLIMATE THROUGH THE AGES
pressure at eight kilometres is inversely proportional to the
temperature (on the absolute scale) of the higher layers of the
atmosphere, and since in the upper air temperature increases
from low to high latitudes, at eight kilometres the pressifre
decreases from low to high latitudes. If the surface of the
globe were fairly homogeneous this would result in the forma-
tion of two great systems of westerly winds in the upper air
(the " polar whirls ") blowing completely round the earth
with their centres near the poles. Such a polar whirl may in
fact exist in the Southern Hemisphere, but in the Northern
Hemisphere, with its geographical and meteorological com-
plexities, the circulation is greatly distorted, so that it is more
correct to speak of a " zone of sub-polar whirls." This
condition, however, must be an exception from the geological
point of view, and during the warm geological periods the
polar whirls were probably in existence in the upper air in
both hemispheres. The reason for the exception in the
Northern Hemisphere at present is not the low temperature
over the Greenland ice sheet, since the layer of abnormally cold
air above the ice is comparatively thin ; the circulation
apparently takes place round the low-pressure centre and is
due to the fact referred to previously, that, above the low
pressure the stratosphere extends down to a low level and is
therefore abnormally warm.
In the stratum between the surface and eight kilometres
the average temperature decreases from lower to higher
latitudes, and the pressure at the surface due to this layer of
air alone is therefore greater in high than in low latitudes.
The effect of this layer would be to cause easterly winds at
the surface in all latitudes. Thus we have two apposing
tendencies ; the temperature distribution above eight kilo-
metres tends to produce westerly winds at all levels from the
surface up to nearly twenty kilometres, while the temperature
distribution below eight kilometres tends to produce easterly
winds at the surface. Which of these two directions, ea 4 st or
west, predominates in the resultant surface wind of any
latitude, depends on the vertical temperature distribution.
If the decrease of temperature towards the poles is rapid,
as in the " glacial " periods, the weight of the cold air in high
latitudes in the lowest eight kilometres may be more than
sufficient to counterbalance the pressure difference between
PRESSURE AND WINDS 53
the equator and the poles at eight kilometres, and there will
be a polar cap of east winds as at present. But if the decrease
of temperature towards the poles is slow, as it was during the
" non-glacial " periods, the weight of the lowest eight kilo-
metres of air may not be enough to counterbalance the pressure
difference between the equator and the poles at eight kilometres,
and in that event westerly winds will prevail at the surface
up to the immediate neighbourhood of the poles. This must
not be interpreted to mean that there were no depressions,
or only one stationary depression concentric with the pole.
Some theoretical work of H. Jeffreys (2) leads him to the
conclusion that the friction of the wind against the earth's
surface must inevitably introduce a system of moving cyclones
surrounding the pole. In the absence of glacial anticyclones,
however, it appears that cyclones would be fewer and less
intense than at present, and would occur as a rule in very
high latitudes.
At this point we may pause to consider a possible objection.
Since the atmospheric circulation depends on differences of
temperature, it would seem to follow that the greater the
temperature difference between equator and poles, the stronger
would be the atmospheric circulation, and consequently the
greater the amount of heat carried from low to high latitudes
by the winds and wind-driven ocean currents. This would
tend to restore the balance and keep the temperature difference
between equator and poles more or less constant. This way
of looking at the atmospheric circulation is very plausible, but
it ignores the fact brought out in the preceding discussion that
there is a critical point for the planetary atmospheric circulation
just as for the distribution of temperature in a polar sea.
In the glacial state, with low temperatures near the poles
the excess of air density in the layer of the atmosphere below
eight kilometres causes easterly winds in high latitudes ;
these have an equatorward component at the surface which
hinders the poleward surface component of the westerly winds
from carrying warm air into high latitudes. This shutting
out of the warmth-bringing equatorial air helps to keep the
polar regions cold. In the " non-glacial " state, the excess
of density in the lowest eight kilometres is not sufficient to
counterbalance the warmer stratosphere of high latitudes and
the winds are therefore westerly with a poleward surface
54 CLIMATE THROUGH THE AGES
component, up to high latitudes. The influx of warm
equatorial air then helps to maintain the high polar tem-
peratures. Owing to the existence of this critical point tke
atmospheric circulation, so far from smoothing out the tem-
perature contrast, maintains or even magnifies it.
The boundary between the polar east winds and the
temperate west winds (the " polar front ") is at present
the chief seat of the development of the barometric
depressions of temperate latitudes. There seems to be no
doubt that the presence of two adjacent air currents at
different temperatures which are in motion relative to one
another is an important factor in bringing these depressions
into existence (3). Owing to the frequent passage of depres-
sions the average pressure in about 60-65 latitude is lower
than it would otherwise be. These belts of cyclonic activity
and low pressure limit the poleward extension of the sub-
tropical anticyclones and help to cause the relatively sharp
differentiation of the climatic zones at present ; in the absence
of polar east winds we should expect a gradual fall of pressure
from maxima in middle latitudes to minima near the poles.
The chief centres of storminess would be found over the oceans
in the Arctic and Antarctic regions ; in middle latitudes there
would be only occasional feeble depressions moving slowly
along irregular tracks.
The zone of westerly winds which blow round the world
in middle latitudes is broken up to some extent near the
surface by travelling depressions and anticyclones, but at a
height of a few miles the winds are much stronger and more
steady. A steady west wind blowing round the globe would,
however, be unstable; as described for example by C. G.
Rossby (4), any slight disturbance of the west-east movement
would set up wave-like disturbances, with the crests towards
the pole and the troughs towards the equator. The wave-
length depends on the strength of the winds ; the average
wind velocity in winter in the northern hemisphere at present
is such that the wave-length is about 3,000 miles, and this is
the approximate distance between the Aleutian and Icelandic
low-pressure centres (the corresponding centre in about 60 E.
is masked by the Siberian winter anticyclone). The weaker
the winds the shorter the wave-length ; hence when for any
reason the average speed of the westerly winds falls off, the
PRESSURE AND WINDS 55
circulation tends to break up into a number of smaller cells.
The deep single Aleutian and Icelandic low pressure centres are
replaced by smaller and less stable double centres. The winds
are less strong, but the exchange of air between high and low
latitudes is maintained by larger north-south and south-north
components.
Since the strength of the west-east circulation depends on
the temperature gradient between low and high latitudes, in
the non-glacial periods we should expect a weak circulation
and hence numerous small areas of low pressure. This
would tend to equalise climatic conditions along the same
parallel of latitude, whereas the large semi-permanent Aleutian
and Icelandic lows which predominate at present lead to
great extremes, e.g. between Labrador and north-west Europe.
In the early stages of the Quaternary glaciation, as pointed
out by R. F. Flint and H. G. Dorsey (6), the zonal circulation
must have increased in strength. At present, when the
circulation is especially strong the Icelandic low tends to spread
eastwards ; a similar change in the Quaternary would have
facilitated the eastward extension of the Scandinavian
glaciation. On the other hand strong west winds crossing
the Rocky Mountains would give Fohn effects on the leeward
side and so limit the eastward extension of the Gordilleran
ice-sheets, favouring a separate centre of glaciation in eastern
Quebec and Labrador. When fair-sized ice-sheets had
developed in the north, however, semi-permanent glacial
anticyclones would form which would displace the tracks of
depressions southward. This would weaken the zonal circula-
tion ; depressions would tend to stagnate south of the ice-
sheets, facilitating the extension of the latter southward on
their western margins while farther east comparatively warm
southerly winds checked the southward expansion. These
ideas are interesting and warrant further investigation, but the
pattern suggested by Flint and Dorsey is complicated by
other factors such as isostatic changes due to the weight of
the ice.
In tropical and sub-tropical regions the existing planetary
circulation over the oceans (equatorial low-pressure belt, trade
winds, sub-tropical anticyclones) is clearly the result of zonal
temperature differences and the rotation of the earth. In
warm periods thermal zones must have existed though probably
56 CLIMATE THROUGH THE AGES
somewhat less marked than now, and there is no reason to
suppose that trade winds and sub-tropical anticyclones did not
exist. The absence of the polar fronts, however, and the pole-
ward displacement of the tracks of depressions would permit
the anticyclones to extend farther poleward, just as they do
now in the North Atlantic in summer, when the south-north
thermal gradient is smallest, compared with winter, when the
thermal gradient is greatest.
Fortunately Nature herself carries out for our inspection an
experiment which suggests the way in which the general
atmospheric circulation to be expected during periods with a
small temperature difference between equator and poles would
differ from the circulation during periods with a large temper-
ature difference. In summer the air over the Arctic Ocean
is now more than 30 F. warmer than in winter, while in
equatorial regions there is very little change of temperature
throughout the year. The temperature difference between
equatorial and polar regions is therefore about 50 F. in
summer and more than 80 F. in winter. As a rough approxi-
mation we may say that the temperature difference between
low and high latitudes during the present summer in the
Northern Hemisphere is similar to the temperature difference
which existed during winter in the warm periods. Nature
has a habit of complicating her experiments with irrelevant
details, and in all the oceans but one there are, even in summer,
large quantities of ice and ice-cooled water which, coming into
contact with warm ocean currents, produce great differences
of temperature in short distances, but in the North Pacific
Ocean this complication is reduced to a minimum, and we may
illustrate the probable system of winds during the winter of a
warm period by the system of winds prevailing at present over
the North Pacific in summer (Fig. 6). Of course it is not to be
expected that this will give us an exact picture of what happened
during a warm period ; the differences in other oceans,
and especially in the Southern Hemisphere, would mevit^bly
introduce some modifications, but at least it will serve as a
suggestion.
The distribution of pressure at present has been shown in
Figs. 3 and 4. In January (Fig. 3) the anticyclone is small
and sharply defined ; it does not extend north of 40 N., and
between 40 N. and 60 N. lies a well-marked area of low
PRESSURE AND WINDS 57
pressure, the Aleutian low. In July the anticyclone is less
sharply defined, but extends almost to 60 N., and the Aleutian
low has completely disappeared. This disappearance of the
cyclonic centre from the North Pacific in summer is of great
importance ; the Icelandic minimum in the North Atlantic
does not disappear during summer and the low-pressure belt
in the Southern Hemisphere also persists throughout the year.
There seems no doubt that this peculiarity of the North Pacific
is directly due to the absence of ice in summer both floating
ice in the sea and large ice-sheets on the neighbouring land-
masses. The surface winds over the Pacific corresponding
with the pressure distribution in January and July are shown
' < / /.. f. f f f f f^ r .-f f ^-r-
20
Fig. 5. Winds over North Pacific, January.
in Figs. 5 and 6. In January (Fig. 5), representing winter
conditions at present, a belt of north-easterly trade winds
extends completely across the ocean south of latitude 25 N.
and is almost entirely separated by a belt of calms and variable
winds from another system of winds, mainly westerly, between
30 and 50 N. North of 50 N. the prevailing wind is from
north-east. Thus the system of winds in January, while it is
calculated to bring mild winters to the American coast south
of 55 N., greatly intensifies the rigour of the winter in sub-
polar latitudes and on the eastern coasts of Asia.
In July (Fig. 6), representing summer conditions at present
and winter conditions during the warm periods, the system
of winds is very different. The direction of the north-east
58 CLIMATE THROUGH THE AGES
trade is more easterly, and west of 1 80 longitude even south-
easterly ; it passes without a break into a great system of
southerly winds which blow from 30 N. to the Arctic Circle.
The south-easterly direction of these winds north of about
latitude 50 N. is due to the monsoonal inflow into the great
land-mass of Asia ; during the warm periods the winds
probably retained their south-westerly direction into high
latitudes. These winds bring high temperatures over the
whole of the North Pacific coasts (except the western coast
of the United States), and are especially favourable to Alaska
and North-eastern Asia, which enjoy a warm climate at this
120
140
160 E 180 W 160
140
Fig. 6. Winds over North Pacific, July.
season. The winds induce favourable oceanic currents, and
in spite of the narrowness and small depth of Bering Strait
some warm water succeeds in penetrating into the Arctic
Ocean through that opening in summer. The Arctic Ocean
retains a large amount of floating ice throughout the summer,
so that the polar cyclone cannot develop properly, and the
southerly winds of the North Pacific are greatly weakened ;
there seems no doubt that if the Arctic Ocean were free of
ice the system of southerly or south-westerly winds would attain
great steadiness. If at the same time the Bering Strait were
replaced by a wide and deep gap, practically the whole surface
of the ocean north of 30 N. would be set in motion in a north-
easterly direction, and an immense volume of warm water
would be driven into the Arctic Ocean.
PRESSURE AND WINDS 59
This picture of the pressure and winds during the geological
periods characterised by widespread warmth, which we have
obtained first from some theoretical considerations, and
secondly from an examination of present summer conditions
over the North Pacific, fits in admirably with what we know
of the climates of these warm periods. The system of stable
southerly winds, extending across the middle latitudes, would
give them fine quiet weather in place of the present succession
of storms. As we saw in the Introduction, apart from the
warmth and rich vegetation of the polar regions, the most
striking feature of these periods was the widespread develop-
ment of semi-desert conditions in temperate regions. True
deserts were probably less extensive than they are to-day,
owing to the smoothing out of the zonal contrasts, but very
large regions had a " Mediterranean " type of climate, with a
small rainfall during the mild winter and a long dry hot
summer. This explains, for example, the widespread dis-
tribution of the characteristic " sub-tropical " (i.e., Medi-
terranean) vegetation of the first half of the Tertiary period.
Let us look at the reverse of this picture, and see what
would happen during an ice-age. An extension of the cold
areas over a large part of the present temperate regions, such
as occurred during the Quaternary, would bring the " polar
front " between the polar east winds and the temperate west
winds nearer to the equator, and by increasing the temperature
contrast between low and high latitudes would increase the
storminess. This means that the sub-tropical high-pressure
belts would be sharply limited on their poleward sides. The
result would probably be an intensification of the present
winter conditions a small but intense anticyclonic belt in
about 20 to 25 latitude, a narrow belt of powerful trade
winds, and a deepened equatorial trough of low pressure.
The circulation would be much less stable than that of the
warm periods, and the anticyclonic belts would be subject to
grea* and rapid displacements. Hence the rainfall would be
increased over all the tropical and sub-tropical regions ;
outside the new storm tracks the increase would be greatest and
most regular near the equator, while towards the tropics the
rainfall would be less in amount and very variable from year
to year. Here again the theoretical conclusions are supported
by geological results ; outside the great ice-sheets there is
6o CLIMATE THROUGH THE AGES
evidence of a much greater rainfall (snowfall on the mountains
in two belts, one along the new storm tracks a short distance
equatorward of the ice-edges, and the other along the equator.
The former gave rise to the great lakes of the Great Basin of
America and of the interior of Asia, and to a large number of
mountain glaciers, the latter to the greatly increased lakes of
Central Africa and to the glaciers of Kenya, Kilimanjaro,
Ruwenzori, and parts of the Andes. Between these two belts
the evidence of greater precipitation is more indefinite and
irregular.
We must now return to the geographical circulation.
This we have seen is characterised by the presence over the
continents of high pressure in winter and low pressure in
summer, resulting in monsoonal winds. The intensity of
the monsoons over a continent depends on a number of
factors the limits of latitude, the size, the presence of large
arid basins surrounded by mountain ranges, and the strength
and direction of the winds over the neighbouring seas. The
intensity of the Siberian winter anticyclone is due partly to
the great size of Eurasia, and very largely to the way in which
its surface is broken up by mountain ranges. The highest
pressure occurs over the great enclosed basin south of Lake
Baikal, from which the cold air finds difficulty in escaping.
The winter cooling is of course essential to the development
of the anticyclone, but the existence of the anticyclone in turn
intensifies the cold. In North America, where owing to the
absence of transverse mountain ranges the cold air finds less
difficulty in escaping, the winter anticyclone is comparatively
feeble. A good example of the effect of favourable orographical
conditions on the pressure distribution is the Iberian Peninsula,
which is occupied by a well-marked anticyclone in winter.
In North-west Europe, on the other hand, the influence of the
Icelandic minimum and the barometric depressions which
originate in the Atlantic make the maintenance of a winter
anticyclone very difficult.
The establishment of continental low pressure in summer
depends on similar factors, but the centres occur nearer the
equator than do those of the winter anticyclones. In Asia the
area of low pressure in July extends over a large part of the
continent, but the actual minimum occurs comparatively near
the sea in North-west India, and is very intense ; this position
PRESSURE AND WINDS 6 1
of the minimum is due largely to the position of the mountain
ranges which interfere with the free circulation of the air (6)
and entirely inhibit the North-east Trade, which is the natural
wind of that latitude. Directly to the westward over the
Sahara, where the air is as hot or hotter, pressure is much
higher, and in fact throughout the year the Sahara is practically
a continuation of the belt of the North-east Trades, probably
because the mountain ranges are not high enough to shut out
these winds entirely.
These scattered instances show how difficult it is to analyse
the geographical circulation exactly, but they do give us some
basis for estimating the results of various geological changes on
the local wind circulations. Consider, for instance, a warm
period in which there are extensive oceans and some flat
continents of moderate size, little diversified by mountain
ranges. It seems that these continents would not greatly
modify the planetary circulation described above. In summer,
when the winds are generally weakest, the continents in low
and middle latitudes would be hot, and there being no
mountain ranges to cause the ascent of air, the only rainfall
would occur in sporadic thunderstorms and squalls. In
winter they would be cooler than the oocans, but in the absence
of a polar reservoir of cold air, they would not be intensely
cold, especially since the prevailing winds in temperate zones
would be from the equator. The centres of the anticyclones,
as now, would probably lie over the oceans, but there would
be a tendency for the high-pressure areas to extend nearer the
poles over the continents, giving southerly winds on the west
coasts and westerly winds on the east coasts. A diagrammatic
reconstruction of the pressure and winds during winter and
summer in a warm period is shown in Fig. 7.
There are two special parts of the geographical circulation
which demand further notice. One of these is the south-west
monsoon of the Asiatic continent ; the other is the circulation
over ice-sheets. The south-west monsoon (6) is remarkable
because it involves a large transference of air from one hemi-
sphere to the other ; there is a continuous pressure gradient
from the sub-tropical anticyclone in the South Indian Ocean
to the minimum over Asia, and the surface winds are south-
easterly south of the equator and south-westerly north of the
equator. The air-flow actually in places surmounts the
62
CXIMATE THROUGH THE AGES
mighty barrier of the Himalayas, nowhere less than 12,000 feet
in height, and arrives in Tibet as a dry descending current.
Now it is possible that under certain conditions there may be
a quite considerable difference between the mean annual
temperatures of the two hemispheres, leading to a more or
less permanent circulation of the type of the south-west
monsoon. A circulation of this type affords a possible
Summer
Fig. 7. Reconstruction of pressure and winds during
a warm period.
explanation of the peculiar climate of the Upper Carboniferous
period (see Chapter XV.).
The circulation over a large ice-sheet is of great importance
for the study of the meteorology of glacial epochs. A surface
of snow and ice reflects a large part of the solar radiation
which falls on it, and, owing to the dryness of the air above it,
also radiates freely and receives little return radiation from
the air. Hence during most of the year the surface is very
cold, and this cools the lower layer of the air in contact with it.
Since cold air is relatively heavy, barometric pressure must be
higher over a large ice-sheet than at the same level over
PRESSURE AND WINDS 63
neighbouring seas. This is interpreted by W. H, Hobbs (7)
to imply that an ice-sheet is the site of a nearly permanent
anticyclone, with outflowing winds on all sides, supplied by
air which flows in at higher levels and descends over the
ice-sheet. This raises the difficult question of the supply of
moisture to maintain the flow of ice, for descending air is
warmed by compression and is normally dry, but Hobbs pointed
out that the surface of the ice is intensely cold, and may
be much colder than the air at a height of several thousand
feet. The moisture of the upper winds reaches ground level
in the form of vapour, but a large part of it is immediately
condensed as ice-mist or deposited directly as hoar-frost. The
outflowing winds sweep these crystals before them and so
maintain the marginal parts of the ice-sheet.
There is undoubtedly a good deal of truth in this theory.
Owing to difficulties in reduction to sea-level, pressure on
the surface of a large ice-sheet cannot be compared directly
with that over the surrounding ocean, but the winds do on the
whole tend to blow outwards and must be made good by
descending currents, while the vertical distribution of tempera-
ture is such that there must be some condensation as ice-
crystals or hoar-frost. On the other hand, the supposed
high pressure may be entirely due to a quite thin layer of air
near the ground, above which the structure ceases to bear any
relation to an anticyclone, while the outflowing winds may be
purely katabatic rivers of cold air such as flow down any
slope on a cold clear night and have no relation to the general
pressure distribution. In such a case the normal processes
of precipitation could go on unchanged above the surface
layers of air, the only difference being that precipitation would
fall entirely as snow. It is very unlikely that the deposit of
hoar-frost could suffice to supply the enormous quantities of
ice which issue from an ice-sheet each year as glaciers, icebergs
and glacial streams.
F. . Matthes (8) discussed the light thrown on this question
by observations at Eismitte near the centre of Greenland
during the Wegener expeditions of 1929 and 1930-31, the
latter covering more than a year of continuous instrumental
readings. These observations show clearly that quiet fine
conditions are the exception and stormy cloudy conditions
the rule, that the katabatic winds are feeble and easily
64 CLIMATE THROUGH THE AGES
overpowered by storm winds, and that by far the greatestfactor
in the nourishment of the ice-sheet is ordinary snow. The
great oscillations of pressure and temperature closely resemble
though much lower in the scale, those in typical cyclonic,
regions such as New England. The winds are strong, but
blow mainly from east, with a secondary maximum from
S.S.E. True precipitation is actually more frequent than on
the west coast of Greenland and not much less common than
on the rain-swept coast of Norway, but owing to the low
temperatures the total amount is not large, the average being
estimated from the firn layers as 12-4 inches of water a year.
The frequency of easterly winds is not due entirely or even
mainly to katabatic winds from the ice divide to the east. It
means that depressions pass mainly to the south, but extend
their influence right to the centre of the ice-cap. There is
now no difficulty about the supply of moisture, which is
derived from the Atlantic Ocean and carried inland, over-
riding the shallow katabatic winds of the coast, to be condensed
into snow over the ice-shed and swept on to be gradually
deposited on the leeward slopes. Atmospheric pressure is
undoubtedly higher than it would be if the surface, at the same
level, were unglaciated, but it appears that the sub-continent
is not big enough to form the basis for a self-supporting
glacial anticyclone.
The Antarctic presents a more difficult problem. W.
Meinardus (14) considered that the Antarctic anticyclone is
limited to the lowest 2,000 metres, above which the circulation
is cyclonic, and since the greater part of the Antarctic continent
is above this level, he supposed that the greater part of the land
is subjected to cyclonic air motion. Sir George Simpson
pointed out (9) that an extensive ice-covered plateau must
be occupied by a glacial anticyclone just as if it were at sea-
level ; he accordingly divides the Antarctic continent into two
parts, a plateau at about 3,000 metres (10,000 feet) and a
plain near sea-level. The latter is occupied by an anti-
cyclone at sea-level, but owing to the rapid decrease of pressure
with height, conditions in the free air are cyclonic at a height
of about 3,000 metres. Thus at the latter height there is an
anticyclone at the surface of the plateau and a cyclone in the
free air above the plain.
It appears that an ice-sheet does not develop a stable
PRESSURE AND WINDS 65
glacial anticyclone until it attains a certain size. The
Greenland ice-sheet is not broad enough to prevent the
influence of large depressions from extending even to its
centre, but these depressions are deflected southwards by
the ice-sheet so that their centres rarely pass directly over
the central regions of Greenland. The greater part of the
Antarctic continent is immune from travelling depressions.
On the other hand, the smaller ice-masses of Iceland and
Spitsbergen appear to have little effect on the pressure
distribution. Thus the critical point comes at a diameter
somewhat larger than the width of Greenland. In 75 N.
Greenland is about 650 miles across. From studies of the
January temperature distribution over land areas in latitude
50 to 70 N. (10), which are normally snow covered in winter
and are therefore similar to ice-sheets in their effect, I think
the diameter which a circular ice-sheet must reach before it
begins to dominate the pressure distribution is between seven
hundred and a thousand miles. When the diameter is less
than 700 miles, the normal winds of the region sweep over
the ice-sheet without much hindrance, and the only effect
of the ice is to cool the air slightly by conduction. When the
diameter reaches, say, 1,000 miles, a glacial anticyclone
develops, with clear skies and intense cooling by radiation.
The outwardly-directed winds spread Arctic conditions in a
broad zone round the margin of the ice, and may even result
in the " sympathetic " glaciation of a neighbouring mountain
range. It is probable that the great development of Alpine
glaciers during the Quaternary was partly due to cooling by
the winds blowing off the Scandinavian ice-sheet. It will
also be remembered that during the earlier stages in the final
retreat of the Scandinavian ice-sheet the ground vacated by
the ice was occupied by a dwarf flora of Arctic plants, but
later, when the ice-sheet was smaller, the retreating edge was
immediately followed by a temperate flora. This probably
indicates the stage at which the glacial anticyclone broke
down.
Every cause or factor which is put forward to explain
climatic changes has to take into account the modifications
which would be introduced into the atmospheric circulation
by its operation. Even the possible occurrence of modifications
of the circulation sufficient by themselves to give rise to great
66 CLIMATE THROUGH THE AGES
climatic changes, without the intervention of any other
factor, has been discussed. Thus W. H. Dines remarks (i i) :
" There seems to be no particular reason why the winds known
as the c trades ' should not be westerly and the winds of
temperate latitudes easterly. Perhaps such a system is possible
and might be stable if once established. It would explain
the glaciation of North-western Europe, for it would very
greatly lower the temperature of that region, but it is not
feasible as an explanation of the glacial epoch, because it
would raise the winter temperature of North America."
A restoration of the meteorological conditions of the Quatern-
ary Ice-Age was attempted by the late F. W. Harmer (12)
on the assumption that the glaciations of North America
alternated with those of Europe. It is hard to conceive of
great changes such as these without some ulterior reason,
such as a change in the land and sea distribution or in the
solar radiation, and we must regard the part which the
atmospheric circulation plays as that of a regulator, at times
perhaps an amplifier, but probably not an originator of major
climatic oscillations.
It must be admitted, however, .that the part played by the
circulation of the atmosphere in climatic changes is not yet
fully understood. In the past hundred years, for example,
there has been a marked recession of glaciers in all parts of
the world, accompanied by a large rise of temperature in the
Arctic and a rise of winter temperature over a much wider
region. R. Scherhag (13) attributes these phenomena to a
strengthening of the atmospheric circulation and consequently
of the Gulf Stream, but this only pushes the problem one stage
further back, i.e. 9 to the cause of the stronger circulation.
The answer certainly does not lie in a change of land and sea
distribution, and to the best of our knowledge there has been
no appreciable change of solar radiation. It is not unlikely
that the cause lies in the atmosphere itself, or in its inter-
actions with the oceans, owing to some process initiated almost
by " accident " in the constant turmoil of depressions and
anticyclones, but which, once begun, will automatically
increase in intensity until it becomes unstable or is reversed
by some other " accident." The atmosphere and hydrosphere
are so vast that such self-reinforcing actions may well persist
for many decades. It is possible that the majority of temporary
PRESSURE AND WINDS 67
swings of climate are of this nature. If so, they cannot be
said to have a " cause," any more than can a run of luck in a
game of pure chance.
REFERENCES
(1) SHAW, SIR NAPIER. " The air and its ways." London, 1924.
(2) JEFFREYS, H. " On the dynamics of geostrophic winds." London, Q. J* R
Meteor. Soc., 52, 1926, p. 85.
(3) BJERKNES, J., and H. SOLBERG. " Life-cycle of cyclones and the polar front
theory of atmospheric circulation." Kristiania, Geofysiske PubL, 3, No. i,
1922.
(4) ROSSBY, C. G. " The scientific basis of modern meteorology." Washington,
Tearb. Agric., 1941, p. 599.
(5) FLINT, R. F., and H. G. DORSEY. " lowan and Tazewell drifts and the North
American ice-sheet." New Haven, Amer. J. Sci., 243, 1945, p. 627.
(6) SIMPSON, G. G. " The south-west monsoon." London, (. J. R. Meteor.
Soc., 47, 1921, p. 151.
(7) HOBBS, W. H. " The glacial anticyclones. The poles of atmospheric
circulation." New York (Macmillan), 1926.
(8) MATTHES, F. E. " The glacial anticyclone theory examined in the light of
recent meteorological data from Greenland." Trans. Amer. Geoph. Union,
27, 1946, Pt. I, p. 324.
(9) BRITISH ANTARCTIC EXPEDITION, 1910-1913. Meteorology, vol. i. Dis-
cussion, by G. G. SIMPSON. Calcutta, 1919.
(10) BROOKS, C. E. P. " Continentality and temperature." London, Q,. J. R.
Meteor. Soc., 43, 1917, p. 164.
(11) DINES, W. H. "Circulation and temperature of the atmosphere."
Washington, D.C., Monthly Weather Review, 43, 1915, p. 551.
(12) HARMER, F. W. " The influence of the winds upon climate during the
Pleistocene epoch : a pabeometeorological explanation of some
geological problems." London, Q,. J. Geol. Soc., 57, 1901, p. 405.
(13) SCHERHAG, R. " Die Erwarmung der Arktis." Copenhague, J. Cons.
int. Explor. Mer., 12, 1937, p. 263. See also Meteor. Mag., London, 73,
'938, p. 29.
(14) DEUTSGHE-SttDpoLAR-ExpEDiTiON, 1901-1903. Ill Bd., Meteorologie.
Berlin, 1911.
CHAPTER III
THE CIRCULATION OF THE OCEANS
OWING to the high specific heat of water, the great
oceanic currents and the variations in the surface
temperature of the sea to which they give rise are of
very great climatic importance. The classic example of this
is the Gulf Stream Drift and the high winter temperatures of
North-west Europe, but there have been still more notable
instances in the geological past, in the highly favourable
climates of the polar regions during the warm periods. It is
estimated that at present about half the transfer of heat from
low to high latitudes is due to ocean currents, the remaining
half being due to interchange of air. Ocean currents are due
chiefly to two causes, differences of density, and the winds,
which drive before them the surface layers from which motion
is imparted by friction to the underlying layers. If two
masses of water of different densities lie side by side a circula-
tion will be set up between them, resulting in a surface flow
from the lighter to the heavier mass, and a flow at a greater
depth from the heavier to the lighter, and if the earth were at
rest this would continue until the horizontal differences of
density had been removed and all the heavy water lay at the
bottom, with all the light water on top. Owing to the earth's
rotation, currents of water are deflected to the right in the
Northern Hemisphere and to the left in the Southern Hemi-
sphere just like currents of air, and the ultimate flow is at
right angles to the gradient of density, giving two currents
moving side by side in opposite directions.
Density depends almost entirely on two factors, the
temperature and the salinity. With falling temperature,
water increases in density until it reaches a temperature of
4 C. (39 F.) if it is fresh, but about -2 C. (28 F.) if it is
average sea water. Since 28 F. is also the freezing point of
sea water, the latter on being cooled will always sink through
water of equal salinity, while fresh water at a temperature
68
THE CIRCULATION OF THE OCEANS 69
of 39 F. will sink through fresh water having either a
higher or lower temperature. The temperature of the oceans
almost everywhere falls with increasing depth, and the lower
parts of the oceans are occupied by a stratum of water at about
39 F.
Density also increases with salinity ; the average salinity
is greatest in the sub-tropical open oceans where evaporation
is great and rainfall slight. Since these are also in general
the areas where the temperature is high, the effect of salinity
on density partly balances that of temperature. Density
becomes especially great where an ocean current from the
tropics penetrates into high latitudes, losing much of its heat
but retaining a high salinity. Thus the relatively warm salt
water which originates in the Gulf Stream and penetrates into
the Arctic Ocean is heavier than the colder but much fresher
water of local origin ; the latter remains on the surface and
freezes readily, giving rise to great quantities of floating ice.
Fresh water is always lighter than sea water of average salinity
(35 P ai% ts per thousand) at temperatures which are met with
in nature.
In the oceanic circulation as it is developed at present
(Fig. 8), the winds apparently play a much greater part than
differences of density, especially in tropical and temperate
latitudes, where the direction of the ocean currents is almost
everywhere the same as that of the air currents. On a non-
rotating globe this agreement would be easy to undertsand,
but as it is, the matter is more complex. The wind drives the
surface layers before it, the movement being communicated by
friction and vertical interchange of water to the underlying
layers, but owing to the rotation of the globe, the surface
current caused by a steady wind is inclined to the wind
direction at an angle of 45 degrees, to the right in the Northern
Hemisphere and to the left in the Southern Hemisphere. As
we go below the surface the currents deviate more and more
from the winds, and the main mass of the water moves at right
angles to the direction of the wind. Consider now the case
of a centre of high pressure in the Northern Hemisphere round
which the winds circulate in a clockwise direction. All these
winds will be driving the water to the right, that is, towards
the centre of the system. The result will be that water is
piled up in the centre ; we shall have an oceanic " hill "
CLIMATE THROUGH THE AGES
THE CIRCULATION OF THE OCEANS JI
down the slopes of which the surface water will commence to
run. The rotation of the earth will deviate this water to the
right, and when a steady state is reached it will be flowing
round the central " hill " in a clockwise direction. The final
result will be in fact as we now find it, a system of oceanic
currents surrounding a central area of stagnant ocean, closely
resembling the system of winds blowing round a central area
of calm. The tendency of the winds to drive water towards
the centre is just balanced by the tendency of the accumulation
of water in the centre to flow outwards. Winds irregular in
direction and velocity are less effective than steady winds in
causing ocean currents, and with variable winds the angle
between the resultant wind and the surface current is generally
less than 45 degrees (i).
The best known system of currents is that in the North
Atlantic (2, 3), (Fig. 8). Commencing with the tropical
part of the Atlantic Ocean, we find that the North-east and
South-east Trades give rise to currents which turn more and
more to the eastward and increase in volume as they approach
the equator, until they unite to form the Equatorial Current.
For convenience, the northern and southern portions of this
current are called the North and South Equatorial Currents,
but there is no definite dividing line except that from May or
June to November a very shallow current, the Equatorial
Counter Current, sets eastward where the winds are weakest
between about 3 and 10 N., ultimately entering the Gulf
of Guinea. The greater part of the North Equatorial Current
turns north-westward as the Antilles Current, which passes
between Cuba and the Bahamas, and unites with the Gulf
Stream flowing through the Strait of Florida. The Antilles
Current is estimated to convey nearly forty cubic miles of
water an hour past Porto Rico.
Owing to the greater strength of the South-east Trades,
the South Equatorial Current is stronger and steadier than the
North Equatorial. It is directed slightly north of west ;
striking Cape San Roque on the Brazilian coast in 5 S., it
divides into two branches, of which the southern turns south-
westwards as the Brazilian Current, while the northern and
more extensive passes along the coast of Guiana and unites
with the western branch of the North Equatorial Current.
The combined current flows towards the coasts of Honduras
72 CLIMATE THROUGH THE AGES
and Yucatan, and thence mainly through the Yucatan Channel
into the Gulf of Mexico. Here it spreads out and turns
eastward, passing between Florida and Cuba as the Florida
Current.
In Florida Strait the Gulf Stream moves with a speed of
80 nautical miles a day in the centre, and conveys about 22
cubic miles of water an hour. The combined Florida and
Antilles Currents move northward to Cape Hatteras with an
average velocity of 70 miles a day in the centre, and half
this amount on the edges and convey about 47 cubic miles of
water an hour. Off Cape Hatteras is the so-called " Delta
of the Gulf Stream," where it begins to break up into several
branches. South of Nova Scotia the velocity is about 38 miles
a day. At the south-eastern and southern edge of the Grand
Banks of Newfoundland the Gulf Stream comes into conflict
with the cold southward-flowing Labrador Current, which
greatly lowers its temperature. The average temperature of
the water flowing towards Europe after passing the Grand
Banks is 10-15 F. lower than the temperature of the Gulf
Stream off Cape Hatteras, and the greater part of this cooling
must be due to the Labrador Current, either directly by
intermingling of the warm and cold waters and the melting
of icebergs which enter the warm current, or indirectly by
the winds which blow from the cold to the warm water. The
salinity of the water is also lowered somewhat.
Off the Newfoundland Banks the Gulf Stream divides into
several branches ; the most northerly flows towards West
Greenland, another flows towards Iceland, and a third,
containing the main body of the water, flows towards Europe
and the Mediterranean. The West Greenland Current is
felt as far as 66 N. ; it is this current which causes the decay
of the ice brought round Cape Farewell by the East Greenland
Current. North of 66 it appears to curve round and join the
Labrador Current, but part of it may continue northward
beneath the surface and cause the " North Water," the .wide
sheet of navigable water found in the upper end of Baffin Bay
in summer and autumn. The branch of the Gulf Stream
which passes directly towards Iceland usually reaches the
south-west coast, where it ameliorates the climate somewhat,
after which it is lost.
The third or main branch of the Gulf Stream passes directly
THE CIRCULATION OF THE OCEANS 73
eastward, again dividing in about 45* N., 40 W, into two
branches. The southern branch turns south-eastward, skirting
the coasts of South-west Europe and Africa as a cold current,
and ultimately re-entering the North-east Trade Current.
Between Gape Verde and Gibraltar, and even farther north
in summer, it sets off the coast, and is separated from the land
by a belt of cold upwelling water, to which we will refer again
later. The northern branch, gaining renewed velocity from
the prevailing south-west winds, crosses the Atlantic with an
average speed of 12 miles a day, and bathes the shores of
Western and North-western Europe from the Bay of Biscay
to the North Sea. This current is banked up against the
coast, greatly ameliorating the climate. The bulk of the water
passes north of Ireland and Scotland to the North Sea, from
which one arm passes west of the Faroes to Iceland, turning
east again north of Iceland and mingling with a south-
easterly branch of the East Greenland Current in a series of
great whirls.
The larger arm, under the influence of the prevailing
southerly winds, drifts along the Norwegian and North
European coasts to Novaya Zemlya, where it is largely
overlain by colder but fresher water and loses its identity.
From North Cape an arm goes northward to Spitsbergen,
where it mingles with the westward-flowing Arctic Drift
in another series of whirls ; this branch gives rise to the
relatively favourable climate of Spitsbergen, and keeps the
western coasts of this archipelago almost free of ice. In the
Faroe-Shetland channel the volume of the warm current
is about 2 cubic miles an hour, and it has decreased to less
than i cubic mile off the Lofoten Islands. Even this amount,
small compared with the volume of the Gulf Stream off the
Atlantic coast of Florida, is of very great climatic importance,
and its variations from year to year have important effects
on the Norwegian harvests. An increased volume and high
temperature (the two usually go together) of the Atlantic
Current off Norway in May gives good harvests in the autumn
of the same year, and diminishes the amount of drift ice in
the Barents Sea one or two years later.
We have seen that in the Arctic Ocean the last remnants
of the Gulf Stream are finally lost beneath a layer of colder,
fresher water. The latter originates chiefly in the great
74 CLIMATE THROUGH THE AGES
rivers of Eurasia and North America which discharge into
the Arctic Ocean, and this surface stratum, in which ice-
formation is very active, forms the mainspring of the return
cold circulation. Passing north of Spitsbergen it continues
towards the east coast of Greenland ; the main mass of the
water follows this coast southwards as the East Greenland
Current, bearing great quantities of ice. Owing to the
earth's rotation, this current is banked up against the coast ;
it rounds Cape Farewell and passes up the west coast of
Greenland as far as Disco Island. Here it turns westward
under the influence of the prevailing easterly winds, and
finally, mingling with the West Greenland Current, it flows
southward as the Labrador Current, gaining important
accessions from Smith Sound and other channels in the
Northern Archipelago. Off the Newfoundland Banks the
Labrador Current meets the Gulf Stream, as we have seen,
and helps to lower its temperature. When the Labrador
Current and Gulf Stream meet, their densities are approxi-
mately the same, and they mix along the junction. The
mixture is, however, slightly heavier than either of the original
currents, and this produces a " density wall," on either side
of which the currents are opposed. The maximum density
is from 20 to 30 miles inside the cold wall, so that there is a
cold current flowing alongside the Gulf Stream in the same
direction. There are also continual eddies breaking off from
the cold wall and drifting eastwards.
Before leaving the subject of the North Atlantic and Arctic
circulation it is necessary to emphasise the part played by
floating ice. It has been pointed out that cold water can only
remain at the surface above warmer water by virtue of being
lighter, because it is less saline. This relative freshness can
be brought about in three ways : by heavy rainfall, as in the
doldrums, by great rivers, or by the addition of ice. Ice,
even when formed in the sea, is fresh, and though some salt
water is usually mixed up with the ice at first, this tends to
drain out. Hence, when the ice melts it decreases the salinity
of the neighbouring sea water. A thin layer of relatively
fresh water is constantly gaining salt from below by mixing
and diffusion, and unless it continually received accessions
of fresh water, by the time it had travelled a thousand miles
or so it would differ little in salinity from the underlying
THE CIRCULATION OF THE OCEANS 75
water. Hence it could no longer exist as a cold surface
current. There are not likely to be great differences of rainfall
over adjacent parts of the ocean (if there were, the heavier
rainfall would most likely be over the warmer water), and the
accession of river water is only possible near the coast, so
that the only way in which a current can remain relatively
fresh while traversing the open ocean is by bearing with it
large quantities of floating ice, the melting of which con-
tinually renews the surface layer. In the absence of ice,
the current would either lose its identity, or become heavy
and sink below the surface. Thus, for instance, in the Arctic
Ocean the fresh water from the great rivers is conserved
by being frozen, instead of mixing with the more saline
underlying water, and helps to form the floating ice-cap or
Palaeocrystic ice. The East Greenland Current is initiated
by this floating ice-cap and maintained by the ice which it
carries with it, and which gradually melts. In the same way
the Labrador Current is supplied partly by the remains of the
East Greenland Current and partly by the ice from the
innumerable channels of the Arctic Archipelago of America.
It seems highly probable that if there were no floating ice in
the Arctic Ocean the East Greenland and Labrador Currents
would not exist ; the water as it cooled would sink, and the
return to lower latitudes of the water brought by the warm
currents would take place not at the surface, but below the
surface, if not actually at the bottom.
I have described the North Atlantic circulation, with its
extension into the Arctic, in some detail, because a good
grasp of it is necessary in order to understand the way in
which changes of the oceanic currents controlled the warm
periods. The other oceans may be dismissed more briefly.
The western halves of the North Pacific, South Atlantic,
and, to a less extent, the South Pacific all have warm currents
resembling the Gulf Stream. In the North Pacific the warm
current is unable to penetrate the Bering Strait, and there-
fore turns south-eastward and washes the western coast of
North America. In the South Atlantic and South Pacific
the warm currents enter a great stream of water which
circumnavigates the globe in the Southern Ocean, picking
up ice from the Antarctic and sending branches northward
along the western coasts of all the continents. In the tropical
76 CLIMATE THROUGH THE AGES
Indian Ocean the currents are largely controlled by the
monsoons.
I have referred to the importance of upwelling cold water.
Cold water, owing to its greater density, tends to sink towards
the bottom, so that, except in the presence of ice, there is
generally a steady decrease of temperature as we go deeper
below the surface. Since the maximum density of sea water
occurs at its freezing point of about 28 F., so long as there is
a plentiful supply of water at this temperature the bottoms of
the great oceanic basins will be occupied by water not much
above the freezing point. The warm surface layer may be
likened to a skin, and wherever this skin is broken , the colder
underlying layers will be exposed. This will happen whenever
there are winds over adjacent areas blowing away from each
other (divergent winds), or whenever the winds blow off
the coast. It will also happen whenever a current flowing
along a coast-line is deflected away from it by the earth's
rotation. The latter happens with the currents on the west
coast of South America and South Africa the Humboldt
and Benguela Currents which turn away from the coast
and cause belts of upwelling cold water to form between
them and the shore. The low temperature of these currents
is due quite as much to this upwelling of cold water as to
the original low temperature of the surface water. The
temperature of the surface layers is also lowered slightly by
breaking waves, which mix up the surface " skin " with the
underlying colder layers and so cause a diffusion of heat
through the whole depth affected by the waves.
Along the edges of an ocean current travelling across
the open sea there is usually a certain amount of eddy motion.
This is especially noticeable off the Newfoundland Banks,
where the Gulf Stream meets the Labrador Current. This
must result in a certain amount of mixing, and a decrease
in the volume or temperature of the warm current. It also
lowers the average velocity, and therefore increases the* loss
of heat by radiation and conduction while the current is
travelling a given distance. The loss both of volume and of
heat is greater in proportion from a weak current than from
a strong one. Thus we may sum up the causes which lead
to the decrease of temperature or volume in a warm ocean
current as follows :
THE CIRCULATION OF THE OCEANS 77
1. Mixing with colder surface water by eddy motion along
the edges.
2. Melting of floating ice which drifts on to the warm
current.
3. Mixing with colder underlying water by
(a) upwelling due to divergent winds or motion
directed away from a coast ;
(b) breaking waves.
4. Cooling by conduction to the air, especially to cold winds.
5. Cooling by radiation.
We have next to consider the variations which these factors
may have undergone in the geological past, and especially
during the warm periods. In Chapter II. we found that
during the warm periods, on the poleward sides of the sub-
tropical high-pressure maximum, the winds tend to blow
directly towards the poles over the whole surface of the ocean.
These winds would act on the water in the way described in
the first paragraph of this chapter ; that is, they would drive
a body of water towards the right (in the Northern Hemisphere)
or towards the eastern shores of the ocean. There would be a
piling up of the water in the east, so that the surface of the
ocean would slope downwards towards the west. In the
steady state this would give rise to a wide oceanic surface
current directed from south to north, in the same direction
as the winds. There is no reason to suppose that the inter-
tropical circulation during the warm periods differed from
that found at present, and the warm currents due to the winds
of middle latitudes would be reinforced by the warm inter-
tropical water driven westward in the Equatorial Currents
and rounding the western ends of the sub-tropical highs.
Under these conditions, and with the complete or almost
complete absence of floating ice, the occurrence of adjacent
are^s of water at different surface temperatures would be
reduced to a minimum. Thus, the cooling under headings
i, 2, and 4 would be much less than at present.
The temperature of the water at the bottom of the deep
oceanic basins cannot be lower than that of the coldest part
of the sea surface, in fact, owing to earth heat, it must be a
few degrees higher. During the warm periods, therefore,
78 CLIMATE THROUGH THE AGES
when the surface waters of the polar oceans were well above
freezing point, there must have been a corresponding rise in the
temperature of the bottom layers. This implies a marked
decrease in the vertical temperature gradient, and while
there must always have been upwellings of underlying water,
due to off-shore winds and currents leaning away from the
coasts, their cooling effect must have been much less than at
present. Further, with the decrease of storminess consequent
on the absence of the polar fronts, steady light or moderate
winds would prevail in middle latitudes, and there would be a
great diminution of divergent winds and of wave motion.
Thus the cooling of surface ocean currents under headings
3(0) and 3 (b] would also be less than now.
Finally, we come to 5, cooling by radiation. As will be
seen in Chapter VI., the higher temperature of the air implies
a greater amount of water vapour, especially above the
oceans, but probably not an increase in the cloudiness, and
this means that a larger part of the earth's radiation would
be absorbed by the air, part of it being returned to the surface
of the sea and helping to maintain the temperature. Thus
we see that during the warm periods all the circumstances
worked together to maintain the temperature of the warm
ocean currents into high latitudes. Since these warm currents
were also accompanied by warm winds, it will be seen that
with large, open oceans and low, level continents, the extension
of warm temperate oceanic climates into the immediate
neighbourhood of the poles does not involve any insuperable
difficulty.
An investigation of the probable systems of ocean currents
in the northern hemisphere during the various geological
epochs was made by P. Lasareff (4). He placed plaster
models of continents in a circular plane basin filled with water,
and directed streams of air obliquely towards the circum-
ference to represent the trade winds. When the model
reproduced the present land- and sea-distribution, the currents
produced resembled existing currents even in detail. The
horizontal temperature gradient was simulated by passing a
heating coil round the edge, which represented the equator.
The results are of great interest ; in the models representing
the warm periods ocean currents passed across the pole,
whereas in the cold periods no current crossed the pole.
THE CIRCULATION OF THE OCEANS 79
The results were especially effective in reproducing the
variations of climate in Europe. The currents shown by the
models agree with the directions of migration of marine
animals.
The picture we have drawn of the oceanic circulation
during the warm periods warm currents extending from
shore to shore of the oceans and steadily drifting poleward,
to return to low latitudes beneath the surface is not the
only one which has been presented. T. C. Chamberlin (5)
has arrived at very different conclusions ; he supposes that
during the warm periods there was very great evaporation
in low latitudes, so great in fact that the increased salinity of
the water caused it to become heavy enough to sink to the
bottom. Here it travelled slowly north and south towards the
poles, retaining its heat, and rising to the surface in high
latitudes, where it caused highly favourable climates. An
illustration of this type of circulation on a small scale has been
described earlier in this chapter (the " North Water " in
Baffin Bay). But I do not think that this " reversal of the
oceanic circulation " is a practicable explanation of climatic
changes, for at least two reasons. In the first place, it will be
seen in Chapter VI. that greater warmth does not necessarily
mean greater evaporation ; once the air is saturated, it
cannot take up any more moisture unless either the tem-
perature rises still further or some of the water vapour it
already contains is first condensed as rain. The temperature
cannot go on rising indefinitely, and the conditions during
the warm periods were less favourable to rainfall in low and
middle latitudes than at present ; hence, evaporation was
probably less active rather than more active than now.
Secondly, there does not seem to be any obvious reason why
the saline water should rise at the poles when it got there.
In the absence of a wind-driven circulation, we should expect
the heavy water to remain at the bottom and accumulate
there, until it occupied all the oceans except a thin surface
layer. Here there would be a slow drift of water from the
regions where precipitation exceeded evaporation, and from
the mouths of the great rivers, to the regions where evaporation
exceeded precipitation, the drift being just enough to maintain
equilibrium. The example of the " North Water " is not to
the point ; the warm water here comes to the surface either
8O CLIMATE THROUGH THE AGES
in an eddy or because the prevailing off-shore winds drive
away the surface layer of colder but fresher water.
During the Quaternary Ice-Age the warm currents stood
less chance than now of carrying an appreciable portion of
their original warmth to high latitudes. Owing to the
enormous quantities of floating ice which existed in the oceans,
and which have left traces of their existence in the submarine
accumulations of glacial material which have been dropped by
icebergs, the surface waters must have been very cold. We
have evidence of great icebergs in the English Channel,
which dropped boulders weighing many tons on to the sea-
floor at Selsey, and there are glacial accumulations off the
west coast of Ireland and in many other localities. The
winds from the glacial anticyclones must have driven these
icebergs and their cold thaw water far across the oceans.
This water, being light because of its freshness, spread over
the warmer but more saline water of the warm currents in
mid-Atlantic, just as it does to-day in the Arctic, so that the
Gulf Stream, for instance, must have lost its identity in
relatively low latitudes.
In addition to Chamberlin's hypothesis referred to above,
changes in the oceanic circulation induced by alterations of
the land and sea distribution have often been suggested to
account for climatic changes. F. Kerner has been especially
active in explaining the warm periods in this way, but I am
deferring a consideration of his work until Chapter VIII.,
since his method is to analyse the distribution of temperature
resulting from the present land and sea distribution, and to
apply the results to the geography of former geological epochs.
We may refer here to a very old idea that the Quaternary
Ice- Age was brought about by the omission of the Gulf Stream
from the economy of the North Atlantic. Four ways have been
suggested in which this may happen ; the opening of a wide
gap between North and South America by the submergence
of the Isthmus of Panama, allowing the Gulf Stream proper
to pass into the Pacific instead of being bent back into the
North Atlantic ; the northward extension of the eastern
shore-line of South America in such a way as to deflect the
greater part of the Equatorial Current to the south instead
of to the north ; an increase in the velocity of the North-east
Trades in the Atlantic relatively to the South-east Trades,
THE CIRCULATION OF THE OCEANS 8 1
shifting the whole system of Equatorial Currents southward
with the same result ; and the formation of an extensive
" Antillean Continent " across the path of the Gulf Stream
and Antilles Current, forcing them to pass eastward much
farther south than at present and form a closed circulation
in tropical and sub-tropical regions. Any one of these changes
might modify the surface circulation in the North Atlantic
and introduce corresponding climatic changes in eastern North
America, and especially in Europe, and we must examine them.
The separation of North and South America by a strait
across the Isthmus of Panama occurred during the greater
part of the Tertiary period, and was responsible for the
great difference in the faunas of these continents, but it does
not appear to have persisted into the Quaternary. Hence
from this factor alone we should have expected a cold climate
in North-west Europe during the Tertiary, becoming warmer
in the Quaternary, which is the reverse of what actually
happened. Evidently the opening and closing of this gap
did not greatly affect the Gulf Stream. South America
stands in the course of the South Equatorial Current like a
mighty wedge, with its apex at Cape San Roque in 5 S., and
all that portion of the current which lies between the equator
and 5 S., and which would normally be deflected southwards
by the earth's rotation, is turned to the northward, and enters
the North Atlantic as the Guiana Current. It is the Guiana
Current which mainly supplies the warm water in the Gulf of
Mexico. If, owing to geographical changes, the apex were
shifted two degrees farther north, the amount of water which
it deflects from the Southern to the Northern Hemisphere
would be decreased by about forty per cent., an event which
would appreciably affect the warmth of the North Atlantic.
The north-eastern part of South America appears to have
been slightly lower during the Quaternary than at present,
but the configuration both of the land surface and of the
ocean floor is such that a change of a thousand feet or more,
whether elevation or depression, would make very little
difference in the latitude of the apex of the wedge. There is,
therefore, no reason to suppose that the geographical changes
in this region during and since the Quaternary have been
sufficiently great to introduce any important modifications
in the volume of the Guiana Current.
82 CLIMATE THROUGH THE AGES
There is a considerable amount of evidence that during
at least the early part of the Quaternary period the Gulf of
Mexico was largely dry land ; this would merely turn the
waters of the Guiana Current north into the Antilles Current,
and would not greatly affect the temperature of the Gulf
Stream off the east coast of the United States. In fact, so
far as the geography of the Quaternary can be reconstructed,
it was as favourable as the present for the existence of a
powerful warm current in the North Atlantic.
This leads us to ask if the effect of minor geographical
changes on the great oceanic circulations has been over-
estimated. When the project of a Panama Canal was first
mooted, there was some popular outcry that it would allow
the Gulf Stream to pass through into the Pacific and so
interfere with the climate of Europe. This fear was quite
unnecessary, but it illustrates the importance which the
" man in the street " attaches to the slender barrier of the
Isthmus of Panama. Actually, as we have seen, only about
one-third of the water which forms the Gulf Stream off the
east of Florida passes through the Gulf of Mexico at all ;
two-thirds of it is derived from the Antilles Current, which
takes the whole of the water from the North Equatorial
Current. But the real reason for the existence of the Antilles
Current is not the chain of islands known as the Antilles, it
is the limitation of the sub-tropical antic yclonic centres to the
eastern halves of the oceans, combined with the rotation of
the earth, which deflects the North Equatorial Current to
the right, i.e., northwards.
In the North Pacific there is a gap between the Philippine
Islands and China which is wide open to the waters of the
North Pacific Equatorial Current, but the latter ignores
the invitation, and instead turns northward in the open
ocean to form the warm current which gives its favourable
climate to Japan. In fact, under normal conditions, water
which is travelling westward north of the equator must turn
north, and water which is travelling westward south of the
equator must turn south, unless hindered from doing so by
some geographical obstacle. This flight from the equator
will take place most readily where the isobars also trend
away from the equator at the western ends of the sub-tropical
oceanic anticyclones.
THE CIRCULATION OF THE OCEANS 83
Finally, there remains the fourth consideration, the unequal
strength of the Trade winds in the two hemispheres. At
present the atmospheric circulation over the whole Southern
Hemisphere is stronger than that over the Northern Hemi-
sphere, partly because the greater area of the oceans leads to
a smaller loss of energy through friction, and partly because,
owing to the very low temperatures over Antarctica, the
temperature gradient between equator and pole is greater in
the Southern Hemisphere. Hence the South-east Trades
are the stronger, and owing to their momentum are able
to blow right across the equator into the Northern Hemisphere.
The difference is greatest in June to July arid least in December
to January, but the doldrums lie north of the equator through-
out the year. Hence the greater part of the Equatorial
Current, both in the Atlantic and Pacific Oceans, lies north
of the equator, and is deflected northward by the earth's
rotation.
Now we know that the glaciation of the Antarctic Continent
began during the Tertiary, while the Northern Hemisphere
was still enjoying genial climates in high latitudes. Hence
we may suppose that at this period the South-east Trade
crossed the equator to an even greater extent than at present,
and that this helped to maintain the temperature of the
Northern Hemisphere and to depress that of the Southern
Hemisphere. Then the Northern Hemisphere also became
glaciated, and, owing to the greater land area in the glaciated
regions, these northern ice-sheets outweighed the southern,
and caused the North-east Trades to become as strong as or
stronger than the South-east Trades. This caused the
Northern Hemisphere to receive a smaller share of the
equatorial warm water, intensifying the glacial conditions in
that hemisphere still further, while the glaciation of the
Southern Hemisphere remained relatively slight. Hence we
see that during the Quaternary period the variations of the
oceanic circulation must have tended to exaggerate the
climatic oscillations in the Northern Hemisphere and moderate
them in the Southern Hemisphere.
The partial closing of the gap between Greenland and
Europe by the elevation of the submarine ridge which passes
through Iceland and the Faroes to Scotland, which occurred
during the Quaternary Ice-Age, must have deflected the
84 CLIMATE THROUGH THE AGES
Gulf Stream Drift into lower latitudes and displaced the
Icelandic minimum southwards, altering its alignment to
west-east or even north-west-south-east, instead of south-
west-north-east as at present. This must have profoundly
modified the climate of the countries bordering on the North
Atlantic, and probably increased the severity of the glaciation
in these regions. These changes have been discussed by the
late F. W. Harmer (6, 7), who attempted to reconstruct the
pressure distribution and storm tracks which would prevail
under these conditions. His papers were written before the
work of G. de Geer on annual clay varves had demonstrated
the contemporaneity of at least the Wurmian glaciation in
North America and Europe, and he supposed that the
glaciations of these two continents alternated, but I think
the pressure distribution which he deduces would have
favoured increased winter snowfall over the north-eastern
parts of North America, and hence brought about glaciation
rather than deglaciation.
H. J. E. Peake and H. J. Fleure (8) in some comments
on Harmer's papers suggest a complete explanation of the
Quaternary glacial sequence in Europe in terms of the elevation
and depression of a Labrador-Greenland- Iceland-Scotland
land-bridge. They point out that with this bridge complete
there would be little or no ice in the North Atlantic in summer,
and the climate of the British Isles would be dry and sunny,
similar to that of the coast of British Columbia, and un-
favourable for glaciation. A somewhat smaller elevation,
however, which left some gaps in the land-bridge, " would
probably increase the amount of ice in the North Atlantic
so long as the northern lands remained much higher than at
present," and would cause a deterioration of the climate of
Western Europe. They point out that the first or Guiizian
glaciation was limited to Scandinavia and the Alps, and did
not extend to France, the North Sea, or the English plain,
so that it might well have been the result of elevation only,
and they suggest that during this glaciation the land-bridge
was complete and the climate of England favourable. The
cold period in Eastern England, represented by the Weybourne
Crag and Chillesford Beds, which appears to correspond with
the Gunzian glaciation, would then fall either just before or
more probably just after the time of maximum elevation.
THE CIRCULATION OF THE OCEANS 85
The later glaciations occurred during periods of lesser elevation,
when the land-bridge was not complete and there was much
ice in the North Atlantic ; the interglacial periods occurred
during the intervals of subsidence in which the land fell below
its present level.
There is quite a lot to be said for this interpretation of
the Quaternary sequence in Europe. The suggestion that
a large amount of cold water was accumulated in a closed
Arctic basin, and that the level of the Arctic Ocean may
even have risen well above the general level of the remaining
oceans, this mass of cold water being subsequently released
by a depression of the land and flooding southwards into
the Atlantic, may be especially fruitful. It gives a plausible
explanation of the sudden appearance of the Arctic fauna
in middle latitudes, for example, in the Sicilian (3OO-foot)
raised beaches of the Mediterranean, which contain a fauna
now found only in the northernmost parts of Europe. But
we must not forget that the Quaternary sequence in Europe
was paralleled by that in other parts of the world, such as the
Himalayas, and that any explanation of the phenomena
found in Europe must fit into place in a larger scheme which
takes account of the whole world.
The final role of the oceans in climatic changes to which
we have to refer is that of regulator. It is well known that
owing to the high specific heat of water and to the fact that
the changes of temperature penetrate to a greater depth,
a large sea or ocean takes much longer to warm or to cool
than does a land surface in the same latitude. Hence the
annual range of temperature on islands or windward coasts
is much less than that in the interior of great continents. But
this conservation of heat is not limited to periods of a year
or even a few years. During a period of cooling climate,
the coojed sea water sinks to the bottom of the oceans and the
warmest water remains at the top.
If the ocean covered the whole surface of the earth it
would have an average depth of 2,600 metres (8,500 feet) ;
if we take the amount of heat reaching the outer limit of
the earth's atmosphere from the sun as 720 calories per
square centimetre per day (see next chapter), we find that
the whole of this solar heat for a year would have to be
absorbed and retained by this universal ocean, to raise the
86 CLIMATE THROUGH THE AGES
temperature by one centigrade degree. Again, suppose
that at the end of a long warm period the mean temperature
of the oceans is 10 C. (18 F.) higher than at present. When
we remember that a large portion of the oceanic water is
now little above freezing point, it is seen that this is not an
exaggerated assumption. Then if for some reason the
equilibrium temperature sank to its present level, the heat
conserved in the oceans would suffice to maintain the average
temperature 2 C. (3-6 F.) above the present for a period
of nearly 250 years. The retardation of warming up after
a cold period would be less effective, since the cold water
would remain at the bottom of the ocean without greatly
affecting the higher layers.
R. Spitaler (9) goes even further than this. In his theory
of the astronomical cause of ice-ages (Chapter V.), he attempts
to get over the difficulty that increased eccentricity of the
earth's orbit would act oppositely in the Northern and Southern
Hemispheres by supposing that the regulating effect of the
oceans could maintain glacial conditions during a period of
10,000 years while conditions were otherwise not specially
favourable for glaciation. He does not give any calculations
in support of this figure, and it seems to be excessive. Spitaler's
claim would perhaps have been based more soundly on the
consideration that owing to the creep of cold water along
the ocean floor, severe glaciation in one hemisphere would
suffice to maintain low temperatures throughout the bottom
layers of the whole ocean, and so to some extent lower the
temperature in the other hemisphere also, though the process
would not be very effective.
Huntington and Visher (10) suggest that the growing
salinity of the oceans during the course of geological time
may have had some climatic effect. This may be so, but I
doubt if the effect can have been noticeable during the greater
part of geological time. The accession of salt to the ocean is
at present derived almost entirely from the sedimentary rocks,
that is, it has previously been withdrawn from the oceans.
The very great estimates of the duration of the pre-Cambrian
period now current nearly a thousand million years
suggest that even at the beginning of the Palaeozoic the ocean
had a long history behind it, and was almost as salt as it is now.
This is borne out by the relatively advanced organisation of
THE CIRCULATION OF THE OCEANS 87
the earliest fossils, which also suggest life in salt water rather
than in fresh. It is possible, however, that a smaller salt
content may have been a contributory cause in the pre-
Gambrian glaciations. The fact that fresh water reaches its
greatest density at a temperature above its freezing point,
while ordinary sea water freezes before it cools to its maximum
density, is the reason why fresh-water lakes freeze more
readily than ocean inlets of the same depth. In very early
geological ages it is possible that the sea was less salt than
now, and if the difference was sufficient to bring the tempera-
ture of maximum density above the freezing point, which
would happen if the salinity were less than 24-7 parts per
thousand compared with the present value of about 35 per
thousand, the surface of the ocean would have frozen more
readily than at present. Other conditions being equal,
therefore, glaciation of coastal mountains would have been
easier in very early geological ages than at present.
The fluctuations of salinity from one geological epoch to
another may also have affected the capacity of the air to
absorb moisture from the oceans. The withdrawal of a great
volume of fresh water during the glacial periods, to be locked
up in the form of ice, must have increased the average salinity
slightly. Moreover, there are variations in the amount of
soluble matter locked up in salt and gypsum beds, etc. ;
at the close of a long warm period with shallow seas and
numerous lagoons this amount must have been appreciably
greater than at present. Thus we may suppose that there
have been small fluctuations of salinity, with minima at the
end of the long warm periods and maxima during the ice-ages,
superposed on a very slow secular increase. Decreased
salinity would increase the vapour pressure over the oceans,
and it will be seen in Chapter VI. that an increase of water
vapour in the atmosphere tends to raise the mean temperature.
In this way there would be a tendency for both the warm
periods and the ice-ages to be intensified with also a slight
secular fall of temperature. I think, however, that at least
since the middle of the Palaeozoic period the variations of
temperature due to this cause alone must have been so small
as to be negligible compared with the other causes of variation.
It seems highly improbable that at any stage in the known
geological record, with the possible exception of the early
88 CLIMATE THROUGH THE AGES
pre-Cambrian, was the main mass of the sea water sufficiently
fresh for its temperature of maximum density to be above
its freezing point.
REFERENCES
(1) DURST, C. S. " The relationship between current and wind." London,
Q,. J. R. Meteor. Soc., 50, 1924, p. 113.
(2) HEPWORTH, M. W. CAMPBELL. " The Gulf Stream." London, Geogr. J.,
1914, p. 431.
(3) SVERDRUP, H. U. " Oceanography for meteorologists." New York, 1942.
(4) LASAREFF, P. " Sur un methode permettant de demon trer la d^pendance
des courants occaniques des vents aliz^s et sur le role des courants
oceaniques dans le changement du climat aux epoques gcologiques."
Beitr. Geoph., 21, 1929, p. 215.
(5) CHAMBERMN, T. C. " An attempt to frame a working hypothesis of the
cause of glacial periods on an atmospheric basis." J. GeoL, Chicago,
7> '899, PP. 545 and 667.
(6) HARMER, F. W. " The influence of the winds upon climate during the
Pleistocene epoch." London, Q.J. Geol, Soc., 47, 1901, p. 405.
(7) HARMER, the late F. W. " Further remarks on the influence of the winds
upon climate during the Pleistocene epoch." London, Q,. jf. R. Meteor.
Soc., 51, 1925, p. 247.
(8) PEAKE, HAROLD J. E., and H. }. FLEURE. " The Ice-Age." Man, 1926,
p. [4]-
(9) SPITALER, R. "Das Klima des Eiszei takers." Prag, 1921. (Litho-
graphed.)
(10) HUNTINGTON, E., and S. S. VISHER. " Climatic changes, their nature
and cause." New Haven, 1922.
CHAPTER IV
RADIATION FROM THE SUN
IT has been shown in the preceding chapters that a com-
paratively small initial change in the mean temperature
of the polar regions might be so magnified by secondary
effects, especially in connexion with the polar ice-caps, that
the final result would be a very great change of climate,
sufficient to account for the genial polar climates of the
" warm " periods. We have now to begin our search for such
possible initial causes, and the most obvious place to look is
the great source of all warmth and life on the earth the sun.
Careful studies are being made, especially by the Astrophysical
Observatory of the Smithsonian Institution (i), of the amount
of heat radiated by the sun, and of the variations to which it is
subject. The measurement aimed at in the first place is the
amount of heat which would reach a unit area of the earth's
surface exposed to the solar beam at right angles, if none
of it were intercepted by the earth's atmosphere. As it is
impossible to get outside the atmosphere to obtain these
measurements, the result has to be arrived at indirectly.
Observatories have been established at several points on high
mountains, where the air is normally very dry and the sky
clear ; the chief of these observatories are at Montezuma in
Chile (8,895 feet), Table Mountain, California (7,500 feet)
and Mount St. Katherine, Egypt (8,500 feet). Elevated
stations are chosen because the mass of air through which the
sun's rays have to penetrate is less ; dry regions because part
of the solar radiation is absorbed by water vapour and also
because a clear sky is essential for regular observations. At
these places the direct heating power of the sun is therefore
greater than it is on low, humid and cloudy plains, but there
is still a large amount of absorption. This is calculated in
two ways. When the sun is nearly vertical, the thickness of
air through which its rays have to penetrate is much less than
when it is near the horizon, and the heat received is
9 CLIMATE THROUGH THE AGES
consequently greater. Making a number of observations at
intervals during a morning when the meteorological conditions
remain practically uniform is therefore almost equivalent to
making observations at the same moment at different heights
above the ground. It is found that with radiations of certain
wave-lengths which are only absorbed slowly by the
atmosphere, the logarithm of the amount lost is proportional
to the air mass through which the rays have passed, and it is
possible to calculate the amount of heat which these rays would
deliver if there were no atmosphere. By means of a spectro-
scope these measurements are taken for a number of different
wave-lengths, and the results are plotted. In general, they
show a smooth curve which is, however, interrupted by a few
marked depressions corresponding with the wave-lengths of
radiations which fail to penetrate the air at all. These rays
are almost or quite absorbed in the high levels of the
atmosphere, and in order to include them in the estimate of the
sun's total radiation an allowance has to be calculated from
the values of neighbouring wave-lengths. As a result of a series
of measurements and calculations carried out by the Astro-
physical Observatory at various stations from 1920 to 1939, the
mean value of the intensity of the sun's radiation outside the
limits of the earth's atmosphere is given as i 945 calories per
square centimetre per minute. This means that a layer of
cold water a centimetre deep exposed to a vertical sun,
absorbing all the solar radiation and giving out none in
exchange, would warm up at the rate of nearly 2 C. a
minute. This value is termed the " solar constant," but the
name is somewhat misleading, as it seems probable that the
mean value of i 945 is subject to small variations on either
side of the mean.
The range of these variations is still subject to doubt, owing
to uncertainty in the daily values. The annual means for
the 20 years from 1920 to 1939 range from 1-928 in 1922 to
i -950 in 1921, or just over one per cent, of the mean, but both
of these are early values and somewhat uncertain. Abbot
finds a number of short periodicities in the data, the longest
being 23 years ; he estimates the amplitude of this cycle
as less than 0-5 per cent, of the average value of the solar
constant. It is clear that so small a change, even if maintained,
would not suffice to bring about great changes of climate.
RADIATION FROM THE SUN 9 1
The possibility of much greater changes of solar radiation in
geological time cannot, however, be ruled out. Sir George
Simpson (2, 3) has discussed the probable effects of a large
oscillation of solar radiation, and has shown that they agree
with the observed climatic changes during the Quaternary.
The first effect of an increase in the solar radiation would
be to raise the temperature everywhere on the surface of the
earth, but more in low than in high latitudes. This would
immediately increase the amount of evaporation from water
surfaces, and also the strength of the atmospheric circulation.
More evaporation means more cloud and more precipitation.
But as will be seen in Chapter VII. , cloud reflects back to
space a large part of the solar radiation which falls on it.
Hence an increase of cloudiness lowers the temperature.
The final result of a large increase of solar radiation would
therefore be a slight rise of temperature and a great increase
in cloudiness and precipitation. The increase in total
precipitation with increasing radiation is shown in curve I of
Fig. 9, reproduced by permission of Sir George Simpson
and the Manchester Literary and Philosophical Society
from (2).
In high latitudes and especially on high ground, a large
part of the total precipitation falls as snow. As the radiation
increased, the proportion of precipitation falling as snow
would decrease, but for a time this decrease would be slower
than the increase of total precipitation. Hence the snowfall
would increase at first, but with a further increase of radiation
the general rise of temperature would cause so much less of
the precipitation to fall as snow that the total snowfall would
begin to decrease. The curve of snowfall is shown as II in
Fig. 9-
The accumulation of snow to form ice-sheets results from
the excess of snowfall over melting. The melting is shown in
curve III ; so long as the summer temperature does not rise
above freezing point it is inappreciable (point A on the curve) .
With rising temperature it grows slowly at first, so long as the
snow cover is complete, but as soon as the melting is sufficient
to expose part of the underlying surface it proceeds rapidly.
At point B the curves of snowfall and melting intersect and
above that point any accumulation of snow or ice will dis-
appear. Curve V shows the accumulation of snow during
92 CLIMATE THROUGH THE AGES
a period of increasing radiation. With decreasing radiation
the changes would proceed in the reverse order.
In low latitudes where there is no snow on low ground the
result of increasing radiation would be a progressive increase
of rainfall, but on equatorial mountains of sufficient height
there would be a glacial cycle resembling that in high latitudes.
RADIATION
Fig. 9. Effect of increasing radiation on precipitation and
accumulation of snow.
Fig. 10, also reproduced by permission from (2), shows the
effect of a double oscillation of solar radiation. The first
period of increasing radiation causes an ice-age in high
latitudes and increasing rainfall in low latitudes. At the
maximum radiation the ice melts and we have a warm moist
interglacial period in high latitudes coinciding with the peak
of a pluvial period in low latitudes. As radiation decreases,
there is a return of glaciation at first, but as radiation and
RADIATION FROM THE SUN
93
precipitation decrease still further, the latter, even though it
falls entirely as snow, is insufficient to balance the loss of ice
by outflow and the ice-sheets finally disappear. We now
enter a long cool arid interglacial period, which lasts until
a new increase of solar radiation brings about a renewal of the
glacial and pluvial cycle.
3
"c
zn
Fig. 10. Effect of two cycles of solar radiation on glaciation.
As shown in the lowest curve of Fig. 10, this reconstruction
fits in admirably with the classical sequence of glacial and
interglacial periods worked out by Penck and Bruckner (4).
The first crest of solar radiation represents the Gunz and Mindel
glaciations and the relatively short Gunz- Mindel inter-
glacial. The trough of low solar radiation represents the long
94 CLIMATE THROUGH THE AGES
Mindel-Riss interglacial. Finally the second crest of radiation
represents the Riss and Wurm glaciations and the short
Riss-Wurm interglacial.
Sir George Simpson worked out the geological and climatic
implications of his theory iri greater detail in another paper
(3), and showed that they are consistent with the general
climatic and biotic history of Europe during the Quaternary.
There are, however, certain difficulties.
In the first place, the assumption is made that the sun is a
variable star. If such variations are periodic, glaciation
should have recurred at short intervals throughout geological
time ; the theory cannot account for the long genial periods.
Sir George Simpson tends to meet this difficulty by continental
drift (see Chapter XIII.), but even so, such marked changes of
precipitation as the theory requires should have left their
records in the character of the sedimentary rocks, but there
is no trace of such a regular cycle in the geological record.
This difficulty was to some extent met by F. Hoyle and R. A.
Lyttleton (5), who supposed that at intervals of time of the
order of 100 million years the sun passes through clouds of
interstellar matter. The particles on and near the track fall
into the sun, their kinetic energy being converted into heat
and giving rise to increased solar radiation. Since the cloud
would in general be densest near the centre, radiation would
rise to a maximum and then decrease again, the time of the
passage being of the order of 100,000 years. Further, since
many such clouds are irregular, the one causing the Quaternary
ice-age may have had two centres, so giving two maxima
of radiation and four glaciations.
Secondly, the glacial sequence was far more complex than is
represented in the simple fourfold scheme of Penck and
Bruckner. The Riss glaciation had two or three distinct
maxima and the Wurm three, in addition to retreat stadia.
This objection is not very serious, however, because there is
no difficulty in postulating minor but still large fluctuations
of solar radiation superposed on the smooth curve of Fig. 10.
The third difficulty concerns the climates of the interglacial
periods. By the solar radiation theory the Gunz-Mindel and
Riss-Wurm should have been mild and very wet, the Mindel-
Riss cool and very dry. The available evidence suggests,
however, that the general succession of vegetation was very
RADIATION FROM THE SUN 95
similar in both Mindel-Riss and Riss-Wurm interglacials,
indicating that the recession of the ice in Europe was followed
in each case by a rise of temperature to a level somewhat
higher than the present. This rise in turn gave place to a fall
and finally to the onset of another glaciation. The post-
glacial period has so far followed a similar course, a rise to a
maximum in the " Climatic Optimum " followed by a fall,
but it is too early yet to say that we are now in the second
half of an interglacial period.
Finally, the available data do not support the hypothesis
that in low latitudes one pluvial period represents two
glaciations. The pluvial sequence has been worked out in
detail in tropical Africa, where the rises and falls of the great
lakes are believed by E. Nilsson (6) to have been contem-
poraneous with the advances and retreats of the mountain
glaciers. For example, in the last interpluvial period, when
the prehistoric Lake Kamasia dried out completely, the
mountain glaciers also melted completely. Subsequent oscilla-
tions of climate also run closely parallel in both lakes and
glaciers.
The evaporation from the tropical oceans, and hence the
total rainfall over the globe, depend on the wind velocity as
well as on the temperature. Large ice-sheets in high latitudes
increase the temperature gradient and hence the strength of
the atmospheric circulation, while at the same time, owing
to the existence of large semi-permanent glacial anticyclones
and the equatorward deflection of the storm tracks, they
decrease the area over which most of the rain falls. Hence,
other things being equal, each glacial period must have been
a pluvial period in tropical and sub-tropical regions and each
interglacial an interpluvial period. Whether this oscillation
was superposed on a longer oscillation in which one crest
covered two glaciations is not yet clear.
We may sum up by saying that if the radiation of the sun
has varied greatly in the past the variations would have had the
effects postulated by Sir George Simpson, but that there is
no direct evidence of such variation, and the indirect evidence
only partially supports the theory.
F. Kerner-Marilaun in his book on Palaeoclimatology (7)
takes the standpoint that variations of solar radiation should
only be brought in as a last resource, i.e., if it is impossible to
96 CLIMATE THROUGH THE AGES
account for the climate of a period of geological time by the
known factors of land and sea distribution, volcanic dust,
etc. As an example he considers the temperature of Messel,
near Darmstadt, Germany, during the Eocene. From geo-
graphical and astronomical factors he calculates the probable
temperatures as : January, 50 to 56 F., July, 70 to 74 F.
From the plant remains and the nature of the deposit the
temperature is estimated as 56 in January and 68 in July.
The differences are within the limits of probable error and it
is not necessary to bring in unknown factors such as a change
of solar radiation. In other examples the agreement is less
good but it is not yet possible to say whether the discrepancies
are due to errors in the calculations, in the interpretation of
the gcologiccil data or to changes of solar radiation. Never-
theless the author foreshadows the time when, from a large
number of such calculations in different geological periods,
it may be possible to reconstruct the variations of solar
radiation. Till then the question must be left open.
As another example of such a calculation I may mention
a reconstruction of the probable climatic conditions in the
middle Eocene of south-east England which I made for
Mrs. E. M. Reid and Miss Chandler (8). I estimated the
probable upper limit of the mean annual temperature as
65 F. (a remarkable agreement with Kerner-Marilaun's quite
independent figure) whereas Mrs. Reid and Miss Chandler
inferred from the vegetation that the mean temperature was
probably not below 70 F., and I suggested that the dis-
crepancy might possibly be due to increased solar radiation.
In all other respects the climatic reconstruction agreed closely
with the inferences from the vegetation.
Brief reference may be made here to a suggestion by K.
Himpel (9) that the sun suffered Nova-outbreaks in Mid-
Algonkian (pre-Cambrian), late Upper Carboniferous and late-
Pliocene, the average interval between outbreaks being 250 to
350 million years, which he claims is about right for Novae. Each
outbreak caused a thousand-fold increase in radiation which
resulted in a corresponding increase of precipitation, followed
by rapid cooling to a level below the present. At one stage
during the cooling there was a maximum snowfall and con-
sequently glaciation. Further, each outbreak diminished the
mass of the sun and consequently its normal radiation, leading
RADIATION FROM THE SUN 97
to a general level of climate somewhat cooler than before.
It seems impossible, however, that even a short-lived thousand-
fold increase of solar radiation could have occurred without
devastating animal and plant life, and there is certainly no
trace of such a catastrophe in late Pliocene.
Total radiation is not the only solar characteristic which
probably affects terrestrial climate ; we have also to take
account of the nature of the radiation. An index to the latter
is found in the spotted area of the sun. The spottedness is
generally expressed as the " relative number" (10), which
is obtained by an arbitrary formula, but which has been
found by photographic comparisons to be closely proportional
to the spotted area. A relative number of 100 corresponds
with about one five-hundredth of the sun's visible disc covered
by spots, including both umbra and penumbra. Since 1749
the monthly mean relative sunspot numbers have varied
between o and 206.
The sunspot " relative number " does not bear any simple
relation with the solar constant. Sunspots show a very marked
and persistent oscillation of approximately 11-2 years, falling
to an annual mean of 10 or less at sunspot minimum and rising
to a maximum which has varied since 1749 from annual
means of 46 to 154. This i i-year oscillation is not recognisable
in the values of solar radiation, so it must represent a variation
of some other solar characteristic. It seems to be an almost
permanent characteristic of the sun, for a cycle of ten or
eleven years has been found in the thickness of annual layers
in laminated deposits of various geological ages, including the
Upper Carboniferous glacial clays of Australia.
This eleven-year oscillation is not the only one shown by
sunspots ; more important for our purpose is the fact that
both the eleven-year means and the heights of the maxima
undergo large variations over decades. Thus the annual
maxima varied from 154 in 1778 to only 46 in 1817, and back
again to 139 in 1870. Previous to 1749 we have no systematic
observations, but there are a number of scattered references
in the Chinese archives dating back nearly to the beginning
of the Christian era and a few records from Europe going back
to medieval times. From these there appears to have been
an important maximum of solar activity towards the close of
the eleventh century, and another, which R. Wolf believed to
08 CLIMATE THROUGH THE AGES
y
be the absolute maximum over the whole of the Christian era,
about 1372.
There is little doubt that some relation exists between the
sunspot cycle and terrestrial^conditions, but it is very obscure.
In tropical regions temperature averages about 2 F. higher
at spot minimum than at spot maximum ; this relation
disappears in temperate regions but reappears in the Arctic,
where it is clearly shown both in the temperature of Spitsbergen
and in the amount of ice in the Barents Sea, indicating that at
sunspot maximum the temperature falls in the Arctic and the
area of the floating ice-cap increases. Over the world as a
whole rainfall seems to have a tendency to be greatest at spot
maximum, but there are many exceptions. One of the most
interesting relations is between sunspot relative numbers and
the frequency of thunderstorms, which are most frequent at
sunspot maxima. The agreement is close in inland and
tropical regions, especially Siberia, the West Indies and the
tropical Pacific, but almost or quite vanishes in the stormy
regions of western Europe (u). For the world as a whole
the variation from sunspot minimum to sunspot maximum
amounts to more than 20 per cent, of the mean number of
thunderstorms.
These various relationships suggest that at periods of great
solar activity as marked by large sunspot numbers, the earth
as a whole should be somewhat cooler, rainier and more
stormy than at times of little solar activity. Ellsworth
Huntington and S. S. Visher (12) have based a complete
theory of climatic change on these relationships. According
to their view, the climate of the earth is largely governed by
the relations between sunspots and storminess. Increased
solar activity is considered to result in increased storminess,
together with some displacement of the storm tracks. The
greater vertical movement of the air associated with the
increased storminess carries greater quantities of heat from the
earth's surface to the higher levels of the atmosphere, where
nearly half of it is lost by radiation to space, and when such
a period of increased activity occurs with extensive and high
continents, and perhaps with other favourable conditions, such
as a paucity of carbon dioxide, a glaciation results. This
combination of circumstances is considered to account for
the Quaternary glaciation, and perhaps also for that of the
RADIATION FROM THE SUN 99
Per mo-Carboniferous period. In the latter, the storm tracks are
placed very far south, and higher latitudes are supposed to have
remained unglaciated because they were occupied by deserts, a
conclusion which ignores the extensive development of coal
measures. Periods of slight solar activity and few sunspots
had slight storminess and steady winds from low to high
latitudes, hence they were periods of mild and equable climate
over the whole earth. The variations of solar activity in the
geological past are connected by Huntington with changes
in the distance of the nearest fixed stars, especially double
stars, a conclusion which few, if any, astronomers endorse.
It will be seen in Chapter XXII. that the variations of
rainfall in the temperate zone during the Christian era have
run fairly parallel with the variations of solar activity shown
by the records of sunspots and aurorae. But the phenomena
of the Quaternary Ice-Age were on a scale many times greater,
and would require enormous and prolonged outbursts of
sunspots, which seem quite improbable. Moreover it has
been shown (14) that the terrestrial changes are not pro-
portional to the sunspot relative number ; the effect falls off
rapidly as the relative number increases. Hence, while
variations of sunspot activity may account for some of the minor
glacial oscillations, it is unlikely that they played any appre-
ciable part in the four main glacial advances and retreats of the
Quaternary Ice-Age, and still less likely that they caused that
Ice-Age as a whole. The hypothesis also breaks down
completely over the Permo-Carboniferous glaciation.
One other speculative work may be briefly referred to,
because it is typical of a number of early hypotheses which
rely on a diminution of the sun's radiation to explain the
occurrence of ice-ages. E. Dubois (13) assumed that the sun
has passed through a series of stages represented now by
various fixed stars. At first, when the sun was in the stage
represented by blue-white stars, radiation was more intense
and the terrestrial climate was warm. The sun then changed
to yellow, its radiation diminished, and the earth cooled.
This was during the Tertiary period, and at intervals the
sun passed into a third stage, intermediate between the yellow
and red stars, during which its radiation was still feebler.
These intervals formed the various glacial epochs of the
Quaternary. The only " evidence " adduced in support of
IOO CLIMATE THROUGH THE AGES
the theory concerns the frequency of blindness to certain
colours, which is regarded as a reversion to previous geological
epochs, the spectroscopic analysis of the light of phosphorescent
animals, and similar peculiarities. This theory is quite
untenable. In the first place the postulated changes in
radiation would not have had the effects attributed to them.
Secondly, the age of the sun is now estimated as between a
hundred thousand million and a million million years, while
the age of the oldest known rocks is not much more than a
thousand million years (Appendix I.), so that it is highly
improbable that the later stages of the geological record
should have seen any appreciable change in the sun's constitu-
tion of the type supposed by Dubois. Such a conclusion
is in fact borne out by the record of the rocks. The warm
climates of the early Palaeozoic period might have lent colour
to a belief in a hotter sun at that time (some 400 to 500 million
years ago), but it happens that the earliest reliable climatic
record we possess shows us an ice-age.
Variations in the distance of the nearest " fixed " stars have
been suggested as a possible source of climatic changes, through
variations in the radiation received from this source, but this
suggestion must be ruled out for two reasons. At present
the heat received from all the fixed stars together is not one
ten-millionth of that received from the sun, and there is thus
no possibility of explaining colder periods than the present
on these lines. The close approach of a large star might
account for a warmer period, but such an event would have
left its traces in a derangement of the solar system, and
astronomers say that it could not have happened at any time
in the geological past.
We may end this chapter with a reference to a curious
hypothesis put forward by R. L. Ives (15) to explain the
Permo-Carboniferous glaciation. He suggests that a small
satellite of the earth, either original or captured, was dis-
rupted by tidal and other forces in the late Palaeozoic to form
a ring of fragments round the equator, similar to Saturn's
rings. This caused low equatorial temperatures, with storms
and heavy snowfall along the boundaries of the shadow.
Multiple glaciation is accounted for by the occurrence of a
series of stages in the break-up. Such an occurrence seems
intrinsically improbable, however, and in any case it can be
RADIATION FROM THE SUN IOI
readily calculated that a ring of this nature, if it cast an
effective shadow, would lower the temperature of the whole
earth to such an extent as to make life impossible.
REFERENCES
(1) WASHINGTON, SMITHSONIAN INSTITUTION. ''Annals of the Astrophysical
Observatory," vol. vi, 1942. By C. G. Abbot, F. E. Fowle, and W. H.
Hoover.
(2) SIMPSON, G. C. " Past climates." Mem. Manchr. Lit. Phil. Soc., 74,
i9 2 9-3> no - x > PP- 34-
(3) SIMPSON, G. C. " The climate during the Pleistocene period." Proc.
Roy. Soc., Edinburgh, 50, 1929-30, p. 262.
(4) PENCK, A. and E. BRUCKNER. " Die Alpen in Eiszeitalter." Leipzig
3 Vols., 1901-9.
(5) HOYLE, F. and R. A. LYTTLETON. " The effect of interstellar matter on
climatic variations." Proc. Camb. Phil. Soc., Cambridge, 35, 1919, p. 405.
(6) NILSSON, E. " Quaternary glaciations and pluvial lakes in British East
Africa." Geogr. Ann., Stockholm, 13, 1931, p. 249.
(7) KERNER-MARILAUN, F. " Palaoklimatologie." Berlin (Gebr. Borntraeger) ,
1930.
(8) REID, E. M. and M. E. J. CHANDLER. " The London Clay flora.",
London, 1933.
(9) HIMPEL, K. " Die Klimate der geologischen Vorzeit." Veroff. Astron.
Ges. Urania, Wiesbaden, nr. 4, 1937.
(10) ABBOT, C. G. " The sun." London and New York, 1912.
(11) BROOKS, C. E. P. "The variation of the annual frequency of thunder-
storms in relation to sunspots." London, Q.. J. R. Meteor. Soc., 60, 1934,
P- 153-
(12) HUNTINGTON, E., and S. S. VISHER. " Climatic changes, their nature and
cause." New Haven, 1922.
(13) DUBOIS, E. " The climates of the geological past." London, 1895.
(14) BROOKS, C. E. P. "Non-linear relations with sunspots." London,
Q.. J. R. Meteor. Soc., 53, 1927, p. 68.
(15) IVES, R. L. " An astronomical hypothesis to explain Permian glaciation."
Philadelphia, J. Frankl. Inst., 230, 1940, p. 45.
CHAPTER V
ASTRONOMICAL FACTORS OF CLIMATE
THE radiation emitted from the sun is not the only
factor in determining the solar climate of the earth.
Whether or not the total heat received by the earth
in the course of a year has remained constant, its distribution
among the belts of latitude during the different months has
certainly varied from time to time, and this distribution can
be calculated. There are three variables to be considered
in this respect. The first is the obliquity of the ecliptic, or
the angle which the plane of the equator makes with the
plane of the earth's orbit round the sun. It is this which
causes the seasons ; the greater the obliquity of the ecliptic
the greater is the contrast between the heat received in summer
and that received in winter. In 1910 the obliquity was
23 27' 3-58", and it was decreasing at the rate of 0-47" a
year, but the limits of its variation are difficult to calculate.
Lagrange found a maximum of 27 48' in 29,958 B.C., a
minimum of 20 44' in 14,917 B.C., and a maximum of 23 53'
in 2167 B.C. J. N. Stockwell gives much narrower limits,
ranging from 24 36' to 21 59', with a maximum of 24 17' in
8150 B.C., since when there has been a steady decrease.
Drayson, on the basis of a theory not accepted by the majority
of astronomers, supposed the obliquity to range from 35 to
11, the period being 31,680 years. The latest work by M.
Milankovitch (i) assumes a variation between 22 and 24^ in
a period of 40,400 years.
The second variable is the eccentricity of the earth's orbit.
This orbit is elliptical, with the sun at one of the foci, and the
distance between the centre of the ellipse and this focus,
expressed in terms of the major axis of the ellipse, is termed
the eccentricity. It varies in a period of about 100,000 years
from zero to a value of about 0-07. When the earth is nearest
to the sun it is in perihelion, when most distant, in aphelion.
At perihelion the earth travels along its orbit more rapidly
than at aphelion. Thus the season which coincides with
ASTRONOMICAL FACTORS OF CLIMATE 1 03
perihelion will be short and relatively warm, that which
coincides with aphelion will be long and relatively cold, but
the total amount of heat received on each hemisphere in the
course of a year will be the same. At present, the Northern
Hemisphere has its winter in perihelion and its summer in
aphelion ; with the Southern Hemisphere, of course, the
reverse is the case. Hence the solar climate of the Northern
Hemisphere is less extreme than that of the Southern Hemi-
sphere ; the fact that, actually, the climate is much more
extreme in the Northern Hemisphere is due to the preponder-
ance of land there. The season in which perihelion falls is
not constant but undergoes a cyclic variation with a period
of 21,000 years. Thus 10,500 years ago the Northern Hemi-
sphere had its winter in aphelion and its summer in perihelion,
consequently a more extreme solar climate. This regular
variation is termed the precession of the equinoxes ; it is the third
of our astronomical variables.
The variation of the eccentricity and the precession of the
equinoxes form the basis of CrolPs famous astronomical theory
of the Quaternary Ice- Age (2). He supposed that at periods
of great eccentricity the hemisphere with its winter in aphelion
had a. climate so severe that, if geographical conditions were
favourable, the snowfall during the long cold winter was
heavy enough to persist through the short hot summer arid
thus develop ice-sheets. At the same time the opposite
hemisphere was enjoying a genial or interglacial period. Croll
justly points out that the power of a snow surface in reflecting
the sun's rays back to space without their having any warming
effect on the earth is of great importance in the heat economy
of ice-ages, but, apart from this, his discussion of the
meteorological changes associated with periods of maximum
eccentricity is probably unsound. Moreover, recent geological
investigations have shown that the glacial periods and other
climatic changes are practically synchronous in the two
hemispheres, while de Geer's absolute dating shows that the
periods at which glaciations occurred do not fit in with those
required by droll's astronomical theory.
Rudolf Spitaler (3) re-investigated the astronomical theory
of climatic changes. His first step was to relate the mean
temperature of any latitude in any month to the amount of
heat received from the sun in that latitude. For this purpose
IO4 CLIMATE THROUGH THE AGES
he analysed the existing mean temperatures of the different
latitudes between 60 N. and 60 S. in January, July, and
the year, and obtained an expression for the mean tem-
perature in any latitude in any month, which may be rewritten
as follows, to give Fahrenheit degrees :
'=(-17+156 S +2 9 SJ (W)-H- 3 6+26 4 S m ) (L).
Here S is the average daily heat received on a horizontal
surface at the limit of the earth's atmosphere in the latitude
in question during the year, S m is the average daily heat
received under the same conditions during the month m
(the units being so chosen that the value of S at the equator
during an equinoctial day is approximately 0*5), and L is
the fraction of land and W the fraction of water covered by
the line of latitude. By making the appropriate changes in
the values of S and S m the temperatures under various
astronomical conditions can be calculated from this formula,
and by varying L and W the effect of varying land and sea
distribution can be introduced.
Spitaler rejects CrolPs theory that the conjunction of a long
cold winter and a short hot summer provides the most
favourable conditions for glaciation, and adopts the opposite
view, first put forward by Murphy (4) and now generally
adopted, that a long cool summer and short mild winter are
the most favourable. Spitaler's attempt was rational, but
fails to fit in with the actual sequence of events ; in particular
his time-scale is hopelessly impossible. For example he ends
the Wurm glaciation at 89,680 B.C. whereas geological
evidence puts it at a mere 18-20,000 B.C.
W. Koppen and A. Wegener (5), while accounting for the
Quaternary Ice-Age as a whole by the latter's theory of
continental displacements (Chapter XIV.), bring in astro-
nomical causes to account for the succession of glacial and
interglacial periods. Like Spitaler, they emphasise the
importance of a low summer temperature, but they employ
a different method, based on work by M. Milankovitch (i),
in which the amount of radiation received at any point during
the summer half-year is expressed as the equivalent latitude
of that point with respect to present astronomical conditions.
With perihelion in June and a great obliquity of the ecliptic,
the radiation received by any point in the north temperate
ASTRONOMICAL FACTORS OF CLIMATE 105
or Arctic zones will be greater than the present, so that the
equivalent latitude will be lower. Thus, in 65 N., the
summer radiation in 9,500 B.C. was as great as that now
received in 60 20' N., while in 20,400 B.C. the summer radiation
was as low as that now received in 68 N. It is pointed out
that a decreased obliquity of the ecliptic must increase the
temperature contrast between pole and equator in summer
without making much difference in winter. Hence the
atmospheric circulation in summer would be strengthened,
giving an increased frequency and intensity of cyclones in
the temperate belt, and the summer rainfall of Southern
Europe would be greater.
The plane of the ecliptic varies in a period of 40,400 years,
while perihelion makes the circuit of the seasons in 20,700 years.
In each hemisphere the coldest summers occur when the
smallest obliquity corresponds with summer in aphelion
during a period of maximum eccentricity. Hence the authors
suppose that the glacial periods occur at maximum eccentricity,
and consist of two periods of cold summers 40,000 years apart,
which are united in one glaciation through the power of
persistence of an ice-sheet. Any retreat stadia formed during
the intervening period of warmer summers are masked by the
readvance of the second phase, which starts from the remains
of the first ice-sheet and therefore attains a greater develop-
ment. The glaciations in the two hemispheres are therefore
roughly synchronous, but a maximum in the Southern
Hemisphere occurs about 10,000 years before or after the
corresponding maximum in the Northern Hemisphere. On
these lines the authors give in Table 3 a chronology of the
Quaternary Ice-Age.
This theory marks a great advance on Croll and Spitaler
but the chronology is still a long way from fitting the geological
evidence, ending the Wurm glaciation, for example, about
66,000 B.C. and dating the post-glacial climatic optimum at
9,100 B.C. instead of 5-3,000 B.C. when the astronomical
climate differed from the present by an insignificant amount.
The latest and most elaborate reconstruction of glacial
history from astronomical data, also based on Milankovitch,
is by F. E. Zeuner (6). Zeuner studies the glacial history of
Europe in minute detail, including also the periglacial regions,
from which he makes inferences as to the character of the
I06 CLIMATE THROUGH THE AGES
Northern Hemisphere. Southern Hemisphere.
(The figures are in thousands of years before the present.)
Climatic Optimum, 9-1.
Pre-Baltic Stadium, 33-30.
Post-Wurm I., 110-103.
Baltic Stadium, 25.
Wurm II., 74-66.
Wurm L, 118-110.
RissIL, 193-183.
Riss L, 236-225.
Nameless, 1 305-302.
Mindel II., 434-429.
Mindel I., 478-470.
Gunz II., 550-543.
Gunz I., 592-585.
Post-Riss II., 200-195.
Post-Riss L, 226-218.
About (442), 389, 350,
312, 270.
Post- Mindel I., 468-462.
Pre-Gunz II., 560-554.
Table 3. Koppen's interpretation of Quaternary sequence.
climate. He follows Koppen and Wegener in expressing
variation of summer radiation as equivalent latitude. He
shows that the variation of the present snow-line with latitude
closely follows the variation of radiation received in the
summer half-year. He argues that with changing astro-
nomical conditions a rise of the winter temperature increases
the snowfall and the corresponding fall of summer temperature
enables the snow to persist through the year. He also follows
up various secondary effects of glaciation such as the reflecting
power of the snow surface, shift of tracks of depressions, and
the periglacial belt of east winds, as well as the effect of changes
of sea level caused by the locking up of great quantities of
water in the ice-sheets, and the delayed effect of the weight
of the ice in depressing the land.
The result is a very detailed scheme of changes of both
climate and sea-level which fits in well with the most recent
geological interpretations, as exemplified by the curve of
1 H. Gams and R. Nordhagen consider that between the Mindel and Riss
glaciations in the Alps, but nearer the latter than the former, there was an
additional glaciation, which they term the Muhlbergian.
ASTRONOMICAL FACTORS OF CLIMATE
107
equivalent latitude of 65 N. His dating is briefly as follows
(dates in thousands of years before present day) :
Zeuner.
Late Glaciation III.
II.
I.
Penultimate
Glaciation
II.
I.
Antepenultimate 1 1 .
Glaciation I.
Early Glaciation II.
I.
25
. 7 2
"5
187
230
435
476
550
590
Penck and Bruckner.
Wurm 40-18.
Riss
Mindel
Gunz
130-100
430-370
520-490
Table 4. Zeuner's interpretation of Quaternary sequence.
On Zeuner' s view the long interglacial between the Ante-
penultimate and Penultimate Glaciations (or Mindel-Riss)
was on the whole about 4 F. warmer than the present, with
a rather oceanic climate, but it was interrupted by a minor
cold period (see footnote to Table 3).
Zeuner recognises that the astronomical theory does not
account for the Quaternary Ice- Age as a whole, only for the
details within the period. His scheme comes much nearer to
the geological dating than do those of Spitaler and Koppen
and Wegener. The astronomical dates still tend to be earlier
than those favoured by the geologists, but this may reasonably
be attributed to the natural lag in the accumulation and
disappearance of great ice-sheets, which Zeuner considers
may have amounted to some thousands of years.
Whatever we may think of the parallelism between
Milankovitch's curve and the succession of glacial stages, it
is clear that astronomical changes, and especially changes in
the eccentricity of the earth's orbit, must have had quite
appreciable effects on the climates of the past. At periods
of maximum eccentricity the hemisphere with winter in
aphelion must, other things being equal, have more pronounced
seasons than that with winter in perihelion, and these pro-
nounced seasons must have increased the strength of monsoons
and other seasonal changes.
108 CLIMATE THROUGH THE AGES
According to W. H. Bradley (7) the Eocene of Colorado,
Utah and Wyoming includes beds with annual layers covering
a duration of five to eight million years, which show periodi-
cities of 1 1\ years (sunspot cycle), 23 years, 50 years and about
21,000 years, the latter presumably representing the cyclic
changes of eccentricity of the earth's orbit and the precession
of the equinoxes. Alternations of layers in the Cretaceous of
U.S.A. also suggest a cycle which is estimated as about
21,000 years, but there are no annual layers.
It is possible that the coal seams in the Upper Carboniferous
represent periods of great eccentricity with winter in perihelion
in the Northern Hemisphere and a small obliquity of the
ecliptic giving a climate in the Northern Hemisphere with
little annual range of temperature, and consequently no
annual rings of growth in the woody stems. In that case the
intervening beds of sandstone and clay would represent the
other halves of the periods in which winter in the Northern
Hemisphere was in aphelion and the climate consequently
more extreme, and the whole succession from the base of one
coal seam to the base of the next would represent a period of
21,000 years. Nearer the present, the alternation of brown
coal and bauxite beds in the Tertiary of South-eastern Europe
may be due, as suggested by Koppen and Wegener, to a
similar alternation of equable and extreme astronomical
climates. Similarly, as suggested by Dacque (8), the extreme
climate of the Old Red Sandstone may be due to the deposits
Raving been formed during a period of great eccentricity,
but in the absence of absolute dating of these deposits, both
these suggestions must remain speculative. There is more
support for the supposition that during the closing stages of
the retreat of the Quaternary ice-sheet of Scandinavia, the
extreme " continentality " of the climate was due partly to
the greater obliquity. The maximum obliquity of 8500 to
7500 B.C. (according to the work of Stockwell) would have
had the effect of lowering the winter temperature and raising
the summer temperature by about i F. Even in this example,
however, the changes due to purely geographical causes were
probably much greater than those due to the increased
obliquity, and the astronomical effect by itself would have
been hard to distinguish.
ASTRONOMICAL FACTORS OF CLIMATE log
REFERENCES
(1) MILANKOVITGH, M. " Th^orie math^matique des phnomenes thermiques
produits par la radiation solaire." Paris, 1920.
(2) GROLL,J. " Climate and time in their geological relations." London, 1875.
(3) SPITALER, R. " Das Klima des Eiszeitalters." Prag, 1921 (Lithographed).
(4) MURPHY, J. J. " Glacial climate and polar ice-cap." London, Q,. jf.
GeoL Soc., 32, 1876, p. 400.
(5) KOPPEN, W., and A. WEGENER. " Die Klimate der geologischen Vorzeit."
Berlin, 1924.
(6) ZEUNER, F. E. " The Pleistocene period ; its climate, chronology and
faunal successions." London, Ray Soc., 1945.
(7) BRADLEY, W. H. " The varves and climate of the Green River epoch."
U.S. Geol. Surv., Prof. Paper, 158, 1929, p. 87.
(8) DACQUE, E. " Grundlagen und Methoden der Palaeogeographie." Jena,
CHAPTER VI
THE ABSORPTION OF RADIATION BY THE ATMOSPHERE
THE mean temperature of the earth is determined
by the balance between the radiation received from
the sun and that given out by the earth. The solar
radiation entering the earth's atmosphere undergoes various
transformations before it finally leaves the atmosphere again
as radiation to space. These changes have been set out very
clearly in a diagram by W. H. Dines (i) (Fig. n, reproduced
by permission of the Royal Meteorological Society). In this
diagram, A is the radiation reaching the limit of the earth's
atmosphere from the sun. The numerical value of A
(measured in calories per square centimetre per day) is about
720. Part of this radiation is reflected from the surfaces of
clouds, snowfields, and to a lesser degree from all parts of the
earth's surface, both land and water, without undergoing any
change. Another part of the solar beam is scattered by the
molecules of the gases and the particles of dust in the atmos-
phere ; some of this scattered radiation ultimately reaches the
surface of the earth, but some of it is entirely lost to the earth.
The amount lost by reflection and scattering is represented by
D, which has a value of about 320. [The numerical values
are taken partly from a later source (2).] Another part of
the radiation, G, is absorbed by the air, and the remainder,
B, is absorbed by the earth. The average value of B is about
350, leaving only 50 for C.
The surface of the earth is losing heat in three ways. An
amount, G, which depends on the temperature, is radiated
outwards ; of this a small part, M, is reflected back to the
earth, mainly from the under surfaces of clouds, without
undergoing any change ; another part, H, is absorbed by
the air, and the remainder, K, is lost to space. Another
part of the surface heat is transferred to the atmosphere by
evaporation of moisture from the surface, the heat of evapora-
tion being given up to the air when the moisture is again
THE ABSORPTION OF RADIATION BY THE ATMOSPHERE III
condensed, and a third part is communicated from the earth
to the air by conduction and carried upwards by convection,
but this is partly balanced by the reverse process, conduction
from the air to the earth, and the net result is probably small.
The heat transferred by evaporation and by the net conduction
together is indicated by L.
The air is constantly receiving heat, C, from the sun, H
and L from the earth. This heat is in turn radiated by the
air, part E going back to the earth and part F to space. Thus
the incoming radiation A is finally given out again in three
forms, D by direct reflection, K by radiation from the earth,
and F by radiation from the air, and since, practically speaking,
V
1
wt
t
k )
A
Outer Limit of
Atmofphci*
^
(
C
A,r t
i*.
*~ H
C-*
Ground
LK/
n
c
XT _
) (
)G ,
nr
Fig. 1 1 . Heat exchange of atmosphere.
there is no gain or loss of heat from one year to the next,
A-D+K+F.
The value of G, the radiation from the earth's surface,
is related to the temperature in accordance with the well-
known Stefan-Boltzmann law, which states that the radiation
from a black body is proportional to the fourth power of its
absolute temperature. The earth's surface is not quite a
black body, but the radiation must be greater the higher the
temperature. G and L are together equal to B+E + M, hence
the temperature of the earth's surface must be determined by
the sum of these three quantities (radiation from the sun
reaching the surface, radiation from the air to the earth,
and terrestrial radiation reflected back to the earth), less the
heat L lost by evaporation and conduction. A change in
any of these quantities will therefore bring about a change
in the mean temperature of the earth. W. H. Dines (i) and
112 CLIMATE THROUGH THE AGES
(2) assigns numerical values to all the quantities A to M ;
some of these values are only rough estimates, but they will
serve for a discussion of the possibility of appreciable climatic
changes being brought about by variations in their amount.
The amount of solar radiation absorbed by the air is
probably small ; Dines gives it a value of 50 calories, or
one-fourteenth of the whole solar radiation, so that for the
present we can ignore it. Variations in the value of A have
already been discussed in Chapter IV., and here A is con-
sidered as a constant. Variations of B, the solar radiation
reaching the earth's surface unchanged, therefore depend
chiefly on variations of D, the solar radiation reflected back
to space or lost by scattering ; Dines gives a value of 320 for
D. The chief reflecting surfaces are clouds and snowfields,
and the loss by scattering may be greatly increased by the
presence of dust. Clouds are very efficient reflectors, as is
shown by the intense brightness of cumulus clouds in sunlight ;
A. Angstrom estimates that clouds reflect 75 per cent, of the
solar radiation falling on them, while L. B. Aldrich (3) puts
the figure at 78 per cent, and, calculates that on the average
slightly over 40 per cent, of the sun's radiation is lost to the
earth by reflection from clouds. Hence we should expect an
increase of cloudiness to lower the average temperature.
At present the average cloudiness over the whole world is
just over five-tenths ; if it increased to six-tenths without
any corresponding increase in the amount of water held in
the atmosphere as vapour (clouds are water droplets or ice
crystals, not water vapour, so that the supposition is con-
ceivable), the mean temperature would be considerably
lower than it is now. This effect of cloudiness is of great
importance ; it is discussed in greater detail in the next
chapter. If the increase of cloudiness were accompanied
by an increase in the amount of water vapour, the fall of
temperature would be less, as will be seen.
An increase in the area covered by snow and ice would
increase the heat lost by reflection, and in this case there are
no compensating circumstances. Large areas of snow and
ice usually have clear skies and dry air above them, and there
is nothing to check the wastage of o solar heat. If we assume
on the basis of some work by A. Angstrom (4) that over an
ice-sheet about four-fifths of the solar radiation is reflected
THE ABSORPTION OF RADIATION BY THE ATMOSPHERE I 1 3
back to space, compared with one-fifth in an unglaciated
region, the lost solar radiation in temperate latitudes would
be sufficient to melt more than 30 feet of ice in the course of
a year. This must have been a powerful factor in increasing
the rigour of the great ice-ages in the glaciated regions, and
in facilitating the extension of the ice-sheets. According to
E. Antevs (5), the ice-covered area at the maximum of
glaciation was about 13 million square miles, compared with
6 million square miles at present. The increase of 7 million
square miles represents 3 5 per cent, of the earth's surface.
Allowing for the fact that glaciated regions are not entirely
cloudless, we may estimate the increased loss of solar radiation
by reflection from the ice-sheets as at least 2| per cent, of that
reaching the whole earth. W. Wundt (6), allowing for the
snowfall and drift ice of the peripheral regions, arrives at a
figure of 3 per cent. The effect would be to lower the
temperature of the whole earth by at least 4 F. (Wundt gives
7 F.).
A variable which may be more generally effective is the
volcanic dust in the atmosphere. In the years following
great volcanic eruptions of the explosive type, such as those
of Krakatoa (1883), Santa Maria and Pelee (1902), Colima
(1903), and Katmai, Alaska (1912), the solar radiation
reaching the earth's surface (Dines 5 quantity B) may be 15
or 20 per cent. (45 to 60 calories) below the normal value.
This radiation is not all lost to the earth ; part of it goes to
warming the upper air and therefore increases the value of
E, but we shall see later that the net effect is an appreciable
lowering of temperature, and that volcanic dust has possibly
played a part in causing ice-ages.
The quantity M, terrestrial radiation reflected back to
the earth, is relatively small (60 calories) on Dines' calculation,
and as its variations depend on some of the factors, cloudiness
and dust, which control D, it need not be considered further
here. That leaves for discussion E, the radiation from the
air to the earth, and this quantity is not only large (540
calories) but probably very variable. It depends on several
factors, but by far the most important is H, the terrestrial
radiation absorbed by the atmosphere, to which Dines assigns
a value of 600 calories per square centimetre per day. The
action of the atmosphere in raising the temperature of the
114 CLIMATE THROUGH THE AGES
earth by absorbing its radiation is similar to the action of the
glass roof in a greenhouse. In the atmosphere the place of
the glass is taken by certain gases, notably water vapour,
ozone, and carbon dioxide. W. H. Dines points out (2) that
the present mean temperature of the earth's surface, 288 A.
(59 F-)> * s a bout 4 A. (7 F.) above the mean temperature
which would prevail if the atmosphere had no power of
absorption and if there were no reflection or scattering. In
addition to raising the mean temperature over the earth as a
whole, atmospheric absorption, in conjunction with the
mobility of the air, has an important effect in bringing about
a greater approach to uniformity in the temperatures of
different latitudes than would exist on a dry earth. The
importance of this effect in controlling variations of climate
has been emphasised by Sir George Simpson.
Water vapour has a very high coefficient of absorption of
radiation in the wave-lengths chiefly emitted by the earth.
Moist air is warmed by radiation from the earth and
immediately returns to the surface part of the heat gained in
this way. Hence on a clear night the surface loses heat more
rapidly if the air is dry than if it is moist, and the amount of
water vapour in the lowest layers is one of the most important
elements in calculations for forecasting the occurrence of
frosts. Except over cold dry continental regions, however,
there is always enough water vapour present in the whole
thickness of the atmosphere to absorb practically the whole of
the radiation in most parts of the spectrum of terrestrial
radiation. An increase in the amount of water vapour
would therefore have little effect on the proportion of
terrestrial radiation transmitted unchanged to space. It
would, however, have some effect on the vertical distribution
of temperature in the atmosphere. Consider the atmosphere
as divided into a number of concentric shells, and suppose for
a moment that each of these shells absorbs all the radiatiom
reaching it from the shell on either side, while there is a fall
of temperature from the inner to the outer shells. Then it
will be seen that the earth's surface is receiving radiation only
from the innermost shell, while only the outermost shell is
radiating to space. Hence the radiation from the air to the
earth is greater than the radiation from the air to space.
Although these conditions are not fully realised in the earth's
THE ABSORPTION OF RADIATION BY THE ATMOSPHERE 115
atmosphere, they are sufficiently near the truth for the above
conclusion to hold. An increase of water vapour, by increasing
the completeness of absorption in each of our hypothetical
shells, would make this difference somewhat greater, and
so raise the temperature of the earth's surface, while it
would increase the temperature of the air at a height of
four or five miles more than that at the surface, and so lessen
the decrease of temperature with height.
More aqueous vapour in the air may be due either to
greater evaporation at the same temperature or to a higher
initial temperature. Greater evaporation may be due to a
greater expanse of sea in low latitudes, to a greater average
wind velocity, or to a greater vertical interchange of air.
In any case more heat is taken from the surface and carried
to the level of condensation, whence it is partly radiated to
space. The result is an increase of L, the heat lost to the
earth's surface otherwise than by radiation, and it therefore
causes a decrease of the earth's radiation G, i.e., a fall of the
surface temperature, in spite of the greater humidity. On
the other hand, if the mean temperature of the air rises from
some cause unconnected with the water vapour content, the
latter will automatically increase, since warm air has a greater
capacity for water vapour than cold air has. If all other
conditions of land and sea distribution, relief, etc., remained
unchanged, an increase in the water-vapour content due to
increased solar radiation would be accompanied by an increase
in cloudiness and precipitation as well as by a rise of tem-
perature ; this is the basis of Sir George Simpson's theory of
glaciation described on pages 91 to 96. If, however, an
increase in the water vapour content is brought about by a
large increase in the area of the oceans, accompanied by a
decrease in the average height of the land, the solar radiation
remaining unchanged, there will not necessarily be an increase
of cloud and precipitation.
The most important way in which cloud is formed, and
practically the only way in which an appreciable fall of rain
can be produced, is by the ascent of moist air, which is cooled
by expansion until condensation takes place. Except in
limited regions of high relief, the air must become unstable
before is can be forced to rise. The stability of a column of air
depends on the temperature gradient ; the less rapid the fall
I 1 6 CLIMATE THROUGH THE AGES
of temperature with height, the more stable is the air. We
have seen that an increase in the water vapour, other things
remaining unchanged, increases the absorption at high levels
in the atmosphere, and therefore decreases the fall of tem-
perature with height and makes the air more stable. The
ascent of air, both in the great cyclonic storms and " barometric
depressions " and in the smaller thunderstorms and local
showers, becomes less frequent and extensive, and there may
be an actual decrease in the cloudiness. These conditions are
illustrated in the trade wind belts. The air crosses large
stretches of ocean ; it speedily becomes nearly saturated and
takes up further moisture very slowly, yet owing to the stability
of the conditions there is very little ascent of air, the amount
of cloud is small, and rainfall is very scanty. With a generally
higher temperature over the earth, these conditions would
extend to higher latitudes. During the hot summer of 1921
the air over the British Isles contained more water vapour than
during the cool summer of 1924, but in 1921 the air was stable
and there was little cloud and rainfall, while in 1924 the air
was unstable and there was much cloud and a rainfall above
normal.
The development of barometric depressions and conse-
quently the formation of cloud are facilitated by the presence
of horizontal temperature differences in masses of air which
are moving relatively to each other. A more uniform dis-
tribution of temperature over the earth, such as prevailed
during the warm periods, would therefore largely remove one
important source of cloudiness at the present day. Thus it
is possible that a general elevation of temperature over the
globe could increase the amount of water vapour in the air
without increasing the cloudiness ; the result would be a
further rise of temperature, intensifying the original increase.
In certain theories of climatic change (See Appendix II.,
Section IX.) extraordinary importance is attached to variations
in the amount of carbon dioxide in the atmosphere. v Periods
during which the atmosphere was rich in this gas are con-
sidered to have been uniformly warm, those in which it was
poor to have been cold or even glacial periods. Chamberlin
has devoted great ingenuity to a discussion of the probable
variations in the quantity of carbon dioxide in the atmosphere
during different geological periods, and though his conclusions
THE ABSORPTION OF RADIATION BY THE ATMOSPHERE I 17
are probably somewhat exaggerated, there appears to be little
doubt that the variations have been considerable. Recent
physical researches have shown, however, that the part of
the terrestrial radiation which is taken up by carbon dioxide
is almost completely absorbed by water vapour, and no
increase in the amount of the former gas could increase the
total absorption appreciably. W. J. Humphreys pointed out
(7, p. 567) that the only way in which an increase of carbon
dioxide could affect the temperature would be by absorption
at high levels in the atmosphere where water vapour is nearly
absent. As explained in the case of water vapour, this would
increase slightly the proportion of radiation from the air which
is directed towards the earth, and decrease that which is
directed towards space, and in this way, the mean temperature
of the earth may have been modified to the extent of a few
tenths of a degree.
In 1939, however, the question was taken up again by
G. S. Callendar (8) who relates the cold of the Permian
to the exhaustion of carbon dioxide by the Carboniferous
forests. During the Mesozoic the relatively small develop-
ment of plant life allowed the amount of CO 2 , steadily
replenished by the animal life of the seas, to increase again,
but a great deal was locked up in Tertiary lignite formation,
especially in western North America, and this may have
brought about a progressive cooling which ended in the
Quaternary Ice-Age. This theory cannot account for the
oscillations of the individual glaciations, the time-scale of
which is too short. Callendar ends by pointing out that the
great coal consumption in the twentieth century has raised
the amount of CO 2 in the atmosphere from -028 per cent,
about 1900 to -030 per cent, in the i93o's, and that this
increase has been accompanied by a small but steady rise in
the mean temperature of the colder regions of the earth.
This argument has rather broken down in the last few years,
however, for the rise of temperature seems to have reached
its crest and to have given place to a fall. The possibility
that changes in the amount of CO 2 have been responsible for
some small part of the climatic changes of geological time
seems to remain open however.
The idea that volcanic dust may have important climatic
effects is very old. In 1784, B. Franklin suggested that the
Il8 CLIMATE THROUGH THE AGES
hard winter of 1783-84 was due to the great quantities of
dust in the air, and that the source of this dust might be
either the destruction of meteorites or the great volcanic
eruptions in Iceland. Other references to this possibility
appeared from time to time, but it was not until the great
diminution of radiation received at the surface after the
eruption of Katmai in 1912 was noted that the subject received
exhaustive discussion by Abbot and Fowle (9), and W. J.
Humphreys (10, also 7, pp. 569-603). This discussion has
shown that the effect of dust is due, not to absorption, but
to the scattering and reflection of the solar radiation to which
it gives rise. When a volcanic dust cloud is thrown to a great
height in the air, as in the eruption of Krakatoa, it takes from
one to three years to settle. Humphreys calculated the average
diameter of the particles from the optical effects, and found
that it was i -85 microns (-00185 mm.). This is greater than
the wave-length of solar radiation in the region of maximum
intensity, and the particles therefore greatly interfere with
the passage of the solar radiation. On the other hand, the
wave-length of terrestrial radiation is six or seven times the
diameter of the particles, and the terrestrial radiation passes
through dusty air with little loss. The action of dust in the
two cases may be compared to that of a number of wooden
balls floating in a pond. Against a barrier of such balls
small ripples break up and are lost, but larger waves to which
the balls can rise and fall are hardly affected. Humphreys
calculates that the reduction by volcanic dust of solar and of
terrestrial radiation is in the ratio 30 to i. Observations in
1912 showed that the Katmai dust reduced the solar radiation
reaching the earth by about 20 per cent., which, if maintained
through a long period of years, would lower the mean tem-
perature of the earth by about 10 F., an amount quite
sufficient to initiate an ice-age. The compensating processes
reflection of terrestrial radiation, and radiation from the
dust itself are negligible in comparison. During the past
1 60 years the average temperature of the earth has been
lowered by volcanic dust possibly as much as i F. The
most notable cold years since the beginning of the eighteenth
century have all followed great volcanic eruptions, especially
1784-86, following the eruption of Asama (Japan) in 1783 ;
1816 (" he year without a summer"), following especially
THE ABSORPTION OF RADIATION BY THE ATMOSPHERE
the eruption of Tomboro (Sumbawa) in 1815 ; 1884-86,
following Krakatoa in 1883 ; and 1912-13, following Katmai
in 1912. H. Arctowski (n), who has studied the temperature
variations in great detail in connexion with the solar control
of terrestrial temperatures, admits the great influence of the
Krakatoa eruption, but considers that the effects of the
eruptions of 1902 and 1912 on temperature were very small.
The direct effect of volcanic dust on temperature is felt over
the whole globe (according to Arctowski only over the hemi-
sphere in which the eruption occurred) ; it is not localised
in the regions which were glaciated during the Quaternary
Ice-Age, but in addition to, or in consequence of, the direct
effect, there may be a secondary effect on the centres of high
and low pressure. A. Defant (12), from a study of the strength
of the atmospheric circulation during the two years following
each of the four great explosive volcanic eruptions of 1883
(Krakatoa), 1886 (Tarawera), 1888 (Bandai San) and 1902
(West Indies), considered that the result of each eruption
was a strengthening of the atmospheric circulation for the
next two years. C. E. P. Brooks and T. M. Hunt, however,
(13), taking into account also eruptions in 1875, *9 12 an d 1914*
found that the strengthening of the circulation was much
more transient, persisting for only six months. We are left
therefore with a general cooling as the only appreciable
climatic effect of volcanic dust.
It may be remarked here that a large increase in the number
of meteors entering the earth's atmosphere would have a
similar effect to that of volcanic dust, but so far as I know,
there is no evidence of such an increase during geological time.
Volcanic dust appears to be a possible explanation of
climatic periods colder than the present. The variation of
this element in geological time is not yet well known ;
Humphreys calculates that the amount of dust required to
maintain an ice-age would amount to a layer only one-fiftieth
of an inch thick in 100,000 years, so that it would hardly be
noticeable in the sedimentary rocks. We should expect
volcanic activity to be greatest during the great periods of
earth-movement and mountain-building, when the continents
were highly emergent and the land and sea distribution
favourable for glaciation. The explosive stage of activity
generally comes later in the life-history of a volcano than the
I2O CLIMATE THROUGH THE AGES
stage of fluid eruptions, so that the maximum amount of dust
might be delayed for some time after the continents first
became highly emergent. In the Quaternary, at least, there
was in fact an appreciable lag between the first great emergence
of North America and Scandinavia and the beginning of
widespread glaciation, and this difficulty is met by the
hypothesis that the final lowering of temperature necessary
for ice-formation was given by the occurrence of widespread
explosive eruptions. On the other hand, the complete
absence of volcanic dust would not raise the mean temperature
more than about i F., which is inadequate to account for the
temperature of the warm geological epochs.
E. and A. Harle (14) attributed the general warmth of
the Mesozoic period to the presence of a much denser atmos-
phere than now exists on the earth, which helped to conserve
the heat of the sun and so raised the general temperature.
They base their argument on the existence of great flying
reptiles in the Mesozoic, which they consider were too heavy
to have flown in the present atmosphere, but as we are ignorant
of the muscular development of these reptiles, this argument
carries no weight. From the physical side there seems to be
no reason why the earth should be losing its atmosphere,
and the variations of climate during geological time are very
far from suggesting the action of any irreversible force.
REFERENCES
(1) DINES, W. H. " The heat balance of the atmosphere." London, Q,. J. R.
Meteor. Soc., 43, 1917, p. 151.
(2) " Dictionary of Applied Physics," edited by Sir RICHARD GLAZEBROOK,
vol. iii., London, 1923. Article, " Radiation," by W. H. DINES.
(3) ALDRICH, L. B. " The reflecting power of clouds." Washington, D.C.,
Ann. Astrophys. Obs., Smithsonian Inst., 4, p. 375.
(4) ANGSTROM, A. " The albedo of various surfaces of ground." Stockholm,
Geogr. Ann., 7, 1925, p. 323.
(5) ANTEVS, E. " The last glaciation." New York, Amer. Geogr. Soc.,
Research Series, no. 17, 1928.
(6) WUNDT, W. " Anderungen der Erdalbedo wahrend der Eiszeit." Met.
#., Braunschweig, 50, 1933, p. 241.
(7) HUMPHREYS, W. J. " Physics of the air." 3 ed. London (McGraw-Hill),
1940.
(8) CALLENDAR, G. S. " The composition of the atmosphere through the
ages." Meteor. Mag., London, 74, 1939, p. 33.
(9) ABBOT, C. G., and F. E. FOWLE. " Volcanoes and climate." Washington,
D.G., Ann. Astrophys. Obs. Smithsonian Inst., 3, 1913, and Smithsonian Misc.
Coll., 60, 1913, no. 29.
( 10) HUMPHREYS, W. J. " Volcanic dust and other factors in the production of
climatic changes and their possible relation to ice-ages." Philadelphia,
J. Frankl. Inst., 176, 1913, p. 131.
THE ABSORPTION OF RADIATION BY THE ATMOSPHERE 121
(u) ARCTOWSKI, H. "Volcanic dust veils and climatic variations." New
York, Annals JV. Y. Acad. Sci., 26, 1915, p. 149.
(12) DEFANT, A. " Die Schwankungen der atmospha rischen Zirkulation u'ber
deni nord-atlantischen Ozean im 25-jahrigen Zeitraum, 1881-1905."
Stockholm, Geogr. Ann., 6, 1924, p. 13.
(13) BROOKS, C. E. P., and T. M. HUNT. " The influence of explosive volcanic
eruptions on the subsequent pressure distribution over western Europe."
Meteor. Mag., London, 64, 1929, p. 226.
(14) HARLE, E., and A. HARLE. " Le vol de grands reptiles et insectes disparus
semble indiquer un pression atmosphe"rique eleve"e." Paris, Bull. Soc.
Geol. France, n, 1911, p. 118.
CHAPTER VII
THE EFFECT OF CLOUDINESS ON TEMPERATURE
IN the last chapter we saw that one of the most potent
factors in modifying the distribution of solar radiation
over the surface of the earth was the reflection from the
upper surfaces of clouds. For this reason, cloudiness must
obviously be of great importance as a factor in the mean
temperature of any region. In Africa, for example, the highest
mean annual temperatures are found, not over the equator
where the solar radiation at the limit of the earth's atmosphere
is greatest, but over the deserts to the north and south of the
equator where the skies are clearest. The mean annual
temperature (corrected to sea-level) over the equator in the
interior of Africa is 82 F., while over the Sahara in latitude
20 N. it is 89 F. In spite of the lower mean altitude of the
sun, the small degree of cloudiness over the Sahara (2-4 tenths
of sky covered in latitude 20, compared with 5 5 tenths at
the equator) is sufficient to make the former 6 to 7 F. warmer
than the latter. It is worth while to investigate this effect a
little further, and attempt to make some estimate of the change
of mean temperature which would result from small variations
in the mean cloudiness, all other factors being regarded as
constant.
We have seen that the mean temperature of the earth's
surface is closely related to the amount of radiation which
it gives out, and that the latter is equal to the amount of
radiation which it receives, minus the loss of heat by
evaporation and convection. If the earth were a perfect
radiator, and if its temperature were the same in all parts
of the surface, the fourth power of its temperature would
be proportional to the amount of heat which it radiates
outward. Although neither of these conditions is quite
fulfilled, it is obvious that the higher the radiation the greater
the mean temperature, and vice versa. We have also seen
that the incoming radiation from the sun undergoes a number
THE EFFECT OF CLOUDINESS ON TEMPERATURE
of changes, as a net result of which the earth's mean tem-
perature is higher than it would be if there were no atmosphere.
These changes are very complex, but we can say that an
increase in the radiation received from the sun would increase
all the quantities concerned. The radiation reaching the
earth's surface would be greater, the return radiation from
the earth to the atmosphere would be greater, consequently
the amount of terrestrial radiation absorbed by the atmosphere
would be increased, which would in turn increase the radiation
from air to earth, and so on. An increase of 10 per cent, in the
radiation received from the sun would therefore result in an
increase of not exactly 10 per cent., but something of the order
of 10 per cent., say, between 5 and 15 per cent., in the radiation
from the earth's surface. This result would be independent
of the local modifications in the distribution of temperature
due to the readjustment of the wind systems, and also supposes
that there is no change in the constitution of the atmosphere
such as the formation of ozone in the stratosphere.
Clouds being effective reflectors of solar radiation, the
presence of clouds means a loss to the earth of a certain
amount of energy. It is true that clouds also reflect a part
of the terrestrial radiation back to the earth, and that this
partly compensates for the lost solar radiation, but the com-
pensation is not nearly complete. For one thing, it seems
probable that the percentage of the long-wave terrestrial
radiation reflected from the under surfaces of clouds is smaller
than the percentage of the short-wave solar radiation reflected
from their upper surfaces ; the action of water droplets in
this respect is probably similar in kind, though perhaps not
equal in degree to the action of volcanic dust particles described
on page 118. The temperatures of the surface and of the
atmosphere are so intimately connected that it is impossible
to conceive of a great change in one without a change in the
other. Hence we may say that it is immaterial whether the
radiation is lost owing to a decrease in the amount of heat
radiated by the sun or a decrease in the amount which
penetrates the mantle of clouds.
A. Angstrom ( i ) estimated the reflection of solar radiation
falling on clouds as 75 per cent. This figure probably applies
to rather dense heavy clouds ; more recently B. Haurwitz (2)
investigated the insolation received at Blue Hill, Mass., with
124 CLIMATE THROUGH THE AGES
given cloud amount, cloud density and elevation of the sun.
Cloud density is expressed on a scale of o (very thin) to 4 (very
dense cloud), and is as important as cloud amount in deter-
mining the insolation, but as we have no means of estimating
variations of cloud density during geological time, it seems
best to consider the results with the average density of 2 6
found by Haurwitz. He gives a table of annual insolation
with various cloud amounts and cloud densities as a percentage
of the insolation with cloudless skies. Interpolating for
density 2-6, we have approximately :
Cloud amount (tenths) . o 1-3 4-7 8-9 10
Insolation, per cent. .100 93 82 68 41
At present the average cloudiness over the earth is 5 4 tenths,
giving an average of 82 per cent, of the radiation received with
a cloudless sky. An increase of one tenth in the cloud amount
would reduce this figure to 78 per cent., and a decrease of
one tenth of cloud would raise it to 85^ per cent. Let us
make the simple assumption that the radiation from the earth's
surface, and therefore the fourth power of the mean temperature
(absolute degrees) are proportional to the amount of in-
solation. Then, starting with a figure of 59 F. or 288 A.
for the mean temperature of the earth's surface at present, we
have the following results :
4-4 5-4 6-4
Cloudiness (tenths of sky)
Insolation (per cent, of cloudless
sky) 90 86 82
Mean temperature, A. . . .291 288 284
Mean temperature, F. . . . 65 59 53
These figures show that a decrease in the mean cloudiness by
only one tenth of the sky would result in an increase of the
mean temperature by as much as 6 F., while an increase of
cloudiness by the same amount would lower the mean
temperature by 6 F.
It may be noted here that large ice-sheets, by deflecting
depressions into lower latitudes, would decrease the extent and
permanency of the sub-tropical anticyclones, especially over
the oceans. This in turn would lead to an increase in the
cloudiness of these areas, and would result in an appreciable
THE EFFECT OF CLOUDINESS ON TEMPERATURE 125
loss of heat to the world. This may have been a contributory
cause of the rapid expansion of the ice-sheets.
We saw in the Introduction that one of the salient features
of the " warm " periods was their dryness. From this we
naturally infer that they were favoured by unusually clear
skies, and this gives us an explanation of their warmth or
rather, it pushes the explanation one step farther back, for
we still have to account for their clear skies.
Up to the present we have been assuming, for the purposes
of the argument, that the cloudiness of the sky is the same in
all latitudes. That, of course, does not correctly represent
the conditions prevailing at present. We have a belt of cloudy
skies near the equator (about 5! tenths), then two belts of
clear sky along latitudes 20 to 30 in each hemisphere,
with an average cloudiness of 4 to 5 tenths, decreasing to
less than 2 tenths in the centres of the great deserts, and
outside these clear belts cloudiness increasing again to 6| or
7 tenths in cold temperate and sub-polar regions. This
distribution has a considerable effect on the mean tempera-
ture of the earth, for two reasons. First, owing to the greater
average elevation of the jsun, the radiation received (at the
limit of the atmosphere) is much greater in low latitudes
than it is near the poles. The radiation received on a hori-
zontal surface of one square centimetre area in the course
of a year on the Arctic Circle is about half that received on
the equator. Hence a cloud on the equator reflects back to
space about twice as much radiation as a similar cloud in
latitude 66. The same average cloudiness for the whole
earth would result in a much higher mean temperature if
the clouds were concentrated in high latitudes than if they
were concentrated in low latitudes.
Secondly, although clouds are much more effective as
reflectors of solar radiation than as reflectors of the long-
wave terrestrial radiation, they are not without a certain
effect on the latter. The figure given by W. H. Dines
(Chapter VI., i) is equivalent to a reflection of 8 per cent,
of the terrestrial radiation compared with 60 per cent, of
the solar radiation according to Haurwitz or 75 per cent,
according to A. Angstrom. There is a large transference of
heat from low to high latitudes by winds and ocean currents,
and the temperatures of the polar and sub-polar regions are
126 CLIMATE THROUGH THE AGES
raised considerably by the heat conveyed in this way. Hence
the outgoing radiation from the earth's surface in high latitudes
is much greater than the incoming radiation ; let us suppose
that in some particular locality it is ten times as great, and that
the sky is half covered by clouds. Then it is easily seen that
the gain of heat by the reflection of terrestrial radiation back
to the earth exceeds the loss due to reflection of solar radiation
back to space. Beyond the latitudes 67 N. and S., during the
polar night, the incoming radiation from the sun is zero, and
the effect is pure gain. Thus in high latitudes in winter,
cloudy skies are actually effective in raising the mean
temperature.
The qualification " in winter 5> suggests that in addition
to the mean cloudiness and its distribution according to
latitude, the seasonal variation is important. A locality in
which the sky is generally overcast in winter and clear in
suirtmer would have a higher mean temperature than another
place in the same latitude subjected to similar conditions, but
with its skies clear in winter and overcast in summer. This
is brought out very clearly by A. Angstrom (i) in an analysis
of the annual variation of temperature at Stockholm in
relation to the radiation. At Stockholm the mean annual
cloudiness is 6-4 tenths, and it o varies from 5- 1 tenths in June
to 7 9 tenths in December. Angstrom calculates that if the
mean annual cloudiness remained at 6*4, but instead of being
greater in winter than in summer was the same in all months
of the year, Stockholm would be colder than it is at present
in every month, the average difference being 2-2 F.
The remains of desert deposits formed during the warm
periods are mainly limited to middle and low latitudes,
while in high latitudes we find the remains of a rich vegetation
requiring a considerable rainfall. This distribution suggests
that while the cloudiness was small between about 10 and
55 latitude, it increased very rapidly beyond 55, thus giving
the most favourable conditions for a high temperature in all
parts of the world.
Although the popular conception of a geological land-
scape is a steaming jungle rather than, as it should be, an
arid plain, variations of cloudiness have played compara-
tively little part in theories of climatic change. Marsden
Manson (3) has, however, seized on the dual role of clouds
THE EFFECT OF CLOUDINESS ON TEMPERATURE 127
as reflectors alike of solar radiation and of terrestrial radiation,
and has constructed an elaborate theory of geological climates
on this basis, in conjunction with the gradual waning of the
internal heat of the earth. He points out that the surface of
the larger planets is covered by an unbroken layer of cloud,
and assumes that this must have been the condition of the earth
in past times, and in fact comparatively recently. Through
this cloud canopy the sun's rays could not penetrate, and as a
factor of climate the sun was almost inoperative. The earth
was radiating more than it was absorbing, and the sources of
this outgoing energy were the original supply of earth-heat
and radio-active minerals. Owing to the poorly conducting
crust, earth-heat was liberated, not in a steady stream, but
in spasms during periods of volcanic action and crustal
movement.
Let us start with one of these liberations of earth-heat.
Within its protecting cloud canopy the surface, oceans and
continents alike, was warm from equator to poles, but the land
surfaces cooled more quickly than the heat-conserving oceans,
and in due course, while the warm oceans were still supplying
enough moisture to maintain the cloud canopy intact, the
land surfaces began to freeze, and ice-sheets developed.
Apart from some local glaciations in the centres of the larger
continents this stage was first reached on a planetary scale
in the Permo-Carboniferous period ; this glaciation coincided
more or less with the present sub-tropical high-pressure belts,
and the reason is stated to be that "cold an ti cyclonic winds"
cooled the land most rapidly in those belts. The cooling of the
oceans continued, and with decreasing evaporation a stage
was reached in which these high-pressure areas, to-day
possessing the clearest skies of the world, ceased to be mantled
in clouds the sun broke through and deglaciation commenced.
Now followed a period of dual control, solar energy
prevailing near the equator, earth-heat towards the poles.
In spite of fluctuations, the latter gradually diminished, and
just before the Quaternary glaciation the polar oceans became
cold for the first time. Then the second planetary glaciation
occurred, centred in the cold temperate belts of greatest
precipitation, at this time the only regions which were per-
manently overcast. The cooling of the oceans continued,
and evaporation ceased to supply enough moisture for even
128 CLIMATE THROUGH THE AGES
this limited cloud belt, the sun shone over the whole world,
deglaciation again commenced and is still continuing.
The theory is interesting, but there are some insuperable
difficulties. With warm oceans and an unbroken cloud
canopy, the land surfaces, unless at a great altitude, would
not be likely to freeze ; the conditions are most nearly realised
at present in the equatorial rain belt, in which the land is
maintained at the same temperature as the neighbouring
oceans. " Cold anticyclonic winds " presuppose cooling by
radiation ; even if under world-wide isothermal conditions
the pressure distribution could remain unaltered, which is
highly improbable, we must suppose either that the anticyclone
would break down the cloud canopy, in which case the
tropical sun would certainly prevent glaciation, or that the
clouds would remain in spite of the anticyclone, in which
case the descending air would not be cold. Finally, the moist
conditions supposed by Marsden Manson to have prevailed
during the warm periods are in direct opposition to the dry
conditions demonstrated by the geological evidence set out
in the Introduction.
L. J. Krige (4) suggested that increased cloudiness and
precipitation would occur during periods of mountain building
because of unusually high evaporation from ocean basins
due to heat entering them from below. This type of suggestion
is very difficult to discuss quantitatively but it seems that to
make an appreciable difference to the amount of evaporation
the quantity of earth-heat would have to be an improbably
high multiple of the present supply.
REFERENCES
(1) ANGSTROM, A. " On radiation and climate." Stockholm, Geogr. Ann., 7,
1925, p. 122.
(2) HAURWITZ, B. " Insolation in relation to cloudiness and cloud density."
jf. Met. Amer. Met. Soc., 2, 1945, p. 154.
(3) MANSON, MARSDEN. " The evolution of climates." Baltimore, Md., 1922.
(4) KRIGE, L. J. " Magmatic cycles, continental drift and ice-ages." GeoL\Soc.
S. Africa, 1929.
CHAPTER VIII
CONTINENTALITY AND TEMPERATURE
IN the last few chapters we have discussed the factors
influencing the distribution of temperature, namely, winds
and ocean currents, the heat received from the sun, the
heat transmitted by the earth's atmosphere, and the reflection
from cloud surfaces. We may divide these factors into those
which are constant along a given line of latitude and give rise
to the " solar climate," and those which vary from place to
place even in the same latitude, and so give rise to the local
distribution of climates. The latter may be termed the
" geographical climate," since it depends mainly on the dis-
tribution of land and sea and partly also on the relief of the
land surface. Hence there is an intimate relationship between
temperature and land and sea distribution, and we shall expect
to find that the changing outlines of the geological continents
have been reflected as changes in the distribution of
temperature.
If we look at a map of the mean annual temperature, we
notice first of all that the isotherms run roughly parallel
with the lines of latitude. There is a belt on both sides of
the equator in which the temperature is above 80 F.,
extending from America across the Atlantic, Africa, India
and the Indian Ocean, the East Indies, and part of the
Western Pacific. This belt broadens out greatly over the
continents and narrows over the oceans ; over the Eastern
Pacific it thins out altogether. Between 30 N. and 30 S.
the eastern sides of the continents are warmer than the
western sides. Surrounding each pole is an irregular area
over which the mean temperature is below freezing point ;
the average position of the isotherm of 32 F. is north of
75 N. in the Arctic, but about 60 S. in the Antarctic. In
the Arctic the isotherm of 32 F. lies farther north over the
oceans and the western coasts of the continents than it does
over the central and eastern parts of the continents, that is,
the land is generally colder than the sea. Hence in temperate
199
130
CLIMATE THROUGH THE AGES
.2
*.*-
rO
e
->
C/2
4-4
s
SP
d
HH
bb
CONTINENTALITY AND TEMPERATURE
132 CLIMATE THROUGH THE AGES
regions the isotherms are widely separated over the oceans
and crowded together over the continents. The course of the
warm Gulf Stream Drift is marked by poleward bends of the
isotherms ; the course of the cold currents and the up-welling
of cold water is marked by equatorward bends, especially
along the California, Humboldt, and Benguela Currents.
The charts for the extreme months January and July
(Figs. 12 and 13) show that in temperate latitudes the sea
is much warmer than the land in winter ; in summer the
land is warmer, though not to the same extent. The larger
the continent, the greater the depression of temperature in
its centre, though this is controlled also by the roughness of
the relief. The chart for January also shows the extraordinary
effect which is exercised by the Gulf Stream Drift on the coast
of Scotland and Norway ; in this month the Arctic Circle
between longitude o and the Norwegian coast is actually
more than 40 F. warmer than the average of the whole
parallel, and 90 F. warmer than the " cold pole " of Siberia.
The last point calling for special notice is the low temperature,
in summer as well as winter, experienced in Antarctica and the
neighbouring parts of the Southern Ocean, Greenland, and
most of the Arctic Ocean, all those parts of the world, in fact,
which are permanently covered with ice-sheets or closely
packed floating ice.
In studying the climates of past times the geography of
which has been reconstructed, 1 it is useful to have some
mathematical expression of this relationship between the
temperature and the distribution of land and sea. Such an
expression can be calculated in two different ways. We can
either calculate the mean temperatures of each of a number
of different parallels of latitude, and express the mean for any
parallel in terms of its latitude <f> and the fraction n of the
parallel which is covered by land, or alternatively, we can
start with the mean temperatures of a number of individual
points and represent them in terms of the latitude and the
local land and sea distribution. The first method was initiated
by J. D. Forbes (i) and further developed by R. Spitaler (2),
who obtained an expression which, converted into Fahrenheit
degrees, becomes :
T (F.) =27 -6+32 cos^ + i3 cos 2^+35 n cos 2<.
1 Sec Chapter XII.
GONTINENTALITY AND TEMPERATURE 133
Since cos 2 <f> is positive between 45 N. and 45 S., negative
from 45 to the poles, the term 35 n cos 2<f> indicates that the
effect of land in low latitudes is to raise the temperature and
in high latitudes to lower the temperature. This is simply a
mathematical expression of the fact that an extensive land-mass
tends to give a hot desert near the equator and a cold tundra
near the poles.
It is to be noted that Spitaler's formula is derived entirely
from the present distribution of temperature and of land and
sea, and therefore implicitly assumes the existence of the
present system of winds, of ocean currents, and of ice. The
fact that the formula applies reasonably well to both hemi-
spheres, in spite of their very different configurations, shows
that this is not necessarily a serious objection, but it is obvious
that three conditions must be fulfilled before it can be applied
to determinations of the temperature in other geological
periods. First, there must be large areas of open ocean
within the tropics, with free communication between low
and middle latitudes, in order to allow for the great trans-
ference of heat by ocean currents. Secondly, the land and
sea distribution must not be of such a nature that the system
of pressure and winds is radically different from the present
system ; in the language of Chapter II., the planetary cir-
culation must not be dominated by the geographical
circulation. Thirdly, there must be extensive areas of
floating ice. For example, Spitaler's formula gives for the
Arctic Ocean a mean temperature of about 15 F., which is
well below the freezing point of sea water. But we have seen
in Chapter I. that if the " non-glacial " temperature of the
polar regions could be raised by some 5 F. there would be no
floating ice, and the mean annual temperature would be well
above 32 F. The formula in its present form, therefore,
does not apply to the " non-glacial " periods ; a better
representation of the zonal distribution of temperature during
these periods would be given by a formula of the type :
T=T +# cos <f> b cos 2<f>-\-cn cos 2<,
the negative sign of the term cos 2 < allowing for the decrease
of the zonal contrast during these periods.
For small changes of geography, however, Spitaler's
formula gives a useful means of estimating the resulting
134 CLIMATE THROUGH THE AGES
changes of temperature. The result of a decrease of ten
per cent, in the area of land in any latitude on the mean
temperature of that latitude would be as follows :
Latitude (degrees) o 10 20 30 40 50 60 70 80
Change of tem-
perature (F.) . -3-5 -3-3 -2-7 -1-7 -0-6 +0-6 +1-7 4-2-7 +3*3
We will return to these figures later.
We must now return to the second method of calculating
geographical factors of temperature, in which the basis is
the temperature distribution at a number of individual
points instead of the mean temperature along whole parallels
of latitude. This method has been extensively employed
by F. Kerner and later by myself ; it can be used, not only for
the whole world, but also for restricted areas, though as
F. Kerner von Marilaun (Kerner- Marilaun) (3) points out
with justice, the temperature of any given point depends
not only on the geography of the region immediately
surrounding the point, or even on the distribution of land and
sea along the whole line of latitude ; we have to take into
account conditions over the whole globe, as far as they affect
the air and water circulation. He accordingly divides the
geographical factors into local, or stenomorphogenous, and
general or eurymorphogenous. Kerner has written a large
number of palseoclimatological papers, of which I have
selected three for discussion here. He subsequently put
together his results in an important book (3).
The first paper (4) deals with the winter climate of Europe
during the Tertiary period. At present this temperature is
governed on the west coast by the temperature of the Gulf
Stream Drift, and decreases eastward in accordance with the
distance from the Gulf Stream and the increasing continentality.
He accordingly represents the temperature along any latitude
by the equation :
where t is the winter temperature of a European locality,
T the winter temperature of the Gulf Stream Drift in the
same latitude, d the distance of the point from the Gulf
Stream Drift along that latitude (the " linear continentality "),
L and / the percentages of land in large and small areas
surrounding the place. A and B are constants, which were
CONTINENT ALITY AND TEMPERATURE 135
evaluated from the mean January temperatures reduced to
sea-level taken at the intersections of every fifth degree of
latitude and longitude from 35 to 55 N., and 20 W. to
70 E. The best representations of the general and local
continentalities L and / were determined empirically, and the
definitions finally adopted were : for L the percentage of land
in a twenty-degree " square " of latitude and longitude
surrounding the point (/ 20 ), and for / the average of the
percentages in five-degree and ten-degree squares (/ 5 and
/ I0 ). A square in this sense is an area bounded by lines of
latitude and longitude each covering the same number of
degrees. The formula as finally calculated took the form
^T-A( 4 +o- 2 AE)/ 20 -B- 5 (/ IO +/ 5 ),
where AE is the longitude in degrees east of Greenwich.
The values of A and B were calculated separately for each
fifth degree of latitude.
Fig. 14. Isotherm of 32 F. in Tertiary and Quaternary.
After F. v. Kerner.
By means of this formula and reconstructions of the land
and sea distribution during six stages of the Tertiary period,
the " stenomorphogenous " isotherms were reconstructed.
The results are' summarised in a small figure showing the
position of the January isotherm of 32 F. in each period,
which is reproduced in Fig. 14. The results indicate a more
favourable climate from the beginning of the Eocene up to
and including the Miocene, a Pliocene climate differing little
from the present and a less favourable climate of the early
Quaternary in Western Europe. The differences from the
present rarely exceed 10 F. in Central Europe, but east of
136 CLIMATE THROUGH THE AGES
the Caspian during the Middle Eocene and Oligocene the
calculated January temperatures are nearly 30 F. above the
present temperature. In the early Pleistocene the calculated
temperatures are nowhere 5 F. below the present. The
mean temperatures in January over the area covered by
the figure, expressed as differences from the present mean
temperature, are as follows :
Latitude N 55 50 45 40
F. F. F. F.
Early Eocene . . . +2 + 6 f 10 +7
Eocene +4 +9 +13 +-io
Oligocene .... +5 +12 +13 f 9
Miocene +r +3 +5 +3
Pliocene 2 3 4 3
Pleistocene . . . . i + i 4~ i - i
Table 5. Kerner's calculated temperature differences,
January.
With regard to the distant or eurymorphogenous component,
only a few qualitative remarks are possible. The submergence
of the greater part of Florida would have increased the strength
and velocity of the Gulf Stream somewhat, but the effect on
the winter climate of Western Europe would have been slight.
The existence of a land-bridge between Greenland and Europe,
according to Semper, would raise the temperature off the
west coast of France by some 13 F., but would lower the
winter temperature of the Arctic regions. Semper thought
that the Tertiary polar floras developed in a continental
polar climate with hot summers, but this view is not tenable.
For the warming effect of the " Indian Drift " which reached
Central and Southern Europe from the Indian Ocean during
the older Tertiary, Heer's simple assumption of a warming
effect of 7 F. (5) is probably the best approximation that can
be made at present.
The winter temperatures deduced by Heer from the fossil
plants in the Miocene are from 5-io F. higher than the
temperatures due to the local geography during that period,
but this difference is sufficiently accounted for by the effects
of more distant changes, and particularly by the warming
effect of the Indian Drift. Kerner does not consider the
possible effect of a change from " glacial " to " non-glacial "
conditions.
CONTINENTALITY AND TEMPERATURE 137
The second paper by F. Kerner (6) extends this study
to the Arctic regions. The distribution of the January
temperatures along each parallel of latitude in the Arctic
Ocean is expressed in the form
t=awbk,
where w is the warming effect of a gap ten degrees of longitude
broad, open to the world oceans, and k is the cooling effect
of a ten-degree barrier between the Arctic and the open ocean
farther south. The values of the terms w and k are deduced
from the distribution of temperature in January under present
geographical conditions, and from these values and the land
and sea distribution reconstructed for the Middle Eocene the
isotherms for that period are reconstructed. Some attempt
is made to allow also for a decreased cooling of the Gulf
Stream by the cold Labrador Current, but the calculations
implicitly assume a large area of floating ice in the Arctic
Ocean. The reconstructed temperatures are still below
freezing point over most of the region north of 70 N., but they
are well above the present temperatures, and it seems probable
that the improvement was sufficiently great to raise the mean
" non-glacial " temperature at the pole above the freezing
point. In accordance with the argument of Chapter I.,
this would mean that there would be no floating ice-cap,
and a totally different climatic regime would come into force.
This question can be better investigated on the basis of
some more recent work of Kerner' s (7) in which he returns
to the discussion of Arctic temperatures. The first part of
this paper, dealing with the akryogenous or " non-glacial "
marine climate of an open polar ocean, has already been
referred to in Chapter I. In the second part, Kerner analyses
the distribution of January temperature at present along the
75th parallel of latitude. This parallel runs mainly over the
ocean, the only land which it crosses being Novaja Zemlya,
the Taimyr Peninsula, the New Siberian Islands, parts of the
Arctic Archipelago, and Greenland, but over the greater part
of its course it has extensive land-masses Europe, Asia, and
North America within five degrees to the southward. There
are only two gaps in this surrounding land-ring, the very
narrow and shallow Bering Strait, through which very little
warm sea water penetrates at any season of the year and none
138 CLIMATE THROUGH THE AGES
at all in winter, and the broad Atlantic gap through which
the Gulf Stream Drift makes its way. The surface of the
Arctic Ocean is cooled in winter by the cold winds from the
winter anticyclones over the great continents, and warmed
by the Gulf Stream Drift, and the January temperature at
any point along the 75th parallel therefore depends on the
amount of land to the southward and the distance from the
Atlantic gap through which the warm water enters. These
two factors are named by Kerner the " Continental " term
K, and the " Separation " term S.
To get the matter clear, imagine a one-roomed cottage
with thin walls, against the outside of which snow is banked,
making them very cold, while in one wall there is a fire.
The temperature at any point near one of the walls is then
determined by the distance from the wall and the distance
which separates the point from the fire. The walls represent
the cold continents and the fire the Gulf Stream Drift. Kerner
finds that the January temperature of a point on the 75th
parallel is given (in Fahrenheit degrees) by the expression :
T=45-i-o7 S 1-9 K.
The precise evaluation of the " Separation " term S and
the " Continental " term K is somewhat complex and need
not be gone into here.
According to this formula, the most favourable distribution
of land conceivable for high polar temperatures would be a
number of long, narrow islands extending from low to high
latitudes, separated by wide, deep channels. This distribution
may have actually occurred at some stage in the middle of the
Palaeozoic period, and it was approached to some extent during
parts of the Mesozoic and Tertiary periods. Two examples
are considered by Kerner, the Middle Eocene and the Upper
Jurassic. In the Middle Eocene, according to a recon-
struction by Matthew, the circum-polar ring was broken by
three broad gaps, an enlarged Bering Strait, the present
Atlantic gap, and the Obic Sea which separated Europe from
Asia. In the Upper Jurassic, according to a reconstruction
by Uhlig, the Atlantic gap was replaced by the " Shetland
Strait " farther west, and there was in addition a fourth gap,
the Jana Sea, between 120 and 140 E.
CONTINENT ALITY AND TEMPERATURE 1 39
From his formula, Kerner calculates the mean January
temperatures for the 75th parallel to have been as follows :
Present. Mid-Eocene. Upper Jurassic.
F. F. F.
-20-7 +7-8 +18-5
All the figures for individual longitudes are higher than
the present ones in the same longitudes, but none of them
exceed 32 F., and they still represent a very severe climate.
We saw in the Introduction that during both these periods,
vegetation of a sub-tropical or warm-temperate aspect
flourished at several points north of 70 N., and Kerner
considers that either the plant evidence is not reliable or the
solar heat must have been greater.
The real reason for the discrepancy is that, in spite of
his discussion of akryogenous (" non-glacial ") temperatures
in the first part of the paper, Kerner employs the actual
distribution of temperature as the basis of his calculations,
instead of the " non-glacial " distribution. Thus his calcu-
lated figures imply the presence of a large amount of floating
ice and ice-cold water in the Arctic Ocean, whereas we have
seen that, given a " non-glacial " temperature only five
degrees above the present, there would be no ice, and the
water cooled by radiation would at once sink to the bottom.
Evidently we have to recalculate his figures on a " non-glacial "
basis.
Kerner's Middle Eocene mean January temperature is
28-5 F. above the present mean in 75 N. Of this increase
we find that 6-9 F. is accounted for by the decrease in
continentality and the remaining 2i-6F. by the increased
influx of warm ocean currents (Separation effect). The small
continentality effect may be allowed to stand, but the Separa-
tion effect requires some modification. Suppose a warm current
at temperature t introduced into a mass of water at tempera-
ture ?, the resulting mean temperature of the water-mass
being T. Then we can suppose that the warming effect (T t 1 )
is proportional to (tt 1 ), and we can write T^-f+c^ t)
where c is a constant fraction, depending on the volumes
of the mass of cold water and of the warm current. Hence
the present Gulf Stream Drift would have a smaller warming
effect on a non-glacial Arctic Ocean than on the present
140 CLIMATE THROUGH THE AGES
glacial Arctic Ocean. On the other hand, if there were no
Arctic ice there would be no cold East Greenland and
Labrador Currents. From a study of the sea surface isotherms
of the North Atlantic, we find that in January the temperature
along the centre of the Gulf Stream is 71 F. in latitude
30 N. and 64 F. in 38 N., a fall of 0-9 F. per degree.
From 38 to 43 N. on the other hand temperature falls by
about 22 F. in only 5 degrees. Of this fall, only about 5 F.
can be due to the normal fall with latitude, and the remaining
17 F. is due to admixture with the cold water of the Labrador
Current. That is, the present January sea surface isotherm of
about 32 F. in 75 N., 10 E., would be replaced by one of
49 F. Now we have the following data for a recalculation
of the warming effect of the Gulf Stream Drift in a non-glacial
polar basin with the present configuration :
" Glacial " " Non-glacial "
Conditions. Conditions.
F. F.
Temperature of Gulf Stream Drift, / 32 49
Temperature of Arctic Ocean, T . . - 18 25
Difference (/ T) 50 24
Table 6. " Glacial " and " Non-glacial " temperatures.
The total difference (tt 1 ) is proportional to (/- T), so
that we have to multiply the coefficient i 07 of Kerner's
"Separation" effect S by 24/50 or approximately 0-5 in
order to correct this factor for a non-glacial ocean with the
present land and sea distribution. If, now, we introduce a
second current equivalent to the Gulf Stream Drift, the
additional warming effect will be proportional, not to (t t 1 }
but to (/ T), and the resulting temperature T 1 will be equal
to ^+2^(/ / x ) c*(t t 1 }, i.e., the increase is something less
than twice that due to a single Gulf Stream Drift. The
constant c is small, so that the additional term is not im-
portant ; moreover, Kerner's method of calculation makes
some allowance for it, but in order to be on the safe side I
have reduced the factor 0-5 to 0-4. The increase in the
non-glacial temperature of the Middle Eocene, due to the
change in the value of S, should therefore be only 8 '6 F.,
instead of 21 -6 F., making, with the increase of 6*9 F. due
to the decreased continentality, the total increase in the
CONTINENT ALITY AND TEMPERATURE 14!
non-glacial January temperature during this period 15*5 F.
This has to be added to the mean January " non-glacial "
temperature at present, 25 F., raising the Middle Eocene
temperature of the Arctic Ocean in 75 N. to 40-5 F. This
is well above the freezing point of sea water, so that there is
no ice, and 40-5 F. is also the real January temperature.
For the Upper Jurassic, Kerner obtains a January tem-
perature in latitude 75 N. of 39-2 F. above the present.
Of this amount, his calculations show that 6-3 F. is due
to the change in the Continental effect and 32 9 F. to the
change in the Separation effect. Applying the correction
for a non-glacial ocean by multiplying by 0-4, the latter
quantity becomes 13*2 F., making with the change in the
continental effect a total increase of the " non-glacial " tem-
perature of 19-5 F. Added to the present non-glacial
temperature of 25 F., this makes the Upper Jurassic January
temperature 44-5 F. in latitude 75 N., a figure which is
very near the present mean January temperature of South-
west England, so that these temperatures are quite consistent
with the remains of the vegetation discovered by geologists.
While I make no pretence as to the absolute accuracy of these
computed figures, I do claim that they are so far above the
freezing point of sea water that the strictest revision is unlikely
to reduce them below it, and that the case for an ice-free
Arctic Ocean during some periods of geological time is
thereby established.
It has been suggested to me that the figure of 17 F.
adopted for the cooling of the present Gulf Stream Drift by
ice and ice-cooled Arctic water is too great, and this will
serve as an example of the effect produced on the calculation
by a modification of the basis. Let us reduce this figure
from 17 F. to 9 F., which is almost certainly too small.
Repeating the calculations on this new basis, we find that the
mean January temperature in 75 N. comes out as 37-5 F.
during the Middle Eocene and 40 F. during the Upper
Jurassic. These figures are well above the freezing point,
so that the Arctic Ocean would still be non-glacial.
Another objection which may be raised to the way in
which these high polar temperatures have been obtained
is that they depend on the existence of powerful ocean currents,
which in turn depend on the planetary wind system, and that
142 CLIMATE THROUGH THE AGES
a warming up of the Arctic Ocean without a corresponding
change in the temperature of the equatorial zone would
result in a decreased strength of the wind-driven ocean
currents. This possible objection has already been dealt
with in the chapter on Pressure and Winds, page 53, where
it was shown that there is a critical point in the planetary
circulation. With a temperature difference above this critical
value, the winds are largely directed outwards from the pole,
ocean currents have difficulty in penetrating into the Arctic
basin (barely 2 per cent, of the water in the Gulf Stream off
Florida reaches the Arctic Ocean), and their temperature
is also greatly lowered by the cold winds. With a temperature
difference below the critical value, the winds are directed
inwards towards the pole, and the volume and temperature
of the ocean currents are maintained with much smaller loss.
Hence the oceanic circulation induced by high polar tem-
peratures would assist in maintaining those temperatures ;
similarly, the oceanic circulation with low polar temperatures
would help to keep the temperature low.
The interesting question arises, has the temperature of
the Arctic Ocean risen above the critical point at any stage
of post-glacial time ? I think there is no doubt that it has (8).
During the " Climatic Optimum " there was a rich flora in
Spitsbergen, while the fossil marine mollusca indicate a coastal
sea temperature much higher than the present in all the
Arctic lands which are at present dominated by sea ice,
including Iceland and Greenland. The " Climatic Opti-
mum " was experienced also over most of Europe, where
it has been studied more closely than in the Arctic lands.
In Scandinavia the warm period appears to have begun
rather suddenly during a period of increased vigour in the
circulation of the Atlantic Ocean (" Atlantic " or " Maritime
Phase"). During this period, owing to submergence, the
Baltic lay more open to the Atlantic than at present, and a
maritime climate extended as far as the coast of Finland.
Depressions passed readily across Denmark and along the
German coast, so that in Northern Europe the Atlantic period
had a greater rainfall than the present, with milder winters
and cooler summers. About 3000 B.C. the connexion between
the Baltic and the Atlantic again became restricted, and there
was a slight extension of the area of the British Isles. These
CONTINENTALITY AND TEMPERATURE 143
changes were associated with a notable difference of climate.
Depressions seem to have favoured a northerly track into
the Arctic Ocean, and the British Isles, Western and Central
Europe became much more continental, with very warm
summers ; the winters do not seem to have been any colder
than at present. These conditions were very marked in
Switzerland, where settlements occurred at very high levels
and there was much traffic over passes which are now occupied
by glaciers. In Spitsbergen the " ice floor " melted com-
pletely. The mean annual temperature at Green Harbour
is at present 19 F., so that there must have been a rise of
mean temperature by at least 13 F. in this part of the Arctic.
The favourable climate lasted until about 3000 B.C., deterior-
ated slowly until about 500 B.C. and then came to an abrupt
end. The change of climate for the worse was very rapid,
and, according to H. Gams and R. Nordhagen (9), in the
Alps it " had the appearance of a catastrophe."
My reading of this history is that the increased circulation
of the Atlantic period swept away the ice from the Arctic
Ocean (though apparently not from the channels among
the islands north of America and Greenland). After the
passing of the Atlantic period, the Arctic Ocean became
somewhat cooler, but, being still free of ice, was very stormy,
and this storminess itself maintained the winter temperature
above the critical point and prevented the ice-cap from
beginning to form. Then came an unusually quiet cold
winter, the ice-cap obtained a footing, and perhaps in the
course of a single season covered the greater part of the Arctic
Ocean. The result was a sudden great change in the climate
of Europe ; the conditions of to-day came in " with the
appearance of a catastrophe." The ice-cap, once formed,
kept the winter temperature below the critical point by its
own power of persistence.
It is possible that the Arctic Ocean again became free of
ice during historic times, from about the fifth to the tenth
or eleventh centuries of the Christian era. O. Pettersson (10)
makes out a good case for the absence of sea ice in the East
Greenland Current during the latter part of this period.
His map of the old Norse sailing routes shows a track direct
from Iceland to the east coast of Greenland in latitude 66 N.,
then down the coast to Cape Farewell, and up the west coast.
144 CLIMATE THROUGH THE AGES
According to the documentary evidence which he adduces,
this route was followed until nearly A.D. 1200, and for most
of the period there is no mention of ice in any of the numerous
descriptions. From Greenland the Norsemen sailed to
Wineland (on the coast of North America), and again there
is no mention of ice. Recently this question has been re-
investigated by L. Koch (n). From an exhaustive study of
historical records of the ice off East Greenland and Iceland he
concludes definitely that from A.D. 800 to 1200 there was
scarcely any summer ice near Iceland. This is very striking ;
Pettersson's own inference is that the ice did not then come
so far south as it does now, and it seems probable that the
Arctic Ocean was, if not ice-free, at least in the intermediate
or " semi-glacial " condition described in Chapter I., in
which a small cap formed in winter but disappeared completely
in summer. This question is discussed further in Part III.
In 1917 I made my first incursion into the subject of
continentality and temperature (12). This paper dealt with
the region between 40 and 60 N. and between the Atlantic
coast of Europe and 90 E., that is, practically the same
region as the first of Kerner's papers (4), which I had not
then seen. Fifty-six stations were selected in this area, and for
these were found the height above sea-level, the mean tem-
peratures of January and July, the " continentality " and the
radiation receive . The " continentality " was measured
as the percentage of land in a five-degree circle, in a zone
between five- and ten-degree circles, and in a zone between
ten- and twenty-degree circles surrounding each station,
these measures being termed respectively G 5 , C 5 . IO , and C I0 . 20 .
It was also found necessary to introduce a Gulf Stream com-
ponent into the January temperatures north of 50 N., the
temperature decreasing by 0-6 G. for every hundred kilo-
metres, or 1-7 F. for every hundred miles, east of a great
circle through Valentia in South-west Ireland, and touching
the north-west coast of Norway.
The most important result of the investigation was to
show that the January temperature of Europe is much more
closely related to the land and sea distribution than to the
amount of heat received from the sun. The solar heat is
the same at all points on the same line of latitude (apart from
the effect of cloudiness, which was not discussed but which
GONTINENTALITY AND TEMPERATURE 145
itself depends on the land and sea distribution) and decreases
rapidly from south to north, so that if the temperatures were
governed by this cause only, the isotherms should run east
and west. It is found that actually they run nearly north
and south, the rate of temperature decrease eastwards from
the coast towards Russia being much more rapid than the rate
of decrease northwards from Southern to Northern Europe.
In July the effect of land and sea distribution is less marked
+10-
-30
Radius of Inland in Degrees
5 10 16 6
20
-I
-54
500 1000 1500 2000
Ares of Island In Tnousands
of Square Miles
Fig. 15. Change of temperature due to formation
of an island.
and is slightly exceeded by the effect of solar radiation, so
that the isotherms run more east-west than north-south.
Another interesting result concerns the effect of land-
masses of different areas. Suppose a circular island were
to form in mid-ocean in about latitude 60 N., and to increase
gradually in size until it reached a radius of twenty degrees
of arc or an area of about two million square miles. The
resulting changes of temperature at a point on the edge are
shown in Fig. 15. With increasing area the January tempera-
ture (lower curve) would fall and the July temperature (upper
curve) would rise. From the figures obtained it appears thai
10
146 CLIMATE THROUGH THE AGES
the fall of temperature in January would be very slow at first,
being only 0-5 C. or 0-9 F. by the time the island had
reached a radius of five degrees of arc (area about 375,000
square miles). As the island increased in size the fall of tem-
perature would then become very rapid, until by the time the
radius was ten degrees (area 1,500,000 square miles) it would
amount to 22 C. (40 F.). After this the cooling would again
increase more slowly with increasing area.
This curious curve is connected with the influence of the
island on the atmospheric circulation. In Chapter II. we
found that the effect of a small ice-covered island on the
atmospheric circulation is slight the storms sweep across
it with very little hindrance. A larger island modifies the
distribution of pressure, and an island with a radius of about
450 to 500 miles (about seven degrees) begins to develop a
winter anticyclone. With a radius of ten degrees this winter
anticyclone becomes semi-permanent and dominates the
pressure distribution, and any further increase of radius
merely results in a slow increase of intensity. While the radius
is not more than five degrees (350 miles), the general meteoro-
logical conditions are unaltered and the cooling effect of land
is due to its lower specific heat, which causes it to lose its
summer heat more rapidly than does water, but with a radius
of seven degrees (480 miles) or more, the winter anticyclone
prevents the influx of heat from the neighbouring sea, and
cooling by radiation proceeds rapidly.
Summer conditions are very different. As the island
increases in size, the July temperature rises rapidly at first,
until the radius has reached about five degrees (350 miles).
Up to this point the sole effect is probably the absorption
of the sun's heat by the soil and its transference to the lower
layers of air by conduction. When the radius reaches seven
degrees (480 miles) the increase of temperature becomes
slower, and may even cease altogether ; this is probably due
to a sea-breeze effect which lowers the afternoon temperature
considerably. After the radius has exceeded about twelve
degrees (830 miles) the July temperature begins to rise again
more rapidly, but with a radius of twenty degrees the warming
effect at the centre is only about n G. (20 F.).
The broken line in Fig. 15 shows the effect of the island
on the mean annual temperature (mean of January and
CONTINENTALITY AND TEMPERATURE
July). The annual temperature is raised slightly by the
introduction of an island with a radius of less than seven
degrees (480 miles), the maximum effect, a rise of rather
more than i C. (2 F.) in the mean temperature, occurring
in the centre of an island of radius five degrees (350 miles).
With greater areas the mean annual temperature is lowered.
The distribution of land most favourable for high average
temperatures is therefore a number of small islands each
about 700 miles across, a condition which was frequently
Fig. 1 6. Observed, and calculated temperature
changes, Litorina period, January.
approached in Europe during the early part of the Tertiary
period.
This investigation concluded with an attempt to recon-
struct from the changes in the land and sea distribution
during the Litorina post-glacial submergence of Scandinavia
(which occurred during the Atlantic period), the changes
in the mean temperatures of January and July. For this
purpose the present mean temperatures of those months at
a large number of Scandinavian and Baltic stations were
compared with the percentage of land in a five-degree circle,
148 CLIMATE THROUGH THE AGES
and it was found that a decrease of one per cent, in the
continentality raised the January temperature by 0*20 G.
(0-36 F.) and lowered that of July by 0-06 G. (0-11 F.).
On this basis the change of temperature over the Northern
Baltic was calculated as +3 C. (+5 F.) in January and
i C. ( 2 F.) in July. The calculated temperatures in
different districts are compared with those deduced by various
authors from the fauna and flora in Table 7 ; the results for
January are shown graphically in Fig. 16, in which the
calculated changes are shown by lines of equal temperature
change, and the variations from present conditions required by
geologists are shown by the figures. A variation of uncertain
amount is indicated by the sign + or without a figure.
The agreement is good on the whole. The actual type of
change in the direction of a more insular climate, warmer on
the whole, is in perfect agreement. Many of the botanists
comment on the prolongation of the autumn into the present
winter, which is especially characteristic of the change to a
more insular climate. The amounts of the change are also
in good agreement except in the Christiania region and in
North Denmark, where the geologists require a greater change
than would be inferred from the change of continentality.
This is probably accounted for by the greater freedom of
ingress which the more open seas allowed to the warm waters
of the Gulf Drift, but this point is discussed in detail later.
In 1918 I was able to extend the study of " Continentality
and Temperature " to embrace the greater part of the
world (13). The method adopted was extremely simple,
perhaps rather too simple. From various sources I obtained
the average mean temperature in January and July at each
point of intersection of the ten-degree co-ordinates of latitude
and longitude over both land and sea. On a globe 1 a ten-
degree circle was drawn round each of these points, divided
into east and west semi-circles, and the amount of land in
each semicircle was measured and expressed as a percentage
of the area of the semicircle. The area of ice in the whole
circle was also measured and expressed as a percentage of the
area of the whole circle ; this area included both land ice and
sea ice. In winter the area of sea ice is many times the area
*
1 The octagonal globe employed in the Meteorological Office for work con-
nected with the Roseau Mondial was utilised for this purpose.
CONTINENTALITY AND TEMPERATURE
District.
Author.
Inferences from Fauna
and Flora.
Calculated
Differences.
January.
July.
Norway
Rekstadt and
No trace of warm period.
+ 1-0
-0-3
Helgeland
Vogt.
(65 N.).
Trondhjem.
J. Rekstadt.
Climate " not greatly dif-
-f I 'O
-0-3
ferent from present."
Bergen .
C. F. Kolderup.
Climate " somewhat mil-
+ 0-2
o-o
der than present."
Jaederen
K. Bjorlykke.
No marked warm period.
-fo-i
o-o
(S.W. Nor-
way).
Sorland (S.
D. Danielsen.
Climate similar to present.
-j-O'2
o-o
Norway) .
J. Holmboe.
" Somewhat warmer than
present."
Christian ia
C. Brogger.
Rise of 2 in mean annual
-fo-6
-O'l
Region.
temperature.
J. Holmboe.
Rise of 1-9 to 2-2 in
mean annual tempera-
ture. Climate more
maritime.
Denmark
V. Nordmann.
A damp, warm period
+ 0-3
O'O
North.
(warmer winters, sum-
mers unchanged).
Sweden
G. Andersson.
About 2 warmer. More
-f i to
O-2
General.
temperate.
+ 3
to - i
R. Sernander.
Insular climate.
L. von Post.
Warm, moist.
North Germany.
Several authors.
The difference, if any, was
o-o to
o-o to
in the direction of a more
-0-8
-f 0-2
continental climate.
East Baltic.
Kupfer.
Damp, warm (climate of
-f-2'0
-0-6
West European coasts).
H. Lindberg.
Finland had a more in-
-f3'O
- I -0
sular climate.
Table 7. Comparison of actual and calculated
changes of climate.
150 CLIMATE THROUGH THE AGES
of land ice, and the results are taken as applying to the former.
The effects of each of these variables land to the west,
land to the east, and ice, independently of the other two
were then worked out by the method of correlation. The
results can be set out in general terms as follows :
1. In winter, the effect of land to the west is always to
lower temperature. This holds in every latitude except
10 S. and 20 S.
2. In winter, the effect of land to the east is almost
negligible. The only important exception to this
rule is in 70 N. latitude, which may be considered
as coming within a belt of polar east winds.
3. In summer, the general effect of land whether to the
east or west is to raise temperature, but the effect is
nowhere anything like so marked as the opposite effect
of land to the west in winter.
4. The effect of ice, in the few cases in which it is possible
to measure it, is invariably to lower temperature.
5. The temperature even of a point in mid-ocean in any
latitude is modified by the presence of land along other
parts of that parallel of latitude. The January tem-
perature of a point in mid-Atlantic in latitude 60 N.,
for instance, is higher at present than it would be if
the North Pacific were occupied by land.
These general conclusions could have been arrived at
without a laborious statistical analysis, but the latter was
necessary to reduce them to figures, and so make possible
calculations of the thermal effect of changes in the land and
sea distribution. This calculation is carried out by means
of the formula :
where T is the temperature of the point required, Z is the
" zonal temperature " (see below), L w is the percentage of
land in the semicircle to the west, L E the percentage of land
in the semicircle to the east, and I the percentage of ice in
the whole circle, a, i, and c are constants for any particular
latitude.
CONTINENTALITY AND TEMPERATURE
The " zonal temperature " Z is not a fixed quantity for
any particular latitude ; it depends on the amount of land
in the neighbourhood of that latitude, a zone which is mainly
land-covered having a lower zonal temperature than a zone
which is mainly occupied by water, but the relationship is
not simple. 1
The coefficients a, b, and c were obtained by purely statistical
methods. They are as follows :
a
b c
a
b
Land to
Land to
Land to
Land to
January.
west.
east. Ice.
July.
west.
east. Ice.
70 N.
-0-43
0-20 0-46
70 N.
4-0-02
4-0-02 -0-16
60
-0-31
-o-oi -0-07
60
-o-oi
4-O-II
50
-0-29
4-0-09 0-09
5 I
4-0-04
4-o-o6
40
-0-17
4-0-04
4-0-05
4-0-07
30
-0-08
4-0-03
30
4-o-oS
o-oi
20
-o-oi
-o-oi
20
4-0-07
4-0-02
ioN.
o-oi
4-0-03
10 N.
4-0-03
o-oi
4-0-01
o-oo
4-0-02
o-oi
ioS.
4-0-04
-o-oi
! 10 s.
4-0-04
-0-03
20
4-0-07
o-oo
20
4-0-02
-O-O2
30
4-o-o6
4-0-03
30
o-oi
o-oi
40
4-0-09
-0-03
40
o-oo
-0-03
Table 8. Effect of one per cent, of land to the west, of land to the
east, and of ice on temperature.
It should be noticed that in this table the unit area of
ice, one per cent, of the whole circle, is twice that of the
unit area of land, one per cent, of a semicircle. The two
figures can be made comparable by adding together the
two land coefficients, thus :
January. July-
70 N. 60 N. 50 N. 70 N.
Effect of land . . 0*63 0-32 0*20 40*04
Effect of ice . . . 0*46 0*07 0*09 0*16
The cooling effect of ice in winter is apparently less than
that of land. This is because most of the area shown as
occupied by ice is sea which is covered by more or less
Winter :
Z - 70(0-95-003 0) log L-
tan* L
Summer :
100
= 30(1 -05- cos 0) log L4-
tan 2
100
'5*
CLIMATE THROUGH THE AGES
cattcred drift ice and icebergs, that is, the surface is partly
ice and partly water. In high latitudes in winter the land is
usually snow covered, and it is easy to see that this snow
surface must have a greater cooling effect than scattered
sea ice. For ice-sheets over land it would be best to adopt
the coefficients of land in winter.
These formulae, being based on the present land and sea
distribution, could not be employed in calculating the
distribution of temperature during periods with a radically
different distribution of the continents, but it seems legitimate
to employ them to calculate the differences from the present
Fig. 17. Lines of equal change of land level in Quaternary,
and changes of land and sea distribution. Additional land
shaded, additional sea black.
of the temperature distribution in a geological period during
which the main outlines of the geography were the same as
at present. In doing this it is best to measure the differences
in the land and sea distribution from the present, and to
calculate from these the differences in the temperature dis-
tribution, rather than to attempt to work ab initio. By working
only with differences, we preserve intact the local peculiarities
of climate and minimise the risk of error. The formulae were
applied in this way to a reconstruction of the land and sea
distribution during the early part of the Quaternary period.
This was the culmination of a great period of elevation in
cold temperate regions, with corresponding depression in
CONTINENTALITY AND TEMPERATURE 153
the tropics. I have represented the differences in level and
land and sea distribution between that time and the present
in Fig. 17. The lines of equal change of height were drawn
from plotted figures accumulated from a great variety of
sources. The restoration of the land and sea distribution is
based mainly on the change of height used in conjunction
with relief and bathymetrical maps, but in a few cases the
actual ancient shore line has been traced ; it depends on the
assumption that the continents held their present positions.
The hypothetical restoration of Antarctica is based on well-
known bio-geographical data, much of which has been admir-
ably summarised by C. Hedley (14). Bio-geographical data
have also been used as additional criteria in a few cases, such
as the separation of Madagascar from Africa and New Zealand
from Australia, or the union of Siberia to Alaska and of Japan
to the mainland. It is also necessary to remark that there is
not always evidence that the changes were strictly contem-
poraneous, but there is enough to show that the map is
sufficiently correct to form the basis for a discussion. From
this map and the formulae it is evident that, even without
an increase in the glaciation, the fall of temperature in winter
outside the latitudes of 40 must have been very considerable*
This fall was still further augmented by the great increase in
the altitude of these regions. Now we know that, except in
parts of Asia, practically the whole land surface north of 50 N.
was glaciated, so that as a rough approximation we may
assume that glaciers formed wherever the mean annual tem-
perature fell below 32 F. (the problem of precipitation is
dealt with in Chapter IX.). At first the increase of the land
area would have raised the summer temperatures, but as the
snow-cover began to persist through the year and form per-
manent ice-sheets, this excess disappeared and was replaced by
a deficit. Taking the coefficient of land ice in summer as
0-16 C. for one per cent, of a ten-degree circle, and the
average coefficient of land as +0-05 C., every increase of four
per cent, in the portion of a ten-degree circle of land covered
by ice lowered its temperature by 0-8 C. (1-4 F.), and where
the ice extended on to the sea the lowering was 0*6 C.
(i-iF.); consequently, within the borders of the great
ice-sheets the lowering of temperature in July amounted to
nearly 20 C., an amount sufficient to enable the accumulation
154 CLIMATE THROUGH THE AGES
of ice to continue in summer as well as in winter. In January
the change probably made little difference.
In Figs. 1 8 and 19 are shown the calculated differences
of temperature from the present, both before the formations
of the ice-sheets and at their maximum extension. The lines
are lines of equal difference of temperature from present
conditions in the same months. The calculated differences
at a few points in the Northern Hemisphere are set out in
Fig. 1 8. Changes of temperature due to geographical
changes, January.
Table 9, with, for comparison, the differences at the same
points deduced from the geological and biological evidence :
Locality.
Author.
Inferred Fall.
Calculated Fall.
Jan. July. Mean.
27
Scandinavia.
J. Geikie. More than 20 36
18
East Anglia.
C. Reid.
20
18
i3
Alps.
Penck and
ii
13
9
Bruckner.
Japan.
Simotomai.
7
9
5
Table 9. Comparison of calculated change of temperature
with that inferred from geological evidence.
The agreement is quite good, and seems to show that the
decrease of temperature during the Quaternary Ice-Age was
completely accounted for by the changes in the distribution
of land and sea and the effect of the ice itself. This does not
CONTINEN*TALITY AND TEMPERATURE
J 55
necessarily mean, however, that an increase in the land
area in high latitudes would by itself suffice to initiate a
glaciation ; the winter temperatures would be lower than
at present, but the summer temperatures would be higher,
and on low ground the winter snowfall would not
survive the hot summer that is why Siberia is not glaciated.
Probably, glaciers can only originate on an area of high ground,
but the presence of a large continental region with a low winter
temperature is essential if a local glaciation of the mountain-
valley type is to develop into a regional glaciation of the ice-
sheet type. This is clearly illustrated in the glacial history of
Fig. 19. Changes of temperature due to geographical
changes, July.
Europe ; the Quaternary glaciation began in the mountains
of Norway, but the extension of the ice half-way across
Europe was possible only because, owing to elevation, there
was a large area of high continentality to the eastward. The
beginning of glaciation in Norway was due to increased
elevation bringing a larger area above the snow-line, and
perhaps also to the shutting out of the Gulf Stream Drift.
Once the glaciers had reached a certain size they became
independent of the elevation of the ground in their centres,
and their extension was governed, first, by the amount of
snowfall available for their nourishment, and secondly, the
snowfall being sufficient, by the balance between the cooling-
power of the ice and the natural or " non-glacial " temperature
of the regions into which they intruded.
156 CLIMATE THROUGH THE AGES
It will be interesting to compare the results of a ten per
cent, decrease in the amount of land in each zone of latitude,
calculated from my formula, with those obtained by means
of Spitaler's formula given on page 132. According to my
formula, the change of temperature falls into two parts, a
local part due to the local changes in the land and sea
distribution, and a general part due to the change in the
general zonal temperature. For example, if owing to local
elevation the Faroe group became a single large island of the
size of Iceland, the result would be two-fold a large local
decrease in the winter temperature of the site of the new island
and of the surrounding area, and a small general decrease
in the temperature of the whole zone between 50 and 70
North latitude.
The effect of a change from 50 to 40 in the percentage of
land in any zone of latitude on the mean temperature of the
whole zone would be as follows :
Latitude .... o 10 20 30 40* 50 60 70
January, F. . . . 0-5 0-4 -fo-i -f i i +2-6 4-4-7 +7*3 (+16)
July, F 0-3 0-4 i -a 1-3 2-2 2-4 3-3 3'4
Mean, F 0-4 0-4 0-5 o-i 4-0-2 4-i*i +2*0 ( + 6)
Spitaler's Mean, F. 3-5 3-3 2-7 1-7 0*6 -fo-6 4-1*7 4-2-7
Table 10. Effect of a decrease of 10 per cent, in the amount
of land in a zone of latitude.
The figures for 70 calculated by my formula are uncertain,
but in 60 my result is in good agreement with Spitaler's,
which was obtained by quite different methods. The chief
difference in the results occurs in low latitudes, where the
effect of land area is much less important according to
my results than according to Spitaler's.
The essential point for the theory of climatic variations
is that, according to either computation, the warming effect
of a decrease in the land area becomes greater as we get
nearer to the poles. In summer, a decrease in the area of
(unglaciated) land results in a slight decrease of temperature,
and the net increase during the year is entirely due to the very
large increase in winter. If the percentage land covering
north of 60 N. decreased by ten, the result would be a rise
in the " non-glacial " January temperature by at least 7 F.,
which would be sufficient to bring it above the freezing point
and so introduce a " non-glacial " climate. This is the
CONTINENTALITY AND TEMPERATURE 157
general case, of which the two examples Middle Eocene and
Upper Jurassic discussed by F. Kerner are special instances.
The conclusion to which we are brought, therefore, is that
moderate changes in the land and sea distribution, such as
have occurred frequently enough in geological times, are
amply sufficient to bridge the gap between non-glacial and
glacial climates, or between warm and cold geological periods,
and that extraneous aids, such as variations of solar radiation
or changes in the astronomical climate, while possible causes,
are not necessary conditions.
REFERENCES
(1) FORBES, J. D. " Inquiries about terrestrial temperature." Edinburgh,
Trans. R. Soc. y 22, 1861, p. 75.
(2) SPITALER, R. " Die Warmevertheilung auf dcr Erdobcrflachc." Wien,
Denkschr. K. Akad., 51, 1886, Abt. 2, p. i.
(3) KERNER- MARILAUN, F. " Palaoklimatologie." Berlin (Gebr. Born-
traeger), 1930.
(4) KERNER, F. " Synthese der morphogenen Winterklimate Europas zur
Tertiarzeit." Wien, 1913.
(5) HEER, O. " Untersuchungen iiber das Klima und die Vcgetationsverhalt-
nisse des Tertiarlandes." Winterthur, 1860.
(6) KERNER, F. " Klimatogenetische Betrachtungen zu W. W. Matthews,
' Hypothetical outlines of the continents in Tertiary times V Wien,
Verh. k. k. geol. Reiclisanstalt, 1910, p. 259.
(7) KERNER, F. " Das akryogene Seeklima und seine Bedcutung fur
geologischen Probleme der Arktis." Wien, Sitzungsber. Ak. Wiss., 131,
1922, p. 153.
(8) BROOKS, C. E. P. " The problem of warm polar climates." London,
Q,. J. R. Meteor. Soc., 51, 1925, p. 83.
(9) GAMS, H., and R. NORDHAGEN. " Postglaziale Klimaanderungen und
Erdkrustenbewegungen in Mitteleuropa." Miinchen, Geogr. Gesellsch.
Landesk. Forschungen, H. 25, 1923.
(10) PETTERSSON, O. " Climatic variations in historic and prehistoric time."
Svenska Hydrogr.-Biol. Komm. Skriften, 5, 1914.
(n) KOCH, L. "The East Greenland ice." Copenhagen, Komm. Viden-
skabelige Under so gelser i Grdnland, Kobenhavn, 1945.
(12) BROOKS, C. E. P. " Continentality and temperature." London,
Q,. J. R. Meteor. Soc., 43, 1917, p. 169.
(13) BROOKS, C. E. P. " Continentality and temperature." (Second paper).
" The effect of latitude on the influence of Continentality on temperature."
London, Q. J. R. Meteor. Soc., 44, 1918, p. 263.
(14) HEDLEY, C. " The palaeographical relations of Antarctica." London,
Proc. Linnaan Soc., 124, 1911-12, p. 80.
CHAPTER IX
PRECIPITATION RAIN, SNOW, AND HAIL
THE various forms in which water falls from the sky,
of which the most frequent are rain, snow, and hail,
are conveniently grouped together under the term
precipitation. The precipitation is occasionally slightly aug-
mented by other forms, dew and hoar-frost, and on the arid
western coast of South America there are mountain plants
which live on the moisture they derive from mist, but,
practically speaking, the three forms first mentioned are
the only ones which need be considered. Precipitation is
formed by condensation of the water vapour in the air, which
has been derived by evaporation from the surface of the sea,
lakes, vegetation, soil, etc. Air at a certain temperature
can only hold a certain amount of water vapour, and this
amount decreases very rapidly with falling temperature.
Hence, if saturated air is cooled, some of the water vapour
which it contains is condensed to form cloud, and, if the
condensation is continued far enough, rain or snow. Hail is
formed when a column of air rises very rapidly to great
heights, as in thunderstorms. The cooling of moist air is the
only way in which an appreciable amount of precipitation
can be produced. If the pressure on a mass of air or any
other gas is lessened, the gas will increase in volume, and in
doing so will become colder, unless heat is supplied from
without. At sea-level the air is subjected to the pressure of
the whole of the atmospheric column above it, equivalent in
weight to about fifteen pounds per square inch. As we go to
higher levels and leave part of the atmosphere below us, the
mass of superincumbent air becomes less, and the pressure falls.
Hence any sample of air which rises to higher levels in the
atmosphere will expand and cool, while a sample which
descends to lower levels will be compressed and warmed.
That is the reason why, generally speaking, the air is colder the
higher the level.
It will be understood from this that very nearly all
138
PRECIPITATION RAIN, SNOW, AND HAIL 159
precipitation falls from air which is rising and therefore
expanding and becoming cooled. From the circumstances
under which the air rises, precipitation is classified into three
types Orographic, Cyclonic, and Convectional or Instability
precipitation.
Orographic precipitation falls where a current of air
encounters high ground and is forced to rise. The hilly parts
of Western Britain, standing in the path of the moist south-west
winds from the Atlantic Ocean, receive a great deal of
Orographic rain, and are, in fact, the wettest parts of these
islands. The west coast of Norway is similarly situated ;
other regions are the eastern end of the Black Sea, the
mountains of Lebanon, the coast of Honduras in Central
America, the Western Ghats of India, and especially the
Khasi Hills of Assam, one of the wettest spots on the globe.
Orographic rain may be very heavy in warm countries where
the air contains a great deal of moisture, but in this country
it is persistent rather than heavy. Much depends on the
topography ; a long range of hills of uniform height is more
effective as a rain-maker than an isolated mountain, since,
unless it is already in an unstable condition, air will always
go round an object rather than rise above it. Orographic
precipitation in cold regions frequently falls as snow in winter.
When we study the distribution of precipitation in a
mountainous region, such as the Alps, we find that the amount
is moderate in the lowlands and valleys, and becomes greater
as we ascend the slopes of the mountains. At a certain height,
however, termed the " level of maximum precipitation,"
this increase with height ceases, and above this level the
precipitation becomes less as we ascend. This level depends
on the temperature and relative humidity of the air over the
lowlands, and the vertical decrease of temperature ; in the
Alps it occurs at a height of about 7,000 feet, where the pre-
cipitation is between two and three times that over the lowlands.
We can also distinguish the level of greatest rainfall in the
Alps between 4,000 and 5,500 feet, and the level of greatest
snowfall in the Alps at about 8,000 feet. The latter must
not be confused with the snow-line, which is about 2,000 feet
higher in the Alps ; the level of maximum snowfall depends
on the winter conditions, while the snow-line is determined
very largely by the conditions in summer. The relative
i6o
CLIMATE THROUGH THE AGES
position of the snow-line and the level of maximum snowfall
are of great importance for the development of glaciers, as we
shall see in Chapter XVI.
Cyclonic precipitation falls during the passage of barometric
depressions, cyclones, and other forms of atmospheric dis-
turbance which depend on general rather than on local
conditions. The hurricanes of tropical and sub-tropical
regions bring torrential rains which often cause widespread
Fig. 20. Tracks of depressions.
floods and add to the havoc of the winds, but it is only in
temperate and sub-polar regions that cyclonic rain forms an
important part of the total average rainfall. In the eastern
half of Britain the greater part of the winter rainfall is cyclonic,
and the same is true of the winter precipitation of all parts
of the temperate belt except the high ground near the sea.
The distribution of cyclonic precipitation is largely governed
by the tracks followed by barometric depressions. Although
the paths of individual depressions in temperate regions often
appear to be erratic, it has been found possible to classify
them into a number of tracks, which are more usually followed
PRECIPITATION RAIN, SNOW, AND HAIL l6l
than the intervening regions (Fig. 20). These tracks have a
preference for moist areas, especially inland seas such as the
English Channel, the Baltic, and the Mediterranean, or for
well-watered plains such as Hungary and Poland. Track I,
the favourite track at present in all seasons but spring, runs
from Iceland or the Faroe Islands north-eastward, some
distance off the coast of Norway, to the Arctic Ocean, or across
the north of Norway to the White Sea.
This question of the tracks of depressions is important
for palaeometeorology, for a considerable degree of permanence
has been attributed to them. During the Quaternary Ice-Age,
when the northern tracks were closed by the ice-sheets and the
glacial anticyclones which occupied them, Track V, which
runs south of the main glaciated area, was the favourite track.
Many depressions passed from end to end of the Mediterranean,
and the rainfall associated with them caused the north of
Africa to be much moister than at present ; this was a " pluvial
period " in that region. Track Vb, passing northwards
across Central Europe, was probably also extensively followed
during the glacial period, and caused heavy snowfall over the
south-eastern margin of the ice-sheet. At present it is followed
chiefly in spring, and is associated with high pressure and low
temperature to the north-west ; it brings cold spells in Central
Europe.
Marsden Manson ( i ) supposes that the cyclone tracks across
North America have been fixed in their present position
throughout the whole of geological time, and that the dis-
tribution of precipitation has always resembled that existing
at present. He supports this theory by the coincidence that
the pre-Cambrian glaciations of that continent occurred in
the present storm belt, but the geological evidence does not
warrant the generalisation, since North America had an arid
climate during a large part of geological time. During the
Eocene period, when the plant-bearing deposits of the Arctic
Circle were formed, the winds on the west coast of Greenland
in 70 N. appear to have been mainly south-westerly, indicating
that the area of lowest pressure and, presumably, of heaviest
rainfall, lay still farther north, and other evidence will be
adduced later. It will be an important part of future work to
lay down the storm tracks of past ages as closely as possible,
since this will provide a large amount of information as to
11
1 62 CLIMATE THROUGH THE AGES
the barometric distribution and the position of the belts of
rainfall.
The precipitation associated with barometric depressions
falls as rain or snow according to the temperature at the level
of condensation. Not infrequently in winter the northern
half of a depression brings snow, while the southern half brings
rain. During the Quaternary Ice-Age this distinction between
the southern and northern halves was probably very pro-
nounced, and depressions skirting the ice-sheets must have
caused a large annual snowfall on the ice. As explained in
" The Evolution of Climate/' this tendency to a maximum
snowfall near the margin of the ice-sheet appears to have
played a large part in causing the successive development
of centres of glaciation more and more to the south-west
Scandinavia, Scotland, Ireland.
Convectional or Instability precipitation is typified by the
hail or heavy rain of thunderstorms. It is due to the warming
up of the lower air, by contact with ground warmed by the
sun. When the warming has proceeded far enough, the
lowest layer of air becomes potentially lighter than the air
above it (i.e., its temperature is so much higher than that of
the layer above that even after the expansion and cooling
consequent on lifting it to the level of the latter it would still
be warmer). Under these conditions the surface air begins
to rise through the air above it, at first in thin threads, but
if the process goes far enough, in thicker columns. The
first result of this process is the formation of the small cumulus
clouds so commonly seen in England on a summer afternoon ;
very often in this country the process goes no farther and the
clouds die away in the evening without producing rain,
but under favourable conditions of vertical distribution of
temperature (and in the presence of sufficient moisture),
sudden thundery showers or true thunderstorms may result.
The mechanism of a thunderstorm is complex, and need
not be discussed ; here it is sufficient to remark that the
typical thunderstorm is essentially the product of hot, relatively
calm weather and moist air. The thunderstorms associated
with " cold fronts " during the passage of depressions have
a different origin, and the rainfall associated with them comes
under the heading of cyclonic rain.
On tropical coasts, the rising of the warmed air over the
PRECIPITATION RAIN, SNOW, AND HAIL 163
land is facilitated by the onset of the relatively cool sea breeze,
and the progress inland of the latter is marked by a line of
thunderstorms. In many parts of the tropics the greater part
of the rainfall is caused in this way.
Thunderstorm clouds often extend to a great height, and
the temperature of the upper part of the cloud may be below
freezing point. Under these conditions the precipitation
often takes the form of hail, which is typically associated
with thunderstorms. It is an important point in the theory
of the climate of the " warm " periods that the vegetation
of Europe during the Tertiary period often shows evidence
of damage by hail, and of torrential rains of the instability
type, even when the stage reached by the plants shows that
the deposit was formed quite early in spring. During these
warm periods the relief was generally low, giving little oro-
graphical rain, and depressions were probably weak and
sporadic, but conditions were especially favourable for the
occurrence of thundery showers, and most of the precipitation
was of this type.
The origin of the snow required for the nourishment of
ice-sheets is a difficult problem. The snowfall of Greenland
appears to be due almost entirely to the winds from the
ocean, which blow from the sea on to the ice during the passage
of intense barometric depressions, but for the far larger ice-
sheet of Antarctica this explanation is only tenable for the
margins. W. H. Hobbs (2) considered that the process was
as follows : the air which flows outward on the surface of the
ice-sheet must be replaced by inflowing moist air at higher
levels. These upper air currents are indicated by clouds
which can be seen passing inland across the coast. In the
centre of the anticyclone this air descends, and is of course
warmed by compression, but owing to the intense radiation
the surface of the ice is intensely cold, colder in fact than the
air at considerably higher levels (this Antarctic inversion of
temperature is a well-known phenomenon). The descending
air is cooled by contact with this intensely cold ice surface,
and deposits its moisture in the form of small granular masses
of ice. The formation of an ice-mist of small crystals of ice
in this way has been observed in Siberia, but apparently the
precipitation was not sufficient to be termed snow.
Sir George Simpson (3) showed that this ingenious
164 CLIMATE THROUGH THE AGES
mechanism cannot be the whole story, since the amount of
snowfall which it would yield would be insignificant. He
therefore carries the argument a step farther. The moist
air which flows in at high levels is brought down to the surface
of the ice by the anticyclonic circulation, and is cooled by
radiation and by contact with the ice. In this way it becomes
about as cold as when it crossed the margin of the continent.
If it becomes colder, some snow will be precipitated in the way
supposed by Hobbs, but, in any event, it will be almost or
quite saturated. Owing to local circumstances there are
irregularities in the temperature distribution ; the coldest
air tends to spread out on the surface and flow down slopes,
undercutting and lifting up the air which is less cold. The
latter is forced to rise again ; it is cooled still further by
expansion, and consequently snow is formed. This account
agrees with the fact that by far the larger part of the snowfall
of the Antarctic continent occurs in blizzards.
We are now in a position to discuss in general outline the
distribution of precipitation over the globe. The local
details are so complex that it is not possible to present the
distribution adequately on a small-scale map ; this distribution
is intimately connected with the local geography, and for our
purpose it is more important to have generalisations applicable
to any land and sea distribution. In the first place, we may
distinguish four zones of precipitation in each hemisphere :
1. The equatorial belt of heavy rainfall.
2. The sub-tropical dry belt.
3. The temperate rain belt.
4. The polar cap of generally light snowfall.
These four rainfall belts correspond with the four pressure
belts described in Chapter II., the equatorial belt of low
pressure, the sub-tropical anticyclonic belt, the temperate
storm belt, and the polar caps of relatively high pressure.
The belts of pressure are best developed over the oceans, and
it is probable that the same is true of the rainfall. The most
detailed estimate of the zonal distribution of rainfall was made
by C. E. P. Brooks and T. M. Hunt (4), and the results are
shown in Fig. 21.
PRECIPITATION RAIN, SNOW, AND HAIL 165
The rainfall in inches for the different 10 zones of latitude
are as follows :
Latitude N.
Land (in.) .
Oceans (in.)
Latitude S.
Land (in.) .
Ocean (in.)
9O-80 80-70 70-60 60-50 50-40 40-30 3O-2O 20- 1 IO-O
5'8 I2'I 19-3 20-2 23*2 26'6 32'I 56-3
(5) 8-0 27-2 56-4 51-1 44-2 38-6 48-0 63-3
0-10 IO-2O 2O-3O 30-40 40-50 50-60 60-70 70-8O 80-90
60 -I 42-7 26'0 22-2 31-3 38-4 6-9 2'4 (2-0)
54'3 42'5 36-7 43*4 47'9 37'8 15-0 3*7
Table 1 1 . Distribution of precipitation according to latitude.
The rainfall zones are clearly shown in the figures for the
sea and in the land over the Southern Hemisphere. In the
Northern Hemisphere the great continental masses of Eurasia
<fc
Fig. 2 1 . Variation of rainfall with latitude.
and North America are largely beyond the influence of winds
blowing directly from the oceans, and except near their coast-
lines they are relatively dry.
The total amount of precipitation over the whole earth,
including both land and water, in the course of a year averages
40-9 inches. This must be equal to the total amount of
evaporation, since the amount of water vapour and of water
droplets or ice-crystals in the form of cloud held in the air
at any time is equivalent to only a few millimetres of rain
less than one per cent, of the average annual fall. Evaporation
and precipitation are intimately related, since both depend
largely on vertical movements in the atmosphere. Rising
air, as has been pointed out, is responsible for very nearly
all the condensation of water vapour to form rain, snow, or
1 66 CLIMATE THROUGH THE AGES
hail, and every ascent of air must be counterbalanced some-
where by descending air. This is warmed by compression
as it descends ; its capacity for holding water vapour is
increased, and it arrives at the surface with a low relative
humidity. Evaporation takes place chiefly into air which
has recently descended to the surface from higher levels in
this way ; it also occurs where air is blowing from a colder
to a warmer surface and is being warmed by contact with the
latter. The total amount of precipitation, therefore, is not
necessarily proportional to the area of the oceans even in low
latitudes, an assumption which is sometimes made. A
narrow tropical sea with mountainous shores, across which a
steady wind is blowing, would be subject to intense evapora-
tion, but a wind of the same velocity blowing over a stretch
of ocean a thousand or more miles in length, even if initially
dry, would at the conclusion of its journey be nearly saturated,
and would have almost ceased to evaporate.
Vertical motion of the air, whether caused by winds blowing
against a range of mountains, by great cyclones or barometric
depressions, or by local convectional movements, always
reduces to a question of temperature contrasts between regions
either remote or near at hand. In particular, the presence of
ice is a potent factor in causing vertical movement of air.
Hence a period with warm climates extending over the greater
part of the world would probably be a period of less evaporation
and therefore of less total rainfall than a glacial period, in spite
of the increased amount of water vapour which the warmer
air can take up ; conversely, during a glacial period, even
if the temperature is lower, the total rainfall may be increased.
This was exemplified during the Quaternary Ice-Age, when
the rainfall over the non-glaciated regions was heavier than
the present rainfall, and probably much heavier than that of
any part of the Mesozoic or Tertiary periods.
We have to take account of the evaporation in another
way. The biological effectiveness of rainfall depends not
only on the total fall, but much more on the amount which
becomes available for plant life. In regions subjected to
great evaporation the effectiveness of the rainfall is greatly
diminished, and a hot country with the rainfall of South-
eastern England would be regarded as dry. The rainfall of
Jerusalem, for instance, is heavier than that of London, but
PRECIPITATION RAIN, SNOW, AND HAIL
i6 7
owing to the greater evaporation it is less effective. R. Lang
(5) expresses the effectiveness of the rainfall as the " Rain-
factor/ 5 which he obtains by dividing the annual precipitation
in millimetres by the mean temperature in centigrade degrees,
and he finds that the character of the soil is closely related
to this factor in the way shown by the accompanying diagram
(Fig. 22). According to this diagram, with a mean annual
temperature of 30 G. (86 F.), desert formations may occur
with a rainfall as high as 1,200 mm. (48 inches) a year, but if the
mean temperature is only 10 C. (50 F.), the rainfall cannot
be more than 400 mm. (16 inches). With a rain-factor
100
Salt, Dust
and Sand
Earths
10 20
40 60 80
Annual rainfall in inches.
100 120
Fig. 22. Relations of soil to temperature and rainfall.
After R. Lang.
between 40 and 60, the deposits are still coloured entirely by
iron oxides, but the chemical composition and the colour
vary according to the mean annual air temperature. When
the latter lies between 32 F. and 54 F., yellow earths are
formed ; between 54 F. and 68 F., red earths ; and above
68 F., deep red loams or laterite. The latter, therefore,
requires a rainfall of at least 800 mm. (32 inches) a year,
and may be taken as evidence of a fairly moist warm climate.
When the rain-factor is between 60 and 160, the colour of
the deposit is due to red or yellow iron oxides and partly to
black humus, the resulting mixture giving the deposits a brown
colour (brown earths). With the rain-factor between 100
and 1 60, the colour is determined entirely by humus, and
1 68 CLIMATE THROUGH THE AGES
" black earths " result ; above 160 the earth is bleached of all
colouring matter by the rich vegetation and the heavy rain,
and the result is a white subsoil surmounted by a deposit of
pure humus.
When the mean temperature is below o C. (32 F.), all
chemical action ceases, and the purely mechanical deposit
takes the colour of the rock from which it was formed.
Deposits formed from igneous rocks, or from a mixture of
rocks of different colours, are generally grey, and the deposits
formed near the edge of an ice-sheet are often of this colour.
A more recent table by E. M. Crowther (6) is based on a
" leaching factor " R 3-3 T, where R is the rainfall in cm.
and T the mean annual temperature in G. The results,
based on work in U.S.A., may be summarised as follows :
Leaching factor Temperature increasing
above 70 Podsol ; Brown Forest Soils ; Ferruginous
laterites.
Transitional
(Leaching factor
below 70, rain-
fall above 70 cm.) Prairie soils.
Leaching factor Rainfall increasing
and rainfall both Grey Chestnut Brown Tchernosem
below 70 desert ; soils semi-desert
soils soils ;
Table 12. Relations of soil to temperature and rainfall.
After E. M. Crowther.
The biological effectiveness of the precipitation also depends
on the proportion of it which sinks into the soil. This is
governed by a number of factors, especially the character of
the fall and the nature of the soil and vegetation. A per-
sistent " soaking " rain of moderate intensity is much more
effective than a torrential downpour which runs quickly off
the land and floods all the streams, although the actual
amounts may be the same. A snow-cover which accumulates
during the winter and melts gradually in spring may be very
effective. Thick vegetation covering a soft soil checks the
rate of run-off and allows a larger proportion of the rainfall
to be absorbed than does hard bare earth. This aspect of
PRECIPITATION RAIN, SNOW, AND HAIL 1 69
the rainfall has been brought out by the discussions of the
desiccation of South Africa (7). In the past fifty years the
country has been suffering increasingly from drought, but the
conclusion from expert evidence is that this is not due to an
actual decrease in the amount of rainfall, but to a change
in the nature of the soil and vegetation. When South Africa
was first settled, the country was covered by a rich vegetation,
the rainfall was steady and persistent, and a large proportion
of it was absorbed. The effect of over-pasturage has been
to destroy much of the protective vegetation, and the soil has
been washed away or trampled hard. The temperature
contrasts have been increased owing to the heating effect of
the sun on the patches of bare ground, and the rain now falls
largely in heavy " instability " showers, including destructive
thunderstorms. The run-off is proportionally greater, owing
to the more torrential nature of the fall and the loss of the
vegetation, so that with nearly the same rainfall the amount
of water available for use has decreased. The possibility of
changes of this nature brought about by human activities has
to be remembered in all discussions of the vexed question of
" desiccation " in historic times ; in fact a passage in Plato's
" Critias " suggests that the decadence of Greece may have
been due to such a change.
The distribution of the precipitation among the seasons
is almost as important as the total amount. We have to
distinguish between regions with their precipitation almost
equally distributed throughout the year, regions with their
rainy season in winter and dry summers, and regions with their
rainy season in summer. The character of the seasonal
distribution governs the type of vegetation on the one hand,
while on the other hand it is intimately related to the general
meteorological regime. This often enables us to derive
important information as to the general meteorology of a
period from a study of the plant remains ; for instance, the
presence of annual growth rings in tree stems may mean a
seasonal alternation of temperature, but associated with
evidence of a high degree of warmth, it implies the alternation
of dry and rainy seasons and a monsoon type of climate.
The limits of the various rainfall types have been set out
in great detail by W. Koppen (8). Along the immediate
neighbourhood of the equator is a belt of heavy rain in all
I7O CLIMATE THROUGH THE AGES
seasons. This is the region of the dense forests of the Amazon
and Congo River basins, and it is known as the " tropical
rain-forest region." It also includes most of the East Indies
and the Malay Peninsula, and extends into Cambodia and
Assam. On low ground the tropical rain-forest belt does
not usually extend more than 10 on either side of the equator,
but under favourable conditions it may extend to 20 or even
to nearly 30 latitude. These outlying areas include the
eastern coast of Madagascar, the eastern slopes of the Andes,
and the " everglades " of Southern Florida, another region
of tropical swamp.
The soil is usually a rich dark humus. The following
graphic description is given by E. Warming (9) : " Forest is
piled upon forest. The trees forming the highest storey have
tall thick trunks, which are unbranched up to a height of 120
to 150 feet or more. Beneath them are trees of moderate
stature with branches not reaching those of the higher tier.
Beneath these in turn succeed slender thin-stemmed low palms,
tree-ferns, and shrubs . . . scattered about are huge herbs
which reach 12 or 15 feet in height. If there still remain
space available on the ground that is reached by the light, it is
occupied by dark green ferns, Selaginella, mosses, and similar
scrophytes. But often the light is too feeble to permit of more
than a very small number of plants developing on the ground,
which then may be almost bare of vegetation, with its black
humus covered only by fallen decaying wet leaves, twigs, and
remnants of fruits, between which only bizarre saprophytes
find places . . . but there are hordes of epiphytes clothing
trunks and branches ... as well as ferns, mosses, and so
forth. Trees of the forests situate in the cloud-belt of Java
and the Moluccas are enveloped in a soaking mossy felt, which
may be thicker than the trunks themselves and imparts to
them a peculiar dark appearance. . . . Finally, there is
a wealth of lianas, whose flowers and fruit one can rarely see,
and whose long, often curiously shaped stems, span the distance
between soil and tree-crowns, or hang down from the latter
or partly trail along the ground. . . . The twilight prevailing
is much less dark than in European beech-forest. All the
species . . . seem to abhor a vacuum and to combine in
an endeavour to utilise all the space available."
Many of the species show protective devices against the
PRECIPITATION RAIN, SNOW, AND HAIL 171
very heavy showers, especially a smooth cuticle which cannot
be wetted, drip tips and channelled nerves, but paradoxically
some plants of the highest storey show xerophytic characters.
A highly important feature for our purpose is that the trees
show no annual growth rings.
Where the contours of the ground are favourable, as in
Eastern Sumatra, great tropical swamps are formed, which
appear to reproduce the conditions prevailing during the
formation of the coal measures, and H. Potoni6 (10) believes
that the coal measures were in fact formed under similar
conditions in a very warm rainy climate. The forests which
formed the coal measures seem to have been similar in many
respects to the present tropical rain-forests. As described by
David White (n), the Carboniferous forests showed " rankness
of terrestrial vegetation ; great size of trees, plants, and leaves
. . . great size of fronds, and absence of annual rings," while
" fairly well-developed palisade tissue points towards sunlight."
The lianas of to-day were represented by " many long slender
clambering or climbing ferns and fern-like types."
A. Wegener, in discussing the movements of the poles
according to his theory of " Continental Drift " (see Chapter
XIII. ), makes great use of the beds of coal for determining
the position of the equator in the successive geological periods.
It should be remarked, however, that although peat is forming
at present in the tropics in one or two isolated regions, it is rare,
while in the temperate rain belts peat bogs occur wherever
there is sufficient moisture.
Nearer the poles, in temperate latitudes, are other areas
in which rain or snow falls in sufficient quantities in all seasons.
These areas include Northern and Eastern North America,
where they pass into the tropical rain-forest area, all Northern
and Central Europe, and a large part of Asia ; in the Southern
Hemisphere they are limited to relatively small areas in Chile,
South-eastern Australia, and New Zealand. In Europe the
area occupied by these temperate rains was the site of great
peat formation during the post-glacial period, and certain
coal beds of earlier geological periods are attributed by
Wegener to the temperate rain belt.
On either side of the equatorial " rain-forest " belt, and
extensively developed on the eastern sides of the continents, is
the monsoon or summer rain region, best known from its
172 CLIMATE THROUGH THE AGES
occurrence in India and China. This type of distribution
is due to the extensive alternate heating and cooling of the
interiors of large continents and the consequent alternation
of monsoon winds ; it occurs on either side of the tropical
rain-forest belt in South America and West Africa, and is very
extensively developed in Eastern Africa and in Southern and
Eastern Asia ; it is also found in the north and east of
Australia. The type of vegetation associated with it is the
savannah or meadow land, passing into open forest with
increasing rainfall. Where the rainfall is especially heavy and
the temperature steadily high, it may give dense tropical
rain-forests.
On their poleward sides, the summer rain regions in the
western and central parts of the continents usually pass into
deserts, which are characterised by a slight and irregular
rainfall, many months sometimes passing without even a
shower. In parts of the South American desert it has probably
not rained for centuries. Such plants as there are show
special devices to prevent the loss of water, but in many
deserts the ground is entirely bare of plants. Among animals,
one of the most characteristic is the lung-fish (Ceratodus] , which
is adapted to breathe either air or water, and can lemain
dried up for long periods. This form, which is still living in
Australia, has persisted since the Permian (allied forms are
known since the Devonian), and affords some evidence of
the continuity of desert conditions throughout a large part
of geological time. The best geological evidence of arid
climates is lithological desert sandstones of rounded and
polished grains usually red in colour, " dreikanter," and other
wind-eroded rocks, deposits of gypsum and of salt ; these
abound in many horizons and seem to suggest that the desert
belts were greatly expanded in the past. This was probably
true, but we have to remember that before land plants reached
their present degree of specialisation, large areas which would
now be habitable by plants must have been bare rock, subject
to aerial denudation, so that " desert " sandstones do not
necessarily imply a rainfall as small as that of modern deserts.
On the poleward sides of the deserts the rainfall increases
again, but falls mainly in winter, while the summers are hot
and dry. The best known example of this climate is the
Mediterranean region, whence the type is often known as the
PRECIPITATION RAIN, SNOW, AND HAIL 173
Mediterranean. It is found also in California and in limited
regions in Chile, near Cape Town, and in Southern Australia ;
it is thus practically limited to the belts between latitudes 30
and 45 and, except in Mesopotamia, it never extends far
from the sea. Eastward it usually passes into a semi-arid
or arid climate. It gives a peculiar type of vegetation adapted
to resist the drought of summer and to maintain its moisture
from considerable depths, for example, the vine.
The Mediterranean type of climate appears to have per-
sisted in South-east Europe during a large part of the Tertiary
period (12), though with variations in the total amount of
rainfall. The region was an archipelago of small islands ;
in the Early Eocene it had a rather moist climate with some
rain in all seasons, but in the Middle Eocene the entire absence
of a land flora probably indicates a semi-arid climate. In
the Oligocene there were well-marked dry and rainy seasons,
and in the Early Pliocene the climate was again warm and
rather moist.
On its polar side, the Mediterranean climate passes into
the temperate rain belts, which, except in the interior of the
great continents of Eurasia and North America, have a
sufficiency of rain at all seasons, and are occupied mainly by
forests of conifers or deciduous trees, which in the more
equatorial parts of the belts include some sub-tropical species.
Nearer the poles the winter is severe, with a persistent snow-
cover, and the summer is short ; this is the " boreal " climate.
Near the coast there is abundant rain at all seasons ; in the
interior the winter is usually dry, but the annual variation
of temperature in all cases enforces a period of inactivity in
plant growth. This zone is the peculiar home of peat bogs,
which require a rainfall of at least 40 inches a year and a
mean temperature above 32 F. The great outbursts of peat
formation in the " Atlantic " and " sub- Atlantic " periods
of post-glacial time gives us a measure of the raininess of these
periods.
The distribution of the belts of rainfall is closely related
to the distribution of the belts of pressure and wind described
in Chapter II., and the temperature belts described in Chapter
VIII., and we may set out the general succession of climatic
belts in the way shown in Table 13. The first column gives
the average latitude in which the different belts are found
174
CLIMATE THROUGH THE AGES
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PRECIPITATION RAIN, SNOW, AND HAIL 175
in the two hemispheres, while the second column gives the
" astronomical " zones of the old school text-books, which are
limited by the tropics and the polar circles. The third
column shows the belts of temperature according to Supan,
who limited his " hot belt " by the mean annual isotherm
of 20 C. (68 F.), which approximately fixes the polar limit
of palms, and his cold caps by the isotherms of 10 G. (50 F.)
for the warmest month, which forms the limit of growth of
cereals and forest trees. It will be noticed that the cold cap
extends into much lower latitudes in the Southern than in
the Northern Hemisphere ; this is due partly to the influence
of the great Antarctic ice-cap and the ice-laden Southern
Ocean, and partly to the smaller annual range of temperature
over the oceanic regions of the Southern Hemisphere than over
the continental regions of the Northern Hemisphere.
The rainfall belts have a close relation to the wind systems.
The equatorial rain belt is limited to the Doldrums and part
of the region of the South-east Trades. The dry belts include
most of the trade wind regions, the whole of the sub-tropical
calm belts, and extend a short way into the domain of the
westerly winds, where they give place to the temperate rain
belts, which in turn are limited on their poleward sides by the
polar east winds. We have seen reason to believe that when
the temperature-difference between equatorial and polar
regions was below a certain critical value, the polar caps of
east winds would be suppressed and the westerly winds would
extend almost to the poles. This would result in a spreading
out of all the other zones into higher latitudes, and would carry
the poleward margins of the sub-tropical dry belts into the
regions at present occupied by the temperate rain belts, while
the latter moved northward to occupy the polar " cold caps,"
now characterised by tundras, glaciers, and in Greenland and
the Antarctic by " the climate of eternal frost."
REFERENCES
(1) MANSON, MARSDEN. " The physical and geological traces of the cyclone
belt across North America." Washington, Monthly Weather Review, 52,
19124, p. 102.
(2) HOBBS, W. H. "Characteristics of existing glaciers." New York, 1911.
(3) BRITISH ANTARCTIC EXPEDITION, 1910-1913. Meteorology, vol. i. Discussion,
by G. C. SIMPSON. Calcutta, 1919.
(4) BROOKS, C. E. P., and T. M. HUNT. " The zonal distribution of rainfall
over the earth." London, Mem. R. meteor. Soc. y 3, no. 28, 1930.
176 CLIMATE THROUGH THE AGES
(5) LANG, R. " Verwitterung und Bodenbildung als Einfuhrung in die
Bodenkunde." Stuttgart, 1920.
(6) CROWTHER, E. M. " The relationship of climatic and geological factors
to the composition of soil clay and the distribution of soil types." London,
Proc. R. Soc., B, 107, 1930, p. i.
(7) UNION OF SOUTH AFRICA. " Report from the Select Committee on
Droughts, Rainfall, and Soil Erosion." Cape Town, 1914. " Final
report of the Drought Investigation Commission, October 1923." Cape
Town, 1923.
(8) KOPPEN, W. " Die Klimate der Erde." Berlin und Leipzig, 1923.
(9) WARMING, E. " (Ecology of plants." Oxford, 1909.
(10) POTONIE, H. " Die Entstehung der Steinkohle und der Kaustobiolithe."
5 Auf. Berlin, 1910.
(n) WHITE, D. "Upper Palaeozoic climate as indicated by fossil plants."
Sci. Mori., New York, 20, 1925, p. 465.
(12) KERNER, F. " Bauxite und Braunkohlen als Wertmesser der Tertiar-
klimate in Dalmatien." Wien, 1921.
CHAPTER X
MOUNTAIN-BUILDING AND CLIMATE
IT has long been evident that the succession of climates
in the various geological periods has not been haphazard,
but has followed a certain ordered sequence. The last
climatic episode on the grand scale has been Quaternary
glaciation. Looking back beyond that, we see a long succession
of genial climates in the Tertiary and Mesozoic, until we come
to the great Upper Carboniferous glaciation in the late
Palaeozoic. Beyond that again is another long period of
mainly warm climates Lower Carboniferous, Devonian,
Silurian, Ordovician, and Upper Cambrian, bringing us to
another great glaciation at the close of the Proterozoic,
immediately preceding and perhaps extending into the
Cambrian period. Beyond that again, and almost lost in
the mists of antiquity, deposits of a still earlier glaciation have
been recognised in South Africa, Australia, North America,
and perhaps in Scotland.
The systematic nature of these occurrences is made more
obvious when we consider their probable absolute ages in
years. The Quaternary glaciation is recent, it began perhaps
one million years ago. From the evidence of the radio-active
rocks (see Appendix I.), it is calculated that the upper part of
the Carboniferous system is about 260 million years old.
The base of the Cambrian is placed at 500 million years.
The exact age of the first of the four great glaciations is not
known, but a fair estimate would be 700 to 800 million years.
Thus the great glaciations seem to have occurred at almost
regular intervals of a quarter of a thousand million years.
The last two of the long genial intervals, and for all we know,
the first also, have been interrupted by minor deteriorations
of climate, as in the Silurian and the Cretaceous to Lower
Eocene, which produced local valley and sometimes piedmont
glaciers but not regional ice-sheets, suggesting a shorter cycle
superposed on the longer one.
The same ordered sequence has been observed in the
178 CLIMATE THROUGH THE AGES
evolution of the earth's surface features, where it has been
termed " the rhythm of geological time. 3 ' As was remarked
in the Introduction, there have been long periods in which
the earth's crust was at rest, while the denuding agencies
gradually lowered the surface almost to a uniform plain and
the waves of the sea bit deeply into the continents. Alternating
with these have been relatively short periods of intense dis-
turbance during which the earth's surface was thrown into
great folds and ridges, when the mountain ranges which form
the articulated skeletons of the continents were brought into
being. The greatest periods of mountain-formation occur
in close relation with the greatest periods of glaciation ; thus
the Alpine period of folding in the Tertiary preceded the
Quaternary Ice-Age ; the Hercynian folding in the Upper
Lower Upper Ccstedoruaft HcrcyrAiar\ Cretaceous Alpirxe
Prolcro}0ic u) Prolcro*oic (Silurian) (Upper -Eocene (Quaternary)
' Carboniferous) '
75 O 5OO E5O
Millions of years
airx bu'ildirxq (full line) ar\d cjlaclaJlorv (brokers I'mc) Schematic
Fig. 23. Mountain-building and glaciation (schematic).
Carboniferous preceded the Upper Carboniferous glaciation.
It is known that there was a period of great disturbance and
mountain-building preceding the Cambrian period, and
another, lower in the Proterozoic, probably preceded the
first of the four great glaciations, the deposits of which,
in Australia at least, rest on great outflows of lava. The
minor cold period of the Silurian was also associated with
a period of folding and mountain-formation, the " Caledonian,"
which, however, did not reach the intensity of the Hercynian
and Alpine foldings. Thus we can represent the variations
of mountain-building activity and of climate during geological
time as a series of waves (Fig. 23) in which the long troughs
represent the periods of stability and genial climate, the sharp
crests the periods of mountain-building and climatic stress.
The last two ice-ages at least were not synchronous with
the maximum of mountain-formation, but followed them after
some millions of years, as has been indicated in the diagram.
MOUNTAIN-BUILDING AND CLIMATE 179
This lag has been attributed to two causes. After a long
quiescent warm period the whole mass of the oceans is warm,
and has to be cooled down before general glaciation can begin.
This process may occupy thousands of years, and smooth out
climatic fluctuations within an ice age, but could not cause a
lag of millions of years. The second cause is that mountain
ranges are first elevated as smooth domes, which are worn into
irregular contours by the ordinary processes of erosion. The
lightening of load caused by the removal of this eroded material
causes further isostatic elevation and a greater effective height.
A. Wagner (i) has put forward a third explanation. The
steady warming of the earth's crust by radio-activity is much
greater than the normal escape of earth-heat at the surface
so that the crust becomes continually hotter and more plastic.
This allows folding, mountain-building and volcanic out-
breaks, in which the accumulated earth-heat is liberated.
At present earth-heat raises the mean temperature of the
earth by only 0-3 F., but Wagner thinks that during the
mountain-building of the first half of the Tertiary this figure
may have been nearer 10 F., more in high than in low
latitudes. This radio-active heat warms the earth's crust,
and melts the base of any nascent glaciers. The latter flow
rapidly to lower levels where they are dissipated.
After the end of the main epoch of mountain building the
crust becomes solid and quiescent and cools again. This allows
ice to freeze to the ground and so pile up great ice-
sheets. When the ice reaches a certain thickness the vertical
temperature gradient in the upper part of the crust increases
(at cost of lower layers) and melts the bottom of the ice-sheet,
causing it to flow out to the warmer periphery where it melts.
This is the phase of maximum extension of the ice-sheets.
After the ice-sheets have melted back the earth-flow decreases
again and the cycle recommences. In this way Wagner
accounts not only for the lag of glaciation behind mountain-
building but also for the succession of glacial and inter-glacial
periods. The theory is mentioned here for the sake of com-
pleteness, but it seems improbable that the surface temperatures
can have fluctuated so greatly solely because of earth-heat,
especially during the course of the Quaternary Ice-Age.
H, Jeffreys (2) states definitely that the surface temperature
of the earth must have been almost wholly maintained
l8o CLIMATE THROUGH THE AGES
by solar radiation practically ever since it became solid at
the surface, and certainly throughout geological time.
Conduction from the interior is by comparison quite un-
important.
The dates of the pre-Cambrian glaciations are still uncertain,
but have been provisionally put at 500, 1,000 and probably
1,500 million years ago, with more doubtful occurrences
round 600 or 800 million years ago. The duration of a period
of folding and mountain-building is about 50 to 80 million
years, while that of an ice-age is much shorter, hence the
peaks representing mountain building have been shown
broader than those representing glaciation.
As G. Manley (3) pointed out, the problems of why the
Quaternary glaciation began just when it did, and of why
it was interrupted by interglacial periods, is the same, on a
larger scale, as the problem of the fluctuations of the glaciers
during the historical period. So far as is known, these have
been broadly similar over the whole world. Especially
during the past 100 years glaciers have been in rapid retreat
in both hemispheres alike. The causes of these minor
advances and retreats of the glaciers are still unknown.
The close parallelism between mountain-building and
climate has naturally attracted the attention of geologists.
For example, in 1899 J- Le Conte (4) pointed out that the
Quaternary glaciation was preceded by a period of " almost
universal continental elevation and enlargement," and that
the Quaternary ice-sheets developed on the surfaces of the
plateau so formed. The greatest exponent of the importance
of elevation, however, has been W. Ramsay (5), who, under the
title " Orogenesis and Climate," attempted a complete
solution of the climatic problem on these lines.
In the second paper (5), Ramsay summarises his theory of
the climatic importance of elevation : " It is well known that
a climate is modified by the relief and the elevation of a region,
so that with increasing altitude it gradually becomes cooler
and even glacial. Further, mountains and highlands exercise
a great effect as condensers of precipitation, as boundaries of
climate, etc. But meteorologists and geologists reflect less
often that the relief of the continents influences, not only the
local or regional climate, but the whole economy of the
calories which the sun supplies."
MOUNTAIN-BUILDING AND CLIMATE l8l
We have seen in Chapter VI. that the atmosphere, owing
to the selective absorption which it exercises on radiation of
different wave-lengths, acts like the glass of a greenhouse in
raising the temperature of the earth's surface. Ramsay
points out that this action takes place under most favourable
conditions over extensive low plains where the air mantle is
thickest and most dense. Over the mountains and highlands,
where the air is thinner and less dense, the loss of heat by
terrestrial radiation escaping to space is greater, " the lofty
parts of the continents can be regarded as holes in the glass.
They not only chill the place just beneath them, but more or
less the whole hotbed."
Loss of heat also takes place owing to the vertical movements
which high ground introduces into the atmosphere. " The
air currents, passing over mountains, high coasts, and other
elevations in their way, are forced to rise, and at the higher
position their loss of heat is greater than if they had flowed
at a greater level. 55 It may also be remarked, though this
consideration is not mentioned by Ramsay, that high ground
favours the formation of cloud which, as shown in Chapter
VII., lowers the temperature owing to its reflection of solar
radiation. But the most important effect of high ground,
according to Ramsay, is that it serves as a gathering ground
for glaciers. High land is necessary for glaciation, and high
land in rather high latitudes is especially favourable for the
occurrence of a general cold period. " If there only exist
high enough islands and continents, ice-caps will appear,
extend their glaciers down to the sea, and send out their
armadas of icebergs. To melt them, enormous quantities
of the heat reserve of the sea will be consumed. Cold water
forms extensive superficial layers, and gradually fills the depth
of the ocean right to the equator. 55 As the ice-caps grow,
more and more water is bound up in them ; this water is
taken from the sea and gradually lowers the sea-level, thus
accentuating the elevation of the land. Antevs (6) calculates
that during the Quaternary glaciation the average elevation
of the land above the sea was increased by more than 300
feet owing to the locking up of water in the form of land ice.
Thus Ramsay has built up a very complete qualitative
theory of geological climates on the basis of changes of elevation
alone. His theory contains a very considerable amount of
1 82 CLIMATE THROUGH THE AGES
truth ; he has realised the importance of vertical motion in
the atmosphere for the loss of heat by radiation, and he has
also introduced the effect of changes in the general temperature
of the ocean. On the other hand, I think he has overestimated
the importance of the " holes in the glass," since at the present
day only relatively insignificant areas of land are high enough
to make an appeciable difference to the proportion of their
radiation which is absorbed by the atmosphere. The loss of
heat by radiation from air-masses forced to rise in order to
pass over high ground is no doubt appreciable, but the amount
of air raised in this way must be much less than that raised
in cyclones, thunderstorms, convection currents, and other
phenomena of atmospheric instability. In fact, as pointed
out in Chapter IX., air, unless it is already unstable, much
prefers going round a mountain to going over it. One would
be inclined to say that the " polar front " in the North Atlantic
is responsible for more vertical air movement than the whole
of the mountain regions of the globe put together, but there
are no figures available for a quantitative estimate. On the
other hand, we have seen in Chapter VI. that the loss of
radiation by reflection from ice-sheets, at the maximum of the
Quaternary Ice-Age, would suffice to cool the whole earth
by at least 4 F.
The weakness of Ramsay's theory, as of so many other
theories of climatic change, is the lack of a quantitative basis.
Let us examine the various effects of elevation which he
postulates and attempt to evaluate them numerically. The
climatic effects of high ground are, as we have seen, complex.
The most obvious effect is the decrease of mean temperature
with height, at the rate of about 0*3 F. per 100 feet. This
is due to the fact that air which rises from a low level to a
higher level expands and cools by expansion. But when the
air descends again it is warmed by compression, so that the
direct effect of high ground in causing the air to cool by
expansion is purely local, and cannot influence the temperature
of the sea surface or of the lowland plains. The effects which
we have to consider are those which cause a net loss of heat
from the earth's surface as a whole. These include :
i. Radiation to space from the high ground which is not
absorbed by the air.
MOUNTAIN-BUILDING AND CLIMATE 183
2. Reflection of solar radiation from the surface of clouds
(not postulated by Ramsay).
3. Cooling power of the surfaces of ice and snow.
4. Loss of heat owing to increased evaporation.
Reasons have already been given why the loss of heat under
the heading i is probably negligible, and this factor will not
be considered further.
2. Except in winter in high latitudes, clouds lower the mean
temperature by reflecting the rays of the sun back to space.
Where high ground forces the air to rise above the level of
condensation, clouds will be formed, and the local temperature
will be decreased. The level of condensation varies according
to the humidity of the air, but 5,000 feet would be a fair
estimate of the average level over the land. The air above
the sea and the lowlands is not by any means free from cloud,
but it is noticeable that there is a marked increase in the
cloudiness in the neighbourhood of mountain ranges, and
especially on the windward sides, which is not entirely com-
pensated by the somewhat clearer skies on the leeward sides.
Let us say that the net effect of land above 5,000 feet is to
increase the local cloudiness by three-tenths of the sky. Now
we find (7) that about 12 J per cent, of the land, or 3-6 per
cent, of the whole surface of the earth, is at present above
5,000 feet in height. The effect of this high ground is therefore
to increase the cloudiness, expressed as an average over the
whole earth, by one per cent, or o- 1 tenth of the sky. We saw
in Chapter VII. that an increase of one-tenth in the mean
cloudiness results in a decrease of the mean temperature by
6 F. The effect of the introduction of the present topo-
graphy into a nearly base-levelled world would therefore be
to decrease the mean temperature by about 0*6 F. owing to
the increase of cloudiness over the mountain ranges.
During the Quaternary, the average elevation of the land
was considerably greater than it is now ; we do not know
exactly how much, so we must guess. Let us put the elevation
at 3,500 feet instead of the present average of 2,500 feet.
Further, let us suppose that the present elevations were all
increased in the same proportion, so that instead of there being
12^ per cent, of the land above 5,000 feet there was the same
amount above 7,000 feet. From the data given by de Martonne
184 CLIMATE THROUGH THE AGES
(7) we then find that the area above 5,000 feet would have been
21 per cent, instead of 12 J per cent. The resulting increase
of cloudiness would have averaged i 8 per cent, instead of
i per cent, over the whole earth, giving an average cooling
of i o F. compared with a warm period or o 4 F. compared
with the present.
3. The cooling power of the surface of ice and snow is difficult
to evaluate. The mean temperature of the earth's surface
reduced to sea-level is at present 59 F. 5 varying from below
freezing point in high latitudes to above 80 F. in parts of the
tropics. The " snow-line " accordingly varies from sea-level
to above 16,000 feet. But this low snow-line in high latitudes
is really due to the presence of the ice-sheets and floating
ice ; the glaciers creeping down the sides of the hills have
brought the snow-line down with them. The cooling power
of these ice-surfaces belongs partly to the account of elevation,
but partly to other causes, such as increased continentality,
shutting out of ocean currents, volcanic action. What we
are seeking for here is the direct effect of elevation alone, apart
from the other geographical factors which are connected with
elevation. Let us put it that, starting with a warm period
in which there was no ice, a burst of mountain-building
rapidly raises the average level of the land to 2,500 feet ;
we require to find the area brought above the snow-line. In
order to do this it is necessary to make some assumptions as
to the distribution of temperature and land. We will suppose
that during the warm period preceding the elevation the mean
temperature over the land areas is 32 F. at the poles and
83 F. at the equator ; further, that from the poles to latitude
45 N. and S. it rises at a uniform rate of 8 F. in each 10 of
latitude, while from 45 to the equator the mean temperature is
33 F.+5O cos <, where </> is the latitude.
Now, suppose that the whole area is elevated in such a way
that the ratio of land to sea and the percentage areas of land
above different heights are the same in all latitudes, having
everywhere the present average value for the whole world,
the average elevation being 2,500 feet. Then a rough cal-
culation shows that rather more than three per cent, of the
land surface, or about one per cent, of the total surface of the
earth, would be brought above the snow-line, more than half
MOUNTAIN-BUILDING AND CLIMATE 185
this area being between latitude 70 and the poles. The
cooling power of ice compared with that of unglaciated land
varies according to the latitude ; from the data given in
Chapter VIII. it appears that if a land surface in high latitudes
which was formerly bare of ice have one per cent, of its area
ice-covered, the mean annual temperature will be lowered by
about 0-3 F. The ice-covered area is not limited to the area
initially raised above the snow-line, since valley and piedmont
glaciers can descend to low levels. The development of
great ice-sheets probably requires a general cooling of the seas
in addition to simple elevation, but we may take the areas of
mountain glaciers below the snow-line as equal to the areas
above the snow-line. This gives us for the total cooling,
averaged over the whole earth, due to the development of
snowfields and glaciers as a result of elevation, an amount of
0-6 F.
When the calculation is repeated, supposing the average
elevation of the continents to be raised to 3, 500 feet, the heights
of all parts being increased proportionally, the cooling due to
the snow and ice surfaces, when averaged over the whole
earth, is rather more than doubled, becoming i'3F. If,
further, we suppose that the land is concentrated in high
latitudes, forming two continents extending from the poles
to about latitude 50, the cooling becomes still greater, ex-
ceeding 6 F. when averaged over the whole earth. Since
during the Quaternary Ice-Age the elevation was greatest in
high latitudes we may assume an intermediate figure, say
3F-
4. The last term to be discussed is the loss of heat due
to increased evaporation. During the great ice-ages the
total precipitation was probably more than at present ;
during the warm periods it was very much less. The
actual figures can only be guessed at, but judging from
the widespread aridity of periods like the Triassic, we may
not be far out if we suppose that the average precipitation
during the warm periods was about half that during the ice-
ages. Since the amount of water held in the air as vapour
or clouds at any time is equivalent to only a fraction of an
inch of rain, the precipitation gives us a measure of the
evaporation, so that the evaporation also during the ice-ages
may have been twice that during the warm periods. Most
1 86 CLIMATE THROUGH THE AGES
of the evaporation takes place from the surface of the sea.
Evaporation cools the evaporating surface, and an increase
in the elevation of the land, by increasing the evaporation,
lowers the temperature of the sea.
The loss of heat due to evaporation at present averages
about 100 gram calories per square centimetre per day over
the oceans, or perhaps 90 gram calories over the land and sea
together. The loss of this amount of heat would lower the
mean temperature by about 15 F., so that the mean tempera-
ture during the ice-ages may have been lowered by as much
as 8 F. compared with that of the warm periods from this
cause alone. But we cannot attribute all this amount directly
to elevation. As explained in the preceding chapter, the
greater part of the world's precipitation is probably due to the
" polar fronts." During the warm periods the polar fronts
were not developed, and the greater part of the rain fell in
instability showers of a thundery nature. Such showers,
though they are sometimes very heavy, are extremely local,
and do not deliver such a large total of rain as the extensive
rain areas associated with cyclonic depressions. Much of
the difference between the total precipitation of the warm
periods and that of the ice-ages was probably due to this
cause, and not directly to the differences of elevation. Let
us put down half the difference to the account of the polar
fronts and half to the elevation. Then we have an elevation
of 3,500 feet, resulting in a cooling of 4 F. due to increased
evaporation.
We can now sum up our three guesses. Comparing an
ice-age with a warm period, we have :
Cooling due to increased cloudiness . . . . i F.
Cooling due to area above the snow-line . . 3 F.
Cooling due to increased evaporation ... 4 F.
The total cooling due to the elevation alone is about 8 F.,
and from the difficulties of the calculation may be anything
from 5 F. to 10 F., but is not likely to be outside these limits.
Evidently, elevation is by itself and directly an important
factor in climatic changes, but it cannot account entirely for
the changes of some 30 F. in the mean temperature of the
Arctic lands at sea-level, or of the surface of the Arctic Ocean.
The truth is that mountain-building is so intimately
MOUNTAIN-BUILDING AND CLIMATE 187
associated with changes in the land and sea distribution
that it cannot really be considered alone. When the con-
tinents are thrown into folds and ridges, so too are the ocean
floors, at least near the continents. If the continents are
more elevated, the oceans are deeper. The waters retire
into these submarine deeps and expose large areas of the
continental shelves as new land ; this at once increases the
continentality and limits the ocean currents. At the same
time volcanoes break out in the mountain ranges, and by
ejecting dust into the atmosphere add to the general cooling.
The polar oceans, cooled below their freezing point, develop
floating ice-caps, while ice-sheets spread from the mountains
over the surfaces of the continents. The temperature difference
between low and high latitudes passes the critical point for
the atmospheric circulation, and the polar front appears.
The oceanic circulation is radically changed. The whole
process is in fact essentially cumulative, and the final lowering
of temperature is out of all proportion to the small initial
causes. That is why the whole problem of climatic changes
is so baffling.
The time has therefore come to gather up the scattered
threads which we have been patiently disentangling in the
preceding chapters, and to attempt to weave them into a
tapestry which will give us a connected picture of the
mechanism of climatic changes. As ultimate causes, we
have a choice between variations of solar radiation, sunspots,
astronomical changes, ocean currents, continentality,
mountain-building, volcanic dust, and carbon dioxide, though
the possibilities of the last-named are limited. As auxiliary
causes, we have variations of water vapour, of cloudiness, of
the wind circulation, and, most important of all, of floating
ice surely enough, between them, to give us sufficient material
for our purpose.
Let us start with the conditions of the present day and
try to forecast the meteorological changes during the next
quarter of a thousand million years, which, I believe, is the
longest-range forecast ever attempted. We are still living
in an ice-age, though not at its maximum ; whether, before
the forces of mountain-building which began their activity
in the late Cretaceous have worn themselves out, there will
be a return of the ice-sheets in Europe and North America,
1 88 CLIMATE THROUGH THE AGES
we cannot say, so we will skip a million years or so and begin
our forecast with the period when the subterranean fires are
banked and the earth begins again to settle down. The
Arctic Ocean is still filled with ice, and the summits of the
high mountains are above the snow-line. We have our
regular succession of storms born on the edge of the polar
front and travelling eastward into Europe altogether, the
weather is pretty much as we know it to-day (unless by that
time the problem of the artificial control of weather has been
solved who can foretell the triumphs of science?).
When the mountain-building forces cease to repair the
ravages of denudation, the average level of the land begins
to fall steadily. The process is most rapid in the high
mountains, where frost is still active, but everywhere the
rivers are carrying sediment into the oceans and the waves
are grinding into the land. The first noticeable change is the
gradual disappearance of the mountain glaciers as their
gathering grounds are worn down beneath the snow-line ;
the ice-sheets of Greenland and Antarctica still persist, but
are beginning to wane. Bering Strait has been widened and
deepened a little ; a small warm current penetrates through
it into the Arctic Ocean for longer and longer periods in each
summer, and finally succeeds in keeping clear of ice a " bridge-
head " on the northern side, and in maintaining its flow
throughout the year. The Atlantic gap is a little wider--
Iceland is smaller and the southern end of Greenland has
retreated a short distance northward, while at the same time
the gap between Newfoundland and Labrador has widened,
allowing much of the ice and cold water of the Labrador
Current to flow directly down the coast of America, instead
of mingling with the waters of the Gulf Stream off the Grand
Banks. Thus the water of the Gulf Stream Drift is a little
warmer when it reaches the Arctic Ocean, and Spitsbergen
is now ice-free throughout the year. Attacked on two sides,
the Arctic ice-cap melts farther and farther back each summer,
and the Palaeocrystic ice begins to disintegrate. Finally,
there comes a succession of years in which the sun's radiation
is unusually powerful the solar constant remains steadily at
2 o calories per square centimetre per minute and one
summer the Arctic ice-cap breaks up completely and disappears.
That summer the polar east winds are greatly weakened and
MOUNTAIN-BUILDING AND CLIMATE 189
the " polar front " hardly develops ; the Azores anticyclone
extends persistently across the British Isles and Western
Europe almost to the Urals ; hardly any cyclonic depressions
occur south of the Arctic Circle, and the summer is almost
rainless. The cold East Greenland Current carries a little
ice in spring, but this fails to reach Cape Farewell, and the
west coast of Greenland is bathed by a warm branch of the
Gulf Stream instead of by cold Arctic water. The Greenland
ice-sheet makes a record retreat, but the level of the sea is
rising still more rapidly, and the ice-edge still reaches the sea
in many places.
The Arctic Ocean remains open far into the following
winter, and when it finally freezes, the ice is thin and easily
scattered ; it begins to break up quite early in spring, so
that by the middle of summer it has completely disappeared.
The " semi-glacial " condition has been reached in the
Northern Hemisphere. In the following years the ocean
becomes steadily warmer ; the Arctic ice-cap, and with it
the polar east winds, the " polar front," and the Atlantic
cyclones develop later and later each winter and break up and
disappear earlier and earlier each spring, until finally there
comes a winter in which the ice-cap does not form at all, and
even in the middle of winter the Atlantic Ocean is almost
free of storms. The air in high latitudes is not cold enough
for its greater density to counterbalance the relatively warm
and light polar stratosphere, and west winds prevail everywhere
outside the tropics, with, at the surface, a component towards
the poles. The late summers in the British Isles and Western
Europe are now intensely hot and dry ; day after day the
skies are almost clear of cloud, while even the slight hindrance
to the sun's rays offered by volcanic dust is absent, and a
powerful sun warms the surface of the land and of the seas.
The temperature becomes very high, exceeding 100 F. on
several days at midsummer ; owing to the high temperature
the air contains a large amount of water vapour which
absorbs the terrestrial radiation. Part of this radiation is
given back to the earth at night, and in spite of the clear skies
the nights are not cool. (In the dry summer of 1921, which
made some approach to these conditions, the mean daily
minimum temperature at Kew Observatory in July was
nearly 5 F. higher than the mean daily minimum during the
1 9O CLIMATE THROUGH THE AGES
much cloudier month of July 1924.) There are a number
of thunderstorms in spring, but by the end of June the higher
layers of the troposphere have become warmed up to such an
extent by this absorption of terrestrial radiation that the air is
stable, and in the late summer and early autumn there are
not even thunderstorm rains to break the drought. The
general warming spreads even to the equator, though the
rise of temperature is less there than in other parts of the
world, and the temperature difference between low and high
latitudes is greatly diminished.
The Greenland ice-sheet is now retreating very rapidly,
and soon breaks up into a number of isolated masses of dead
ice in the valleys. The warming up of the oceans breaks up
the ring of pack ice surrounding the Antarctic ice-mass, and
that, too, has withdrawn within the limits of the continent
and is now in full retreat. All this ice-water added to the
sea raises its level still further, and helps the influence of the
warm oceans to penetrate into the land. As the mountains
are worn down they offer less hindrance to the winds, and there
is less forced ascent of air and less orographic cloud and rain.
Cyclones are now fewer (though destructive hurricanes still
occur in the tropics and sometimes travel into temperate
latitudes), and altogether the general rainfall is decreasing.
Since vertical motion is the most effective agent in causing
the air to condense its moisture, and the diiest air is that which
has parted with its water vapour by being raised, and then
has again descended to the surface of the earth, this general
decrease of vertical motion gives rise to a general increase
in the relative humidity, so that evaporation decreases, and
the surface of the oceans does 'not lose so much heat in this
way. The wind velocity also is generally smaller, and this
again decreases the evaporation. When all the ice has
finally disappeared, the surface of the oceans is everywhere
warm, and owing to the prevailing poleward component of
the surface winds, this mass of warm water is almost everywhere
moving from lower to higher latitudes, carrying genial con-
ditions into the neighbourhood of the poles. At first the
depths are still filled with cold water a relic of the ice-age
but this cold mass receives no fresh supplies, and is gradually
warmed up by earth-heat and by conduction from the surface.
The earth has now entered on the " non-glacial " stage, and
MOUNTAIN-BUILDING AND CLIMATE igi
conditions will remain sensibly unaltered for millions of years,
until a fresh manifestation of the internal forces of the earth
raises new mountain ranges to begin the cycle afresh. The
climatic conditions at the height of a " non-glacial " or warm
period are of such great interest that a fuller description of
them is reserved for the next chapter.
For a fuller description of the cycle of compression, mountain-
building and erosion, the reader is referred to a book by
J. H. F. Umbgrove (8).
REFERENCES
(1) WAGNER, A. " Klimaanderungen und Klimaschwankungen." Braun-
schweig, Die Wissensch.y Bd. 92, 1940.
(2) JEFFREYS, H. " The earth, its origin, history and physical constitution.*'
2nd ed., Cambridge, 1929.
(3) MANLEY, G. " Glaciers and climatic change, some recent contributions."
London, Q. J. R. Meteor. Soc., 72, 1946, p. 251.
(4) LE CONTE, J. " The Ozarkian and its significance." J. GeoL, Chicago,
7, 1890, p. 525.
(5) RAMSAY, W. " Orogenesis und Klima." Ofversigt af Finska Vetenskaps Soc.
Forh., 52, 1910.
. " The probable solution of the climate problem in geology." Geol.
Mag., London, 61, 1924, p. 152, and Washington, Smithsonian Rep., 1924,
P- 237-
(6) ANTEVS, E. " The last glaciation." New York, Amer. Geogr. Soc., Research
Ser., no. 17, 1928.
(7) MARTONNE, E. DE. " Trait de geographic physique." 2nd ed., Paris, 1913
(8) UMBGROVE, J. H. F. " The pulse of the earth." 2nd ed., The Hague, 1947
CHAPTER XI
THE WEATHER OF THE WARM PERIODS
WE have seen that during long periods of geological
time the earth seems to have been free from ice, so
that even if we accept the theory of Continental
Drift (set out in Chapter XIII.) it is still necessary to believe
that the climatic zones were much less marked than at present.
These periods include a large part of the Palaeozoic, almost
all the Mesozoic, and about half of the Tertiary. Remem-
bering that they were also periods of low relief and very slow
denudation, we must suppose that the time intervals represented
by the rocks of these periods were very much greater than the
time intervals represented by the rocks of the more active
periods, and that this type of climate has, in fact, prevailed
during the greater part of geological time. Its most striking
feature is the appearance of vegetation of sub-tropical or warm
temperate aspect in very high latitudes. These periods have
been termed " pliothermal," though I prefer the simpler word
" warm," which means the same.
In the preceding chapters the various meteorological features
of the warm periods have been deduced, and it is only necessary
here to collect these results and so present a more or less
connected picture of their climate and weather.
Let us imagine that we are voyagers in one of these favoured
periods of antiquity, sailing northward from the equator on
a voyage of discovery towards the pole. We have set out
from a low marshy western shore, through which a broad
but shallow river moves sluggishly towards the sea, winding
in endless curves over a vast plain which stretches as far as the
eye can reach. Under the influence of a light easterly breeze
we sail slowly towards the north-west. The sky is half covered
by woolly cumulus clouds, which now and again thicken and
darken to a passing shower, with perhaps a burst or two of
thunder and a slight squall. The air is warm and moist,
but not unpleasantly so, though we are conscious of a feeling
192
THE WEATHER OF THE WARM PERIODS 193
of lassitude which makes us disinclined for effort, either
physical or mental. As we pass northward, the barometer
rises, the wind backs to north-east, the sky becomes ever
clearer, and the air more bracing. Detached cloudlets are
now the rule. Showers still fall at long intervals, but mostly
in the early hours of the night. On one or two islands that
we pass we find evidence in the fallen trees that occasional
hurricanes occur, but they are very rare, and we are not
greatly troubled by the chances of meeting one of them.
The sea is very warm and blue, and is teeming with life ;
the nights are almost as warm as the days. Still farther
north, and we enter a region of light variable winds and
calms. At times we are becalmed for several days, while
overhead the sun blazes through a cloudless sky. Then
there is a breath of wind from the north-east or from the
south-east, and a few clouds gather, with perhaps a shower
of rain. We are now in the western half of the ocean, and
all the time we are drifting slowly northward on a great
ocean current. The barometer falls very gradually, the
southerly winds become more frequent and stronger, they
draw round to south-west, and, finally, with all sails set, we
bear away to the north-east. The weather, however, does
not differ appreciably from that experienced farther south ;
it is still very warm and for the most part sunny, with a few
scattered clouds. The only difference is that now and again,
perhaps once a month, the barometer drops a few tenths of
an inch, the southerly wind freshens, and a uniform but not
heavy cloud canopy covers the sky, while for a few hours it
rains almost steadily. Then the wind veers to west or north-
west, the sky clears, and after a few showers the steady fine
weather sets in again. It is noticeable, however, that even
when the wind blows straight out of the north it is never cold,
although the season is only late winter.
For some time now we have been sailing northward, with
a low palm-fringed coast in view to the eastward. Beyond
the belt of palms, which marks the limit of the occasional
thunder rains brought by the daily sea breezes, we catch
glimpses of a series of low rounded sandy hills, which seem
to be almost devoid of vegetation. Here and there a line
of reeds marks the course of a stream bed. Mostly, they are
dry and withered, and the channel opens out on to a glittering
n
CLIMATE THROUGH THE AGES
level of white, where a layer of salt or gypsum encrusts thedried-
up floor of a temporary pool, but occasionally the reeds are
fresh and green, marking either a more permanent river, or
the channel taken by the waters of a recent storm. Only
once, however, do we see it actually raining over the land.
On that occasion, the low rounded hills which everywhere
form the background of the landscape, and which usually
stand out clearly against a sky of intense blue, seem to support
a mighty column of cloud, in which we can see the play of
lightning flashes and the deluging rain. We know that
some of the dry channels will soon be rushing torrents of
water, and that the apparently lifeless hollows of the plain
will wake to swarming life. One of the fishes the Ceratodus
or lung-fish is specially adapted to the chances of this life,
since it can breathe air or water at will, but how the other
animals survive the periods of desiccation is a mystery.
As we pass northward, the desert character of the land
becomes less marked, and in about the latitude of London
we decide to land and carry out some explorations on foot.
It is now early in March, but the weather has almost the
character of a fine English June. The vegetation is very fresh
and green. As we go inland across Europe the air becomes
cooler and crisp though not cold. Now and again, after
a few hotter days, there follows a heavy thunderstorm, and this
is the only rain that falls at this season. Later in the year
all this expanse of country will be burnt brown in the steady
heat of a rainless summer ; then as autumn draws on, the
sun will lose some of its power, and the year will draw to its
close in a spell of perfect weather, such as even now we
occasionally meet with in September. Just about midwinter
perhaps one or two mild storms from the ocean to the westward
will pass across the land.
Here we must interrupt our voyage for a moment to explain
that the narrative would differ slightly according to whether
we were in the Mesozoic or in the Early Tertiary world. A
vivid picture of Jurassic Oxfordshire has been drawn by
W. J. Arkell (i). In a clear shallow sea grew a complex of
true coral reefs intersected by narrow channels. The reefs,
which are nowhere very thick, are interspersed with beds
of limestone resulting from their erosion, and it is inferred
that the corals grew on a rising sea floor. The corals are
THE WEATHER OF THE WARM PERIODS 195
obviously found where they grew, and it is very improbable
that the sea temperature was less than 60 F., more than
5 F. warmer than the present temperature of the Atlantic
west of Ireland, which itself has the highest sea temperature
for its latitude anywhere in the world. In the Early Mesozoic
even frost seems to have been unknown, and in Europe the
rainfall was not heavy enough to balance the evaporation.
The prevailing continental deposit was a red desert sandstone,
and in our own country we have in Leicestershire a true
" fossil desert," where wind- worn pillars of granite stand up
into the Triassic sands which ultimately buried all but their
highest summits. A very good picture of the conditions in
the type of country which prevailed over a large part of the
Mesozoic world is conjured up by the finds of dinosaur
eggs in Mongolia. The term " desert " is probably not
strictly applicable, for there must have been enough vegetation
to support these great reptiles. Probably, however, the
vegetation was limited to the larger water-courses which
received the drainage from a considerable area, and the
intervening country was a sandy waste, in which the dinosaurs
buried their eggs to be hatched by the heat of the sun.
Still earlier, in the Permian, a large part of Europe had
been occupied by a great salt inland sea with desert shores,
a European Caspian. This lake was saturated with salt, and
a striking witness to the stability of the climate is the fact that
year by year for ten thousand years the excess of salt was
deposited in regular layers. In summer the deposition was
mainly in the form of gypsum, but this mineral is much more
soluble in cold than in warm water, and as the sea cooled in
winter the deposit of gypsum ceased and rock salt took its place.
From the evidence of similar evaporating solutions in the
Sahara, Kubierschky, quoted by Koppen and Wegener
(Chapter XI II.), estimates that the temperature of the lake
varied from about 60 F. in winter to as much as 95 F. in
summer. In addition to the main mass of salts there are some
isolated layers, apparently formed when adjacent hollows
were flooded at high lake stages, the water afterwards
evaporating and the deposited salts being covered by desert
sands.
In the Early Tertiary, after the brief cold spell which
ushered in the Eocene had passed, there was another long
ig6 CLIMATE THROUGH THE AGES
period of warm climate, though not nearly so hot and dry
as the Early Mesozoic. In fact, although summer was probably
dry, the spring appears to have been decidedly wet in
Europe north of the Mediterranean, and beds of brown coal
were formed in many localities. The rainfall was still mainly
of the instability type, however, falling in violent thunderstorms
accompanied by heavy rain or hail, which stripped leaves
and twigs from the trees and washed them into the lakes.
Many of the trees were evergreens, and the deciduous trees
were in leaf by the end of March, as shown by the relation
of the leafing to the flowering period. Early in the Miocene,
however, the leaves of some of the beech trees show signs of
the action of frost. H. von Ihering (2) estimates the mean
temperature of central Europe as about 68 F. in the earliest
Eocene, 74 F. in the main part of the Eocene and Oligocene,
72 F. in the Miocene and 60-55 F. in the Pliocene.
In western U.S.A., according to various papers summarised
by R. W. Chaney (unpublished), the climate in the lower part
of the Upper Eocene was warm and moist, similar to that
now found on the borders of the tropics. The mean annual
temperature was about 68 F. and the annual rainfall 70
inches, rather uniformly distributed through the year. The
high rainfall was probably due to the neighbourhood of the
growing Sierra Nevada. These conditions persisted through
the Lower Oligocene, but in the Upper Oligocene a
progiessive slow cooling and desiccation set in.
We must now resume our interrupted voyage towards
the pole, but this time we will suppose definitely that we
are in the Upper Eocene period. As we pass towards the
Arctic Circle, we are still in a great northward-flowing
warm current, and the vegetation along the shores continues
to be very rich, but its character gradually becomes more
temperate. The skies become cloudier, and steady cyclonic
rain replaces more and more the thunder rains of Central
Europe. By the time we have passed the latitude of 70 N.,
we find that misty rainy weather forms the rule and fine
sunny weather the exception, while the dense forests along the
eastern shore are frequently hidden by fog. The wind is
now mainly from the west, and the western shores of the
ocean, and still more the country some distance inland, begin
to present a bleak appearance. In winter, the low hills are
THE WEATHER OF THE WARM PERIODS 197
probably snow covered, but there are no glaciers. So we
come to the pole, in a great open basin filled with warm sea
water from the south, which circulates slowly round and round
until it cools and sinks to the bottom. There is no great mass
of Palaeocrystic ice such as we find to-day, no icebergs even,
and although the great rivers sometimes bring down a few
fragments of drift ice in the winter, these soon melt. If we
had made this journey in the Jurassic period we should have
seen no ice at all ; instead, we should have found coral reefs
almost to the Arctic Circle and isolated corals even farther
north.
This is perhaps a somewhat fanciful picture of conditions
during the great warm periods, but it is based almost entirely
on geological evidence, or on logical meteorological deductions
from the slight differences of temperature which we know to
have prevailed between different latitudes during those periods.
REFERENCES
(1) ARKELL, W. J. " On the nature, origin and climatic significance of the
coral reefs in the vicinity of Oxford." London, Q. J. Geol, Soc., 41,
!935 P- 77-
(2) IHERING, H. v. " Das Klima der Tertiarzeit." Zj. Geophys., Leipzig, 3,
7> P- 3 6 5-
PART II
GEOLOGICAL CLIMATES AND
THEIR CAUSES
CHAPTER XII
THE GEOGRAPHY OF THE PAST
WHATEVER view we take of the major cause of the
great climatic fluctuations of geological time, there
can be no doubt that the geographic;;:! conditions
have always played an important part in at least the local
distribution of climate. By geographical conditions, we imply
not merely the bare distribution of land and sea, but also other
variables, such as the height of the land, the presence of
important mountain chains, the vegetative % > jvering, the
movements of the sea in ocean currents, and tvf. existence of
volcanoes. Consequently, before we can pass lo a discussion
of the meteorology of the different geological ages, we must
consider to what extent these geographical factors have varied
in the past.
Palaeogeography, or the reconstruction of former geo-
graphical conditions, is a very difficult science. If we find
a marine deposit, we know that that particular region must
have been at sea at the time ; similarly, the presence of a
deposit obviously laid down on l^.i or in fresh water indicates
the presence of land, but unt ^ivocal evidence of this kind
is the exception, especially for the earlier periods, the deposits
of which have largely been destroyed during the course of
geological history, or buried so deeply as to be inaccessible.
We are almost completely ignorant of the sequence of deposits
on the floors of the oceans. Hence most of the reconstruction
must be based on inference from a variety of facts. A break
in a series of marine deposits, otherwise undisturbed, indicates
that for part of the time, at least, the region was above the
sea. The gradual change of a bed of marine deposits from
fine clays to coarse sands as we follow them from one region
to another, points to the existence of land not far beyond the
latter. The discovery of the same marine fauna in two
different localities indicates a sea connexion between them,
while the presence of different marine faunas of the same age
2O2 CLIMATE THROUGH THE AGES
suggests a land barrier. Similarly, the presence of similar
land faunas or floras in distant regions points to a land con-
nexion, while the presence of different land faunas or floras
close together points to a barrier which may be a sea channel,
but may equally be a range of mountains or a desert. It is
by the gradual collection of such diverse facts as these that the
science of palacogeography has grown up.
The earlier geographical reconstructions presupposed that
the various deposits were laid down practically where they are
found to-day. A few minor exceptions were recognised ;
for instance, the intense crumpling of the rocks in the Alps
and Himalayas shows that the lands on either side of these
chains were formerly at a greater distance from each other,
and their approach has forced the intervening rocks to lie
over one another in great heaps, but these shiftings were
matters of only a few hundred miles. Some geophysicists,
notably A. Wegener, have not accepted this limitation, but
consider that continents have drifted like floating islands over
the face of the earth, and that the positions of the poles have
changed greatly during geological time. Discussion of this
theory is postponed to the next chapter ; here it will be
assumed that the various deposits were formed where they are
now found, and that ihe positions of the poles relatively to the
continents have remained practically unchanged throughout
geological time.
We saw in Chapter Vl^ "-hat the mean temperature of
any latitude at the present c'ay depends to a considerable
extent on the percentage of the area, along a belt 20 wide
centred on that latitude, which is occupied by land. Near
the equator the effect of a large land-mass is to raise the
temperature slightly, but in high latitudes land lowers the
mean temperature, and especially the winter temperature,
much more effectively. For reasons to be given later, it is
necessary to limit the discussion to the mean temperature of
the regions between the poles and latitude 40. Hence the
area of land north of 40 N. and south of 40 S. is an important
climatic factor. Owing to the large area of the oceans south
of 40 S., we are almost entirely ignorant of the land and sea
distribution in that part of the world during geological times,
so that this variable becomes in effect the area of land north
of 40 N. This area has been measured from the composite
THE GEOGRAPHY OF THE PAST 203
charts given by Th. Arldt (i). Numerous reconstructions
of land and sea distribution in different periods have been
published by various authors, which naturally differ widely,
and Arldt has combined these, indicating the areas shown as
land by all authors and also those shown as land by some
authors and as sea by others. On examining the measure-
ments, it was found that in the different geological periods the
areas shown as land by all authors were on the average about
the same as the present area of land. Since we have no
reason to suppose that during the Miocene period, for instance,
the land area in the northern temperate and polar regions
was much greater than at present, the conservative measure
given by the areas shown as land by all authors seems the best
measure to adopt.
The effect of land areas on temperature increases very
rapidly as we approach the poles. This was allowed for
by weighting the land areas in different latitudes according
to a scale derived from the investigation of " Continentality
and Temperature" (Chapter VIII.). Giving a unit area
of land in latitude 60 a weight of i, the scale adopted was
80, 3 ; 70, 2 ; 60, i ; 50, 0-6 ; 40, 0-2.
The figure which would be obtained if the whole hemi-
sphere north of 40 N. were occupied by land was given the
value 100, so that the figures under (i) of Table 14 are
percentages of a full land covering.
The second variable is the average height of the land.
There is a consensus of opinion among geologists that this
has varied extensively during geological history. Thus
T. C. Chamberlin (2) writes :
" It is generally agreed that the present altitude of the
continents is greater than their mean elevation during geologic
history. Geologists recognise at least two stages in which
the continents were exceptionally high and broad ; that
which attended the transition from the Palaeozoic to the
Mesozoic Era, and that which attended the transition from
the Tertiary to the present epoch. The existing stage thus
falls in one of the most notable stages when continental
elevation and breadth were greatest, though perhaps not at
its climax. The latest estimate of the present mean elevation
of the land gives 2,500 feet. The mean elevation of the great
204
CLIMATE THROUGH THE AGES
Period.
Upper Proterozoic
Eocambrian . .
Late Cambrian .
Ordovician . .
Silurian ....
(') (a) (3)
Conti- Eleva- Ocean
nentality. tion Currents.
Per (unit Per
cent. 100 feet.) cent.
Lower Devonian
Middle Devonian
Upper Devonian . .
Lower Carboniferous
Middle Carboniferous
Upper Carboniferous
Lower Permian
Middle Permian .
Upper Permian . .
Lower Trias . . .
Middle Trias . . .
Upper Trias . . .
Rhaetic ....
Lias
Middle Jurassic .
Upper Jurassic . .
Lower Cretaceous
Middle Cretaceous
Upper Cretaceous .
Lower Eocene . .
Upper Eocene . .
Oligocene ....
Miocene ....
Pliocene ....
Pleistocene ....
52
43
21
15
'4
36
28
30
49
49
40
52
5i
52
45
49
40
47
69
60
36
5 1
45
47
46
53
53
60
62
40
26
5
8
33
14
35
50
38
Recent 50
26
31
28
23
36
35
24
(45)
53
61
53
65
49
52
39
4i
42
J 7
26
8
4 6
7
4 6
5
27
5
34
8
28
ii
10
26
39
43
21
29
46
38
29
29
46
4
46
43
24
(4)
Vol-
canic
Action
(o-io).
9
4
2
2
2
5
5
5
3
3
6
6
10
6
5
5
5
3
3
i
i
3
5
6
8
4
3
(5)
Mean Tempera-
ture, 4O-90 N.
"Ob- " Calcu-
served." lated."
29 F. 40 F.
33
43
50
5 1
42
50
53
53
42
32
32
42
45
48
5i
53
53
48
53
44
41
4 J
45
47
45
46
37
28
33
53
69
67
60
48
59
61
50
42
37
36
42
56
57
50
54
50
39
52
50
44
39
48
45
47
48
26
37
Table 14. Values of geographical elements and of mean
temperature.
peneplains is a matter of judgment rather than of knowledge,
but no one would probably put the elevation at much more
than a third of this. Probably a third is too high."
Similarly, H. Jeffreys (3) in discussing the discrepancies
between the age of the earth calculated from the rate of
accumulation of sedimentary rocks or of salt in the ocean
THE GEOGRAPHY OF THE PAST 2O5
and the much greater age calculated from the data of radio-
activity, points out that the former " amount to a proof that
the present rate of denudation is several times greater than the
average of the past."
It has been pointed out in Chapter X. that the earth's
surface has passed through a series of cycles, each cycle
consisting of a relatively short stage of intense mountain-
building, in which the rocks were thrown into great folds
and ridges and the average elevation of the land above the
sea became very great, followed by a long stage of quiet
conditions, in which the forces of denudation lowered the
level of the land, rapidly at first, and then more and more
slowly. We are at present living shortly after one of the
periods of mountain-building and elevation, and the average
level of the land is consequently high, though not so high as
it was during the Quaternary period. At the close of one of the
long quiet periods the average level must have been very much
less than it is now, and was probably only a few hundred feet.
At such times, conglomerates and coarse marine sandstones
are almost entirely absent, and the bulk of the sedimentary
rocks is composed of limestones and very fine clays or shales.
Since we cannot hope to know the average height of the
land in the different geological periods directly, we have to
take as a measure the amount of disturbance of the rocks
caused by mountain-building during that period. The
column under (2) in Table 14 and the curve of height in Fig.
24 are based on a diagram given by E. Dacque (4, p. 449).
Dacque places the chief periods of mountain-building in the
Algonkian or Late Proterozoic, the Late Carboniferous, and
the close of the Tertiary, with minor periods at the close of
the Silurian, in the Cretaceous, and in the Early Tertiary.
The longest orogenetically quiet period appears to have fallen
in the interval from Middle Permian to Middle Jurassic.
There are, however, two points in which Dacque's curve
seems to require modification. Arldt points out (i, p. 711)
that the "alpine" character of a mountain chain a large
number of separate peaks of approximately the same height -
occurs only as a result of glaciation ; before glaciation, the
system, though it may be of considerable height, possesses
only rounded contours of the foot-hill type. The conversion
of the rounded contours into the broken alpine type, and the
206
CLIMATE THROUGH THE AGES
9 -d
THE GEOGRAPHY OF THE PAST 207
removal of the resulting detritus, represents a great decrease
in the load on the underlying plastic layer of the earth, and
consequently leads to a further elevation. Hence it seems
probable that the greatest height of the mountains occurred
after the main periods of mountain-building. Of course
this does not mean an increase in the average height of the
earth's surface, but for the meteorological processes involving
forced ascent of air, a broken " alpine " surface is as effective
as a rounded surface of much greater area. The second point
is concerned with the depression of the sea-level during glacial
periods owing to the accumulation of water in the great ice-
sheets, which may add several hundred feet to the effective
height of the land above sea level. For these reasons, and also
because Dacque's curve represents mountain-building activity,
while what we require is the effective average height of the
land, which naturally lags somewhat behind the process of
uplift, I have modified Dacque's curve slightly, especially in
the Quaternary period. The height of this modified curve at
the end of the Quaternary was 19 (the unit being one-thirtieth
of an inch on Dacque's diagram), and this figure represents
the present height of 2,500 feet. The lowest value on the
scale, i, was considered to represent a mean elevation of 500
feet, and the average height of the land during the Quaternary,
44 on the scale, was taken as 3,500 feet. Through these
three points (i, 500; 19, 2,500; 44, 3,500) a smooth curve
was drawn, and this curve was employed to convert the scale
elevations into estimated mean heights in hundreds of feet.
Of course these figures have no pretension to any great degree
of accuracy ; the conversion was undertaken merely because
Dacque's curve of mountain-building, if taken directly as a
curve of height, seemed to exaggerate the heights of the
disturbed periods relatively to the present far too much.
The third geographical variable which we have to consider
is the oceanic circulation. At present the Arctic basin is
nearly surrounded by a land-ring, which is effectively broken
only by the Atlantic gap, the Bering Strait being too narrow
and shallow to admit an appreciable current. Even the
whole of the Atlantic gap is not occupied by the warm current,
its western side being occupied by the cold ice-bearing East
Greenland Current, but this condition is probably only present
when the Arctic Ocean is ice covered. During the warm
2O8 CLIMATE THROUGH THE AGES
periods, the surface cold currents were probably very limited
in area ; the gaps in the circum-polar land-rings were probably
occupied almost entirely by warm currents, directed towards
the poles, strongest near the eastern sides of the gaps and
diminishing in strength towards the west. Near the western
shores of the oceans there might be local cold currents of slight
intensity issuing from rivers or narrow channels between
islands, which would be able to maintain their identity for
a time owing to their slight salinity. In order to obtain
comparable measures of the amount of heat carried to the
north polar lands during the different geological periods, I
measured the width at 60 N. of those gaps in the circum-polar
land-mass which were directly connected with tropical or
sub-tropical seas. For each gap double weight was assigned
to the first 20 of breadth, single weight to the second 20,
and half weight to breadths beyond 40. The results are
shown in Table 14 and in the curve labelled " ocean currents "
in Fig. 24. The figures are expressed in percentages of the
oceanic effect which would be produced if the only land
consisted of five long narrow islands directed from south to
north.
After the publication of the first edition a series of
experimental reconstructions of ocean currents were described
by P. Lasareff (see p. 78). These bear out the general
lines of the estimated effect of ocean currents given in Table 14.
The fourth factor which has been considered as " geo-
graphical " is the amount of volcanic action. Numerous
climatic roles have been assigned to volcanoes by different
investigators, some of which are favourable and others
unfavourable to high temperatures. The cooling effect of
volcanic dust postulated by W. J. Humphreys (Chapter VI.)
seems to be the best founded of these roles. As Humphreys
points out, the amount of volcanic dust discharged into the
upper air depends on the explosive eruptions and not on the
total amount of volcanic action, but it does not seem possible
to obtain a curve of explosive volcanic activity only. Con-
sequently, we have to be content with a measure of the total
volcanic activity in each period, based on the thickness of
volcanic rocks, especially lavas. For this purpose the
admirable summary of volcanic activity given by Arldt (i)
was converted into figures on a comparative scale of o-io.
THE GEOGRAPHY OF THE PAST 2OQ
Finally, values were assigned for the mean temperature,
based on a curve given by Dacqu to show the zonal
differentiation of climate. While it is not difficult to accept
Dacque's curve as having some value for the Mesozoic and
Tertiary periods, we get into difficulties as soon as we go back
to the Palaeozoic. The Upper Carboniferous especially,
with its apparently tropical forests in temperate latitudes
accompanied by an enormous glaciation near the present
equator, is a meteorological paradox. For the purposes of
comparison, figures were assigned, but they are very doubtful.
In considering Dacque's curve, it was assumed that the mean
temperature of the equatorial regions (apart from the Permo-
Carboniferous period) had remained constant, and that the
curve, therefore, gave the variations of temperature in the
temperate and polar latitudes in the Northern Hemisphere ;
to be precise, in the area between 40 N. latitude and the
North Pole. This cuts out the areas which were most heavily
glaciated during the Upper Carboniferous, and the tempera-
tures to be assigned to that period were therefore not so low
as those of the Upper Proterozoic and Quaternary periods,
although the total amount of land ice present during the
Upper Carboniferous was probably greater than the amount
at any other stage of the earth's history. On the other hand,
the Upper Carboniferous presents evidence of a considerable
amount of glaciation even in North temperate latitudes, a
point which is discussed further in Chapter XV. Moreover,
the faunal changes at this time, and especially the great
extinction of corals, indicate a great lowering of the temperature
of the sea. It was therefore assumed that there was a con-
siderable fall of temperature during the Upper Carboniferous
even in North temperate and polar latitudes, though not so
much as in the Upper Proterozoic or the Quaternary.
In order to obtain numerical measures it was necessary to
find some means of calibrating Dacque's curve. The mean
temperature of the area between 40 N. and the North Pole
is at present 33 F. For the Middle Jurassic, the January
temperature of the north polar basin calculated in Chapter
VIII. was 44-5 F. and the annual range was 13*5 F., giving
a mean annual temperature of 5 1 F.
The variation with latitude over the oceans was small ;
on the other hand, the winter temperatures over the interior
14
2IO CLIMATE THROUGH THE AGES
of the continents must have been several degrees lower than
those near the oceans. As a rough approximation, a value
20 F. above the present, or 53 F., was accepted as the mean
temperature of the whole region in Middle Jurassic times.
This value seems reasonable from a consideration of the
biological evidence.
The mean temperature in the Pleistocene period was taken
as 5 F. below the present mean. The maximum decrease
in the mean annual temperature calculated from the lowering
of the snow-line was more than 20 F. in Scandinavia, 20 F.
in East Anglia, 11 F. in the Alps, and 7 F. in Japan ; on
the other hand, the Pacific Ocean was little affected, and it is
not improbable that over the interior of Asia the winter
temperatures were higher than now. When the interglacial
periods are taken into account, a mean decrease of 5 F. over
the whole area north of 40 N. seems to be a reasonable
estimate. It happened that the differences of 5 F. between
the Pleistocene and the Present, and 20 F. between the
Present and the Middle Jurassic, were actually proportional
to the differences measured on Dacque's curve, and while
this is probably nothing more than a coincidence, it greatly
facilitated the conversion of the scale of this curve into a
temperature scale.
The next step was to determine how far the various elements
continentality, elevation, ocean currents, and volcanic
activity were responsible for the mean temperature. For
this purpose the figures in the column of Table 14 headed
" Mean Temperature, 4o-go N., Observed," were
" correlated " with the figures in the columns headed
" Continentality," " Elevation," " Ocean Currents," and
" Volcanic Action." The figures were divided into two
groups, the doubtful figures for the Upper Proterozoic and
Palaeozoic being separated from the much more reliable
figures for the Mesozoic, Tertiary and Recent. The cor-
relation coefficients are given in Table 15.
Temperature with
Continentality. Elevation. Ocean Currents. Volcanoes.
Palaeozoic .... 52 66 +*37 --50
Mesozoic to Recent 37 -72 +'51 "ii
Table 15. Correlation coefficients between temperature and
geographical conditions.
THE GEOGRAPHY OF THE PAST 211
A correlation coefficient of + 1 indicates that the fluctuations
of the two variables considered are exactly proportional ;
a coefficient of i indicates that the relationship is exactly
inverse.
These coefficients agree with our expectations in showing
that extensive land areas, a high level, and extensive volcanic
eruptions are all associated with low temperatures, while
open connexions between the Arctic Ocean and equatorial
seas are associated with generally high temperatures. The
chief difference between the two periods Pakeozoic and
Mesozoic to Recent, lies in the importance of volcanic
eruptions, which appear to have been much more effective
in the former than in the latter. This is largely due to the
very great volcanic activity which prevailed during the
Permo-Carboniferous glacial period, which dominates the
first half of the climatic curve.
Correlation coefficients show how closely two variables
are connected, but they do not give immediately the
quantitative effect which one variable has on the other.
This is given by the " regression coefficient/ 5 which is the
average amount of change in one variable associated with
a change of one unit in the other variable. For instance, it
was found that the regression coefficient of mean temperature
(Fahrenheit degrees) in terms of continentality (per cent.)
during the Palaeozoic period was 0-31. This means that
an increase in the continentality by one per cent, is associated
with a decrease in the mean temperature north of 40 N. by
o3iF. The regression coefficients calculated from the
correlation coefficients in Table 15 are shown in Table 16.
Factor .... Continentality. Elevation. Ocean Currents. Vulcanicity.
Unit .... i per cent. 100 feet i per cent, i (scale o-io)
Change of Temperature
Palaeozoic .... 0-31 0-38 +0-28 1-68
Mesozoic to Recent 0-32 0*47 +0*28 0*42
" Theoretical " (see
below) .... -0-35 -0-3 (+0-3) -0-5
Table 16. Effect of a change of one unit on the mean
temperature in F.
With the exception of the figures for vulcanicity, there
is remarkably good agreement between the two periods.
212 CLIMATE THROUGH THE AGES
Let us now consider the factors separately. An increase
of one per cent, in the land area north of 40 N. is found
to decrease the mean temperature by 0-31 F. In discussing
the effect of continentality on temperature at the present
day (Chapter VIII.), I obtained expressions for the effect of
land in different latitudes on the mean temperature in January
and July. The effect of an increase of the land area in any
region is made up of two parts, a general decrease in the mean
temperature over the whole belt of latitude, and an additional
local decrease in the neighbourhood of the new land area.
The average effect of an increase of one per cent, in the land
area, calculated from the present distribution of temperature
in relation to the distribution of land and sea, is a decrease
of 0-22 F. in the " zonal" temperature (p. 150), and the local
effect, if spread out over the whole zone, would be equivalent
to an additional decrease of o- 13 F., making a total lowering
of temperature by o35 F. This is the " theoretical " figure
of Table 16 ; it is in good agreement with the figures 0-31 F.
and 0-32 F. obtained from the regression equations.
The effect of elevation on temperature at present is well
known. It is very close to an average decrease of 0-5 G.
per 100 metres or 0-3 F. per 100 feet of elevation, which is
given as the " theoretical " figure in Table 16.
The effect of ocean currents is complicated by the great
cooling power of floating ice described in Chapter I. For
our representation of the warm periods, we may take the
arithmetical mean of the temperatures over the Arctic Ocean
calculated in Chapter VIII. for the Upper Jurassic and
Middle Eocene periods, viz., 43 F. Thus we have the
following data :
Warm Period. Present.
Ocean Currents. Arctic Temperature. Ocean Currents. Arctic Temperature.
Glacial. Non-Glacial.
44 per cent. 43 F. 17 per cent. 18 F. 24 F.
If we take the glacial temperature at present, we have a
difference of 61 F. corresponding with a difference of 27 per
cent, in the ocean currents ; if we take the non-glacial, we
have a difference of 19 F. Now, in geological time, non-
glacial conditions in the Arctic Ocean have been the rule
and glacial conditions the exception. If we take the ratio
THE GEOGRAPHY OF THE PAST 213
of occurrence of the two conditions as five to one, and combine
the two figures 19 F. and 61 F. in that proportion, we obtain
a weighted mean of 26 F. for a difference of 27 in the ocean
currents. This figure refers to winter over the Arctic Ocean ;
the difference between summer and winter is probably not
great, but the effect diminishes rapidly southward, and is also
less over the land than over the oceans. If the average effect
over the whole area north of 40 N. is one-third of that over
the Arctic Ocean itself, or 9 F., we obtain an increase of
temperature of about 0-3 F. for an increase of one per cent,
in the effect of the ocean currents. The amount given in
Table 1 6 is 0-28 F.
The effect of volcanic action is difficult to discuss because
of the arbitrary nature of our scale of o-io. W. J. Humphreys
(Chapter VI.) considers that during the past 160 years the
mean temperature of the earth has been lowered i F. by
volcanic dust. If the value of 2 for the present vulcanicity
is correctly assigned (a very large "if"), this is equivalent
to a decrease of temperature by 0-5 F. for an increase of
one in the scale of vulcanicity. The corresponding value
found for the Mesozoic to Recent periods is 0-42 F., which
is a good agreement. On the other hand, the figure for the
Palaeozoic, 1-68 F., is very much greater, and suggests that
the amount of volcanic dust present during the Upper
Proterozoic and Upper Carboniferous glaciations was much
greater than is indicated by the values of 9 and 10 on the
linear scale. The figure for the Upper Carboniferous should
probably be nearer 40 than 10.
From the correlation coefficients given in Table 15, we
see that the variations of climate during geological time
have been associated to some extent with the variations
of all four of these factors continentality, elevation, ocean
currents, and volcanic action. But the curves in Fig. 24 show
also that these factors of climate have a close relationship
among themselves. When the continents were generally
lofty they were also extensive, and the passages between
them along which ocean currents could penetrate into high
latitudes were few and narrow, while volcanic action was
greatest in periods of mountain-building. Hence part of
the effect of great continentality in lowering temperature
may be due to the great elevation, weak ocean currents, and
214 CLIMATE THROUGH THE AGES
great vulcanicity which accompany it. In order to determine
the effect which would follow a change in one factor only,
while the others remained constant, it is necessary to calculate
" partial " correlation coefficients. This was done, and the
results are shown in Table 17.
Temperature with
Continentality. Elevation. Ocean Currents. Volcanoes.
Palaeozoic .... 25 -22 -22 ~ *53
Mesozoic to Recent -08 -82 +'63 -15
Table 17. Partial correlation coefficients with temperature.
In the calculation of partial coefficients, small errors in the
original data are apt to make a great difference in the final
result. The figures of continentality and ocean currents
for the Palaeozoic are uncertain, and the partial coefficients
for this period have very little value. In particular, the
negative coefficient between ocean currents and temperature
is obviously wrong, since one cannot conceive a state of
affairs in which a wide connexion between the polar and
equatorial oceans brings about a Jow temperature in temperate
and polar regions. Probably the only significant figure is the
high correlation between volcanic activity and temperature.
For the Mesozoic and later periods our data are more exact
and the partial coefficients show that the most important
geographical conditions which determine temperature are
the average elevation of the land and the volume of the ocean
currents. From these partial coefficients we obtain the
following formula for calculating the temperature in any part
of the Mesozoic or Tertiary :
Temperature (F.)~48 o 13 X Gontinentality (per cent.) 0-45
X Elevation (hundreds of feet) +0-43 X Ocean Currents (per
cent.) o 26 X Vulcanicity (o-io).
It will be noticed that these coefficients differ somewhat
from those given in Table 16. The effect of continentality
appears to be greatly reduced ; this is because the cooling
power of land is due partly to the mountain systems usually
found somewhere in a large land-mass, partly to the barriers
which large land-masses place in the way of ocean currents,
and only partly to actual cooling by radiation from the surface
of the land. All three of these effects are included in the
THE GEOGRAPHY OF THE PAST 215
coefficient in Table 16, but in the equation given above, the
first two have been eliminated, leaving only the purely local
radiation effect. It may be only a coincidence, however, that
the value of this local effect in the equation given above
(0-13) is exactly the same as the local part of the total
effect of continentality at present, as described on page 212.
The theoretical temperatures given by this equation are
shown in column (6) of Table 14. The calculation was
extended to the Palaeozoic, although the equation is based
only on the data since the beginning of the Triassic, because
we cannot suppose that the physical laws of climate have
changed, and the equation deduced from the later periods
agrees with what we know of those laws. The discrepancies
shown by the Palaeozoic are no doubt due partly to our
incomplete knowledge of the geographical conditions of this
era, but probably partly also to errors in the temperatures
deduced from the records of the rocks. The most noticeable
feature is that until the Middle Triassic the " calculated "
curve is generally above the " observed " curve. The three
great glacial periods of the Upper Protcrozoic, Upper Carbon-
iferous, and Quaternary stand out clearly ; following each
one of them the " calculated " curve rises more rapidly than
the " observed " curve, as if the earth took a long time to
warm up again after the crisis of the glaciation had passed.
We have good reason to believe that this is true of the Recent
period, the temperature being kept lower than it should be by
the relics of the Quaternary ice-sheets in Antarctica and
Greenland, and by the low temperature of the great body
of sea water. The delay in warming up after the other ice-
ages may be due to similar causes, in which case the statement
sometimes made, that until the Quaternary the oceans had
never been generally cooled, is incorrect. There is, in fact,
a large amount of biological evidence that the oceans became
cold during the Upper Carboniferous. It is possible, however,
that the delay in these cases is more apparent than real. It
is often difficult to determine the exact horizon of a glacial
deposit, and if a few such deposits are placed too high in the
series, they will make the stage following the glacial period
appear colder than it actually was. This explanation may
apply to the apparently low temperature of the Early
Cambrian, but scarcely to the discrepancy of the Upper
2l6 CLIMATE THROUGH THE AGES
Permian and Lower Trias, and on the whole, I believe this
delay in warming up after an ice-age to be a real phenomenon.
For the Pliocene, the " calculated " temperature is much
lower that the " observed." It is a very striking fact, which
has often been commented on, that the Quaternary glaciation
did not coincide with the period of greatest elevation, but
lagged considerably behind it. The cause of this lag was
discussed on page 179.
There is a steep drop in the " calculated " curve in the
Lias (Lower Jurassic) which is barely shown on the " observed "
curve. The Liassic period, so far as we know, had no glaciers ;
probably the distribution of mountain ranges in relation to
moist winds was not suitable. In the absence of ice, the other
factors of low temperature fail to produce their full effect.
The drop in the " calculated " curve during the Cretaceous,
on the other hand, is almost equally marked on the " observed "
curve ; in this instance we have evidence in the erratics of the
English chalk that either shore ice or glacier ice occurred
somewhere in the Northern Hemisphere and that floating
ice was present on the chalk seas.
The relations between the " observed " and " calculated "
temperatures, given in Table 14, since the beginning of the
Carboniferous have some points of interest. It will be
noticed that when the " calculated " temperature is below
39 F. the " observed " temperature tends to be below the
;c calculated " temperature, the mean values for the five cold
periods (Upper Carboniferous, Lower Permian, Pliocene,
Quaternary, Recent) being : " observed," 32-4 F. ; " cal-
culated," 338F. When the " calculated " temperature
lies between 39 F. and 50 F., the " observed " temperatures
tend to be higher than the " calculated," the mean values for
fourteen moderate periods being: "observed," 47-4 F. ;
:c calculated," 45-4 F. When the " calculated " temperature
is above 50 F., the " observed " temperatures are again
lower than the " calculated," the mean values for the warmest
periods being : " observed," 50-0 F. ; " calculated," 52-3 F.
A. decrease of the "calculated" temperature from 52-3 to
4.5-4 F., or 6 '9 F., is associated with a decrease of the
"observed" temperature from 50-0 to 47-4^., or only
2-6 F., a fall of less than 0-4 F. in the " observed " tem-
perature for a fall of one degree in the " calculated "
THE GEOGRAPHY OF THE PAST 21 J
temperature. On the other hand, a decrease of the " cal-
culated " temperature from 45-4 to 33-8 F., or ii-6F.,
is associated with a decrease of the " observed " temperature
from 47-4 to 32-4 F., or 15 F., a fall of i'3F. in the
" observed " temperature for a fall of one degree in the
" calculated " temperature. This result may be due to one
of three causes : It may be accidental, due to the chance
run of the figures, or it may be due to an error in the scale
of the " observed " temperatures, owing to which the tem-
peratures of the moderately warm periods are overestimated
compared with those of both the warm and the cold periods.
On the other hand, it may represent a real phenomenon, a
unit change in the geographical factors making less difference
to the mean temperature of a warm period than to that of a
cold period. Chapter I., and especially Fig. 2, show very
strong reasons why such a difference should actually occur.
So long as the climate remains " non-glacial," the change
of temperature due to a change in the geographical factors h
limited to the direct effect of those factors. Land in high
latitudes has a lower mean temperature than sea, so that an
increase in the land area lowers the mean temperature some-
what, but this effect at present is partly due to the winter
snow-cover. The more free the oceanic communication
between high and low latitudes, the more heat is carried
by ocean currents, but the effect is also proportional to the
difference between the temperature of the warm currents and
that of the main mass of cooler water in the Arctic Ocean,
and therefore two ocean currents during a warm period do
not raise the mean temperature twice as much as one of them
during a cold period. In Chapter X. we saw that the effect
of an increase of elevation by 100 feet becomes greater the
higher the average level of the land ; the change from an
average elevation of 2,500 feet to one of 3,500 feet is responsible
for nearly as much cooling as the change from 500 to 2,500 feet.
When the temperature of the polar regions falls below a certain
level, the climate becomes " glacial," and the cooling power
"of ice is added to the direct effect of the geographical factors.
The relations between the " observed " and " calculated "
temperatures north of 40 N. may be due to this introduction
of the cooling power of ice when the mean temperature falls
below about 45 F, over the whole area, which may imply a
2l8
CLIMATE THROUGH THE AGES
January mean of 27 F. at the pole. If this explanation is
correct, the ice present in the oceans in high latitudes, and the
Greenland ice-sheet, lower the mean temperature north of
40 N. by nearly 10 F., which is in sufficiently good agree-
ment with the results of the theoretical investigation in Chapter
I. The changes of mean temperature which take place
during the transition from a warm period to an ice-age are
55
Calculated Temperatures
$Q 45 40 35
A
30 r
\
\
so
45
ex,
40 1
1
35 I
30
Fig. 25.^ The transition from a warm period
to an ice-age.
shown diagrammatically in Fig. 25. The mean temperature
north of 40 N. is initially 52 F., as shown at A. As the
continents emerge and the ocean currents become weaker,
the temperature falls slowly, as shown by the full line, until
the point B is reached, when it is 45 F. At this point the
Arctic Ocean becomes glacial. The temperature now falls
much more rapidly along the full line BC. The dotted line
BC X indicates the " non-glacial " temperature due to the action
THE GEOGRAPHY OF THE PAST 2IQ
of the geographical factors alone, without the intervention
of the ice, and the distance between the dotted line and the
full line shows the additional cooling due to the ice itself.
The arrow indicates the amount of cooling by ice at the present
time. The broken line shows the corresponding temperatures
calculated from the regression equation given above, which
assumes that the relations are linear throughout. The
three crosses mark the three points determined from the
comparison of the " observed " and the " calculated "
temperatures. The diagram seems to agree well with all
the results previously obtained ; for instance, it indicates
that the " non-glacial " temperature is now only about two
degrees below the critical point, and that a permanent increase
in the general temperature by more than this amount would
result in the breaking up of the Arctic ice. The good quantita-
tive agreement between the effects of the different geographical
factors calculated from purely geological data and those
deduced from existing conditions, the coincidences in points
of detail between the observed and calculated curves of
temperature in Fig. 24, and the fact that the discrepancies
between the two curves are what we should expect from the
combination of " glacial " and " non-glacial " periods in the
same equation, combine to form a very strong body of
evidence that throughout the greater part if not eill of
geological time the major variations of climate have been
entirely controlled by changes in the geographical factors.
Since a careful calculation of the effects of known causes
suffices to explain the facts, it is unnecessary to introduce
hypothetical causes such as variations of solar radiation or
continental drift to explain the long-period oscillations of
climate. The more rapid oscillations within the major
climatic chapters, such as the succession of glacial and inter-
glacial periods within the Quaternary, may not be explicable
by changes in the geographical factors. Zeuner's recon-
struction of the Quaternary sequence (p. 107) fits in well
enough with the facts to lend some support to the theory
that such secondary oscillations are due to astronomical
causes, but variations of solar radiation also remain a
possibility.
The topsy-turvy Permo-Carboniferous period, in which
the greatest glaciation occurred not far from the equator,
22O CLIMATE THROUGH THE AGES
demands a special investigation, which is given to it in
Chapter XV.
REFERENCES
(1) ARLDT, TH. " Handbuch der Palaeogeographie." 2 vols. Leipzig, 1919.
(2) CHAMBERLIN, T. C., and Others. " The age of the earth." Philadelphia,
Proc. Arner. Phil. Soc., 61, 1922, and Washington, Ann. Rep. Srnithson. hist.,
1922, p. 246.
(3) JEFFREYS, H. " The earth, its origin, history, and physical constitution."
Cambridge, 1924.
(4) DACQUE, E. " Grundlagen und Methoden der Palaeogeographie." Jena,
CHAPTER XIII
THE THEORY OF CONTINENTAL DRIFT
IN the calculations discussed in the last chapter, the
assumption was made that deposits were laid down not
far from where we now find them, or in other words, that
the positions of the continental massifs relative to each other
and to the poles have not changed during geological time.
That assumption has been challenged from time to time,
but was not seriously countered until A. Wegener ( i ) published
his well-known theory of continental drift, and supported
it with a wealth of detail and acute reasoning. For a time
Wegener's theory was in considerable favour, but it has
been found to introduce so many difficulties that opinion
now seems to be that if continental drift ever occurred on the
scale postulated by Wegener it was long before the beginning
of the geological record. The final acceptance or rejection
of Wcgener's theory is a matter for geologists, but inasmuch
as palaeoclimatological evidence plays a considerable part in
the working out of the theory, which in turn, if accepted,
completely alters the aspect of the problem of climatic changes,
this book would not be complete without a discussion of what
the theory implies. The theory of continental drift falls into
two parts, and the truth of one part does not necessarily imply
the truth of the other part. The first contention is that the
positions of the continents have changed relative to each
other during the course of geological time ; that at first there
was a single large continent (" Pangaea "). This original
continent split up into various parts which gradually drifted
asunder, the latest division being the separation of America
from Europe. The second contention is that there have also
been radical changes in the positions of these land-masses
relative to the poles. Of course, if the relative positions of
two continents change in any way except by means of a
direct east-west movement, there must necessarily be some
change of latitude, but the movements postulated by Wegener
go far beyond this. Regions like Brazil and Central Africa,
222 CLIMATE THROUGH THE AGES
now near the equator, are supposed to have been formerly
near the South Pole, while other regions now far to the north
were once close to the equator.
This power of free movement of the continents depends
on the difference of constitution between the continental
massifs and the mass of the earth's crust. The former are
composed mainly of silicates of alumina, termed Sial for short,
and are lighter than the rest of the crust, which is mainly
composed of silicates of magnesia (Simd). The sima is
continuous and thick, and forms the floor of the deep oceans ;
the floor of the Atlantic, however, is believed to be covered
by a thin layer of sial, and this difference between the Atlantic
and Pacific has given rise to much speculation. The sial
now consists of a number of separate masses (continents and
continental shelves) with some smaller detached portions
(oceanic islands). Under the action of any long continued
force, however small, the sima acts as a very viscous fluid,
while the masses of sial are rigid, and may be compared
to slabs of wood floating in a sea of treacle. This distinction
between the continental masses and the rest of the crust is
based on a number of converging lines of evidence, and is
now generally accepted.
The argument that the individual continents have been
formed by the splitting up of an original Pangaea, starts
with the notable similarity in the shape of the opposite coasts
of the Atlantic Ocean. Not only is there a great similarity
of shape, but the structural features on either side of the
ocean show a considerable degree of resemblance, and through-
out geological time there has been also a strong likeness
between the animals and plants. Hence it is supposed that
America split oft from Europe during the Tertiary period,
the rift beginning in the south and gradually extending
northwards. This movement is considered to be still in
progress, and to have been sufficient to become evident in the
successive determinations of the longitude of Sabine Island in
North-east Greenland, which is thus shown to have moved
westward by nearly a mile in eighty-four years. These
differences in the longitude, however, as Sir Charles Close
has pointed out (2), are all within the limits of the probable
error, and are not sufficient to constitute a proof of the westerly
drift of Greenland.
THE THEORY OF CONTINENTAL DRIFT 223
It is true that the distribution of a number of animals
and plants which are found on either side of the Atlantic
could be explained much more readily on the hypothesis
that America and Europe-Africa were in contact not long
ago, but this involves the supposition of a formerly wider
Pacific Ocean, against which must be set the distribution of
a number of animals and plants common to both shores of
this ocean.
The second part of the theory is that independent of the
drift of the continents the earth's axis of rotation is under-
going a progressive change, which since early Palaeozoic
times has brought the North Pole from high southern latitudes
via the Pacific Ocean to its present position. The arguments
rest entirely on deductions from the distribution of climatic
zones in past times, and especially on the location of the
Permo-Carboniferous glaciation. We will return to this
point in the next chapter.
The chief weakness of Wegener's theory is the inadequacy
of the forces which he postulates to move the continents.
These are twofold a force directed towards the equator and
a force directed towards the west. The force directed towards
the equator depends on the facts that the earth is not a true
sphere, and that a continent consists of a floating mass of sial,
the centre of gravity of which is higher than the centre of
gravity of the displaced sima, or centre of buoyancy of the
sial. Anywhere except on the equator and at the poles, a
plumb-line on the surface of the earth points, not directly
towards the centre of the earth, but to a part of the equatorial
plane at a somewhat lesser depth than the centre. If the earth
were a true homogeneous and non-rotating sphere, a plumb-
line in latitude <, if produced downward, would make an
angle < with the plane through the equator, but on the earth
as actually constituted the angle would be greater than </,
say, </>*. In a very deep mine the plumb-line, if produced,
would pass nearer to the equator than if produced from the
surface, that is, it would make with the plane through the
equator an angle between <f> and <f>\ In a large mass of sial
floating in a layer of sima, therefore, the downward force
due to the attraction of the main mass of the earth on the sial,
which can be considered as concentrated at the centre of
gravity of the sial, is not exactly opposite in direction to the
224 CLIMATE THROUGH THE AGES
upward force due to the displaced sima, which can be con-
sidered as concentrated at the centre of buoyancy. The
resultant of these two forces is a small component towards
the equator, which reaches its maximum in latitude 45 and
vanishes at the poles and the equator.
There is no doubt that such a force actually exists, and
if the sima is a true fluid, however viscous, it would produce
a slow movement of the continents towards the equator.
The movement has been calculeited from the not very complete
data available, and has been found to amount to 20 cm.
(8 inches) a year in latitude 45, where it is greatest. But
all this rests on the assumption that the sima really is a fluid,
and that this fluidity persists even in the uppermost coldest
layers. The equatorward force is so small that the resistance
of a quite thin non-fluid layer at the surface would suffice to
overcome it. Jeffreys (3, App. G.) is of opinion that while
the earth may be considered to be a plastic body of zero
strength at depths greater than 450 miles, there is some
evidence that the cooler surface rocks have in fact a finite
though small strength to depths of a few hundred miles. For
instance, the floor of the ocean is strong enough to maintain
the Tuscarora deep. This surface strength would probably
be great enough to overcome the force due to the difference
between the centres of gravity and of buoyancy. According
to Jeffreys, then, it is doubtful whether the force available
is strong enough to move the continents at all, and it becomes
highly improbable that such a small force can raise enormous
mountain-chains. To raise a slice of the earth's crust thousands
of feet against gravity requires an enormous force, much
greater than the small forces due to the difference between
the values of gravity in different parts of the continents. The
forces postulated by Wegener are of the order of one hundred-
thousandth (io~ 5 ) of a dyne per square centimetre, whereas
to elevate the Rocky Mountains a force of about one hundred
million (io 9 ) dynes per square centimetre would be required.
According to Jeffreys' calculations, therefore, the forces
available on Wegener' s theory are about one hundred billion
times too small for the effect which is attributed to them.
Jeffreys sums up " Secular drift of continents relative to the
rest of the crust ... is out of the question. A small drift
of the crust as a whole over the interior, which would be
THE THEORY OF CONTINENTAL DRIFT 225
shown as a displacement of the poles relative to the earth's
surface, is not impossible, but the maximum amount seems
too small to be of much interest."
The forces which tend to produce motion in an east-west
direction are much less clearly defined by Wegener than the
force directed towards the equator ; they appear to depend
mainly on the effects of tidal friction both on the floor of the
sea and within the earth's crust, and to be of the same order
of magnitude as the forces directed towards the equator.
From the point of view of palaeoclimatology, the question of
the east-west movement is of less importance than the question
of the north-south movement. East-west movements of
some of the continents relative to others may affect the annual
range of temperature or the distribution of rainfall to some
extent, but cannot radically change the mean temperature
of the whole belt in any latitude, but rearrangements of the
positions of the continents relatively to the poles can obviously
be made to produce almost any changes of mean temperature
which may be required to fit the evidence.
This, in effect, is what Wegener does. Taking as a basis
the power of free movement of the continents over the face
of the earth, Wegener and Koppen (4) proceed to study the
distribution of climates during the various geological periods,
and to map out the positions of the continents relative to each
other and to the poles which will best fit in with the distribution
of climates, while preserving some continuity from one period
to the next. It is assumed that the earth has been under
solar control throughout, and that the distribution of climatic
zones relative to the poles has always been similar to that
found at present. On either side of the equator there has
always been on the land a belt of rich vegetation represented
by thick coal formations, and in the oceans a high-water
temperature represented by reef-building organisms such as
corals and Rudistes. On either side of this tropical belt there
has always been over the land a zone of deserts. Nearer
the poles another belt of vegetation in temperate latitudes has
formed other coal beds less well developed, with annual
growth rings in the tree stems. Finally, the sites of the poles
have generally, if not always, been occupied by ice, either
inland ice-sheets or floating ice-caps according as the pole
lay on land or in the ocean. These zones at present do not
15
226 CLIMATE THROUGH THE AGES
run strictly parallel with the lines of latitude, and no doubt
there were similar irregularities in the past, but Koppen and
Wegener consider that these were never sufficient to mask the
zonal distribution. It is proposed, first, to run briefly through
the distribution of zones in the different periods according to
this theory, reserving criticism to a later stage.
The Late Carboniferous is the earliest period for which a
good cartographical basis is available according to the con-
tinental drift theory, but some attempt is made to reconstruct
the earlier periods. Thus, in the Algonkian (Late Proter-
ozoic), there was an intense glaciation of North America,
which is therefore considered to have been the site of one of
the poles, while the corresponding dry belt is represented by
the Torridon Sandstone in Scotland and the Dala Sandstone
in Central Norway. In the Early Cambrian [now believed
to be mainly Late Proterozoic] there are more or less doubtful
glacial deposits in Norway, Yangtse (China), South Australia,
India (Salt Range), and South Africa. In Australia the
glacial deposits are followed by thick limestones with reef-
building Archaocyathina, indicating a rapid warming. The
authors find that they cannot indicate the position of the poles
and equator during the Cambrian and Ordovician periods.
In the Silurian, the evidence is a little more definite, but
orientation is still difficult. The occurrence of glacial deposits
is doubtful, but there may have been ice in South Africa.
The equatorial belt, represented by corals in the marine
deposits and poor coal seams over the land, passed through
North America, the British Isles, Central Europe, Northern
Siberia, and possibly Australia. The northern desert zone
lay near Leningrad, in Baffin Bay, and especially in North
America. In the Devonian, there was still ice in South
Africa, but Europe lay farther north than in the Silurian,
the equatorial belt passing through France and Spain, Central
Asia and China. Desert formations such as the Old Red
Sandstone were extensively developed in the northern con-
tinent, and the fauna includes the famous " lung-fish "
(Ceratodus), and a " lung-snail. 55 This continent must therefore
have " had a hot desert climate, whose dry periods were only
occasionally interrupted by thundery rains. 55
^ During the Carboniferous period the conditions are known
in much greater detail, partly because of the interest aroused
THE THEORY OF CONTINENTAL DRIFT 227
by the great glaciation which occurred during this period,
and partly because the majority of the workable coal beds
are of Carboniferous age. The greatest development of
ice deposits occurred in South Africa, India, Australia, the
Argentine and Eastern Brazil, and the Falkland Islands.
These are so thick and extensive that they must be due to
great inland ice-sheets, which Koppen and Wegener consider
can only have formed in the neighbourhood of the poles. If
we suppose the South Pole to have been in South Africa at
that time, while the continents still had their present positions
relative to each other, the most remote of them would still
lie too near the equator to be readily glaciated. Hence the
authors suppose that at this period Pangaea, the original
continent, had not yet been split up into its component parts,
and the glaciated continents were all grouped in contact with
each other round the South Pole. The area covered by the
ice-sheets was so extensive, however, that even this rearrange-
ment does not suffice, and it is considered that the various
glacial deposits are not all of the same age, but that glaciation
followed the location of the moving pole. The Brazilian
deposits are the oldest, the Australian and Indian the youngest.
The South Pole travelled from Antarctica via South America
to South Africa, and thence in a great arc across Australia
back to Antarctica. The position of the equator is deter-
mined by the gieat coal beds in North America, Europe,
and China, and it is found that these lie on a great circle
the centre of which falls in the glaciated region. These
coal beds, therefore, represent the tropical rain-forest, a
conclusion which will be discussed later. Other coal measures
in Alaska, South America, South India, Australia, and
Antarctica are attributed to the temperate rain belts. Between
these temperate coal beds and the main mass of coal are a
number of desert deposits.
During the whole of the Mesozoic period there was little
if any ice action, and the development of coal was also
restricted. On the other hand, there was a great expansion
of the dry belts, especially in North America and Africa.
In the sea, great coral reefs were formed. The authors tacitly
admit the generally accepted opinion that during the Mesozoic
the development of climatic zones was less marked than at any
other period since at least the middle of the Palaeozoic. It was
228 CLIMATE THROUGH THE AGES
during the Jurassic that the continents first began to drift
apart, India and Antarctica splitting off from Africa, and
Australia separating from Farther India. South America did
not become an independent continent until the Cretaceous.
Throughout the Mesozoic, the South Pole lay in Antarctica
and the North Pole in the North Pacific. It may be remarked
here that according to the reconstruction of Cretaceous
geography the British Isles lay in about 20 N., and therefore
had a tropical climate. The British chalk, however, contains
a number of erratic pebbles, which are so alien to the general
character of that deposit that it is difficult to attribute them
to any other agency than floating ice, and this fact seems to
be fatal to Wegener's reconstruction of that period.
The interval of rest during the Mesozoic did not extend
into the Tertiary, which was a period of great mountain-
formation and of great and rapid shiftiiigs of the earth's axis,
which brought the North Pole over the land and caused a
great ice-age in the Northern Hemisphere. There are no
certain traces of ice in the Eocene ; there was a considerable
development of brown coal formation, especially in North
America and Europe, but whereas the European beds are
attributed to the equatorial rain belt, the North American
beds are placed in the north temperate zone. (This should
be compared with Berry's description of the Eocene floras
quoted in the Introduction.) The Oligocene was generally
similar to the Eocene, but in the Miocene we have the beginning
of the ice-age in Alaska, North-east Siberia, and the New
Siberian Islands. This is a very important point which is
referred to again later. In the Pliocene, the Alaskan glaciation
spread over the greater part of North America, including
Greenland. At this time the North Atlantic existed only as a
very narrow rift, and Greenland lay to the north or north-east
of the British Isles. The east winds shown by the late F. W.
Harmer to have blown across the North Sea are attributed to
the glacial anticyclone associated with the American and
Greenland ice-sheets. In the Miocene, the pole lay just
north of Alaska. In the Pliocene it moved rapidly across the
Canadian Arctic Archipelago, and at the beginning of the
Pleistocene it lay near Disco Island off West Greenland.
During the greater part of the Quaternary the North Pole
lay in Central Greenland. In the Baltic Readvance it was
THE THEORY OF CONTINENTAL DRIFT 22 9
near Spitsbergen, and then gradually assumed its present
position. Ice-sheets were formed over the parts of the
continents which lay nearest to the poles. The theory,
therefore, requires a revision of the generally accepted
correlation of the Quaternary deposits ; the Alaskan glaciation,
as we have seen, is placed in the Miocene, along with a some-
what hypothetical glaciation of British Columbia. In the
United States the two earliest glaciations, Kansan and
Jerseyan, are attributed to the Pliocene, the Illinioan is
paralleled with the Gunz, the lowan with the Mindel, the
Earlier Wisconsin with the Riss, and the Later Wisconsin
with the Wurm. Correlation between American and Euro-
pean glacial deposits is admittedly difficult, but this
arrangement of the American sequence cuts right across the
present opinions of most American geologists, which are set
out on page 242.
While the general course of the ice-age depended on the
successive positions of the moving pole, the alternation of
glacial and inter-glacial periods cannot be explained in this
way, and the authors have recourse to the variations in the
obliquity of the ecliptic and in the eccentricity of the earth's
orbit. This part of the theory, and the objections to it, were
dealt with in Chapter V.
We may sum up the results of the investigation of climatic
changes by Koppen and Wegener by giving the positions
which they assign to the North Pole relative to the present
position of Africa. Since Europe has always had almost its
present position in relation to Africa, these figures give also
the various positions of the North Pole relative to Europe.
Latitude of
Period. Position of North Pole. England. Antarctic.
Carboniferous . . 30 N. 145 W. o 75 S.
Permian .... 35 N. ii5W. 15 N. 70 S.
Trias 50 N. 125 W. 20 N. 85 S.
Jurassic .... 47 N. 132 W. 20 N. 90 S.
Cretaceous . . . 47 N. 140 W. 15 N. 85 S.
Eocene 45 N. 160 W. 15 N. 90 S.
Miocene .... 75 N. 150 W. 40 N. ca. 90 S.
Late Pliocene and
Early Pleistocene 70 N. 60 W. 60 N. ca. 85 S.
Table 18. Changes of latitude according to Wegener.
23O CLIMATE THROUGH THE AGES
In the third column I have added the corresponding latitude
of England, and in the fourth column the latitude of the
centre of the Antarctic continent.
The zonal arrangement of climates has persisted throughout
geological time, and though there have probably been minor
fluctuations in the average rainfall over the globe, the
variations of climate in any one area have been governed almost
entirely by the variations of latitude which it has undergone.
In the next chapter we will examine the latter contention in
greater detail.
REFERENCES
(1) WEGENER, A. " The origin of continents and oceans." Transl. by J. G. A.
Skerl. London, 1924.
(2) CLOSE, SIR CHARLES. " The geodetic evidence for the supposed westerly
drift of Greenland." London, Geogr. J., 63, 1924, p. 147.
(3) JEFFREYS, H. " The earth, its origin, history, and physical constitution."
2nd ed. Cambridge, 1929.
(4) KOPPEN, W., UND A. WEGENER. " Die Klimate der geologischen Vorzeit."
Berlin, 1924.
CHAPTER XIV
AN EXAMINATION OF THE CLIMATIC EVIDENCE
FOR CONTINENTAL DRIFT
WE will begin the critical discussion of the views set
out in Koppen and Wegener's book, " Die Klimate
der geologischen Vorzeit," with some further analysis
of the climatic conditions during the Upper Carboniferous
period, beginning with the United States, the British Isles,
and Central Europe. According to the " drift " theory,
these coal beds represent a luxurious tropical rain-forest,
and the equator is therefore drawn as nearly as possible through
the middle of them. The evidences of glacial action which
have been adduced from time to time in close proximity, both
in space and time, to these coal beds are dismissed out of hand
as not genuine. The American evidence, however, seems to
be too well founded to be dealt with in this summary fashion.
Thus S. Weidmann (i) describes conglomerates of Upper
Carboniferous to Permian age in the Arbuckle and Wichita
Mountains of Oklahoma and in Kansas, associated with all
the paraphernalia of glaciation scratched boulders, erratics,
fluted and polished floors, and U-shaped valleys. Some of
the boulders in marine deposits have apparently been carried
by icebergs, and the author attributes the phenomena to
islands in the Late Palaeozoic sea bearing local valley glaciers.
J. A. Taff (2) found boulders up to 20 feet across and 5 or 6
feet thick, 50 miles or more from their source, in the marine
Caney shales of Eastern Oklahoma. " No other competent
means of their transportation than ice presumably heavy
shore ice has been suggested. 55 Similarly, A. P. Coleman
(3) considers that there is good evidence for glaciation in
Oklahoma, Nova Scotia, and Alaska (Thousand Isles). As
regards Nova Scotia, Coleman writes, u It is ... probable
that there were moderate elevations from which, under a cool
climate, glaciers spread out over the plains on which coal
forests had been growing not long before. 55
232 CLIMATE THROUGH THE AGES
In the Squantum tillite near Boston, Mass., there are
massive conglomerates 2,000 feet in thickness, which cover a
considerable area. The chief interest of these beds, apart
from the presence of striated boulders, lies in the associated
" varve " beds (4) banded clays which are similar in all
respects to those formed during the retreat of the Scandinavian
and North American ice-sheets at the close of the Quaternary
glaciation, and also similar to those formed in Australia during
the Upper Carboniferous glaciation. These clays owe their
banding to the seasonal variations in the rate of melting of
glaciers, and are therefore incompatible with an equatorial
climate. In places the banding is disturbed during the
deposition of the shales, probably by the grounding of floating
ice-masses.
Wegener recognises that the Squantum tillites demand
serious consideration, and the effort he makes to explain
them away tacitly implies that they form a very serious
objection to the " continental drift " theory. He considers
the possibility that they are real, but were formed at a very
high level in the Appalachian mountain system, then young
and vigorous, and agrees with the general opinion of geologists
that the chances of preservation of high-level glacial deposits
over a wide area would be very slight. As we have seen, there
are other indications that the glaciers reached low levels.
He therefore concludes that as smoothed floors have not been
found beneath the moraines, the remaining phenomena,
although very suggestive of glacial action, could have
originated in other ways. As all the other evidence indicates
that Boston lay in the equatorial rain zone during the
Carboniferous and in the region of hot deserts during the
Permian, " the glacial nature of these tillites is in irreconcilable
opposition to the numerous climatic traces of another kind which
surround it both in space and time." He therefore says that the
burden of proof that the deposits are really glacial rests with
the opponents of the " drift " theory. The Squantum tillites
are, however, accepted by all American geologists, while
the Caney shales in the Arbuckle and Wichita Mountains
were examined by an impartial observer, J. B. Woodworth,
who concluded (5) that while the striae on the boulders which
he observed were probably not glacial, the transport of the
boulders was almost certainly effected by floating ice,
CLIMATIC EVIDENCE FOR CONTINENTAL DRIFT 233
Farther west, in Colorado (6), there are Middle Carbon-
iferous conglomerates some 6,000 feet in thickness, said to
contain boulders up to 50 feet in diameter, and although no
striated blocks have yet been found, the great size of the
boulders strongly suggests ice action.
C. A. Slissmilch and Sir T. W. E. David (6) discuss in
detail the deposits in Europe which suggest ice action. The
evidence, while not so strong as that from North America,
yet has some interest. Sir Andrew Ramsay was of opinion
that glaciated pebbles occurred in the Permian conglomerates
of England, but this interpretation is not now accepted. The
Millstone grit, although it contains no striated material,
points to enormous denudation. In France, M. Julien has
described large masses of angular breccia in the St Etienne
coal basin, with a thickness up to 800 feet. Striae are extremely
rare, but some have been found. " Vertical roots of
Calamites are seen in the sandstones underlying the breccias,
while their stems, as they pass upwards into the breccia, are
crushed, a phenomenon very suggestive of glacial action. . .
He considers that these c morainic breccias ' were deposited
by glaciers having their origin in the great early formed folds
of the Hercynian ranges which were already rising to the
north." In Germany there is also some rather doubtful
evidence of glacial action, the strongest being a shale bed con-
taining occasional boulders up to a foot or more in diameter,
suggesting the action of shore ice or river ice. The European
evidence, taken by itself, is not very convincing, but in con-
junction with the much stronger American evidence it throws
considerable doubt on the theory that the coal measures aie
the remains of equatorial rain-forests.
So great an authority as A. P. Coleman (7) examined the
distribution and sequence of both Permo-Carboniferous and
Pleistocene glacial deposits from the point of view of the
continental drift hypothesis, and concluded that the latter
fails completely to account for them. The extent of the
Permo-Carboniferous glaciated area was so great that even
if the continents were joined up round the South Pole ice-
sheets would still extend into sub-tropical latitudes. There
is, however, evidence that in all the glaciated areas the ice
reached the sea, and in South America and Australia there
was open sea on both sides of the continent. In any case
234 CLIMATE THROUGH THE AGES
such a gigantic continent as that postulated by Wegener
would be highly unfavourable to glaciation owing to the
difficulty in the supply of moisture.
There is one other piece of evidence in connexion with
the climatic zones of the Permo- Carboniferous which may
be referred to, not so much for its intrinsic importance,
which is small, as because it illustrates the methods too often
adopted by Koppen and Wegener in dealing with items which
do not quite fit their theory. Salt beds occur in Angola,
formerly attributed to the Carboniferous. The authors class
these as Permo-Triassic, because during the Carboniferous
" Angola was too near the South Pole for salt beds to form."
The drift hypothesis has certainly not reached a stage of proof
in which it can be asserted that evidence which does not fit
it is thereby proved to be false.
The next point in the discussion of the " drift " theory
concerns the relative ages of the ice-sheets in the Southern
Hemisphere. According to Koppen and Wegener, the
Brazilian deposits are the oldest, the Australian and Indian
the youngest, and L. Waagen is quoted as the authority for a
statement that the glaciation of Brazil and South Africa
occurred before the development of the Glossopteris flora, that
of Australia after it. The age of the glacial deposits in different
parts of the world is discussed in great detail by Sussmilch
and David (6), and they arrive at very different conclusions.
The succession in New South Wales is taken as the standard
of reference, and from the generalised section given, we may
make out the following simplified series :
p ^ . M ( Glossopteris coal measures, etc.
JL ermian ..... x /-+ i i i i
Crmoidal shales.
Branxton glacial horizon.
Greta coal measures.
Sandstone.
Basalt and tuff.
Horizon of Eurydesma cordatum.
Upper Carboniferous
Gangamopteris muds tone.
Brandon conglomerate.
Tillites, etc.
Varve beds.
Rhacopteris horizon.
Tuffs, fluvio-glacial conglomerates, etc.
CLIMATIC EVIDENCE FOR CONTINENTAL DRIFT 235
There are thus two main glacial series, the older and more
important falling in the lower part of the Upper Carboniferous,
while the upper horizon falls probably at the top of the Upper
Carboniferous. Between the two glacial series we find
Gangamopteris and Eurydesma, while Glossopteris first occurs
above the upper glacial horizon. In Victoria, Tasmania, and
South Australia the main glacial horizon lies below the
Gangamopteris horizon, and probably on the same horizon as
the lower tillites of the New South Wales series. Tillites on
the Irwin River in Western Australia are probably somewhat
younger, falling in the Upper Carboniferous near the zone of
Eurydesma cordatum. In Western Australia there are two
glacial horizons, both older than the Glossopteris flora.
In India (Salt Range) the tillites are associated with
Enrydesma, and may be referred to the Upper Carboniferous.
In South Africa the Dwyka tillites are likewise associated
with Eurydesma, and the thick series probably belongs mainly
to the Upper Carboniferous. In the Falkland Islands the
boulder beds are Upper Carboniferous and occur beneath
beds containing Gangamopteris and Glossopteris., and appear to
come at the base of the Per mo- Carboniferous. Finally, the
South American tillites occur just beneath coal measures with
an exclusively Gangamopteris flora, and may be attributed to
Upper Carboniferous. There was very little glaciation in the
Lower Permian anywhere.
Thus the evidence, as set out in 1919 by two unbiassed
observers, and still further emphasised by Sir T. W. Edgeworth
David in a series of lectures in 1926, is to the effect that there
was very little difference in age between the main glaciations
of the different areas in the southern group. All of them are
certainly older than the Glossopteris flora, and the only feature
which supports the theory of a moving pole is the slight
recrudescence of glaciation in the highest Carboniferous or
possibly lowest Permian of New South Wales (Bolwarra and
Branxton beds).
Sussmilch and David note the close relationship between
the areas of folding and the glaciated areas, and also the
enormous thicknesses of volcanic tuff. They further note
that the period of glaciation of Eastern Australia was almost
exactly synchronous with the period of great orogenic move-
ments, also in Eastern Australia, and they suggest that the
236 CLIMATE THROUGH THE AGES
glaciations were due largely, if not entirely, to the presence of
high mountains and of great quantities of volcanic dust.
The recognition of glacial deposits and of ice floating in the
sea in the middle of the northern belt of coal measures, pro-
foundly modifies the problem of the habitat of coal-building
plants. For a long time it was believed that peat, which
is the first stage in the formation of coal, could not form in a
hot country owing to the rapidity of decomposition at high
temperatures, and that the modern representatives of the coal
measures are the peat-bogs of moist temperate regions. The
reversal of this view is mainly due to the influence of H.
Potonie, who described a swamp in Eastern Sumatra in which
peat is actually being formed at the present moment, almost
entirely from the fallen leaves of evergreen trees. Koppen
and Wegener consider that the coal beds extending through
the Eastern United States, the British Isles, Central Europe,
and China represent the remains of similar peat formed by
tree ferns and other highly developed vegetation in equatorial
swamps of the Carboniferous period, while the coal beds of
Spitsbergen in the north, and of Australia, South Africa,
South America and Antarctica in the south, represent the peat
of the temperate rain belts. It seems very doubtful whether
we are entitled to draw this conclusion solely from the nature
of the vegetation composing the coal beds. The absence of
annual rings of growth in the Carboniferous vegetation of the
north temperate belt may be due, as E. Antevs (8) points out,
to the comparatively low organisation of the flora, which only
formed annual rings under extreme conditions. If, as
suggested in Chapter V., the coal beds of the Northern
Hemisphere were formed during a period of high eccentricity
of the earth's orbit, at the times when northern winter was
in perihelion, we need not expect to meet annual rings until
we reach very high latitudes, such as Spitsbergen. Through-
out the Lower and Middle Carboniferous, almost to the
beginning of the ice-age in Australia, the flora was extra-
ordinarily uniform over the whole world (9), Europe, Asia,
Africa, America, Australia all had the same flora. The
Glossopteris flora is of later date than the northern coal beds ;
it represents a different stage in the evolution of plant life,
but not necessarily a difference of climate. This flora was
able to spread across the equator into the Northern
CLIMATIC EVIDENCE FOR CONTINENTAL DRIFT 237
Hemisphere, a fact which suggests a higher organisation than
that of the older flora rather than a special adaptation to cold
climates. The position is similar to that at the end of the
Mesozoic, when a flora of modern type originated in high
northern latitudes and spread over the world, and in the latter
example there is no suggestion of a sweeping change of latitude.
The distribution of desert deposits salt, gypsum, and
desert sandstone also requires a closer examination. In
Fig. 26 the occurrence of these indications of desert action in
D desert sandstones. S salt. G gypsum. Dotted area present deserts.
Fig. 26. Carboniferous arid Permian deserts in relation to the
present desert areas.
the Carboniferous and Permian periods is shown by their
initial letters S, G, and D. The distribution of present deserts
is indicated by stippling. It is seen that with the exception
of a few salt deposits in Europe, Western Asia, and the east
of North America, the late Palaeozoic desert deposits in the
Northern Hemisphere fall entirely in the present desert areas.
The present deserts of the Southern Hemisphere were, however,
unrepresented in the Carboniferous and Permian. This
presumably indicates that during those periods the present
Southern Hemisphere was moister than at present, either
because of the shifting of the continents relative to the poles,
or because the existence of the ice-sheets caused a " pluvial
period >J in the surrounding lands.
These considerations show that the theory of " continental
drift " is not so complete and irresistible an explanation of the
peculiar distribution of climate in the Carboniferous and
238 CLIMATE THROUGH THE AGES
Permian periods as Koppen and Wegener seem to think.
The greatest difficulty is presented by the glacial deposits of
North America, especially the " varve clays," and these form
almost as great an obstacle to the " drift " theory as the glacial
deposits of equatorial Africa form to the assumption that the
continents held their present positions. The only complete
answer to the " drift " theory, however, would be a demons-
tration that the climatic events of the Carboniferous and
Permian were the logical result of the distribution of land,
especially high land, and sea during that period, the poles
being supposed to have kept their present positions.
In the Mesozoic and Tertiary periods the discrepancies
between the past and present distribution of climatic zones
D desert sandstones. S salt. G gypsum. Dotted area present deserts.
Fig, 27. Mesozoic deserts in relation to the present desert areas.
are far less striking. There was little ice, except towards the
close of the Tertiary, and on any theory the climatic zones
were much less developed than at present. It is not possible
to discriminate between equatorial and temperate coals, and
hence the desert deposits offer practically the only evidence
of zoning. But as Fig. 27 shows, the distribution of deserts
during the Mesozoic was very similar to the present distribution.
The chief exceptions were in Europe and the South-eastern
United States. The rainfall of Europe would be greatly
decreased by the displacement of the Icelandic minimum
far to the north, while the present comparatively heavy rainfall
of the South-eastern United States is due almost entirely to
CLIMATIC EVIDENCE FOR CONTINENTAL DRIFT 239
moisture derived from the warm Gulf of Mexico. In fact,
the distribution of deserts shown in Fig. 27 is very nearly
what we should expect during a period in which low continents
and a free oceanic circulation reduced the temperature
gradient between low and high latitudes almost to its least
possible value. The distribution of Rudistes in the Cretaceous
period is the strongest point in favour of the displacement of
the poles. Reef-building forms reach their greatest develop-
ment in the deposits of the great Tethys Sea, the Mediterranean
of the Cretaceous, which, in contrast with the present Mediter-
ranean, was open to the Indian Ocean and received a constant
supply of warm water. They are also found in the north of
South America, Mexico, and the West Indies, the range
extending to the north coast of the Gulf of Mexico, the limits
in America so far discovered being 5 and 30 N. The most
interesting point is that outside the normal range dwarf forms
are found, in latitudes 50 to 55 N. in Europe, but in latitudes
5 to 20 S. in East Africa. If the distribution of Rudistes was
really governed by the sea temperature and we have no
reason to suppose that it was not the assumption of the
present position of the continents requires a much greater
northward displacement of the thermal equator than occurs at
present, and especially a cold current along the east coast
of Africa instead of along the west coast (see Fig. 28) . Probably
the only cause which could produce such a great displacement
of the thermal equator would be an extensive glaciation of the
Antarctic continent, while the north polar regions were prac-
tically free of ice. The glaciation of the Antarctic continent,
according to C. S. Wright and R. E. Priestley (10), began at
least as early as the beginning of the Tertiary, since immediately
above the Cretaceous beds at Cape Hamilton, Graham Land,
there occurs a "moraine-like mass, some metres in thickness,"
which contained angular fragments of crystalline rocks foreign
to the locality. This deposit is evidently glacial, though it
may mean nothing more than a local valley glacier ; it
cannot be younger than Eocene and may be uppermost
Cretaceous. The onset of cold conditions during the
Cretaceous is also indicated by the Cockburn Island sandstone,
which was " crowded with pygmaean forms of life such as
might be expected to result from the encroachment of colder
conditions upon a marine fauna long developed in, and
240
CLIMATE THROUGH THE AGES
CLIMATIC EVIDENCE FOR CONTINENTAL DRIFT 241
habituated to, more genial conditions." There was shore ice
or perhaps icebergs in the Cretaceous sea over South Australia,
the erratics covering a wide area.
In the Northern Hemisphere there is no direct evidence of
the occurrence of extensive ice-sheets during the Cretaceous.
There is, however, some evidence of drift ice in the remarkable
erratic blocks found in the English upper chalk. These blocks
are probably due to transport by shore ice rather than by
icebergs, but in any event their presence is a serious obstacle
to the belief that at that time England lay in about 20 N.
latitude. The problem is very similar to that of the Upper
Carboniferous, since in both periods the biological evidence
points to high temperatures, while the character of the deposits
indicates ice action in the neighbourhood.
The most complete refutation of the drift hypothesis is
given by the fossil plants of the Mesozoic and early Tertiary.
E. W. Berry (n) states that " the distribution of the known
fossil Arctic floras with respect to the present pole proves
conclusively that there could have been no wandering pole."
R. W. Chaney (12) gives maps of Eocene " isoflors " in the
northern hemisphere, showing the limits of sub-tropical,
temperate and cool temperate floras as irregular lines
surrounding the pole. These enclose elliptical areas which
approach the pole most closely round about longitude o and
1 60 W., z.., in the openings between the circum-polar
continents. The limit of the " cold-temperate " flora has an
average latitude of about 78 N. centred four degrees from the
pole. The limit of the " temperate " flora has an average
latitude of 62 N. and is also centred 4 from the pole, that
of the sub-tropical flora has an average limit of 42 N.
and is centred 3 from the pole. The displacements of
the first two are towards Bering Strait, that of the third
towards Asia. The present mean annual isotherm of 40 F.
is also displaced about 4 from the pole in the direction
of western Siberia. The agreement could hardly be better,
and practically amounts to proof that in the Eocene the
North Pole occupied its present position and not a point in
the North Pacific as shown by Wegener.
The correlation of the glacial stages during the last ice-age
in America and Europe according to Koppen and Wegener
differs from that generally accepted. The most important
16
242 CLIMATE THROUGH THE AGES
point deals with the date of the main glaciation in Alaska,
which Koppen places in the Miocene. W. H. Dall, the
representative of the American Geological Survey in Alaska,
does not accept this correlation as possible. He gives the
following sequence at Nome (13) : coal formation during
Eocene and Oligocene, locally covered by marine fossiliferous
Miocene, indicate a mild climate in the early Tertiary. During
the Miocene, the land sank for the most part below sea-level,
and there was much volcanic activity ; the climate became
cool temperate in the Early and Middle Miocene, but warmed
up again in Late Miocene. Since the Miocene, the land has
risen continuously. In the Pliocene the climate was moderate,
and it was not until the Quaternary that Arctic temperatures
set in, to persist until the present day.
The direct correlation of glacial deposits in North America
and Europe is difficult, and we have to rely mainly on such
features as the determination of ages by the relative depths
to which the various deposits have been weathered. A careful
study of all lines of evidence has recently been made by
Osborn and Reeds (14), who accept in the main the results of
F. Leverett (15), based on a comparison of the depth of
weathering of glacial deposits and on the texture and fauna
of the loess. This correlation gives :
I. Glaciation Gunz, Scanian, Nebraskan, Jerseyan.
1 . Interglacial Gunz - Mindel, Norfolkian,
Aftonian.
II. Glaciation Mindel, Kansan.
2. Interglacial Mindel-Riss, Yarmouth.
III. Glaciation Riss, Illinoian.
3. Interglacial Riss-Wurm, Sangamon.
IV. Glaciation Wurm, Wisconsin.
This correlation is in fact almost inevitable. In Europe,
the Mindel-Riss interglacial is distinguished from the Gunz-
Mindel and the Riss-Wurm by its much greater length, and
by a temperature which probably rose higher than the present
temperature, by being in fact an " interglacial " rather than
an " intraglacial " period. Similarly, in America, the interval
between the Kansan and Illinoian was much greater in length
than the remaining interglacial periods. Since the work of
CLIMATIC EVIDENCE FOR CONTINENTAL DRIFT 243
de Geer and Antevs has shown that the latest glaciations in
North America and Sweden were contemporaneous, this
estimation of the age of glacial deposits by the depth of
weathering seems to require that the long Yarmouth stage
be correlated with the long Mindel-Riss interglacial. The
astronomical correlation of the glacial stages has been dis-
cussed in Chapter V. ; it is not an essential part of the theory
of continental drift and need not be referred to again here.
It is always a useful test of a new theory to examine how
far facts which come to light after the theory has been completed
fit into place. An opportunity for such a test is afforded by
the study of the climatic history of Antarctica given by Wright
and Priestley in the volume of results of the British Antarctic
expedition dealing with glaciology (10). According to
Koppen and Wegener's reconstructions, Antarctica as a whole
has been in high latitudes since at least the beginning of the
Carboniferous period. Of course the Antarctic continent is
large, and if the pole was at one side of the continent the
opposite side would extend into temperate latitudes, so I
have tried to pick out on Wegener's charts the particular
point to which Wright and Priestley's climatic indications
refer. In this way I have obtained Table 1 9 (see page 244) .
From this table we see that during the Upper Carboniferous
Antarctica had more or less the climate appropriate to its
latitude according to Wegener. In the Permian and Triassic,
however, while the continent was drifting into continually
higher latitudes, the climate was steadily ameliorating. The
flora of the Jurassic rocks of Graham Land is extremely rich,
and closely resembles that of the Jurassic rocks of Europe,
which according to Wegener then lay in about 15 N. latitude.
The Cretaceous fauna, while still rich, contains at least one
bed of dwarf forms suggesting the oncoming of colder con-
ditions. The only determinable plant fossil, a Sequoia, finds
its nearest relative in the Cretaceous of Europe and Greenland.
It is evident that the climate of Antarctica throughout the
Mesozoic was quite incompatible with its latitude according
to Wegener's theory, unless we assume in addition a great
amelioration of the polar climate. For the Mesozoic, there-
fore, the theory of continental drift presents no advantage over
any other theory. The same applies to the warm climate
of the Oligocene. The doubtful glaciation of the Eocene
244
Period.
Upper
Carbon ifcrous .
Permian .
Triassic.
Jurassic.
Cretaceous.
Eocene.
Oligocene.
Miocene.
Pliocene.
Pleistocene.
CLIMATE THROUGH THE AGES
Locality.
South Victoria
Land
and
Adelie Land.
Graham Land.
Graham Land.
Climate.
Temperate to hot or
cold desert. No
definite evidence of
glacial conditions, 1
but strong evidence
of seasonal climate.
Sub-tropical to warm
temperate.
Temperate to warm
temperate.
Graham Land. First Glariation (?).
Seymour Island.
Cape Hamilton.
Cape Adare.
Campbell Island.
Cockburn Island.
General.
Sub-tropical to temper- f
ate becoming frigid. J
Temperate (?).
Frigid.
Maximum extension of
ice.
Latitude
according
to Wegener.
65 S.
75 S-
85 S.
70 S.
75 S.
6o
80 S.
50 s.
65 S.
Table 19. Variations of climate in Antarctica.
is not very good evidence ; from the description given it
resembles the moraine of a mountain glacier descending a
narrow valley, and it is certainly of far less importance than
the Carboniferous glacial deposits of North America. Campbell
Island is not really Antarctic at all, and the cold climate of the
Pliocene and Quaternary would be expected on any theory.
We may also compare the climatic history of Australia
according to C. A. Sussmilch (16) with its position according
to Koppen and Wegener. Taking the mean of their figures
for Perth, Cape York and Hobart, we find that the present
latitude is 29 S. In the Carboniferous the latitude of
Australia was 59 S. and the climate, warm at first, became
very cold by Mid-Carboniferous. In the Permian the
latitude was 71 and the climate cold at first, becoming
1 Wegener fits the Antarctic continent into the Great Australian Bight.
According to Sir T. W. Edgeworth David, the Australian ice-sheet radiated from
a point to the south-west of Tasmania, which according to Wegener 's recon-
struction would be in the Antarctic. The non-glaciation of the Antarctic in the
Upper Carboniferous, if confirmed by further research, will be a strong point
against Wegener 's reconstruction.
CLIMATIC EVIDENCE FOR CONTINENTAL DRIFT 245
warmer. So far the agreement is satisfactory. In the
Trias the latitude was 68 and in the Jurassic 65, but
according to Siissmilch the climate was probably warmer than
to-day. In the Cretaceous Australia had moved north, to
57, but the climate was colder. In the Miocene the latitude
was 43 but the climate was at least 10 F. warmer than
to-day. In the early Quaternary Australia moved to 54 S.
and the climate became colder, but not very cold. There
is in fact no agreement between the climatic changes in
Australia and the path of the South Pole according to Koppen
and Wegener.
The evidence of the pre- Carboniferous deposits is still
very meagre. The constitution of the slate-greywacke
formation of Robertson Bay, South Victoria Land, which is
of Late Proterozoic or very early Palaeozoic age, strongly
suggests the action of alternate freezing and thawing, and
these deposits may be the Antarctic representatives of the
Late Proterozoic-Early Cambrian glaciation. Later in the
Cambrian we have evidence of a moderately warm sea
stretching nearly or right across Antarctica, in the form of
thick limestones very rich in reef-building Archtfocyathina.
Compared with the forms from Australia, however, all the
Antarctic forms are either embryonic or dwarfed, indicating
that they lived in colder and presumably more southern seas.
No reliable evidence is yet available from the Silurian or
Devonian periods. It seems, therefore, that the chief, perhaps
the only, justification for the theory of continental drift rests
on the distribution of climatic zones during the Upper
Carboniferous period.
REFERENCES
(1) WEIDMANN, S. " Was there a Pennsylvanian-Permian glaciation in the
Arbuckle and Wichita mountains of Oklahoma?" J. Geol., Chicago,
31, 1923, p. 466.
(2) TAFF, J. A. " Ice-borne boulder deposits in Mid-Carboniferous shales."
Bull. Geol. Soc. Amer., 20, 1908, p. 701.
(3) COLEMAN, A. P. " Late Palaeozoic climates." Amer. J. Set., 9, 1925, p. 195.
(4) SAYLES, R. W. " Seasonal deposition in aqueoglacial sediments." Cam-
bridge, Mass., Mem. Mm. Comp. <W., 47, 1919, No. i.
(5) WOODWORTH, J. B. " Boulder beds of the Caney shales at Talihina,
Oklahoma." Bull. Geol. Soc. Amer., 23, 1912, p. 457.
(6) SUSSMILCH, C. A., and T. W. E. DAVID. " Sequence, glaciation, and
correlation of the Carboniferous rocks of the Hunter River district, New
South Wales." Sydney, J. R. Soc. N.S. Wales, 53, 1919, p. 246.
246 CLIMATE THROUGH THE AGES
(7) GOLEMAN, A. P. " Ice ages and the drift of continents." J. Geol., Chicago,
4*> 1933, P- 4<>9-
(8) ANTEVS, E. " The climatologic significance of annual rings in fossil woods.
Amer. J, Sri., 9, 1925, p. 296.
(9) ZEILLER, R. " Les provinces botaniques de la fin des temps primaires."
Rev. gen. sciences, 8, 1897, p. 5.
(10) BRITISH (TERRA NOVA) ANTARCTIC EXPEDITION, 1910-1913. " Glaciology,"
by C. S. WRIGHT and R. E. PRIESTLEY. London, 1922.
(i i) BERRY, E. W. " The past climate of the North Polar region." Washington,
Smithson. Misc. Coll., 82, 110. 6, 1930.
(12) CHANEY, R. W. "Tertiary forests and continental history." New York,
Bull. Geol. Soc. Amer., 51, 1940.
(13) DALL, W. H. " Pliocene and Pleistocene fossils from the Arctic coast
of Alaska and the auriferous beaches of Nome, Norton Sound, Alaska."
U.S. Geol. Survey, Prof. Papers, 125 C., 1920, p. 23.
(14) OSBORN, H. F., and G. A. REEDS. " Old and new standards of Pleistocene
division in relation to the pre-history of man in Europe." Bull. Geol.
Soc. Amer., 33, 1922, p. 411.
(15) LEVERETT, F. " Comparison of North American and European glacial
deposits." s. Gletscherk., 4, 1910, p. 241.
(16) SUSSMILCH, C. A. " The climate of Australia in past ages." J. Proc. R.
Soc. N.S. Wales, 75, 1941, p. 47.
CHAPTER XV
THE CLIMATE OF THE UPPER CARBONIFEROUS
GLACIAL PERIOD
WE have now to look more closely at the geographical
and climatic conditions of the Upper Carboniferous,
in order to see if the distribution and eJevation of
the land-masses were such that ice-sheets might conceivably
have developed in low latitudes, while at the same time a
comparatively mild climate obtained farther north. Fig. 29
gives a rough reconstruction of the geographical conditions on
the supposition that the continents were in their present
positions. This reconstruction is based on that given by
Th. Arldt (i) with some alterations to include the results of
later work. In the Northern Hemisphere we find three small
continents : Nearctis, a primitive North American continent ;
North Atlantis, including Greenland and Western Europe ;
and Angaraland, occupying part of the present Siberia.
Nearctis and North Atlantis were connected by a land-bridge
in about latitude 50 N. South of these three continents the
Tethys Sea, the forerunner of the Mediterranean, extended
east and west from New Guinea to Central America, sending
an arm between North Atlantis and Angaraland to the Arctic
Ocean. This Tethys Sea was bounded on the south by the
great continent of Gondwanaland, extending in a huge
irregular crescent from South America to Australia, and from
20 N. to 40 S. In connexion with the great extent of
Gondwanaland from west to east, it may be remarked that the
Upper Carboniferous marine fauna of South-eastern Australia
resembles that of South Africa, while the fauna of Western
Australia is quite different and resembles that of the Tethys
Sea. This indicates that there was a continuous land barrier
separating the gulf west of Australia from the seas south-east
and east of Africa.
The principal difference between the land and sea dis-
tribution of the Middle and that of the Upper Carboniferous
seems to be that in the former Gondwanaland was not
247
248
CLIMATE THROUGH THE AGES
CLIMATE OF UPPER CARBONIFEROUS GLACIAL PERIOD 249
continuous from South America to Australia, but was probably
broken up into three or perhaps four separate land-masses
by straits leading from north to south. These are indicated
by the broken lines of Fig. 29. By allowing free circulation
between the waters of the Tethys Sea and those of the Southern
Ocean, these breaks in the land barrier would raise the tem-
perature of the Southern Hemisphere considerably, and help
to account for the great climatic difference between the
Middle and Upper Carboniferous.
We know that the Carboniferous was a period of great
mountain-building. The mountain ranges followed two
main directions, north-south and east-west. A range followed
the south coast of North Atlantis into the Mediterranean
region. The site of the Alps was occupied by the Carnic
range of Mont Blanc and there were other ranges in the
Caucasus and the Dobrudja. In the west, the Pyrenees and
Asturia were mountainous. Farther north a range or a series
of ranges ran from Bohemia first northward through the
Sudetes and then westward through Germany. In the west
we have the Armorican chain through Brittany, South
England, Wales, South Ireland, and beyond into North
Atlantis. In North America the Appalachian Mountains
were forming, turning westwards in the south to the mountains
of Oklahoma and South Arkansas. Other north-south
ranges formed the west coast of Nearctis, from Colorado
northwards. In Asia there were a number of east-west ranges
somewhat similar to the present systems, bounded on the
west by the Urals and the Volga Sea, on the south by the
Tethys Sea. The mighty continent of Gondwanaland was
bounded on the west by the Proto-Cordilleras and the
mountains of the Pampas, extending to the Falkland Islands.
In the south of Africa there was another chain, and a great
range ran along the whole eastern coast of Australia and the
present East Indies. These mountain systems are shown by
the heavy lines in Fig. 29. Between the mountain ranges in
the Northern Hemisphere there were, for the most part, wide
moist valleys open to the sea, the home of a rich vegetation.
Opinions differ about the structure of the main area of
Gondwanaland, i.e., whether it consisted of an extensive
high plateau or a series of mountain ridges. It is generally
agreed, however, that the Upper Carboniferous was a period
250 CLIMATE THROUGH THE AGES
of great mountain-building and the general elevation was
probably high. The great thickness of the Upper Carbon-
iferous of South Africa, for example, points to rapid
denudation, suggesting a large area of high ground in the
interior of that continent. The fact that the ice-sheets
spread out from a line near the equator shows that initially
at least the ground was highest there, and may well have been
a ridge 10,000 or 15,000 feet above sea-level.
Finally, it has to be remarked that the Upper Carboniferous
was a time of intense volcanic activity, and especially in
Australia, great thicknesses of agglomerates point to numerous
explosive eruptions from which we may infer the presence of
great quantities of volcanic dust in the atmosphere, forming
a veil which, as Humphreys has shown, would be very effective
in shutting out the solar radiation, while it would allow the
terrestrial radiation to escape with little hindrance (see
Chapter VI.).
At the beginning of the Upper Carboniferous there appears
to have been a general decrease in the temperature of the
seas, indicated by an impoverishment of the fauna and flora
resulting from the extinction of a number of animals and the
withdrawal of the coral boundaries towards the equator.
The land plants also suffered changes, and the introduction
of holometabolism in insects (i.e., the pupa stage) is attributed
by A. Handlirsch (2) to this decrease of temperature. Sir
T. W. Edgeworth David considers that the mean temperature
of the tropical oceans decreased by about 10 F., and it
is probable that the oceans were ice-covered in high latitudes.
We can now attempt to reconstruct the distribution of the
meteorological elements. Over the open Pacific Ocean there
is no reason to suppose that the system of pressure and winds
was appreciably different from that prevailing now. Hence
we postulate a great equatorial current setting westward
towards the eastern coast of Gondwanaland, with a temperature
of about 70 F. The configuration of this coast of Gond-
wanaland appears to have been very favourable for concen-
trating the warm current and directing it into the Tethys Sea,
more favourable even than the present configuration of the
coast of America for concentrating the equatorial currents of
the Atlantic in the Northern Hemisphere. A very warm
current, with a temperature initially in the neighbourhood
CLIMATE OF UPPER CARBONIFEROUS GLACIAL PERIOD 25!
of 70 F., must have flowed through this narrow inter-
continental sea. The supply of warm water would have been
large enough to give this warm current a considerable velocity
perhaps fifty miles a day enabling it to conserve its heat
over a long distance. Part of this warm current turned
northward through the Volga Sea and brought favourable
conditions to Northern Russia, where the Fusulinse appear
to indicate a temperature similar to that of the present
Mediterranean (3), and to Spitsbergen and the Arctic Ocean.
The remainder of the warm Tethys current travelled on
between the Americas, and finally emerged again into the
Pacific Ocean. Evidently there is no difficulty in accounting
for the corals of the Tethys and Volga Seas or the rich
vegetation of the valleys opening off them.
In the Southern Ocean also, between the horns of the great
crescent of Gondwanaland, there is no reason to suppose
that the surface temperatures differed greatly from those
prevailing at present in the same latitudes of the South Indian
Ocean. The absence of the great masses of floating ice
derived from the Antarctic would have tended to raise the
temperature of the whole ocean, but, on the other hand, the
Southern Ocean was mainly limited in the north by land in the
neighbourhood of 40 S. instead of extending to the equator,
and the absence of the supply of equatorial warm water
would tend to balance the absence of the supply of ice. We
have also to take into account the volcanic dust veil. At
present the mean temperature of the ocean surfaces in latitude
40-50 J is about 50 F., and we shall probably not be far out
if we take the same figure for the temperature of the surface
water south of Gondwanaland at the beginning of the Upper
Carboniferous glacial period. When the land ice reached the
sea over wide fronts, the temperature of the surface must
have fallen much lower.
One other point about the distribution of the seas is worthy
of notice, namely, the long gulf extending from the Arctic
regions into the heart of Eastern America, that is, into the
only region outside Gondwanaland which appears to have
been indubitably and severely glaciated during the Upper
Carboniferous.
P. Lasareff (see Chapter III.) made an experimental
reconstruction (see page 78) of the ocean currents of the
252
CLIMATE THROUGH THE AGES
Middle Permian (Fig. 30). This agrees in general with
Fig. 29, especially in the warm current through the Tethys
Sea and the branch across the North Pole. The Gulf between
Nordatlantis and Nearctis is not shown (this of course may be
due to the time-difference) but instead there is a cold current
from the pole running down the western coast of Nearctis,
Fig. 30. Ocean currents of Middle Permian. After Lasareff.
which would be equally effective in giving a more severe
climate to the present North America than that of Europe.
We have now to discuss the system of the winds over
Gondwanaland and the neighbouring seas. The " planetary "
circulation of the atmosphere (Chapter II.) requires a belt
of low pressure over the equatorial regions, a series of
CLIMATE OF UPPER CARBONIFEROUS GLACIAL PERIOD 253
anticyclones in about latitudes 30 north and south, followed
by belts of low pressure and storms in temperate or sub-polar
latitudes. This system is modified by the land and sea
distribution, which gives a tendency for high pressure over the
land in winter and over the sea in summer. The geographical
disturbance of the planetary circulation is now so great over
Asia that the anticyclone does not develop there at a)l in
summer, while in winter it attains a great intensity and is
displaced some distance north of 30 N. The deflection of the
entire Equatorial Current northwards into the Tethys Sea
would probably suffice to maintain the temperature of middle
latitudes north of Gondwanaland permanently above that of
middle latitudes south of Gondwanaland, introducing an
effect of permanent summer in the Northern Hemisphere and
permanent winter in the Southern Hemisphere. We should
expect to find a permanent low-pressure area over the Tethys
Sea and the Volga Sea, while the normal sub-tropical anti-
cyclone was developed oft the southern coast of Gondwanaland.
Under these conditions, with an area of high pressure
to the south of the equator separated by a very long and
lofty continent from an area of low pressure north of the
equator, what would be the wind system over the plateau ?
We have no close parallel at present for guidance ; the
nearest approach is found over Asia and the Indian Ocean
during the south-west monsoon, but most of India is now
at a comparatively low level, forming a plain out of which
the Himalayan ridge rises steeply to a great height. The
highest temperature and lowest pressure are found near
Jacobabad to the south of this ridge. A large portion of the
air which enters India from the high pressure area to the south
is accordingly deflected to flow westward parallel with the
mountain barrier ; it is only in the north-east that the air
crosses at least the Khasi Hills (giving the enormous rainfall
of Cherrapunji) and possibly the main ridge of the Himalayas.
If the area of lowest pressure were to the north of the latter
range, would the stream lines run directly across it ?
The power of air currents to cross high ridges of land
probably depends to a large extent on the steepness of the
slope. The west-south-west winds from the Arabian Sea
are able to cross the Western Ghats, for the greater part of their
length more than 4,000 feet in height (4), and descend on the
254 CLIMATE THROUGH THE AGES
other side as a dry wind. The obstacle presented by the
Himalayas, exceeding 12,000 feet, may be due quite as much
to the steepness of their southern slopes as to their great
height. If the ground sloped more gradually from Southern
India or beyond to the Tibetan plateau, it seems probable
that the height would be a less serious obstacle.
Let us now suppose that the discussion of this first problem
has shown that the air, starting from the Southern Ocean as
a powerful south-east trade wind and changing to south-west
as it crosses the equator, will climb steadily up the surface of
the high ground, and, crossing the crest, will descend its
northern slope. Under these conditions, what would be the
general temperature and weather over the area ? The air
starts at a temperature of about 50 F. and a humidity
approaching saturation ; if the temperature of the plateau
surface is mainly above 50 F., the air will not part with its
moisture readily and the cloud amount will be small, but if
the temperature of the surface is below 50 F., the sky will be
mainly overcast and the precipitation heavy.
Here again much seems to depend on the topography.
If the southern margin of the continent was formed by a
wide plain at a low level, the air would not at once be forced
to rise sufficiently to develop an extensive cloud layer, the
low land surface would therefore be exposed to intense
insolation and would be very hot, and the air would be
warmed up by contact with it to such an extent that it might
rise to very high levels before it again became saturated. If,
on the other hand, the ground rose fairly steeply from the
sea to an elevation of 2,000 feet or so, the formation of
a thick cloud layer would begin before the air had time to
warm up.
In North-eastern India, where the air crosses the Khasi
Hills and enters at least the foot-hills of the Himalayas, the
cloudiness during the south-west monsoon is very great.
The mean cloudiness (8 a.m.) at Cherrapunji, Darjiling,
and Mercara during June, July, and August is nine-tenths
of the sky, while the relative humidity is 95 per cent. Let
us suppose that, the southern coast of Gondwanaland being
sufficiently steep to raise the incoming trade wind above
the saturation level, the mean cloudiness was nine-tenths.
The temperature of the air at any point is governed by
CLIMATE OF UPPER CARBONIFEROUS GLACIAL PERIOD 255
the quantity of heat which it originally contained, plus the
heat which it gains mainly from the surface, minus the heat
which it loses, mainly by radiation. In the conditions
postulated, the cloud would probably be rather thick ; let
us assume that it had a mean density of 3. Then according
to the measurements of B. Haurwitz (see Chapter VIL) the
amount of radiation penetrating a cloud cover of nine-tenths
would be barely half that received with a cloudless sky. This
is less than the average amount received at present in latitude
4O-50. If we take into account the loss by scattering,
especially great during the Upper Carboniferous because
of the great amount of volcanic dust, which would be more
or less proportional to the total solar radiation and therefore
greater in low than in high latitudes, we see that the amount
of solar radiation available for warming the earth's surface
was probably less over Gondwanaland than over the ocean
to the south of it.
To this it might be objected that the under surface of
a cloud layer is also effective in reflecting the terrestrial
radiation back to the surface of the earth, and that this would
redress the balance. But it was shown in Chapter VII.
that the reflecting power of clouds for long-wave terrestrial
radiation is much less than their reflecting power for short-wave
solar radiation.
From this it appears that if the air entering Gondwanaland
from the south immediately formed a cloud layer, it would
not gain any heat from the surface of the plateau. If it
continued to rise along a sloping surface, it would continue
to cool by expansion and form fresh cloud. Taking the
initial temperature of the air as 50 F. and the vertical tem-
perature gradient as 3 F. per 1,000 feet, the snow-line would
be reached at a height of 6,000 feet. There is no evidence
against the supposition that at the beginning of glaciation a
large part of Gondwanaland was above this height, so that
under the conditions postulated, extensive snowfields could
develop. The supply of snow would be ample, since preci-
pitation would go on throughout the year, instead of for a
few months only as in India, although, of course, the monthly
totals would not equal those recorded at the wettest stations
during the height of the monsoon. Hence there would be
a plentiful supply of ice to extend below the snow-line
256 CLIMATE THROUGH THE AGES
on the southern slopes, and to reach the sea along broad
fronts.
It may be remarked here that owing to the absence of
strong seasonal contrasts, the formation of ice-sheets would
probably be very susceptible to slight changes of temperature
and snowfall. At present glaciers form on high mountains
near the equator, but owing to the steepness of the mountain
slopes they descend rapidly to warmer levels and melt. Given
larger gathering grounds, suitable high-level basins in which
the ice could accumulate, and weakened solar radiation to
lessen melting, the rapid development of glaciers into ice-
sheets would appear to be inevitable. Once the snow cover
had been formed and inland ice-sheets had begun to develop,
conditions would at first be very favourable for their rapid
growth. The ice surface would reflect a considerable part of
the weakened radiation which penetrated the cloud, so that
the surface would be very cold. On the southern slopes,
south of the equator, there would be a tendency for the air
drainage to form a north-west wind, which would come into
opposition with the south-east monsoon-trade wind. At
the surface both winds would probably prevail in turn first
the relatively mild and moist south-east wind with a light
snowfall, then an interval of calm, followed by a blizzard
from the north-west, lifting the moist air of the south-east
wind bodily and bringing a burst of heavy snow. But these
changes would be limited to a shallow surface layer, and over
all the south-east wind would blow steadily and the skies
would be heavily clouded. The ice-sheet would merely add
to the effective height of the land.
But we can go much further than this. Once the ice-sheet
had been formed, it would by its own cooling power and by
the reflection of solar radiation from its surface back to space,
effectively lower the snow-line over the glaciated area and its
immediate neighbourhood, and thus enable the ice to spread
over low ground previously unglaciated, or to survive a sub-
sidence of the ground on which it rested below the original
snow-line of 6,000 feet. Once a large ice-sheet has been formed,
its persistence is probably almost independent of the latitude,
and it can only be destroyed by the cessation of the supply
of snow, by a great increase of the ablation, or by a subsidence
of its bed below sea-level. Hence, during the later stages
CLIMATE OF UPPER CARBONIFEROUS GLACIAL PERIOD 257
of the Upper Carboniferous glaciation, the land surface,
worn down by glacial erosion and depressed by the weight
of the ice, may actually have been a low plain, instead of
the lofty plateau which we have supposed necessary for its
inception. So long as the temperature difference was
maintained between the Tethys Sea and the Southern
Ocean, the snowfall would have been sufficient and the
ablation small, so that the destruction of the Upper Carbon-
iferous ice-sheets probably came about by subsidence and,
in fact, the boulder clays are generally overlain by marine
beds.
The greater part of the Gondwanaland ice-sheet was
apparently formed on the southern slopes of the continent,
where conditions were most favourable. There can have
been little, if any, snowfall on the northern slope, and for a
glacier to reach the Tethys Sea an exceptional topography
would be required. In fact, it happened in only one region,
India, and if we may judge from the thinness of the deposits,
for a short time only. We may suppose that there the main
watershed lay very far north, and that the high ground formed
a sort of funnel through which a large amount of rising air
was forced to pass. Somewhat similar conditions give rise
to the abnormal rainfall of Cherrapunji at present. This
high ground would form a very rich gathering ground for
snowfields. If, then, a valley on the northern slope cut back
deeply into this high ground, it might well receive sufficient
ice for a glacier to reach the sea.
The high temperature of the Tethys Sea and the Volga
Sea would give a very favourable climate to Angaraland
and the eastern parts of North Atlantis. The warm seas
would keep the temperature high and the great evaporation
would give rise to a heavy rainfall, making the low broad
valleys among the mountains, open to the warm air from the
sea, very favourable for the growth of a rich vegetation,
able to give rise to the coal measures. These conditions
would extend to the southern parts of Nearctis, where a
small inlet seems to have existed in such a position that it
carried the oceanic influences into the heart of the American
coal district. The glaciers of Europe, if they existed at all,
were probably not more than small mountain glaciers, such
as can develop in any latitude provided the mountains are
358 CLIMATE THROUGH THE AGES
sufficiently high and the precipitation sufficiently heavy,
and we know that in Late Carboniferous Europe there were
both mountain ranges and heavy precipitation. The more
extensive glaciation of North America can be associated with
conditions in the Arctic Ocean.
We have seen that the Volga Sea was occupied by a warm
current which carried mild conditions as far as Northern
Russia, while even in Spitsbergen there was sufficient
vegetation to form workable coal beds. Evidently, where
the Volga Current entered the Arctic Ocean, there was an
ice-free area similar to that now formed by the Gulf Stream
Drift, but larger. The Volga Current was more powerful
than the Gulf Stream Drift, as we should expect it to be
from the more favourable topography. Conditions between
Angaraland and Nearctis are not sufficiently known to decide
whether a similar warm current entered the Arctic between
these continents, or whether they joined or approached so
closely as at present as to prevent any warm current
from passing between them. The map of this region is
rather hypothetical, but the climatic conditions suggest
that there was no such current. The North American
glaciation seems to require the presence of a floating cap
of sea ice to the north of Nearctis. In Chapter I., however,
we saw that at present the Arctic Ocean is only a few degrees
below the critical temperature required for the formation
of such an ice-cap. At present, heat is carried into the
Arctic by the Gulf Stream Drift and by the powerful warm
south-west winds associated with the Icelandic minimum
and its north-easterly extension. In the Upper Carboniferous
the Volga Current probably supplied more heat than the
Gulf Stream Drift, but the barometric distribution was probably
less favourable that at present for the warm south-west winds,
since the alignment of the Volgan cyclone was probably
west to east instead of south-west to north-east. The im-
portance of the latter point for the Quaternary glaciation
has been well brought out by the late F. W. Harmer (5).
We may regard the balance as about even, but the addition
of a Bering Sea Current would other things being the same
certainly raise the Arctic temperature above the critical
point and bring about an ice-free Arctic Ocean. Hence
it seems probable that there was little or no flow of warm
CLIMATE OF UPPER CARBONIFEROUS GLACIAL PERIOD 259
water into the Arctic Ocean through a channel between
Nearctis and Angaraland.
Fig. 29 shows a long gulf extending from the Arctic far
into the temperate zone between Nearctis and North Atlantis.
This gulf, open to the Arctic but closed to the south, must
have exerted a very powerful effect on the climate of the
neighbouring land. It is doubtful if it froze in winter into
a continuous surface of ice ; the influence of the warm sea
to the south of the narrow land barrier may have been sufficient
to keep it open, but if, as we suppose, the Arctic Ocean to
the north was ice-covered, the waters of this gulf must have
carried a great deal of loose floating ice in winter and spring.
That this was so is shown by some of the North American
glacial deposits, which contain large boulders apparently
transported by icebergs or shore ice. Even in summer the
water must have been very cold. There was probably a
slow circulation, southward along the western side of the
gulf and northward along the eastern side, similar to that
now found in Baffin Bay, except that the latter being open
to the south, the ice can escape into the Labrador Current.
The close neighbourhood of this cold water to the north of the
isthmus and the warm water of the Tethys Sea to the southward
must have given rise to a tendency for great storminess,
heavy rain, and dense fogs, weather similar to that now
prevailing on the Newfoundland Banks. Given some mountain-
ous country, such as we know to have been present, conditions
here were very favourable for a moderate glaciation.
Finally, we have to consider the climate of the Antarctic.
According to Wright and Priestley (6), the Antarctic continent
was not glaciated during the Upper Carboniferous ; instead,
there was a dry and wind-swept plateau subject to severe
frost action, and more favourable conditions in sheltered
coastal lowlands in which a fairly rich flora was able to develop.
Of course these conclusions refer only to the coastal parts of
the continent ; the interior may have been glaciated to some
extent, but even so the fact is surprising ; the general con-
ditions over the Antarctic appear to have been no more
favourable than at present, and we should have expected to
find it glaciated. Probably it was not that the temperature
was too high, but that the snowfall was too low compared
with the ablation. With regard to present conditions, it is
26O CLIMATE THROUGH THE AGES
not certain that if the Antarctic ice-sheet could be melted
entirely away, it would be re-established under the present
climatic regime ; it may be simply a survival from the
Quaternary Ice-Age. It is quite possible that if the greater
part of the surface of the Antarctic during the Upper
Carboniferous consisted of a fairly level plateau, the climatic
conditions would resemble those of Northern Siberia, the
very cold but comparatively dry winter not giving enough
snowfall to persist through the summer.
Thus we see that starting from a restoration of the dis-
tribution of land and sea during the Upper Carboniferous
on the basis of the existing positions of the continental massifs,
and deducing the system of winds and ocean currents and the
local climatic conditions in accordance with meteorological
experience as represented by the nearest modern analogies,
we arrive at a very fair reconstruction of the peculiar
climatology of this period. The critical assumption is the
considerable elevation of the central parts of Gondwanaland,
but this is not entirely unsupported by evidence, and is at
worst not less hazardous than the extensive migrations of the
continents through some fifty degrees of latitude. We find
that outside Gondwanaland the only extensive glaciation
occurred in Nearctis exactly where we should expect it,
while on the theory of continental drift this region would lie
on the equator and would be unglaciated. The main difficulty,
the non-glaciation of the Antarctic, is common to both theories.
There is one peculiarity in the action of volcanic dust
which deserves mention. In any latitude its cooling power
is about proportional to the solar radiation received at the
limit of the atmosphere, while its warming power, though
much smaller, is proportional to the radiation from the
earth's surface and the lower layers of the atmosphere. The
cooling varies regularly with latitude, being greatest at the
equator and least at the poles, while the warming is greatest
in the warmest regions. Hence the effect is to increase the
abnormalities of temperature brought about by the dis-
tribution of land and sea ; Gondwanaland was cooled much
more effectively than the Tethys, and in Spitsbergen during
the polar night the effect was pure gain.
Summing up, it appears that extensive glaciation in low
latitudes required at least three, and possibly four conditions :
CLIMATE OF UPPER CARBONIFEROUS GLACIAL PERIOD 26 1
1. The diversion of the whole of the equatorial ocean
current into the Northern Hemisphere, which thereby
became abnormally warm.
2. An extensive elevated continent along the equator,
but extending much farther into southern than into
northern latitudes.
3. A southern ocean shut off by land barriers from all
warm currents.
4. Possibly, a general refrigeration which might be due
to the presence of abnormally large quantities of
volcanic dust.
So far as I can discover, these conditions occurred only
once in geological time, and that occasion coincided with
the only occurrence of extensive glaciation in low latitudes.
Further, the climates of other parts of the world are such
as would be expected from them. The Coal Measures
become, not a violent negation of the possibility of glaciation,
but a necessary complement to it. It seems to me that this
geographical explanation is simple and natural, and does not
violate probabilities as does the arbitrary shifting of continents
and poles.
In the same year (1926) that I first published this geo-
graphical hypothesis of the Carboniferous glaciation, a some-
what similar theory was put forward by C. Schuchert (7)
who believed, however, that the date of the glaciation was
definitely Permian. He writes : c< It is in the youthful
topography, the enlarged continents and the peculiar con-
nexions of the lands that seemingly are to be sought the
reasons for the Permian Ice-Age. . . . This holding in of so
much of the waters of the Antarctic Ocean, combined with
the moist climates in the Southern Hemisphere and the
general highland condition of much of the world in early
Permian time, will be the explanation for the peculiar
position of the continental ice-masses of the Southern
Hemisphere." Later, Bailey Willis (8) attributed the glacia-
tion of South America and South Africa to refrigeration
of the South Atlantic, shut off from warm currents. The
centres of glaciation were near the warm seas, which provided
moisture by rising over wedges of colder air, giving cold
262 CLIMATE THROUGH THE AGES
foggy summer weather, favourable to a low snow-line. He
considers that the glaciation of India was due to local high
mountain ranges.
REFERENCES
(1) ARLDT, TH. " Handbuch der Palzeogeographie." Leipzig, 1919.
(2) HANDLIRSCII, A. '* Die Bedeutung dcr fossilen Insekten fiir die Geologic."
Wien, Mitt. Geol. Ges., 3, 1910, p. 503.
(3) STAFF, H. v. " Zur Entwicklung der Fusuliniden." Zentralbl. f. Min. y
Geol. und Pal., 1908, p. 699.
(4) SIMPSON, G. G. " The south-west monsoon." London, Q,. J. R. Meteor.
Soc., 47, 1921, p- I5 1 -
(5) HARMER, the late F. W. " Further remarks on the meteorological conditions
of the Pleistocene epoch." London, Q,. J. R. Meteor. Soc., 51, 1925, p. 247.
(6) BRITISH (TERRA NOVA) ANTARCTIC EXPEDITION, 1910-1913. " Glaciology,"
by C. S, WRIGHT and R. E. PRIESTLEY. London, 1922.
(7) SCHTJCIIERT, C. " The palseogeography of Permian time in relation to the
geography of earlier and later periods.*' Proc. 2nd Pan-Pacific Set. Congr.,
1926, p. 1,079.
(8) WILLIS, BAILEY. " Isthmian links." New York, Bull. Geol. Soc. Amer.,
43, 1932, p. 917-
CHAPTER XVI
THE CLIMATE OF THE QUATERNARY
OF the four great ice-ages, the first two, the Lower
Proterozoic, the Upper Proterozoic-Lower Cambrian,
and the last, the Quaternary, were developed mainly
in what are now temperate latitudes, while the third, the
Upper Carboniferous, found its maximum extent in regions
now not far from the equator. The Upper Proterozoic-
Lower Cambrian glaciation was apparently similar in many
respects to the Quaternary, but as yet we know so little about
it that no detailed discussion is possible. In this chapter
it is proposed to refer briefly to the meteorology of the
Quaternary period.
At the maximum of the Ice-Age, E. Antevs (i) estimates
that the ice-sheets occupied an area of about 1 3 million square
miles. Of these 4^ million were in North America, i J million
in Europe, i^ million in Asia, and about 5 million in the
Antarctic. The remainder was made up of the expanded
ice-sheet of Greenland and relatively small areas in Australia,
New Zealand and South America. The present ice-covered
area of about 6 million square miles is almost entirely in the
Antarctic and Greenland. It is not certain that all areas
reached their maximum at about the same time, but two lines
of evidence suggest that this is nearly true. The first is the
lowering of sea-level by the abstraction of water, which
according to Antevs's estimate would amount to 305 feet
below the present if all the ice-sheets reached their maximum
extent and thickness together. Various estimates have been
made from the present depths of shore deposits, coral reefs,
etc., which give a minimum figure of about 260 feet. The
second line of evidence is the depression of the snow-line in
North, Central and South America during the latest (Wurm)
glaciation, which has been remarked on by many authors,
and which is almost constant right across the equator,
363
264 CLIMATE THROUGH THE AGES
increasing somewhat in regions of heavy rainfall and decreasing
in dry regions :
Latitude .... 45 N. iyN. 10 N.-20 S. 408.
Depression of snow-
line (feet) . . . 2,300 3,000 1,300-2,000 3,300
The character of the ice-sheets and glaciers varied. In
Northern Europe, North America and presumably also in
Greenland and the Antarctic they were several thousand feet
thick and spread out actively from various centres. Siberia
was for long considered to have had only minor mountain
glaciers, but recent work summarised, e.g., by R. F. Flint and
H. G. Dorsey (2) shows that there was at one time a large
ice-sheet in north-west Siberia extending over the Arctic
shelf, though the ice was thin and inactive. Farther east
and south there were extensive piedmont glaciers. In South
America the glaciers were also mainly of the piedmont type.
The ice-age was divided into glacial and interglacial periods
by a series of large-scale advances and recessions of the ice.
These are best known from Europe and North America,
and appear to run closely parallel in the two continents.
The succession is summarised by F. E. Zeuner (3), K.
Bryan (4) and others as follows (the youngest at the top) :
Alps. N. German Plain. Continental U.S.A.
Wurm Weichsel (including Wisconsin (including
Warthe) lowan)
Riss Saale Illinoian
Mindel Elster Kansan
Gunz ? Nebraskan (Jerseyan)
The Mindel and Riss glaciations were the most extensive.
The Mindel-Riss interglacial (Yarmouth Interglacial in
America) was very long, of the order of 240,000 years, and was
generally mild, but was interrupted by at least one colder
period which did not reach glacial intensity. The Riss
glaciation had a double maximum in Europe at least. The
Riss- Wurm interglacial was short and mild but was inter-
rupted near the middle by a period of sub-arctic conditions
in Jutland.
THE CLIMATE OF THE QUATERNARY 265
The Wurm glaciation was less extensive than the Riss ;
it comprised three maxima, each of less intensity than the
preceding one, separated by sub-arctic or even cold-temperate
conditions. The recession after the third peak was inter-
rupted by several halts or slight readvances.
In other parts of the world the succession is less complete.
In Kashmir and neighbouring territories F. Loewe (5)
recognises three glaciations, which he correlates with Mindel,
Riss and Wurm, decreasing in intensity, and there are probably
traces of the same three in East Africa ; in both cases, however,
the correlation is not certain. In nearly all other parts of the
world the remains of only two glaciations are found, pre-
sumably representing the Riss and Wurm, the former always
being much the more extensive. In Siberia for example there
was no Wurm ice-sheet, only mountain and piedmont glaciers.
It is not yet certain that the earlier glaciations did not occur ;
their moraines may have been destroyed by later advances.
In discussing the cause of the Quaternary Ice- Age, it is
necessary to distinguish between the Ice-Age as a whole,
and the succession of glacial and interglacial periods. In
Chapter XII. it was shown that the most probable cause for
Ice-Ages was elevation and mountain building in extensive
high continents, limited accession of warm ocean currents to
high latitudes, and probably much volcanic dust in the
atmosphere, all of which factors were present at the beginning
of the Quaternary. A decrease in the amount of CO 2 in the
atmosphere (Chapter VI.) may have been a contributory
factor. With the exception of volcanic dust, however, these
are all stable factors, which would not change sufficiently
rapidly to account for the succession of glacial and interglacial
periods. For the latter therefore some other explanation
must be found.
The fact that the last two glaciations at least began and
ended more or less together in all parts of the world is highly
significant. In a minor degree it is paralleled by the recession
of the glaciers everywhere within the past hundred years. It
shows that glaciations in different regions do not depend only
on local conditions, but are mainly controlled by some world-
wide factor such as the temperature of the oceans, the heat
received from the sun, or the circulation of the atmosphere,
or by some combination of them. The late beginning of
266 CLIMATE THROUGH THE AGES
glaciation in the tropics and Southern Hemisphere, if confirmed,
suggests that ocean temperature may be important, because
the lag in cooling the oceans would be greater the farther
removed the region is from the sources of cold water in the
Arctic and North Atlantic.
The first question concerns the lag between the occurrence
of mountain formation and the beginning of glaciation, which
was discussed in Chapter X. There are four possible reasons :
1. The slow cooling of the oceans.
2. The erosion of the mountains.
3. The occurrence of a period of explosive volcanic activity.
4. The occurrence of favourable astronomical conditions.
An important factor in fixing the actual beginning of the
Quaternary glaciation over the land must have been the general
temperature of the sea. At the close of a long warm period
the sea is warm throughout its whole depth ; there is none
of the very cold bottom water which exists at present. This
must be so, for the temperature of the sea depths cannot long
remain lower than the temperature of the coldest part of the
surface. Now the beginning of the Quaternary glaciation
was a period of great elevation in most parts of the north
temperate belt. The gap between Greenland and Norway,
which at present conducts the Gulf Stream into the Arctic
Ocean, was greatly narrowed if not completely closed. Bering
Strait probably differed little from its present condition, and
there may have been an open channel to the west of Greenland.
Now there are some interesting peculiarities in the develop-
ment of the Quaternary glaciation which may have a bearing
on this question of the cooling of the seas. The first glaciation
of Europe was most extensively developed in Scandinavia
and North Russia ; the British Isles were probably not
glaciated until later. The corresponding glaciation in America
was developed in the Rocky Mountains of British Columbia
and in Labrador, but not in the central parts. The glaciation
of British Columbia was apparently an enormous development
of valley and piedmont glaciation due to the great height of
the mountains, but the North European and Labradorean
centres developed true inland ice-sheets. If we suppose that
the elevation of the Wyville Thomson ridge between Greenland
THE CLIMATE OF THE QUATERNARY 267
and Scotland above its present level shut out the Gulf Stream
from the coast of Norway, the Arctic Ocean would lose almost
all the supply of heat formerly carried into it by ocean currents
and its temperature would begin to fall. The ocean south
of the Wyville Thomson ridge would still be very warm,
however, arid the winds must have brought a considerable
amount of heat across the land barrier. It is difficult to
estimate the time which would be required under these
conditions for the thorough cooling of the Arctic Ocean.
Most of the ice formed in the Arctic at present begins with the
freezing of a surface layer of relatively fresh water brought
down by the great rivers which enter the basin, and which,
owing to its smaller density, floats on the main mass of warmer
but more saline water. Probably this water must freeze
fairly near the coast, otherwise the storm winds would break
it up and mix it with the underlying salt water. By analogy
with what happens at present at the junction of the Labrador
Current and the Gulf Stream, we can say that the fresh layer
would become salt more quickly than it would warm up (6).
The resulting mixture would be heavier than both the upper
and lower layers, and would therefore sink. But while the
main oceans were still warm, it seems probable that the heat
transferred by southerly winds would suffice to keep this layer
of fresh water liquid long enough for the mixing process to
destroy it. Thus we conclude that the formation of a cover of
floating ice probably did not follow immediately on the
elevation of the Wyville Thomson ridge, but had to wait until
the cooling of the main oceans had progressed some way.
At first sight it might seem that the accumulation of cold
bottom water could not possibly affect the atmospheric
processes which go on above the surface of the oceans. Such
an influence does take place, however, especially off the western
coasts of the continents, where cold bottom water wells up to
replace the surface water driven away by easterly winds.
Investigations into the effect of the Trade winds on the surface
temperature of the North Atlantic have shown that the
North-east Trade, blowing off the coast of West Africa, does
actually bring up a large amount of cold water from the
underlying layers. This cold water has the effect of lowering
the temperature of the Gulf Stream, and ultimately the
surface temperature of the North Atlantic between the United
268 CLIMATE THROUGH THE AGES
States and Ireland, probably by several degrees. If the
depths of the oceans were much warmer than at present,
this cooling influence would not exist. The meteorological
effects of upwelling cold water on the western coasts of South
America, South Africa, and Australia are extraordinarily
marked, being largely responsible for the desert character of
those coasts. It is probable, however, that this cold water
comes, not from the greatest depths, but from some inter-
mediate layer, and that a certain accumulation of cold water
could take place without affecting surface conditions.
We have no means of knowing how long it took to cool
the main body of the oceans, but it was certainly a very long
time. As an example of the quantities involved, if we suppose
that all the thaw water of the ice, both land and sea ice, which
melts each summer in both hemispheres, sank to the bottom
of the oceans and spread out there, it would take between
ten and twenty thousand years to fill the oceans with cold water.
Immediately after the formation of the Wyville Thomson
ridge, the annual supply of cooled water was probably not
so great as the present annual melting of ice, and at first it
was not ice-cold. When we take into account also the cold
water which wells up in the tropics and becomes warmed
there, so that it has to be cooled again, we see that the thorough
cooling of the oceans must have taken several times, perhaps
many times, ten thousand years. It is unlikely, however, that
this effect could have caused a lag of millions of years.
The second stage in the oncoming of the Quaternary
Ice-Age would occur when the general temperature of the
oceans had fallen low enough for a covering of floating ice
to develop over the Arctic Ocean. This would result in a
great cooling of the lands washed by that ocean Greenland,
Norway, and Northern Russia. The Labrador Current may
have been in existence before, but now it would carry great
quantities of floating ice, and there would be a great lowering
of temperature in Labrador and Newfoundland. The decrease
in the summer temperature would be greater than the decrease
in the winter temperature, and there would also be a marked
increase in the storminess and snowfall. All these regions
would develop glaciers, which would speedily become ice-
sheets. The glaciation of the mountains of British Columbia
may have commenced earlier, but probably increased rapidly
THE CLIMATE OF THE QUATERNARY 269
about this time, while the ice-sheet of the Antarctic probably
reached the sea. This may have been the first or Gunzian
glaciation.
The second cause of lag is the reaction of the elevated land
areas to erosion. The action of frost and running water
removed great quantities of rock, much of which found its
way into the sea. The lightening of the load caused further
uplift, but the topography now being irregular, the higher
peaks were at a greater height than before, while the valley-
heads formed suitable gathering grounds for the accumulation
of snow drifts. It is in such hollows that snow dritts persist
longest in Scotland.
Finally, when all other factors were favourable, it is possible
that either a period of plentiful volcanic dust or a period of
decreased radiation in summer due to astronomical causes,
by keeping down the summer temperature, was the actual
immediate cause of the beginning of glaciation.
Once the ice-sheets had formed, by raising the effective
height still further (both by adding ice to the land and sub-
tracting water from the sea), by reflecting solar radiation
and cooling the area around them, and by shedding ice into
the sea and so depressing ocean temperatures still further,
they would tend to maintain themselves and spread, until for
some reason they became unstable. We must now examine
the possible causes of the break-up of the ice-sheets. These
are :
1. A lowering of the level of the land.
2. A general rise of temperature.
3. A decrease in snowfall.
Since ice weighs about one-third as much as the average
rock, the accumulation of 3,000 feet of ice is equivalent to
adding the weight of 1,000 feet of rock to the land. This
additional weight gradually depressed the land surface,
though with a considerable lag, and brought the margins of
the ice-sheets under the action of the sea, causing for example,
floating ice-barriers which broke away as icebergs. The
area of the ice-sheets and consequently their cooling power
diminished, initiating an amelioration of climate. The
process might be carried far enough for the ice to disappear
27O CLIMATE THROUGH THE AGES
more or less completely. After the load was removed the
land would begin to rise again, causing a return of glaciation.
This process might account for the division of a glacial period
into two or three peaks, and possibly, though this is more
doubtful, for the shorter interglacial periods. Moreover,
since each glaciation would wear down the high ground and
deposit the material round it in the form of moraines, we
should expect each glacial recurrence to be less severe than the
preceding one until the topography became unsuitable for
glaciation. It cannot account for the long Mindel-Riss
interglacial, but during the latter there was a great deal of
earth-movement and volcanism in many parts of the world,
which would eventually cause a return of glaciation. It is
known that after the Wurm glaciation the land in the centres
of greatest ice accumulation continued to sink and in late
Glacial time the central shores of the Gulf of Bothnia were
depressed about 900 feet below their present level ; the
recovery is still in progress. Similar subsidence and recovery
should have followed each glacial advance but the evidence
for interglacial oscillations of the same type was swept away
by subsequent advances of the ice. Also, the oscillations of
level may have been superposed on a steady sinking of the
land, so that each rise was less than the preceding fall. This
would account for the gradual decrease in intensity of
successive glacial peaks.
A world-wide rise of temperature could be due to the
cessation of volcanic activity, to an increase of solar radiation
(Chapter IV.) or to astronomical causes (Chapter V.). The
first two are rather speculative ; moreover it is very doubtful
whether the slight increase of radiation in middle and high
latitudes which would result from a cessation of volcanic
activity would have much effect on a full-grown ice-sheet.
Astronomical effects also seem rather slight, but as the effect
of increased warmth in summer would be reinforced by the
greater cold of winter which would probably result in a
decrease of snowfall, they cannot be ruled out. The good
accord between Milankovitch's astronomical scheme and the
succession worked out by F. E. Zeuner (Chapter V.) supports
the idea that these small astronomical causes may actually
have been the controlling factor in the glaciation of the
Northern Hemisphere. These large ice-sheets would exercise
THE CLIMATE OF THE QUATERNARY 271
a dominant effect on the ocean temperatures and atmospheric
circulation, and so might well control the glaciation of other
parts of the world. There is good reason to believe that the
Pluvial periods of low latitudes were in fact controlled by the
atmospheric circulation. In this connexion it is interesting
to note that according to H. Mortensen (7) there was no
pluvial period in the coastal desert of northern Chile. This
desert exists because of the upwelling cold water of the
Humboldt Current off the coast, which in turn is due to
the south-east trade winds blowing off the coast. A strength-
ening of these trade winds would therefore maintain the
desert conditions.
We come finally to the question of precipitation. The
supply of precipitation would of course follow variations
of solar radiation (Chapter IV.) but it is now generally
recognised that the development of ice-sheets would itself
cause changes in their supply of moisture. This was first
suggested by V. Paschinger (8).
In Chapter IX. we saw that in mountainous country, as
we go upwards the total amount of precipitation increases
to a certain level, above which it again decreases. With
increasing height, also, the proportion of total precipitation
which falls as snow becomes steadily greater. Hence we can
distinguish a level of maximum rainfall, and above that a
level of maximum snowfall. The latter is often very sharply
marked ; it depends on the winter conditions, especially
the general winter temperature of the lowlands, the vertical
temperature gradient, and the relative humidity. The
snow-line, on the other hand, depends mainly on the summer
temperature. At present in the Alps the snow-line is about
2,000 feet above the level of maximum snowfall. Suppose
now the summer temperature decreases while the winter
temperature remains unchanged. The snow-line will descend,
and if the decrease of summer temperature reaches 6 F., the
snow-line will coincide with the level of maximum snow-fall.
The supply of snow available for glaciers will now be greatly
increased, and this stage will see a great development of
glaciers. Even if the cooling is uniform throughout the year,
the snow-line will descend more rapidly than the zone of
greatest snowfall.
At present in polar regions the snow-line is below the
272 CLIMATE THROUGH THE AGES
zone of maximum snowfall, and these regions are widely
glaciated. In the Tertiary period, the snow-line must have
been above the snowfall maximum even in polar regions.
Paschinger considers that the cooling of the temperate regions
spread out from the poles, probably in the form of repeated
cold waves (i.e., outbreaks of the polar front). Owing to the
conservation of heat in the oceans, whence most of the moisture
is evaporated, the total precipitation is not diminished at first,
while the proportion which falls as snow is increased. Glaciers
spread until they reach the sea or some warm lowland where
ablation is rapid. Then as the seas cool, the snowfall
diminishes, while the lowering of temperature due to the ice
itself depresses the zone of maximum snowfall. At the same
time, the development of glacial anticyclones cuts off the
supply of snow in the interior, so that the snow-line rises, until
it is again above the zone of greatest snowfall. The ice-sheets
and glaciers now retreat. When the retreat has proceeded far
enough, the secondary cooling due to the ice ceases to be
effective, the level of maximum snowfall rises to the snow-line
again, and the whole process recommences. This is
Paschinger's conception of the meteorological cycle of a glacial
period ; granted an initial cause, such as elevation, glacial and
interglacial (or " intraglacial ") stages will repeat themselves
regularly until the immense denudation effected by the ice
lowers the mountains or at least the corries and depressions
where snow can gather below the snow-line. He thinks that
this stage has not yet been reached in Europe and that another
glaciation is to be expected in due course.
Paschinger points out that the relationship between the
level of maximum snowfall and the snow-line accounts for
many peculiarities of the Quaternary Ice-Age. In the
continental mountain regions of Asia, with very cold winters
and hot summers, the two levels are many thousand feet
apart, and th$ glacial cooling was not, as a rule, sufficient to
bring the snow-line down to the zone of heavy snowfall.
Hence the development of glaciers and ice-sheets was less
extensive than in Europe or North America. In equatorial
regions, on the other hand, while both snow-line and maximum
snowfall are at a great height, the former lies only a short
distance above the latter, owing to the absence of seasons.
A comparatively slight increase in the snowfall would bring
THE CLIMATE OF THE QUATERNARY 273
them together, and cause a considerable extension of the
mountain glaciers.
This view of the sequence of events in an ice-age
undoubtedly contains many elements of truth, and may
well account to some extent for the alternation of glacial
stages with what I have termed above " intraglacial " stages.
The Mindel-Riss interglacial stands in a different category, and
cannot be accounted for on any purely meteorological cycle ;
it necessarily involves a cessation or great weakening and a
subsequent renewal of the ice-forming factors.
As was pointed out in Chapter II., the development of
ice-sheets would cause changes in the atmospheric circulation
and tracks of depressions, which would react on the supply
of moisture. Besides the work of Flint and Dorsey, referred
to in that chapter, there have been several other studies of
American glaciation on these lines. Thus E. Antevs (9)
considers that the Keewatin and Cordilleran ice-sheets in the
west and centre developed first. The Keewatin and Scandi-
navian ice-sheets caused a southward displacement of the
Icelandic low which caused frequent north-east winds in
Labrador. Once started, the Labrador ice-sheet was fed by
cyclonic snowfall on its southern border. Ultimately the area
of ice grew so large that the supply of snowfall in the central
regions was insufficient to maintain it, and the ice-sheet began
to decay. At this stage, however, depressions were still deflected
southward, and Antevs thinks that the mountain glaciers and
lakes south of the main ice mass may have reached their
maximum during the earlier stages of the retreat. Later,
however, the storm tracks shifted north again and brought
about a rejuvenation of the ice-sheets and a repetition of the
series of events. This process might account for short intra-
glacial periods but not for recurrences after long interglacial
periods as warm as or warmer than the present.
There is no doubt that changes in the atmospheric circulation
must have brought about changes in the centres of the ice-sheets
and it is highly probable that their growth must eventually
have resulted in starvation at the centre, but it seems unlikely
that this would have resulted in their disappearance. It is
also doubtful whether the North American sequence followed
the lines of Antevs's argument ; K. Bryan (10) for example,
states that American geologists believe in a progressive shift
18
274 CLIMATE THROUGH THE AGES
from east to west of the main ice-centre throughout the last
(Wisconsin) glaciation, and R. F. Flint (n) thinks that the
Labradorean and Keewatin areas were both parts of a single
Laurentide ice-sheet fed by maritime air from the south and
south-east and expanding southward and westward.
Summing up, we find that for the occurrence of the ice-age
as a whole the " geographical " theory seems to be the only
adequate one, with possibly some help from CO 2 . The
actual commencement of glaciation may, however, have been
determined by some minor factor such as the astronomical
situation or changes of solar radiation. The interglacial
periods present the main difficulty, because of their close
parallelism in different parts of the world. Astronomical
causes seem to come nearest to filling the necessary conditions,
but alternating depression and elevation, due to the accumu-
lation and removal of the ice-load, are also probable, while
cycles of solar radiation cannot be ruled out. The re-
crudescence of glaciation after the Mindel-Riss Interglacial
was due, at least in part, to renewed mountain building.
Finally, the " intraglacial " oscillations were most probably
caused by reactions between the ice-sheets and the circulation
of the atmosphere.
We must now briefly consider the climate outside the main
areas of glaciation.
The part of Central Europe sandwiched between the
Scandinavian ice-sheet to the north and the Alpine glaciers
to the south must have suffered from a severe climate, which
has been studied by P. Kessler (12). He has three lines of
evidence the climate in the neighbourhood of the present
ice-sheets of Spitsbergen, Greenland, and the Antarctic, the
flora and fauna, and the geological phenomena and all three
present the same picture. The mean annual temperature is
below freezing point, and although the summer may have a
few short spells of warmth, the winters are very cold. On
the margins of the Antarctic continent the summer climate
is especially unpleasant. Although the temperature during
a relatively warm summer month may average above freezing
point, and may go as high as 40 or even 45 F. for a few
hours, yet the persistently overcast sky, the frequent storms
of snow and sleet, and the general unpleasantness of the
weather, are worse than the cold of the interior.
THE CLIMATE OF THE QUATERNARY 275
The conditions in these high latitudes, between ice-sheets
and the sea, however, cannot be regarded as typical of those
in Central Europe far from the Atlantic, especially if, as seems
probable, there was a considerable area of land west of
France.
The study of the flora and fauna gives results of great
interest. The similarity of the plants at high levels in the
Alps to Arctic forms suggests that during the maximum
extension of the ice these cold-loving species inhabited the
low unglaciated ground north of the Alps, and after the ice-age
they followed the retreating glaciers upwards to high levels.
The general picture shows a region of tundra vegetation,
inhabited by the reindeer, the woolly rhinoceros, and the
mammoth. The geological phenomena earth-flows, block-
trains, and mounds, ridges or terraces of angular and sub-
angular material point to frost action on a huge scale, the
earth and rocks moving down the valleys under the action of
repeated freezing and thawing. The general climate of the
region appears to have been highly abnormal ; the prevailing
winds were probably dry glacial winds from the north-east,
but these winds were shallow and were overlain at a small
height by moist winds from the Atlantic, which sometimes
descended to the level of the ground. The snow-line lay
at 3,000 feet in the west and at 5,000 feet in the east, and in
the hills the accumulations of snow carved out cirques or corries
at these levels. These are mainly on the north-eastern side
of the crests, and since Enquist has shown that the greatest
accumulation of snowfall takes place on the lee-side, they
indicate that the snow-bearing winds at a height of 3,000 to
5,000 feet came from the south-west.
The annual precipitation was small at low levels, but
occasionally rain fell in torrential downpours. The evapora-
tion was great, and one of the greatest peculiarities of the cold
periglacial climate was that it could ape the formations ofthe
hottest deserts. An important deposit was the loess, an
accumulation of the finest wind-blown dust, and there were
even small salt lakes in which layers of salt were formed.
It is noteworthy that similar saline deposits are forming at
present in restricted areas in Spitsbergen and Greenland, a
fact which has some bearing on the evidence for Wegener's
theory of polar movements.
276 CLIMATE THROUGH THE AGES
Outside the limits of the ice-sheets and of the peripheral
zone of ice-winds, the weather was probably much as we know
it to-day, but more stormy. This applies especially to the
Mediterranean region, which must have had a heavy rainfall
distributed more or less evenly throughout the year, instead
of a moderate or scanty rainfall limited to the winter months
as at present. These regions probably had the weather now
found on the north-western coasts of Europe. Wandering
storms penetrated into the Sahara, which was then one of the
most genial regions on the globe, and this region, now a desert,
appears to have been one of the main centres in which the
human race rose to a dominant position in the world.
H. v. Ficker (13) calculated that at the time of the maximum
glaciation of the north-west Pamir the rainfall was four or
five times as great as at present.
The equatorial regions in general also had a greater rainfall
than at present, though with local exceptions. Over the
oceans the Trade winds, stronger in consequence of the
greater temperature difference between the equatorial and
polar regions, brought in more warm moist air than at present.
The volume of air ascending in the equatorial belt of low
pressure was therefore greater, and the rainfall in the Doldrums
and over the eastern equatorial parts of the continents was
heavier. The succession of glacial and interglacial periods in
the northern continents was paralleled by a succession of
pluvial and interpluvial periods in tropical Africa, and by
advances and retreats of the mountain glaciers. The exact
correlation is not yet determined, but may be as follows :
A very early lake, the deposits of which have been described
by E. J. Wayland (14) as Kafuan, may correspond with
Gunz and Mindel. Wayland thinks it had two maxima
separated by a period of earth movements. After a long
dry interval a large lake (Lake Kamasia) formed from the
junction of several existing lakes. E. Nilsson (15) calls
this the Great Pluvial and equates it to the Riss. Lake
Kamasia then dried up completely and the mountain glaciers
disappeared. This interpluvial was followed by the Gamblian
period of renewed lake-formation in each of the separate
basins. Nilsson distinguishes four successive lake systems,
the first three representing the three maxima of the Wurm
and the fourth a late Glacial halt or re-advance. Between
THE CLIMATE OF THE QUATERNARY 277
Lakes I. and II. and II. and III. there were lower lake levels,
between III. and IV. the lakes dried completely.
A similar succession can be traced over a large part of
East Africa from the Nile Valley to Rhodesia though the
stages of the last Pluvial have not been distinguished. The
Upper Nile Valley, however, became desert early and K. S.
Sandford (16) considers that in that region there were no
changes sufficiently great to be called " Pluvial " and
" Interpluvial." It is in fact likely that owing to the pre-
ponderance of ice in the Northern Hemisphere the whole
system of climatic belts was shifted southwards and that the
increase of rainfall was much greater south than north of the
equator.
The retreat of the ice-sheets shows a number of halts or
re-advances marked by a series of terminal moraines. These
present a similar appearance in North America and Europe.
There were also a series of fluctuations of lake-levels in East
Africa, which most probably represent the pluvial equivalents.
The variations of lake levels do not necessarily represent
very great changes of rainfall. In the Nakuru catchment
area the present rainfall is about 37.} inches a year. R. E.
Moreau (17) from botanical evidence considers that the
average rainfall in the last of the Wurmian pluvial stages
(Makalian) was about 44-50 inches, while during the arid
Post-Makalian period, when the lakes dried completely, it
cannot have been as low as 27 inches.
REFERENCES
(1) ANTEVS, E. " The last glaciation." New York, Amer. Geogr. Soc.,
Research Series, no. 17, 1928.
(2) FLINT, R. F., and H. G. DORSEY. " lowan and Tazewell drifts and the
North American ice-sheet." Amer. J. Sci., 243, 1945, p. 627.
(3) ZEUNER, F. E. " The Pleistocene period ; its climate, chronology and
faunal successions." London, Ray Soc., 1945.
(4) BRYAN, K., and L. L. RAY. " Geologic antiquity of the Lindenmeier site
in Colorado." Washington, Smithson. Misc. Coll., 99, no. 2, 1940.
(5) LOEWE, F. " Die Eiszeit in Kaschmir, Baltistan und Ladakh." Berlin,
%s. Ges. Erdkunde, 1924, p. 42.
(6) SMITH, E. H. " The international ice patrol." Meteor. Mag., London,
60, 1925, p. 229.
(7) MORTENSEN, H. " Uber den Abfluss in abflusslosen Gebieten und das
Klima der Eiszeit in der nordchilenischen Kordillera." Naturwiss> Berlin,
16, 1929, p. 245.
(8) PASCHINGER, V. " Die Eiszeit ein meteorologische Zyklus." s. Gletscherk.,
i3 1923, P- 29-
278 CLIMATE THROUGH THE AGES
(9) ANTEVS, E. " Correlation of Wisconsin glacial maxima." Amtr. J. Sci. t
243A, 1945, p. i.
(10) BRYAN, K., and R. C. GADY. "The Pleistocene climate of Bermuda."
Amer. J. Sci., 27, 1934, p. 241.
(11) FLINT, R. F. ** Growth of North American ice-sheet during the Wisconsin
age." New York, Bull. geol. Soc. Amer., 54, 1943, p. 325.
(12) KESSLER, P. " Das eiszeitliche Klima und seine geologischen Wirkungen
im nicht vereisten Gebiet." Stuttgart, 1925.
(13) FICKER, H. v. " Die eiszeitliche Vergletscherung der nordwestlichen
Pamirgebiete." Berlin, SitzBer. Preuss. Akad. Wiss., 1933, 2, p. 61.
(14) WAYLAND, E. J. " Rifts, rivers, rains and early man in Uganda." London,
J. R. Anthrop. hist., 64, 1934, p. 333.
(15) NILSSON, E. " Quaternary glaciations and pluvial lakes in British East
Africa." Geogr. Ann., Stockholm, 13, 1931, p. 249.
(16) SANDFORD, K. S., and W. J. ARKELL. " Palaeolithic man and the Nile
valley in Nubia and Upper Egypt." Chicago Univ., Oriental Inst.,
PubL, vol. 17. Prehistoric survey of Egypt and Western Asia, Vol. 2,
Chicago (1933).
(17) MOREAU, R. E. " Pleistocene climatic changes and the distribution of
life in East Africa." London, J. Ecol., 21, 1933, p. 415.
PART III
THE CLIMATES OF THE HISTORICAL
PAST
CHAPTER XVII
THE NATURE OF THE EVIDENCE
IT is not many years since it was generally believed that
variations of climate came to an end with the Quaternary
Ice-Age, a period moreover which was placed hundreds
of thousands of years ago. The post-glacial or " Recent "
period was supposed to show merely a more or less rapid
warming up to the present level, followed by a long period
in which the climates of the different parts of the world were
exactly as we now find them. It was the International
Geological Congress at Stockholm in 1910 which first made the
majority of geologists familiar with the existence of a warm
period intercalated between the ice-age and the present.
About the same time, a number of investigations in different
countries combined to prove that the ice-age itself was not
so remote as it had seemed to be, and that in fact the post-
glacial " geology " of Europe was partly contemporaneous
with the " history " of Egypt. But since the geological
deposits undoubtedly point to changes of climate, slight indeed
in comparison with the preceding ice-age, but still marked
enough to leave their traces permanently written on the face
of the earth, the unvarying climate of history is evidently a
myth. The beginning of the " period of unchanging climate "
has advanced later and later before the attacks of geologists,
and now, in the minds of most of the authors who concern
themselves with the subject, it apparently stands only a few
centuries before Christ. But meanwhile a different, and more
logical, view has arisen, namely, that the present does not
differ from the past, that variations of climate are still in
progress, which are similar in kind, though not in extent,
to the climatic vicissitudes of the ice-age.
There is, however, one point in which the " historical "
period may be said to differ from the " geological " periods ;
during the historical period the distribution of land and sea,
the heights of the mountains, and the positions of the poles have
changed only to a very slight extent. Hence we may regard
282 CLIMATE THROUGH THE AGES
the geographical factors of climate as practically constant
during this period, and any climatic changes which we can
discover and confirm must be attributed to non-geographical
factors, and most probably to variations in solar radiation.
Hence it is in the historical period that we are most likely to
be able to trace the effect of solar radiation on climatic changes.
Of course this difference between the " historical " and the
" geological " periods is more apparent than real ; the
length of the historical period is a few thousand years, while
the length of even the subdivisions of the geological periods is
to be expressed in hundreds of thousands or in millions of
years. Nevertheless, we do seem to be living at present in a
period of quietude relative to the Quaternary period ; the
change from the Ancylus to the Litorina stages in the Baltic,
for instance, represents a greater geographical variation than
anything which has happened since.
The interpretation of the term " historical period " adopted
in this section is a somewhat liberal one ; it is essentially the
period during which the vicissitudes of human life are known
and dated to within a few centuries. Archaeologists are
continually pushing back the boundaries of history, while
astronomers, geologists, and others from time to time supply
new fixed points or new chronologies. At the present time
we have a more or less complete record of human history in
South-western Asia since about 5200 B.C., and that date has
been taken as the point of origin. For a study of the climatic
changes during this period of 7,000 years, we have a variety of
material. Instrumental records are of course of the greatest
value, but reliable meteorological observations go back a
mere three centuries, and for the greater part of the period we
have to make the best of less direct evidence. The various
lines of attack may be summed up as follows :
1. Instrumental records and old weather journals.
2. Literary records (accounts of floods, droughts, severe
winters, and great storms).
3. Traditions, such as that of the Deluge, which can
sometimes be correlated with other data.
4. Fluctuations of lakes and rivers, glaciers and other
natural indices of climate, which can often be connected
with historical events or dated by laminated clays.
THE NATURE OF THE EVIDENCE 283
5. Arguments from the migrations of peoples, for which
climatic reasons may be assigned with some show of
probability. To this we may perhaps add the waxing
and waning of civilisations.
6. The rate of growth of trees, as shown by the annual rings
of tree-growth, which can be correlated with the annual
rainfall.
7. Geological evidence great advances or retreats of
glaciers, growth of peat-bogs, succession of floras, etc.,
which can sometimes be dated approximately.
The first and second sources of data, meteorological and
literary records, and the seventh, geological evidence, are
mainly exemplified in Europe, while the fourth and fifth
sources provide the main mass of information for Asia and
the fourth for Africa ; while the sixth, growth of trees, gives
the only exact chronology for North America, where, however,
it is highly developed.
Instrumental meteorological records even in Europe date
back for only about three centuries, in North America for
two centuries, while in other continents they are practically
confined to the last hundred and fifty years. Moreover,
while old observations are of great interest in discussing
variations of weather from one year to another, they are of
less value in determining changes of climate extending over
a long period. The accuracy of the early instruments is not
always above reproach ; some of the early types of rain gauge,
for example, do not make adequate provision against the
re-evaporation of the fallen water. Defects of exposure may
be a serious source of error. It was a common practice among
early observers to expose their rain gauges on the roofs of
houses, but gauges so exposed do not catch so much water as
gauges exposed on the ground in open sites. Even when
placed on the ground, they may have been too near to buildings
or trees. Most of the sources of error tend to give a rainfall
which is too small rather than too great, so that if the early
instrumental records appear to indicate that the rainfall was
smaller than at present, they must be regarded with suspicion
unless they can be confirmed in some way. The rainfall
minimum in England indicated by Symons in the eighteenth
century was suspected for this reason ; the way in which it was
284 CLIMATE THROUGH THE AGES
confirmed is described in the next chapter, where, in addition,
long rainfall records are discussed from other countries.
There is a curious exception to the comparative modernity
of instrumental meteorological records, namely, the measure-
ments of rain in Palestine in the first century A.D. Hellmann ( i )
states that " the amount of rainfall then considered as normal
for a good crop corresponds pretty closely with that deduced
from the modern observations of Mr Thomas Chaplin at
Jerusalem, whence it can be inferred that the climate of
Palestine has not changed/ 5
There are a number of old meteorological journals in which
the wind and weather are given, but no instrumental readings.
The best known of these are the journal kept by the Rev.
William Merle at Oxford from 1337 to 1344 (2), and that of
Tycho Brahe (3) at Uranienborg on the Island of Hveen in
the Sund from 1582 to 1597. Merle's journal presents a
picture of the weather which would not differ greatly from that
given by a similar journal at the present day. The winters
were certainly not invariably rigorous, for example, 1342 :
" It is also to be noted that there was spring-like weather for
the whole time between September and the end of December,
except on those days to which frost is ascribed, so much so
that in certain places the leeks burst forth into seed, and in
certain places the cabbages blossomed.'' Unfortunately, the
journal is not a day-to-day record so much as a weekly or
monthly summary of the weather, so that it is not easy to
extract numerical data like the frequency of rain-days which
can be compared with similar figures at the present day.
An attempt to count up the rain-days for the two most complete
years, 1341 and 1342, omitting only the " extremely light "
or " very light " rains, gave totals of 152 and 153 respectively,
compared with a present normal of 168, but the difference is
of no significance.
The observations of Tycho Brahe seem to be exceptionally
favourable for determining a difference of climate between
the sixteenth and the nineteenth centuries, because the site
could be accurately identified, and a further series of observa-
tions was made at the same spot from 1881 to 1898. P. la Cour
(3) also has made a careful comparison between Tycho Brahe J s
observations and the mean results at fourteen stations in
Denmark. The most important difference is that the prevailing
THE NATURE OF THE EVIDENCE 285
wind, which is at present from south-west throughout the year,
was in the sixteenth century from south-east, especially in
winter. In winter, south-east winds are cold and dry, whereas
south-west winds are mild and moist, leading to the inference
that the winters were more severe in the latter half of the
sixteenth century than they are at present. The number of
rain-days recorded by Tycho Brahe is about thirty per cent,
below the present mean in winter, whereas in summer the
two figures are nearly the same. There is, however, the
possibility that Tycho Brahe missed some rain-days in winter,
when he would have been out of doors less than in summer.
The number of days with snow is greater than at present,
confirming the view that the winters were colder. H. H.
Hildebrandsson, however, pointed out (4) that the period
1582 to 1597 appears to have had severer winters than the
remaining parts of the sixteenth century ; Tycho Brahe's
observations happened to coincide with a cold spell and were
therefore not representative of the century. This conclusion
from the observations and Hildebrandsson's commentary will
be fitted into their place in the sum total of evidence concerning
climatic changes in Europe in the next chapter.
Observations of wind direction are probably the most
valuable of all the records of old weather diaries, since they
can often be compared directly with present-day records.
An analysis of old wind records in the British Isles, made
by C. E. P. Brooks and T. M. Hunt (5), presented several
results of interest (see Chapter XVIIL). If similar studies
were made for other parts of the world, our knowledge of
climatic changes would be greatly extended.
Some weather journals, apparently from Alexandria,
dating from the early part of the Christian era, described
by G. Hellmann, are referred to in Chapter XX. These
journals would be of the very greatest importance in
demonstrating a change of climate, if it were absolutely certain
that they were made at Alexandria, and not in Greece. That
is the chief difficulty in dealing with early meteorological
observations, whether instrumental or not ; there is generally
an element of doubt somewhere.
Still less satisfactory are inferences drawn from early
descriptions of the climate and physical nature of various
countries. The Roman writers described Britain as damp
286 CLIMATE THROUGH THE AGES
and cloudy, but so would an Italian of the present day, and
we are left in doubt as to whether it was any damper or
cloudier at the beginning of the Christian era than it is to-day.
The general analysis of the literature of the Mediterranean
countries initiated by Arago (6), and continued by a large
number of meteorologists and antiquarians, has shown that
in these countries during the first century of the Christian era
the nature of the vegetation and crops, the dates of sowing
and reaping, and the animal life, all suggest that the climate
differed little from the present. Arago's remarks about the
date and the vine have been quoted by every opponent of
climatic change for the last ninety years the date cannot
ripen its fruit in a mean annual temperature below 21 C.,
the vine cannot abide a temperature above 22 C. ; since
both date and vine flourished in ancient Palestine, the mean
annual temperature must have been 21 C., which is also its
present value. It seems doubtful, however, whether the
solution can be quite so simple as that ; differences of exposure
must come in, and the annual range of temperature from
summer to winter. Even if Arago's strict limits of temperature
be accepted, in a country of such varied relief as Palestine,
the area over which the mean annual temperature at ground -
level (as opposed to mean sea-level) lies between the limits of
21 and 22 C. must be quite a small proportion of the whole.
The effect of a slight change of climate would be nullified by
moving the plantations to a site with a different exposure or
at a different level.
There are two curious features of this mass of anti-variation
literature started by Arago. The first is that it is almost
entirely directed against the idea of a progressive change of
climate, and not against climatic fluctuations. The old
theory of progressive desiccation has been dead for many years,
and all this reiteration is merely killing the slain, for to prove
that the climate of the first century A,D. resembled that of
the present does not prove that the climate of the seventh
century A.D. also resembled the present. The reason probably
is that the progressive theory offers the opportunity for a
definite negation, while the theory of fluctuations does not.
The weather of one year differs from that of another year,
the weather of one decade from that of another decade ; why
should not the climate of one century differ from that of another
THE NATURE OF THE EVIDENCE 287
century ? The question is one, not of fact, but of degree,
which is much less satisfactory. The second point is that
practically the whole of the literature is directed against the
idea that the climate of Europe, Asia, Africa, has become drier.
No one has attempted to prove that the climate has not
become wetter, because the fact is so obvious that no proof
is needed.
The discussion of the literary records of weather follows
a different line of argument. There have at all times been
annalists, who wrote down accounts of the striking events of
their time. They were not concerned particularly with the
weather, but if a great flood or drought, frost or storm, occurred,
they wrote it down. These weather notes have been extracted
by various commentators, who have often been at great pains
to verify the dates and eliminate the errors introduced by
copyists, so that a large amount of fairly reliable material is
now available. It seems a reasonable argument that if a
considerable number of droughts were recorded in one century,
the rainfall of that century was abnormally low ; similarly,
a large number of floods and storms suggest a heavy rainfall.
There are, however, several difficulties to be overcome.
The first is that the completeness of the record changes from
one century to another. Thus the number of records of
droughts may be six in the seventh century and ten in the
thirteenth century, but this does not necessarily mean that the
latter was the drier. The records of storms and floods may
number two in the seventh century and twenty in the thirteenth.
The correct way of stating the evidence would be that of the
total number of records of raininess in the seventh century,
25 per cent, indicate a high rainfall ; of those in the thirteenth
century, 67 per cent., so that the latter century was the wetter.
This gives us a satisfactory method of dealing with records
of raininess, but, unfortunately, records of temperature cannot
be dealt with in the same way, for we have practically only
records of severe winters or hot summers, the mild winters and
cool summers being less often recorded.
The second difficulty concerns the psychology of the annalists.
Vanderlinden (7), in the preface to his " Meteorological
Chronicle of Belgium," divides records of this type into three
stages. In the earliest stage, the authors are concise ; they
merely state " cold winter,' 5 " dry year," etc., without any
288 CLIMATE THROUGH THE AGES
subjective remarks. Later, the records become longer and
more fanciful ; often the chronicler breaks into verse. In
the third stage, there is a certain amount of manipulation of
facts, under the influence of religious or superstitious ideas,
and it is not until the end of the eighteenth century that the
reports again assume a concise and scientific character.
Finally, we have the difficulty that all these annotations tend
to be comparative. Suppose that after a long period of dry
climate there is a change in the direction of greater rainfall,
which accomplishes itself in a period of fifty years, after which
the climate continues at its new level. Obviously the period
of increasing rainfall and the early subsequent years would
suffer by comparison with the dry period which preceded
them, while after the rainier climate had prevailed for one
or two generations it would be accepted as the normal order
of events and would escape comment. Thus, from the records,
the period during and immediately following the change
would actually appear as a rainfall maximum. I think the
maximum rainfall indicated for Europe in the eleventh century
is due in this way to the abrupt change from the dry conditions
of the tenth, while the maximum of the thirteenth century,
which was probably equally if not more pronounced, hardly
appears. The oscillation in the ninth and tenth centuries
also is probably exaggerated from this cause.
There is some discrepancy between the records of the
Classical period and those of the Middle Ages, because the
centre of civilisation moved northward in the interval from
the Mediterranean to Western and Central Europe, that is,
from a generally drier to a more humid climate. The
meteorological events which are considered worthy of record
by the annalists are those which strike them as most unusual ;
in the Mediterranean, a dry summer is taken for granted,
while a wet summer is an event to be recorded ; in North-west
Europe, where the rainfall is usually sufficient at all seasons,
a drought is the more noticeable event. From the records,
one might suppose that the Tiber was more often in flood
than the Thames ; this is because the floods of the Tiber are
short-lived capricious affairs due to sudden heavy storms in the
mountains ; they were considered worthy of record, while
in the Thames the water rises more gradually but also more
regularly, and a certain amount of flooding occurs almost
THE NATURE OF THE EVIDENCE 289
every winter. For these and similar reasons the early records
give an appearance of wetness. Hence the climatic curves
derived from the literary records have to be " calibrated " by
reference to the records of lake levels, advance of glaciers, etc.
Difficulties of chronology may also cause compilations
of historical records to give a false impression of the variations
of climate. When the exact date of say a drought is uncertain,
different annalists may assign it to different years. The
compiler collects all these dates, and quotes a drought for each
of them. In this way a few months of dry weather may become
a drought lasting three, four or five years. Even worse, the
original drought may not have any real existence. Thus
G. E. Britton (8), in his model meteorological chronology of
Britain, considers that the famous three-year drought ended
by St Wilfrid is a later invention to glorify the Saint. For
these reasons, in this revised edition I have attached much
less importance to these early literary records than in the
first edition and I have omitted the diagrams of the frequency
of different phenomena as more misleading than useful.
When we go back beyond the written annals, we come to
the period of tradition. The traditional meteorological event
which will spring at once to the mind is the Noachian deluge,
which finds its parallel in the legends of many other nations
besides the Jews. The Biblical flood was closely similar in
its details to a Chaldaean legend recorded by Berosus, but
most peoples of the Near East, including the Greeks and
Persians, had similar traditions, all of which were probably
derived, in part at least, from the Chaldaean. Curiously, the
Chinese have a flood legend which is remarkably like that of
the Bible. In the Indian version the saving vessel finally
landed on the loftiest summit of the Himalayas. There is
also a flood legend among the Aztecs of Mexico. The meaning
of this widespread tradition is not clear ; if it refers to a single
event in Mesopotamia it must be very old, probably earlier
than 4500 B.C.
Another meteorological legend is that of the twilight of the
Norse gods, when frost and snow ruled the land for generations.
This can reasonably be attributed to a great change of climate
for the worse which occurred about 500 B.C. But since these
traditional meteorological events can only be interpreted in
terms of climatic changes with which we are already acquainted
19
CLIMATE THROUGH THE
through other evidence, they are of little or no help in
elucidating the actual climatic variations ; at most, they can
serve as a confirmation of other evidence.
The fluctuations of lakes and rivers form in general the
most satisfactory evidence for determining changes of climate.
The levels of the Central European lakes during the period
of lake-dwellings are the most reliable source of information
for the long pre-Glassical period in Europe. In Western Asia
the variations of the Caspian form a useful index of the rainfall
during the past 1,500 years or so, eked out by scattered data
from other salt lakes. Lakes without outlet are the most
satisfactory because they respond readily to changes of rainfall.
In both Europe and Asia the fluctuations can generally be
dated by archaeological or historical correlations ; the pro-
nounced variations of level in the salt lakes of Western North
America are of less value because they cannot be dated in
this way. In Europe the variations of rainfall in the basins
of several lakes are recorded in the thickness of annual layers
of sediment, and one of these records goes back to 2300 B.C.
In Africa there is a unique series of actual measurements of
the levels of the Nile, which are dated to within a year.
There is also a large body of evidence about the long-period
variations of level of the Central African lakes, which point
to large fluctuations of rainfall, but unfortunately these
can only be dated very roughly by archaeological means.
Returning to Central Europe, there is a large body of informa-
tion as to the fluctuations of the Alpine glaciers, including
some pre-Classical fluctuations which can be traced and
approximately dated by archaeological evidence. The laws
which govern the movements of glaciers are not yet fully
understood, but a succession of snowy winters and cool wet
summers seem to be most favourable for advance and hot
dry summers for retreat.
The evidence afforded by racial migrations as to climatic
changes depends on the principle that during a period of
increased rainfall there is a movement of peoples from regions
which are naturally moist to regions which are naturally dry,
while during the drier periods the direction of movement is
reversed, the naturally moist regions being occupied and the
dry regions more or less abandoned. E. Bruckner (9) showed,
for example, that emigration from Europe to the United
THE NATURE OF THE EVIDENCE 2QI
States depended on the rainfall. In order that this principle
may be used to determine the course of climatic variations,
certain conditions are necessary. First, there must be large
areas which are on the borderline between aridity and complete
desert ; these areas must be mostly too dry for extensive
agriculture, but with sufficient resources to support under
average conditions a large nomadic population, while a
succession of dry years renders them almost uninhabitable.
In close proximity to this arid region there must be a fertile
well-watered plain, with a long and accurately dated history.
During dry periods the nomads are driven from their homes
by lack of water, but they find little difficulty in moving
from point to point, and the sedentary agriculturists of the
neighbouring plain generally find them irresistible. It is
only in Asia that these conditions are fulfilled in perfection,
the rich plains of the Tigris and Euphrates, the site of a long
succession of civilised states, having on the one side the semi-
deserts of Arabia and Syria, on the other side a great dry
region extending eastwards and north-eastwards as far as
China. We should expect a period of decreased rainfall to
initiate a series of great migrations spreading out from the
dry regions and recorded in the history of the Mesopotamian
states as the invasions of barbarians. The history of Egypt
does not give anything like so complete a record, because
the desert on either side of the Nile valley is too dry, even
under favourable conditions, to support a nomadic population
sufficiently large to have made any impression on the might
of ancient Egypt. The Hyksos conquest of about 1800 B.C.
is the main exception, but the Hyksos themselves probably
came out of Asia. The invasions of China from the west
provide some evidence of climatic fluctuations in the east of
Asia.
The principle that tribal movements were mainly due
to drought is insisted on by H. J. E. Peake in his study of
the migrations of the Aryans (Wiros, as he prefers to call
them). Thus he writes (10, p. 157) : " We have seen
reason for believing that a period of drought, occurring
some centuries before 3000 B.C., drove some of them towards
the Baltic. . . . But the great dispersal was about 2200 B.C.
On this occasion the drought seems to have been more excessive
or more prolonged, for it is believed that the steppe was left
CLIMATE THROUGH THE AGES
for a time almost uninhabited." This evidence, however,
is circumstantial and needs to be used with care. Consider,
for example, the four great outbursts from Arabia, the first
of which occurred during the fourth millennium B.C., the
second, or Amorite, about 2000 B.C., the third, or Aramaean,
from 1500 to 1000 B.C. (according to Peake, mainly 1350 to
1300 B.C.), while the fourth, or Arabian, culminated in the
Islamitic expansion of the seventh century A.D. No reasonable
cause other than drought can be assigned to the first three of
these migrations, but the fourth might be attributed to the
influence of the Moslem religion, were it not that it began
some time before the birth of Mohammed. The Arabs of
the region east of Southern Palestine relate that shortly before
the days of Mohammed, or somewhere about A.D. 600, a
terrible and prolonged drought caused untold havoc, and the
greater part of the tribe migrated to the African coast near
Tunis (n). We have also to distinguish between migrations,
in which large numbers of people (men, women, and children)
moved away from one region and occupied another, and
conquests, in which the ruler of a strong country imposed his
government on his weaker neighbours, and established an
empire. No one would attribute the conquests of Napoleon
Bonaparte, for example, to unfavourable climatic conditions
in France in fact, the reverse conclusion could be argued
more plausibly, namely, that owing to favourable conditions
the state became powerful enough to dominate its neighbours
less favourably situated. The latter is in fact the type of
argument adopted by Ellsworth Huntington in his historical
studies. In compiling a list of migrations, therefore, we must
be careful to omit mere military conquests and raids.
Huntington's contention, as set out in " Civilisation and
Climate" (12), is that a certain type of climate, now found
mainly in Britain, France and neighbouring parts of Europe,
and in the Eastern United States, is favourable to a high
level of civilisation. This climate is characterised by a
moderate temperature, and by the passage of frequent baro-
metric depressions, which give a sufficient rainfall and
changeable stimulating weather. Now it is well known that
the great centres of civilisation in the past lay in more southerly
latitudes than those of to-day, beginning in Egypt, Mesopo-
tamia, and the Eastern Mediterranean, and then passing to
THE NATURE OF THE EVIDENCE 203
Greece and Rome. Huntington attributes these changes in
the centres of civilisation to climatic changes associated with
the northward shifting of the belt of cyclonic activity. In
another volume (13), he gives a detailed comparison of the
history of Rome during the Classical period with the climatic
changes deduced from the growth of the Sequoias in
California (see Chapter XXI.), which he regards as corre-
sponding very closely with the rainfall of the Mediterranean
area. If this principle can be maintained, it obviously
affords a powerful weapon for deducing the existence of
climatic fluctuations during the historical period in other
parts of the world, but S. F. Markham (14) has given an
alternative explanation, namely that the northward migration
of the centres of civilisation has followed improvements in
the methods of heating houses, so that civilised activities
became possible throughout the year in regions with cold
winters, and had no relation to changes of climate.
The width of the annual rings of growth of trees in dry
regions is closely correlated with the rainfall during the
preceding few years, so that old trees offer valuable evidence
as to variations of rainfall during their lifetime. The Sequoias
of Western U.S.A. at present provide the only accurately
dated evidence of climatic fluctuations in that country previous
to the settlement by Europeans. The further description of
the way in which the records are interpreted is postponed to
Chapter XXI. So far, the method has not been applied
to any very old trees outside the United States, but there
seems no reason why it should not be almost equally effective
in other continents.
Geological evidence by itself plays only a very small part
in elucidating climatic changes during the historical period,
because it is only rarely that geological deposits can be dated
with sufficient accuracy. There are considerable possibilities
in the fine seasonally banded clays which have been forming
in lakes and quiet fiords in the glaciated regions. These clays
and the associated annual moraines have yielded valuable
information concerning the rate of retreat of the ice-sheets at
different stages, and in the post-glacial period they have
served to date the peat-bogs and raised beaches which have
supplied abundant evidence of post-glacial climatic changes
in Scandinavia, as described in " The Evolution of Climate/'
294 CLIMATE THROUGH THE AGES
The great deterioration of climate which marked the beginning
of the sub-Atlantic period, about 500 B.C., is dated by
archaeological evidence.
From all this it will be seen that our knowledge of the
climatic changes of the historical period has to be drawn
from a great variety of sources. While this renders the
task of reconstruction more difficult, it has the advantage of
offering frequent opportunities for testing the results by
comparing the conclusions derived from quite different and
independent sets of data. An example of this occurs in the
climatic changes of Europe and Asia. The former are
deduced almost entirely from geological and archaeological
data and from the literary records, the latter from migrations
of peoples and from the levels of the Caspian. The rainfall
curves obtained for these two continents, however, resemble
each other so closely that the fluctuations portrayed are
obviously real.
REFERENCES
(1) HELLMANN, G. "The dawn of meteorology." London, Q,. J. R. Meteor.
Soc. 9 34, 1908, p. 221.
(2) MERLE, REV. W. " Gonsideraciones temperici pro 7 annis." The earliest
known journal of the weather . . . 1337-1344. Reproduced and trans-
lated under the supervision of G. J. Symons. London, 1891.
(3) LA GOUR, PAUL. " Tyge Brahes meteorologiske dagbok, holdt paa
Uranienborg for aarene 1582-1597." Appendix til Collectanea Meteoro-
logica. Kjobenhavn, 1876.
(4) HILDEBRANDSSON, H. H. " Sur le pr^tendu changement du climat europe"en
en temps historique." Upsala, 1915.
(5) BROOKS, G. E. P., and T. M. HUNT. " Variations of wind direction in the
British Isles since 1341." London, Q,. J> R. Meteor. Soc., 59, 1933, p. 375.
(6) ARAGO. " (Evres completes." T. 8. Paris, 1858.
(7) VANDERLJNDEN, E. " Chronique des eVe"nements me'te'orologiques en
Belgique jusqu'en 1834." Bruxelles, 1924.
(8) BRITTON, G. E. " A meteorological chronology to A.D. 1450." London,
Meteor. Off., Geoph. Mem., 8, no. 70, 1937.
(9) BRUCKNER, E. " Klimaschwankungen und Volkerwanderungen." Wien,
1912.
(10) PEAKE, H. J. E. " The Bronze Age and the Celtic world." London, 1922.
(n) HUNTINGTON, ELLSWORTH. "The burial of Olympia." London, Geogr.
J., 36, 1910, p. 657.
(12) HUNTINGTON, E. " Civilisation and climate." 3rd ed. New Haven, 1924.
(13) HUNTINGTON, E. " World power and evolution." New Haven, 1919.
(14) MARKHAM, S. F. " Climate and the energy of nations." London (Oxford
Univ. Press), 1942.
CHAPTER XVIII
EUROPE
WE have no historical records for Europe which go
3ack much more than half-way to the year 5000 B.C.
On the other hand, the geological evidence has been
studied in great detail, and the chronology of the whole
period since the glaciers commenced their final retreat is
rapidly being placed on an exact basis. Since the beginning
of the Christian era, there is a rich European literature which
provide? a wealth of material. The " official " end of the
Glacial period in Sweden, according to the Scandinavian
geologists, is now dated about 6500 B.C. (i), but by this time
the climate of Central and Western Europe had become
definitely temperate, and the latest glacial period really ended
much earlier. The Fenno-Scandian end moraine, dated
about 8300 B.C., which encloses most of Scandinavia and
Finland, would give in some ways a more appropriate date.
After this the rapidly disintegrating remains of the ice-sheet
can have had little effect on the climate of Europe. The
" post-glacial " stages are set out in Table 20, with their
archaeological equivalents in Western Europe, The latter
is only approximate, since cultures do not appear simultan-
eously over the whole area.
The dating of the various archaeological stages is determined
by the known historical sequence in the Eastern Mediterranean,
Egypt, and Mesopotamia, and especially the two latter. In
Mesopotamia, a fixed point is provided by the total solar
eclipse of 1 5th June 763 B.C., which was recorded in the annals.
In Egypt, the dates can be approximately fixed by the heliacal
risings of Sirius and by certain new-moon festivals, but 1580 B.C.
is the earliest date in Egyptian history which can be regarded
as certain within a few years. As regards the dating of earlier
periods, finality is still far from being attained even in Egypt
or Mesopotamia, and this doubt is added to in dealing with
events in Europe which can only be dated by associating them
with some event in the East.
293
296
Date.
6000
5000
4000
3000
2OOO
1000
B.C.
A.D.
IOOO
Climatic stages.
Boreal
Dry, becoming
warmer
Atlantic
Climatic
Optimum
Humid
Sub-boreal
becoming cooler,
drier
Sub-Atlantic
Cool, wet
CLIMATE THROUGH THE AGES
Culture . Vegetation .
Maglemose
Alder, Oak, Elm
Late Tardenoisean Peat
Campignean
Neolithic
Lake-dwellings
Bronze
Early Iron
Romano-British
Lime
Oak giving place
to Pine
Yew
Peat
Beech in Central
Europe
Near present
Table 20. Post-glacial succession, Western Europe.
The broad early stages of the climatic succession are now
very well known from innumerable studies of peat-bogs in
Northern, Central and Western Europe. The successive
stages can be placed in sequence by the microscopical analysis
of the pollen grains which are found in peat-bogs and lake
deposits. Certain pollen grains of some trees and plants
are almost indestructible, and since they are produced in
large numbers and scattered by the wind, they are very
widespread. The technique of their study has been highly
developed by G. Erdtman in a large number of papers, and
has enabled a very close comparison to be made of the forest
succession in different parts of Europe. The absolute dating
is given by the prehistoric objects which are found in the bogs.
Generally speaking, the ground laid bare by the retreat
of the ice was a maze of depressions and ridges. The hollows
were occupied by lakes and ponds, and the ridges first by an
arctic flora, which soon gave place to birch, followed by pine.
By about 7000 B.C. the climate was dry and sufficiently warm
in summer for the rapid spread of hazel. The rise of tem-
perature continued, and with some increase of moisture, by
6000 B.C. all the western half of Europe was occupied by a
rich forest of oak, alder and elm, the alder being favoured by
EUROPE 297
the increasing rainfall. This was the beginning of the
" Climatic Optimum/ 5 with temperatures up to 5 F. higher
than the present, permitting forests to grow much higher up
the mountain sides than is possible now. The heavy rainfall,
however, favoured the growth of peat, and about 5000 B.C.
large areas of forest were killed and buried by peat-bogs.
This phase continued until about 2500 B.C., with gradually
decreasing temperature, when there occurred a rather puzzling
change. The peat ceased to grow and in many places the
surface of the bogs was occupied by forests of pine and yew.
This was the Sub-boreal, which was formerly supposed to be
rather warm and very dry.
It seems certain that at some stage in the Sub-boreal there
was a prolonged drought. Lakes decreased in area and in
a few places trees grew on the floor of dried-up lake basins
below the level of the outlet ; providing definite evidence
that at these places the rainfall was less than the evaporation,
and this enables us to estimate the actual rainfall. The
following table gives estimates for four such lakes in places
for which I have been able to obtain statistics or estimates of
the present rainfall and evaporation. In a drier climate
evaporation was presumably more active than at present and
I have accordingly increased the evaporation figures by
one-third.
Estimated
Present
Estimated
Per cent.
present
rainfall.
sub-boreal
of
evaporation.
rainfall.
present.
inches.
inches.
inches.
I 5 .6
50
21
42
15-6
40
21
52
Donegal . .
Connaught
Sager Lake
nr. Bremen . 13-3 27 18 67
Sechof, Lunz
Austria ... 20-0 56 27 48
The average of the last four figures in this table indicates a
rainfall of only about half the present amount in Western and
Central Europe. This is less than the rainfall of the dry year
1921 and points to a real and very marked change of climate
in Europe.
Botanists, however, have questioned the possibility of a
dry period of such intensity lasting for over a thousand years,
G. Erdtman (3) points out that there was a steady development
298 CLIMATE THROUGH THE AGES
of forests throughout the Sub-boreal, and he considers
that during this period there was a gradual change towards the
cool moist Sub-Atlantic type, but at the very end there was a
relatively short period of perhaps 200 years which might
be described as a dry heat wave, giving place abruptly to much
cooler and moister conditions. Similarly H. Godwin and
A. G. Tansley (4) write of southern England :
" The best opinion of archaeologists and pre-historians
generally is beginning to question the validity, for these
islands at least, of the clear-cut conception of a wet Atlantic
and a dry Sub-boreal period. Evidence of a major climatic
Atlantic-Sub-boreal transition in Britain, comparable with
the thoroughly well established Boreal-Atlantic and Sub-
boreal-Sub-Atlantic changes, is often lacking and there may
perhaps have been several important alterations of climate
between say 5500 and 500 B.C. But it is certain that part at
least of the Bronze Age in England was relatively dry. In
contrast with Neolithic settlements on chalk summits, chalk
uplands seem to have been practically uninhabited during the
Bronze Age, plausibly because of shortage of water.
In the middle of ist millennium B.C. climate became cool
and wet ; of the reality of that transition there is no doubt."
H. Godwin (5) considers that chalk and limestone soils,
now mostly occupied by natural beechwoods, were too dry to
carry close woodland during the Sub-boreal and raised bog
surfaces were much drier than in the Sub-Atlantic. But a
bog-surface is very sensitive to climatic change, and in some
British bogs there are several layers indicating a cessation
and resumption of growth. E. M. Hardy (6) found five dry
phases in the bogs of Shropshire, which he places in the
upper part of the full Boreal, in the Atlantic, in the Sub-
boreal, at the top of the Sub-boreal and about midway in
the Sub-Atlantic. These would be about 6000 B.C., 5000 B.C.,
1200-800 B.C., 700-500 B.C., and at the beginning of the
Christian era.
E. Granlund (7) from an intensive study of Swedish peat
bogs, also found evidence of five dry layers, which ended
about 2300, 1 200, 600 B.C. and A.D. 400 and 1200, that about
600 B.C. being the best developed. This probably represents
the " Grenzhorizont " of the Sub-boreal which is widespread
in Europe.
EUROPE 299
About 500 B.C. there was a very rapid large-scale climatic
change. Over large areas the Sub-boreal forests were killed
by a rapid growth of peat, which was certainly caused by a
great increase of rainfall and probably also a fall of tem-
perature. As many of the bogs formed at this time are now
drying up, the rainfall was probably much greater than at
present. From this peak it has gradually declined, but with
a number of oscillations of smaller amplitude. The general
results of these peat-bog investigations forms the main basis
of the top three curves of Fig. 31.
BC. O A.D.
^/
1
Britain
xP
Get \fral Europe
Swederv
S.E.Burope
50
150
V
IOO O
&.
Fig. 3 1 . Variations of rainfall in Europe.
The records of peat-bogs are borne out by the evidence
of land mollusca as summarised by A. S. Kennard and other
writers in England. These point to a very wet climate in
the Atlantic period. The beginning of the Sub-boreal was
still humid but the rainfall was decreasing and by 1500 B.C.
the wet period had largely passed. About 1000 B.C. the
climate resembled the present.
A detailed study of the changes in level in the Central
European lakes has been made by H. Gams and R. Nordhagen
(2). The earliest part of the Neolithic, while warm, was
decidedly moist, but it appears that the greater part of the lake-
dwelling period was one of low water and of relatively high
temperature. It is supposed, in fact, that the lake-dwellings
were established, not in the lakes themselves, but on peat-bogs
which are now covered by the waters of the lakes. The
3OO CLIMATE THROUGH THE AGES
succession of events has been made out most completely in
the Feder See basin, but other lakes confirm it. During the
Neolithic period the lake was smaller than now, indicating
a dry period which seems to have culminated about 2200-
2000 B.C. This was followed by a period of somewhat greater
rainfall, but drier than the present, and somewhere in this
period was a " high-water catastrophe " a brief regime
of floods which destroyed many of the lake-dwellings. This
flood period cannot be far distant in time from the great
eruption of Bronze Age peoples from the Hungarian plain,
which probably occurred soon after 1300 B.C., and carried
the Phrygians into Asia (see p. 320). It is quite likely, in
fact, that the flood was the stimulus which caused the migration
to a drier climate. After this moist period the lakes again
shrank rapidly to a second minimum, about 1000 B.C., at the
close of the Bronze Age. During this dry period the chief
settlements were located in moist localities, and agriculture
was carried on in places now above the forest level and even
above passes which are now glaciated. Sand dunes on the
lake shores appear to have been formed mainly in this period.
Near the end of the Hallstatt period, about 500 B.C., the
levels of the lakes rose suddenly ; in the Boden See (Lake
Constance) the rise exceeded 30 feet. Most of the lake
villages were destroyed, and settlement in the Alps reached
a minimum, while the occasional remains are concentrated
in the warmest valleys. The Alpine mountain settlements,
including even those where mining for metalliferous ores and
salt was carried on, were abandoned, and Gams and
Nordhagen remark that this climatic fluctuation had the
appearance of a catastrophe. From this peak the rainfall
gradually declined and by the Roman period was very little
above the present, since in the first century A.D. the Boden See
was near its present level, and roads were made across the bogs.
From about A.D. 180 to 350, settlements were again con-
centrated in the driest localities, indicating a return of moist
conditions. In the fourth and fifth centuries A.D. many
German settlements were established on low ground, now
swampy, and this dry period probably continued until the
end of the tenth or the middle of the eleventh century, inter-
rupted in the eighth century by a moist interlude indicated by
a rise of lake levels.
EUROPE 3OI
Traffic across the Alpine passes, as shown by the transmission
of culture, became important about 1800 B.C. (8) when the
Brenner Pass first became traversable, and reached a maximum
at the end of the Bronze Age and in the Early Hallstatt period,
or about 1200-900 B.C. The valley settlements of the Late
Hallstatt period developed independently apparently in
complete isolation, and traffic across the passes was at a
minimum. There was a slight revival at the end of the La
Tene period and in the early Roman Empire (200 B.C. to
A.D. o), but it was not until between A.D. 700 and 1000 that
this traffic again developed on a considerable scale. There
was a re-advance of the glaciers in the western Alps about
A.D. 1300, followed by a retreat to a minimum extent in the
fifteenth century. Near the end of the sixteenth century the
glaciers advanced rapidly and about 1605 they overran settle-
ments which had been occupied since the beginning of history.
About the same time the glaciers advanced in the Eastern
Alps, Iceland, where they almost reached the moraines of the
Late Glacial stages, and probably in other parts of the world ;
and the period from 1600 to 1850 has been termed the " Little
Ice-Age." There were minor maxima of glaciation about
1820 and 1850 ; since then the glaciers and ice-sheets have
been in rapid retreat in all parts of the world. A useful
summary of the advances and retreats of glaciers is given by
F. E. Matthes (9).
From Russia we have a remarkable series of measurements
by W. B. Schostakowitsch (10) of annual layers in the mud
deposits of Lake Saki, a salt lake on the west coast of the
Crimea in 45 7' N., 33 33' E., separated from the sea by a
strip of sandy beach. The total thickness of the deposit is
several metres, most of the individual layers measuring only
a few millimetres. The measurements, in tenths of a milli-
metre, were made partly on photographs and partly on the
original sections ; the earliest layer is dated 2294 B.C., but
in some parts of the sections it was difficult to distinguish the
lines separating the annual layers and there may have been
some errors in calculating the age of parts of the sections.
This series is a valuable climatic record, comparable in
importance with the tree-rings of Western America. The
variations in the thickness of the layers are almost certainly due
to variations of rainfall, and in particular of heavy rainstorms
3O2 CLIMATE THROUGH THE AGES
which would cause rapid run-off. The measurements show
a number of isolated years with very thick layers five or
ten times as thick as neighbouring layers, which is consistent
with this suggestion. A possible connexion with rainfall was
further examined by comparing overlapping five-year means
with the variations in level of the Caspian Sea. Although the
curves show many differences of detail, there can be no doubt
that their general course, from maximum to minimum and
back to a second maximum, is similar. Since rainfall is
presumably an important factor in determining the fluctuations
of the Caspian, it seems probable that the deposits of Lake
Saki are also a rough measure of rainfall. The measurements,
smoothed over 50 years, are shown in the lowest curve of
Fig. 31. This shows the high maximum just before 2000 B.C.,
but the remaining fluctuations do not resemble those to the
north and west very closely. We may complete the picture
for South-east Europe with some data for Lake Ostrovo in
West Macedonia collected by M. Hasluck (n). From
historical evidence he concludes that the lake was high in
Byzantine days and in the eleventh and thirteenth centuries,
after which it was low from at least 1400 to the early nineteenth
century. These details fit in excellently with Schostakowitsch's
data.
When we turn to North-west Europe we find fewer records
to which exact dates can be assigned, but the evidence fits in
well with that from Central Europe. About 1200 to 1000 B.C.
there is evidence of considerable traffic between Scandinavia
and Ireland, probably indicating a minimum of storminess.
There is other evidence that the climate of Ireland during the
Bronze Age was dry and favourable, and that a high civilisation
developed both there and in Scandinavia. The Iron Age was
a time of great peat-formation, and peat-beds on the Frisian
dunes between two layers of blown sand are dated 100 B.C.
Some peat-bogs in Northern France were formed during the
Roman period. This was a time of eclipse for the northern
peoples, who were unable to maintain their high culture.
In about 120 to 1 14 B.C., a period of especially great storminess
in the North Sea (Cimbrian flood) caused the wanderings of
the Cimbri and Teutons, and gave the coast of Jutland and
North-west Germany its present form (18). About A.D. 500
there began a second period of high culture, centred on Tara,
EUROPE 303
in which Irish learning was greatly esteemed in Europe,
and a further great outburst of maritime activity.
The thirteenth century was very stormy in the North Sea,
and many inroads of the sea were reported in the annals.
G. E. Britton (12) gives the following frequencies of " marine
floods " in Britain :
A.D. looi- 1051- noi- 1151- 1201- 1251- 1301- 1351- 1401-
1050 1 100 1150 1200 1250 1300 1350 1400 1450
I I I 4 3 II 2 2 I
The worst years seem to have been 1176 in Lincolnshire,
1250, when much of Winchelsea was destroyed, and 1287-8
when there were three inundations. There was a corres-
ponding maximum on the coast of Holland. Some of these
storms did great damage. In the winter of 1218-19 the
coastal defences of Holland and Frisia were broken through and
large areas inundated.
A study of the history of water mills in East Kent, by
G. M. Meyer (13) points to a period of heavy rainfall in
South-east England, which began some time before 1087
(Domesday Book) and ended during the thirteenth century.
The rainfall about 1303 was much less than in 1217, and is
probably still less to-day.
I have already alluded to the remarkable absence of
mention of ice in the early reports of Norse voyages to Iceland
and Greenland (Chapter VIII.), and this subject is discussed
further in Chapter XXII. It seems probable from the
descriptions that previous to A.D. 1000 the climate of Iceland
was more genial than to-day ; considerable areas cultivated
in the tenth century are now covered by ice. Some remarkable
evidence concerning climatic change in Greenland is discussed
in Chapter XXI. The Iceland glaciers probably reached
their maximum extent of the Christian era in the first half of
the fourteenth century. In the fifteenth and sixteenth
centuries they retreated, to advance again in the seventeenth
century, several farms being destroyed about 1640 or 1650.
Since then there has been a slight retreat.
The records of ice in Danish waters were collected by
C. I. H.* Speerschneider (14), who concluded that they show
no evidence of essentially different conditions in the past.
The first record of a severe ice-year which he could discover
304 CLIMATE THROUGH THE AGES
was for the year 1048. For later centuries he gives the
following table :
Century. Severe Ice. Ice-free.
1 2th . . 3
1 3th 3
1 5th 9 5
i 6th 19 9
i yth 26 12
1 8th 27 33
1 9th 42 58
From the very few occasions when the duration of the
ice could be determined, he forms the following summary :
Period. Gases. Duration (Days).
1296-1546 4 49
1 583- 1 595 5 49
1619-1674 6 76
1700-1750 6 51
In opposition to Speerschneider, I think that both these
tables point to a period of cold winters in the seventeenth
century, and the first also suggests that the fourteenth century
was cold. The detailed records place this minimum of
temperature in the- first half of this century.
Before proceeding to a further discussion of the literary
records, it will be well to summarise the results already gained
(Table 21). In this table the records are collected in chrono-
logical order.
The literary records of Europe provide a great mass of
information which has been described by various authors.
The British records were collected by E. J. Lowe (15) and have
been exhaustively analysed up to A.D. 1450 by C. E. Britton
(12). Records of British droughts have also been collected
by G. J. Symons (16). For Belgium an extensive compilation
has been made by E. Vanderlinden (17). In addition, I
have made use of a compilation by R. Hennig (18) which
relates to the whole of Europe, but may be rather uncritical.
For Britain up to 1450, I have used Britton's work exclusively,
omitting years with both wet and dry periods and for droughts
after that date I have relied exclusively on Symons's work.
EUROPE 305
His rather uncritical assiduity makes his numbers somewhat
excessive, and there is a discontinuity after 1450. From
these sources I have summarised the number of years in each
half-century with
(a) Great storms, floods, heavy rain, or wet summers.
(b) Hot dry summers or droughts.
These are shown in Table 22.
Under (a) an attempt was made to eliminate floods or great
rains associated with isolated thunderstorms in summer.
It will be observed that all the records increase in frequency
as they approach the present day ; this is, of course, simply
because the volume of literature becomes greater. In addition
to this, certain periods stand out because the records include
an abnormal percentage of storms and rain, or of drought.
Thus the period from about 600 to 800 appears to have
been rather dry and, at the beginning, mild, while 800 to
950 and 1050 to 1350 were generally rainy. The period
1701 to 1750 comes out as unusually dry. In order to obtain
the figures of " raininess " of Europe, the three records were
B.C.
5400 Moist and warm.
5000 Drier and cooler.
4500 Moist, rather warm.
4000-3000 Becoming cooler and drier.
2 200 Very dry especially in Central Europe.
2000 Rainy period.
(1275 ?) Short maximum of rainfall (lake villages destroyed) .
1200-1000 Dry and warm, sea traffic.
850 Somewhat moister and cooler.
700- 500 Dry and warm.
500 Sudden increase of rainfall, much cooler. Begin-
ning of Sub-Atlantic.
A.D.
o Climate similar to present.
100 Drier and warmer.
J 8o- 350 Wetter.
600- 700 Dry and warm. Alpine traffic.
800-1200 Little ice. Rainfall heavy in Central Europe.
1200-1300 Great storminess, mild winters, probably rainy.
1600 Beginning of general advance of glaciers.
1677-1750 Dry period generally, mild winters.
1850 Beginning of general recession of glaciers.
Table 21. Fluctuations of climate in Europe.
20
CLIMATE THROUGH THE AGES
Period.
Europe
(General).
Britain.
Belgium.
Total.
" Raini-
ness."
W
(*)
(a)
W
(a)
W
W
W
IOO-5I B.C.
2
O
2
o
50 B.G.-O
5
I
5
i
A.D.
O-5O
3
3
51-100
7
2
7
2
IOI-I5O
4
O
4
I5I-2OO
5
O
5
O
201-250
i
O
-
i
251-300
3
I
. ,
. .
. .
3
I
301-350
i
I
i
I
351-400
3
I
. .
3
I
73
401-450
6
O
. .
6
O
75
451-500
2
I
2
I
82
501-550
5
I
..
5
I
80
551-600
601-650
651-700
16
6 4
3
2
3
i
*
16
t
3
2
4
72
7
60
701-750
3
I
o
2
3
3
50
751-800
4
5
i
O
5
5
50
801-850
8
i
o
O
3
I
ii
2
85
851-900
IO
5
2
O
6
2
18
7
72
901-950
6
i
4
I
o
10
2
83
95I-IOOO
5
8
I
O
3
I
9
9
50
IOOI-I05O
21
8
2
5
2
28
10
74
I05I-IIOO
18
3
5
3
ii
3
34
9
79
IIOI-II50
22
7
6
4
13
3
H
75
H5I-I200
I20I-I250
22
28
9
7
ii
16
6
6
22
19
8
8
i
23
21
75
I25I-I300
25
5
I3
7
8
3
46
15
75
1301-1350
24
4
II
5
8
i
43
IO
81
I35I-I400
17
16
7
4
18
8
42
28
60
I4OI-I450
7
ii
i
16
8
42
16
72
I45I-I500
8
4
3
6
12
8
23
18
56
I50I-I550
2
6
II
12
I3
18
62
I55I-I600
IO
12
22
7
32
19
63
1601-1650
8
X 5
'9
12
27
27
5
1651-1700
. .
20
22
17
8
37
30
55
I70I-I750
J 7
29
23
18
40
47
46
Table 22. Numbers of (a) storms and floods,
(b) droughts, in Europe.
combined in the following way. First of all the numbers
under the heading (a) for Britain, Belgium, and Europe in
each half-century were all added together, irrespective of
whether or no some of those from different sources referred
to the same year. It was considered that if a particular year
appeared as stormy in more than one record, greater weight
EUROPE 307
should be attached to it, and this process of simple addition
answered the purpose. The same process was then followed
with the records under (b). The number of records under
(a), i.e., storms, floods, and great rains, entered to each half-
century was then expressed as a percentage of the total number
under both (a) and (b). The numbers up to A.D. 600 were
then smoothed by adding together the records for five successive
half-centuries and allocating the mean to the middle period
of the five.
On the whole the figures for " raininess " obtained from the
literary records agree with the data from other sources, but
add little to the latter. The apparently rainy character
of the years 45 1 to 650 is due entirely to a large number of
records of floods in the Tiber collected by Hennig for the
last half of the sixth century, and the true figure for raininess
should probably be much lower, comparable with the period
651 to 800. The last part of Table 22 is over-weighted by
Symons's droughts but there seems little doubt that the period
round 1700 actually was abnormally dry (see p. 309).
Actual rainfall records date from the seventeenth century. In
England a record at Townley, near Burnley in Lancashire,
commenced in 1677 and continued with intermissions until
1704. Meanwhile, in 1697, a record was commenced in
Upminster, Essex, which continued until 1716. From this
date there was a gap of nine years, after which records began
at Southwick, near Oundle, in 1726, and at Plymouth in 1727.
Both these are overlapped by a long record from Lyndon,
Rutland, commencing in 1737, and from that date there is no
difficulty in carrying on the story. The records from 1726
onwards were collated by G. J. Symons who published a
well-known table of the annual values in British Rainfall for
1870, but owing to the gap the earlier records could not be
compared directly with his series, while the Plymouth record
came to light after Symons had completed his calculations.
The method employed by Symons was to calculate for each
station the average for the period over which its records
extended, and to express the rainfall for each year as a
percentage of that average. The various series were then
reduced fo a common basis by means of corrections deduced
from the years during which pairs of records overlapped.
This process is sound if the records are truly homogeneous,
308 CLIMATE THROUGH THE AGES
and if the stations are close together and overlap by a sufficient
number of years, but it becomes hazardous when it is applied
several times to isolated records and the overlapping periods
are short. For example, the reduction from the average at
Southwick to that at Lyndon is based on an overlap of only
three years, of which 1737 was much wetter at Lyndon than
at Southwick, while in 1738 and 1739 Southwick was slightly
wetter than Lyndon. Hence some sort of a check seems to
be necessary, and this was provided by expressing the early
figures as percentages of the normal for the period 1881 to
1915, and comparing these figures with the percentages
obtained by Symons. The results show very close agreement :
Percentage given Percentage of
by Symons. 1881-1915.
1726-1736. Southwick ... 99 97
1740-1760. Lyndon .... 78 78
1727-1752. Plymouth ... 83 84
This table is a remarkable testimony to the accuracy and
judgment of the reductions carried out by Symons. It is
reassuring on another point also. We know nothing as to the
construction and exposure of these old gauges, but if they had
recorded less than the correct amounts, the figures in the
second column of percentages would have been lower than those
in the first. The figures may be accepted as reasonably
accurate, and this encourages us to accept also the figures
for Townley and Upminster. These cannot be reduced to
the present-day normal by Symons's method, so they have been
compared directly with estimated normals for the period
1881 to 1915.
In this way we obtain the following figures of rainfall
in percentage of present normal :
Per cent, of normal.
1677-1686. Townley 89
1689-1693. Townley 90
1697-1703. Townley and Upminster ... 91
1704-1716. Upminster 93
1726-1730 100
The rainfalls for the different decades, revised and brought
up to date by J. Glasspoole, are given in the second column
of Table 23, expressed as a percentage of the long period
average for 1726 to 1940. Figures in brackets indicate an
incomplete decade. The third column gives values for Paris
EUROPE
309
Eng-
Hol-
Eng-
Hol-
Years.
land.
Paris.
land.
Mean.
Years.
land.
Paris.
land.
Mean.
1677-1686
89
89
1821-1830
105
97
103
1 02
1689-1703
90
1 08
99
1831-1840
IOO
99
95
98
1704-1720
93
98
95
1841-1850
103
103
107
104
1721-1730
(101)
86
1851-1860
99
101
97
99
1731-1740
92
86
(101)
91
1861-1870
97
95
104
99
1741-1750
87
88
90
88
1871-1880
112
104
no
109
1751-1760
99
109
104
1881-1890
101
90
103
98
1761-1770
102
113
107
1891-1900
98
93
104
98
1771-1780
97
105
95
99
i 90 i - i 9 i o
98
101
99
99
1781-1790
96
98
103
99
191 1-1920
107
1 06
1 02
105
1791-1800
99
79
90
89
1921-1930
107
117
101
1 08
1811-1820
99
96
97
97
1931-1940
103
109
97
103
1941-1947
98
.
Table 23
.Rainfall
means
by decades, percentages of
long
period
average, Western Europe.
collected from various sources and expressed as percentages
of the average for 1771 to 1940. The fourth column gives a
long series for Zwanenburg in Holland corrected to Hoofddorp.
The final column of this table gives the means of the three
series, as far as they go.
Two long records are algo available for Sweden, namely,
Uppsala, near Stockholm (1741-1940) and Lund in the extreme
south (1748-1910), but these differ so greatly in the early
years that they cannot both be correct. They agree in
showing a low rainfall before 1 760, but whereas at Uppsala the
period 1761 to 1820 is recorded as very dry, at Lund the years
1771 to 1810 are excessively wet. These two series have
therefore been omitted.
Table 24 gives figures for Padua and Milan. The record
for Padua runs from 1725 to 1900, that for Milan begins in
1764. The Padua series was corrected to Milan to make one
record. The percentages for Paris, Hoofddorp and Milan
were calculated by Miss N. Garruthers.
Years. Per cent.
1725-1730 (in)
1731-1740 89
I74I-I750 102
1751-1760 112
I76I-I770 113
I77I-I780 87
Years. Per cent.
1781-1790 91
1791-1800 99
1801-1810 104
1811-1820 103
1821-1830 99
1831-1840 105
Years. Per cent.
1841-1850 118
1851-1860 106
1861-1870 92
1871-1880 100
1881-1890 108
1891-1900 105
Years. Per cent.
1901-1910 101
1911-1920 108
1921-1930 79
1931-1936 102
Table 24. Rainfall means by decades, Milan, Italy.
The rainfall minimum in the first half of the eighteenth
century appears to have been widespread. Table 25 gives
the values for places with a record covering at least ten years of
the period, as percentages of recent averages.
310 CLIMATE THROUGH THE AGES
Per cent, of i Per cent, of
Place. Period, normal.
Uppsala . . 1721-1731 87
I739-I749
Lyndon . . 1740-1749 77
South wick . 1726-1736 97
Upminster . 1701-1716 84
Plymouth . 1727-1750 94
Place. Period, normal.
Berlin . . . 1729-1739 91
Paris . . . 1701-1750 86
Bordeaux . 1714-1750 87
Padua . . 1725-1750 103
Charleston . 1738-1750 100
(South
Carolina)
Table 25. Rainfall of first half of eighteenth century.
The figures for Western and Central Europe are remarkably
consistent, and point clearly to a persistent dry period during
these years.
There is one other interesting feature about Table 23.
Rainfall maxima occur in 1761-1770, 1821-1830, 1871-1880,
1921-1930, and possibly 1689-1703, i.e., at intervals of between
50 and 60 years. The Milan series shows maxima in 1725-
1730, 1761-1770, 1801-1810, 1841-1850, 1881-1890, and 1911-
1920, i.e t) an average interval of nearly 40 years. W. H.
Bradley (see p. 108) found a cycle of about 50 years persisting
for a period of several million years in the Eocene of the
West-central United States. The cause is unknown, but if
this cycle is verified from other records it may be of great
economic importance.
Fig. 32 aims at giving a reconstruction of the variations
of temperature in Western Europe. The uppermost curve is
based mainly on the estimates of botanists from plant remains,
especially tree pollen (see p. 296) and is necessarily
generalised. The lower curve is constructed from the estimates
of the character of each winter given by C. Easton (19).
Up to 1 200 records are scanty ; from A.D. 401 to 800 only
4 to 8 per century and from 80 1 to 1200 only 7 to 17 per
half century. These refer mainly to severe winters and the
direct average of Easton's figures is therefore too low. I
applied a rough correction by plotting the mean of the
years given against the number of years and drawing a smooth
curve through them, then reading off the difference between
this curve and the actual mean. The results show cold
periods about 900, noo and 1551 to 1700, and warm periods
about 1175, 1300 and 1725. This curve is repeated in the
right hand part of the upper curve of Fig. 32.
Reliable instrumental records of temperature begin about
EUROPE 311
the middle of the eighteenth century. Series for Lancashire
were discussed by G. Manley (20) and compared with those
for Edinburgh, Oxford, Durham and Stockholm. A. Labrijn
(21) gives a series for De Bilt, Holland, from 1741 to 1940.
All these show a series of oscillations of the order of 30 to 40
years, while the winter temperatures, but not those for other
seasons, have in addition a steady rise which persisted from
about 1810 until it was interrupted by the series of cold
winters in the present decade. This rise of winter tem-
perature has affected the greater part if not the whole of the
Northern Hemisphere and is an important climatic fact.
6 OOP
4OOO
aooo
BC. O AD.
-5'F.
400 AD
BOO
gooo
s~\
/ \ <
/?
/
^ -J'
s
\~' ^-'
L/
r\J C7 ^
Fig. 32. Variations of temperature in Europe.
Upper curve, general. Lower curve, severity of winters (Ea^ston).
A good deal of information about variations of wind
direction in Britain is available from various sources. An
interesting paper by Leonard S. Higgins (22) gives some
inferences as to the prevailing wind directions in South Wales
since about 400 B.C. The sand dunes have formed where the
coast faces west or south-west, and have since moved inland at
intervals. The archaeological and historical evidence, which
is plentiful, is discussed in detail, with the following results :
1 . Blown sand was present, and had recently been increasing,
about 400 to 200 B.C.
2. After that date the area appears to have become stable,
and the dunes fixed by vegetation, until the end of the
thirteenth century.
3. Soon after 1300, references become frequent to moving
sand obliterating roads and pastures and burying
CLIMATE THROUGH THE AGES
buildings. By 1553 an Act of Parliament " touching
the sea sands of Glamorgan " had become necessary.
4. After about 1550, conditions appear to have gradually
become more stable, and the dunes became increasingly
fixed by a plant cover.
The inroads of the sand depend on the conjunction of
several circumstances, especially a period of abnormally high
tides and a period of stormy west or south-west winds. The
results of the investigation, therefore, suggest periods of
stormy winds from west or south-west before 200 B.C. and again
from about 1300 to 1550, while during the intervening and
succeeding periods these winds were less frequent or less
strong.
From 1341 to 1343 there are a number of entries of wind
direction in the weather diary kept by the Rev. W. Merle.
These were analysed by G. E. P. Brooks and T. M. Hunt (23)
and show a dominant wind from west-south-west.
In the latter half of the sixteenth century, however, the winds
appear to have been more easterly. G. M. Meyer wrote to
me : " The following quotation is taken from * A Restitution
of Decayed Intelligence,' by Richard Verstegan, first published
at Antwerp in 1605 ? ^ appears on p. 109 of a copy dated
London, 1634 : "... old shippers of the Netherlands
affirming, that they have often noted the voyage from
Holland to Spaine, to be shorter by a day and halfe sayling
than the voyage from Spaine to Holland."
The natural inference to be drawn from this record is
that, during the sixteenth century, easterly winds were more
prevalent in the English Channel than westerly winds
contrary to modern experience. The following quotation
from a book published in 1579 though ambiguous is to
the same effect : " And the winds in Winter blow about the
morning, but in the Sommer about the evening, and in
the Winter out of the East, and also in the Sommer, out
of the West."
The observations of Tycho Brahe at Uraniborg in Denmark
from 1582 to 1597 also point to conditions over North-west
Europe differing from those prevailing at present. According
to the discussion of these observations by D. La Cour (24) all
easterly winds were more frequent than at present, and the
EUROPE 3 1 3
prevailing direction was actually south-east instead of
south-west.
About the middle of the seventeenth century the winds appear
to have been rather variable in England. Sir Francis Bacon
(25, p. 35) wrote : " In Europe these are the chief stayed
winds, North windes from the Solstice, and they are both
forerunners and followers of the Dog-starre, West windes from
the Equinoctiall in Autumne, East windes from the Spring
Equinoctiall ; as for the Winter Solstice, there is little heed
to be taken of it, by reason of the varieties." On page 39,
however, there is an indication that the wind was mainly
westerly. Probably the winds were more variable than at
present but with some predominance from the west.
From 1667 onwards (23) there are a sufficient number of
old weather diaries to give a fairly complete picture. The
direction and " constancy " (per cent.) for each half-century
are as follows :
London
Direction
Constancy .
Edinburgh
Direction
Constancy .
Dublin
Direction
Constancy .
The resultant directions are shown in degrees from north
through east ; 180 is a south wind, 225 a wind from south-
west and 270 from west. Constancy is the ratio between
the total movement of the air from the resultant direction,
assuming that each wind has unit velocity, divided by the
number of winds and multiplied by 100. A constancy of 33
means that two-thirds of the winds blew more or less from the
prevailing direction and the remainder from opposing direc-
tions. The figures show for London a steady swing from
nearly W.S.Wi to nearly south, and back again. The
most remarkable periods at London were from 1740 to
1747 and from 1794 to 1810, both of which show a
predominance of easterly winds. The Dublin series also
shows mainly easterly winds in 1740 to 1748, but observations
are missing for the second period. The decade 1740 to 1749
1667-
1700
1701-
1750
1751-
1800
1801-
1850
1851-
1900
1901-
1930
237
226
190
221
235
239
19
15
35
21
19
21
243
31
245
15
253
25
257
26
241
30
241
248
2 5 8
262
256
*9
22
27
25
17
3*4
CLIMATE THROUGH THE AGES
was the driest in England since observations began. During
the period 1794 to 1810 the winters in England were
exceptionally severe. The years 1901 to 1930 were remarkable
for the great steadiness of the W.S.W. winds, which is no
doubt related to the rise of winter temperatures.
We are now in a better position to examine Huntington's
contention, set out in the preceding chapter, that the level
of civilisation in Rome fluctuated in accordance with the
average rainfall. The vigorous Roman life of the early
Republic, based on intensive agriculture, was maintained
during the period from 450 to 250 B.C. Towards 250 B.C.
the spirit of discipline and rural simplicity began to decay,
from 225 to 200 B.C. was a period of economic stress, and the
second century B.C. witnessed a great decline of agriculture.
During this period malaria became endemic, according to
E. Huntington (26), because the decreasing rainfall was no
longer sufficient to maintain flowing water in the rivers
throughout the long hot summers, so that stagnant pools and
marshes were formed which provided favourable breeding-
grounds for mosquitoes. After the land law of Spurious
Thorius in in B.C., however, agricultural disturbances
declined and the price of land rose rapidly, but the vine and
olive replaced grain as the main agricultural product. By
90 B.C. there was a marked increase in general luxury and
comfort which reached a high level from 75 B.C. to about
A.D. 50. From A.D. 80 onwards, however, there was a gradual
decline, and A.D. 180 to 190 were years of famine and pestilence.
From A.D. 193 to 210 there was a slight increase in prosperity,
but then began in full force the long " decline and fall of the
Roman Empire."
When we compare these variations in the level of prosperity
with the estimates of rainfall we obtain the following result :
450-250
B.C.
250-200
B.C.
200-100
B.C.
IOO B.C.-
A.D. 50
A.D.
80-200
A.D. 200
onwards.
Civilisa-
tion.
Very high.
Falling.
Low.
High.
Mainly
low.
Very low.
Rain-
fall.
Very heavy
until 300,
then de-
creasing.
Decreas-
ing.
Light until
150 B.C.,
then in-
creasing.
Heavy at
first, then
decreasing.
Light.
tLight and
generally
decreas-
ing.
EUROPE 315
The agreement is surprisingly good, but the climatic
changes seem to precede the changes of civilisation by about
fifty years. This is actually rather more probable a priori
than a direct concordance.
If this principle is sound and if the rainfall curve is correct,
we should expect to find a recrudescence of civilisation in
the Mediterranean from the middle of the eleventh to the
middle of the thirteenth centuries. At that time a large part
of this region was in the hands of the Moslems. It was seen
in the last chapter that the Moslem outbreak from the seventh
century onwards was the fourth of a great series of waves of
emigration from Arabia which are attributed to dry periods.
The Moslem armies rapidly overran North Africa and Spain,
and at first their achievements were largely military and
religious. About the eleventh century, however, they began
to develop a high civilisation, and Egypt for a time took
almost its old place as a leader of thought. As an example of
the high level of Moslem culture in Spain, we have the
Alhambra at Granada (thirteenth century). Italy also
reached a high level during this period, when the city states,
especially Venice and Genoa, became famous for example,
the cathedral of St Mark in Venice was built in the eleventh
and twelfth centuries. The agreement seems to afford
additional support to the rainfall maximum of this period
in Europe, and it justifies us in using, with due caution,
variations in the level of civilisation as indications of climatic
change in other regions also.
It is with regard to ancient Greece that the discussion of
Huntington's theory of civilisation and climate has been
most vigorous. In " The Burial of Olympia " (27), Huntington
first put forward for discussion the theory that up to about
400 B.C. Greece had been well watered and forested, with
perennial streams unsuited to the development of mosquitoes,
but that after that date the rainfall diminished greatly. The
streams were reduced in summer to stagnant pools and
swamps, with the result that malaria became endemic and
undermined the vitality of the population. The driest period
began about the seventh century A.D., and resulted in the
accumulation above the ruins of Olympia of about fifteen feet
of silt. At present the river Lodon is again cutting a channel
through this silt. Unfortunately, the hydrographical system
316 CLIMATE THROUGH THE AGES
of this river is so peculiar that it is doubtful whether any
significance can be attached to this deposit of silt.
The evidence of the Classical writers is very conflicting, but
E. G. Mariolopoulos (28) will not admit that there has been
the slightest change in the climate of Greece since Classical
times, basing his arguments chiefly on the descriptions of the
fertility of the country, the nature of the streams and rivers, and
the dates of sowing and of harvest. He is able to make out
a strong case against climatic change since at least 350 or
400 B.C., and perhaps the best verdict is one of " not proven."
Mariolopoulos gives one interesting quotation from Plato
which shows that questions of climatic change are not new,
but agitated the scientific circles of ancient Greece as well as
those of to-day ; it reads exactly like the report of the recent
Drought Investigation Committee of South Africa, the
decrease in fertility being attributed to the washing away of
the soil.
REFERENCES
(1) ANTEVS, E. " Swedish late-Quaternary geochronologies." New York,
Geogr. Rev., 15, 1925, p. 280.
(2) GAMS, H., and R. NORDHAGEN. " Postglaziale Klimaanderungen und
Erdkrustenbewegimgen in Mitteleuropa." Miinchen, Geogr. Gesellsch.
Landesk. Forschungen, H. 25, 1923.
(3) ERDTMAN, G. " Some aspects of the post-glacial history of British forests."
London, J. EcoL, 17, 1929, p. 42.
(4) GODWIN, H., and A. G. TANSLEY. " Prehistoric charcoals as evidence of
former vegetation, soil and climate." London, J. EcoL, 29, 1941, p. 117.
(5) GODWIN, H. " Pollen analysis and forest history of England and Wales."
Cambridge, New Phytologist, 39, 1940, p. 370.
(6) HARDY, E. M. " Studies of the post-glacial history of British vegetation."
V. " The Shropshire and Flint Maelor mosses." Cambridge, New
Phytologist, 38, 1939, p. 364.
(7) GRANLUND, E. " De Svenska hogmossarnasgeologi." Stockholm, Sverig
geol. Unders., Afh.C., 26, 1932, no. i.
(8) CHILDE, V. G. " The Danube thoroughfare and the beginnings of civilisa-
tion in Europe." Antiquity, i, 1927, p. 79.
(9) WASHINGTON, NATIONAL RESEARCH COUNCIL. " Physics of the air.'*
Vol. IX., " Hydrology," Chapter 5.
(10) SCHOSTAKOWITSCH, W. B. " Bodenablagerungen der Seen und periodische
Schwankungen der Naturerscheinungen." Repr. from Leningrad, Mem.
Hydr. Inst. See London, Meteor. Mag., 70, 1935, p. 134.
(i i) HASLUCK, M. " A historical sketch of the fluctuations of Lake Ostrovo in
West Macedonia." London, Geogr. J., 87, 1936, p. 338.
(12) BROTON, G. E. "A meteorological chronology to A.D. 1450." London,
Meteor. Office, Geoph. Mem., 8, No. 70, 1937.
(13) MEYER, G. M. " Early water-mills in relation to changes in Jjie rainfall
of East Kent." London, Q,. J. R. Meteor. Soc., 53, 1927, p. 407.
(14) COPENHAGEN, DANSK METEOROLOGISK INSTITUT. Medd., No. 2. " Om
isforholdene i Danske farvande i aeldre og nyere tid, aarene 690-1860."
Af C. I. H. SPEERSCHNEIDER. Kjbbenhavn, 1915.
EUROPE 317
(15) LOWE, E. J. "Natural phenomena and chronology of the seasons."
London, 1870.
(16) [SYMONS, G. J.] "Historic droughts." British Rainfall, 1887, p. 22.
(17) VANDERLINDEN, E. " Chronique des e*vnements me'teoroiogiques en
Belgique jusqu'en 1834." Bruxelles, 1924.
(18) HENNIG, R. " Katalog bemerkenswerter Witterungsereignisse von den
altesten Zeiten bis zum Jahre 1800." Berlin, Abh. K. Preuss. Meteor. Inst.,
Bd. 2, No. 4, 1904.
(19) EASTON, C. " Les hivers dans PEurope occidentale." Leyde, 1928.
(20) MANLEY, G. " Temperature trend in Lancashire, 1753-1945." London,
d. J. R. Meteor. Soc., 72, 1946, p. I.
(21) LABRIJN, A. " 200 jaar tempera turwaarnemingen in Nederland." Hemel
en Dampkr. y Groningen, 40, 1942, p. 41.
(22) HIGGINS, L. S. "An investigation into the problem of the sand dune
areas on the South Wales coast." Archaeologia Cambrensis, June, 1933.
(23) BROOKS, C. E. P., and T. M. HUNT. " Variations of wind direction in the
British Isles since 1341 ." London, Q,. J. R. Meteor. Soc., 59, 1933, p. 375.
(24) LA COUR, D. " Tyge Brahes meteorologiske dagbok holdt paa Uraniborg
for aarene 1582-1597." Appendix til Collectanea Meteorologica.
Kjobenhavn, 1876.
(25) BACON, FRANCIS. " The naturall and experimentall history of winds."
1653-
(26) HUNTINGTON, E. " The pulse of progress." New York and London, 1926.
(27) . " The burial of Olympia." London, Geogr. J., 36, 1910, p. 657.
(28) MARIOLOPOULOS, E. G. " tude sur le climat de la Grece. Precipitation.
Stabilit^ du climat depuis les temps historiques." Paris, 1925.
CHAPTER XIX
ASIA
THE interior of the great continent of Asia has for
the last twenty centuries or more been occupied
by nomadic tribes, and we have no great body of
dated literary records such as that which facilitated our
study of the climate of Europe during the Christian era.
On the other hand, south-western Asia includes the sites of
some of the oldest known civilisations of the globe, and we
have a rich mine of historical material from which to draw
conclusions. In Chapter XVIII. we found some evidence
that even in such a climatically favoured continent as Europe,
variations in the rainfall from one century to another strongly
influenced the level of civilisation, and to some extent deter-
mined the wanderings of peoples. During a period of increased
rainfall there is a movement from regions which are naturally
moist to regions which are naturally dry ; this movement
is noticeable both in the locations of Alpine settlements and
in the migrations of whole tribes and nations. During the
drier periods the direction of movement is reversed ; the
naturally moist regions are occupied and the dry regions are
more or less abandoned. In the great Eurasian continent
there is a progressive diminution of rainfall from west to east,
which extends to the eastern boundary of the region of monsoon
rainfall in China, and large parts of the interior of Asia are
on the borderline between aridity and complete desert.
Hence, it is in Asia that we should expect to find droughts
recorded most vividly in history, in accordance with the
principles set out in Chapter XVII.
Evidence from the earlier periods is given by the semi-arid
settlement at Anau on the northern margin of Persia. This
site was occupied from time to time and abandoned during
the intervening periods, and since there is no evidence of
conquest, while the periods of abandonment are represented
by desert formations, it is generally accepted that the inter-
ruptions were due to drought (i). The dating of the earlier
318
ASIA
settlements is by means of the relative thickness of deposits
formed above them, the latest is partly historical. The first
settlement began about 9000 B.C. ; the second, which immedi-
ately succeeded it, about 6000 B.C. The last part of the first
settlement and the whole of the second show evidence of
gradually increasing drought, and the settlement was aband-
oned soon after 6000 B.C. The site was reoccupied, after
an interval of desert conditions, about 5200 B.C. This
third settlement continued until about 2200 B.C. with a short
interruption, probably due to drought, about 3000 B.C.
In 2 200 B.C. there began a period of intense drought, and
about this time not only Anau, but other settlements, such
as Susa and Tripolje, were abandoned (i). The site was
not reoccupied until the Iron Age, probably not much earlier
than Persian times.
The evidence for climatic changes in Asia since 100 B.C.
was summarised by Ellsworth Huntington in his last book (2).
He gives curves of caravan travel in Syria after C. P. Grant
(3), his own reconstruction of the evidence of lakes and
ruins in Asia, and a tabulation of the frequency of migrations
to and from the dry areas since 400 B.C. from A. J. Toynbee
(4), all of which agree in their main features. They point
to rainy periods about A.D. 0-200, 400-500, 700-1100, 1250,
and 1500-1700, and dry periods about 300, 500, noo, and
1400.
The Syrian desert lies across the main land routes between
Asia and Europe-North Africa, and in its present state offers
a formidable obstacle even to motor transport. It would
now be almost impassable by camel caravans, and in dry
periods these went by a circuitous route. At other times,
however, they struck boldly across it, which would have been
impossible without a much heavier rainfall. Huntington
notes that about 550 B.C. Nabonidus set up his headquarters
at Terma in north-west Arabia, now a small village, and his
son sent him couriers and food supplies regularly by camel
across the desert.
In order to carry the curve of migrations farther back,
Table 26 was compiled from three main sources (i, 5, 6), and
was carried from 5200 B.C. down to the year A.D. 50. It
will be observed that the migrations are almost always from
the drier to the wetter regions ; the exceptions are the
32O CLIMATE THROUGH THE AGES
reoccupations of Anau about 5200 and in the first millennium
B.C., and a wave of migration which began in Central Europe
about 1275 B.C. and penetrated as far as the east of Asia
Minor, where it formed the Armenians.
B.C.
5200. Anau reoccupied.
Before 5000. Sumerians and some Semites occupied Mesopotamia.
4000-3000. First Semitic wave from Arabia.
3000. Aryans to Baltic.
2650. Sumer overrun from the north.
2600. Amorites invade Egypt.
2450. Kassites,
2360. Kassites.
2300-2050. Great dispersal of Aryans (or Wiros).
2225. Steppe-folk invade Tripolje area.
2200. Evacuation of Susa and Anau.
Semites in Mesopotamia.
2170. Kassites.
2072. Kassites.
2045. Kassites.
2000. Amorites. Tumulus folk.
1926. Kassites.
1800. Hyksos conquer Egypt.
1750. Eruption from Iran.
1 746. Kassites.
1700. Aryans into Punjab.
1500. Aryans.
1500-1000. Great unrest in Western and Central Asia.
1350-1300. Aramaean wave from Arabia.
1275. Migration eastward to Armenia.
1 1 80. Elamites.
(1050. Dorians.)
ca. 600. Anau reoccupied.
500. Arabs.
300-250. Break up of Hellenised States in West Asia.
1 60. Saka.
A.D. 50. Jafnite wave from Yemen.
Table 26. List of migrations.
In order to construct a climatic curve from this table, a
value was assigned to each century (or other convenient
period according to the detail of the record) based on
the " wet-ward " component of migration. The "numbers
assigned ranged from 5 for the period about 600 (reoccupa-
tion of Anau, apparently complete cessation of migration
ASIA
321
from the dry regions) to +5 for the great drought about
2200 B.C. From these numbers the curve shown in Fig. 33
was constructed up to the beginning of the Christian era.
The later portion of that curve is derived mainly from the two
curves given by Huntington (2) and Toynbee's diagram of
migrations, controlled by the levels of the Caspian (see below).
The climate of Western and Central Asia during the
Christian era is best determined from a study of the fluctuations
of the Caspian and other salt lakes without outlet. The
level of such lakes is determined by the rainfall and evaporation.
If the rainfall increases, the level of the lake rises and it over-
flows its shores until it offers a sufficiently increased surface
for evaporation to balance the greater rainfall over the basin.
6OOO
4-QOO
A.D. 3QO
000
WesTar\d Central
7OO
p.c.
China
AD.
000
1500
ig' 33- Variations of rainfall in Asia.
If the increase of rainfall is very great, the lake may rise until
it finds outlet to the sea. With decreasing rainfall the level
sinks and the area decreases until the evaporation is again
only sufficient to balance the rainfall. A decrease in the rate
of evaporation per unit area would have the same effect
as an increase of rainfall. Only one of these natural rain-
gauges, the Caspian Sea, lies sufficiently near to the world of
antiquity for its variations of level to be brought into our
chronology, but the variations of the remaining lakes have
evidently been similar, and we may infer that the fluctuations
of rainfall indicated by the Caspian represent with fair
accuracy those of the whole of Central and Western Asia, in
spite of the complications introduced by variations in the
course ofthe Oxus River. The fluctuations of the Caspian
were carefully studied by E. Bruckner (7), and have been
further discussed by Ellsworth Huntington (8).
322 CLIMATE THROUGH THE AGES
The first definite reference to the Caspian is given by
Herodotus, about 438 B.C. Huntington interprets Herodotus'
description as implying that the length of the Caspian from
north to south was about six times the breadth from west to
east. At present its length is only between three and four
times its breadth, but if it became deeper, it would expand
very little in an east-west direction and very greatly to the
north. He also considers it probable from the description
that the Sea of Aral was united with the Caspian. For these
reasons Huntington believes that when Herodotus wrote,
the Caspian stood about 150 feet higher than now. Strabo,
in A.D. 20, gave descriptions from which Khanikof has
estimated that the Caspian stood at that time 85 feet above
its present level. On the other hand, L. Berg (9) states that
these former very high levels are contradicted by the fact that
deposits containing Cardium edule, still living in the Caspian,
are found to a height of only 75 feet above the present level.
These early records, therefore, would not carry much weight,
unless they were supported by material from other sources.
This is not the case, for in the discussion on " The Burial of
Olympia " (Chapter XVIII.) Sir Aurel Stein brought forward
evidence that about the beginning of the Christian era the
levels of the salt lakes and marshes in the desert west of Tun-
huang were about the same as to-day. These form part of a
defensive line which was completed by a Chinese wall, built
about 100 B.C. and abandoned early in the first century A.D.
Wherever this wall abuts on any of the lakes or marshes it
can clearly be traced down to within a few feet of the actual
water level in the spring of 1907.
Nothing further is known as to the level of the Caspian
until the middle of the fifth century, but J. W. Gregory (10)
states that in A.D. 333 the Dead Sea stood at its present
level. Between A.D. 459 and 484 the " Red Wall " was built
as a barrier against the Huns. At this time the level of the
Caspian must have been very low, for the wall extends below
water 18 miles from the shore, and a caravanserai at the old
port of Aboskun is now under water ; there are other sub-
merged houses and cities of unknown date in different parts
of the basin. The level was at least 15 feet below the present.
Istakhri, an Arab geographer, in A.D. 920, records that
the old wall at Derbent projected into the sea so far that
ASIA 323
six of its towers stood in the water, and from this Bruckner
concludes that the level was 29 feet above the present. There
is also evidence that Lake Seistan, in Persia, was high about
A.D. 900.
The caravanserai at Baku, which, according to Bruckner,
was built in the first half of the twelfth century, indicates a
level 14 feet below the present. In 1306 to 1307 the level was
37 feet above the present ; this may be partly due to the fact
that the Oxus entered the Caspian instead of the Sea of Aral
at that time, but it is significant that almost in the same year
Dragon Town, on the shores of Lop Nor, was destroyed by
the rising water. In 1325 the level was still high.
Early in the fifteenth century the Caspian swallowed up
a part of the city of Baku (level 1 6 feet above present) . ' The
level was still high in 1559 and 1562.
For the seventeenth to nineteenth centuries Bruckner gives
the following levels :
1638. 15 feet above pre-
sent level.
1715-1720. i foot above.
1 730- 1814. Relatively high.
1815. At least 8 feet
1830. i foot above.
1843-1846. 2 feet below.
1847. i foot above.
1851-1860. i foot below.
1861-1878. i to 3 feet above.
above present.
The changes of level in the Caspian are shown in Fig. 34.
B.C. A.D. 200 400 00 800 1000 1200 1400 1600 1600
Fig. 34. Variations in the level of the Caspian.
The curve of rainfall in Asia since 5200 B.C. agrees well
with those for Europe (Fig. 31). Both rise to a maximum
between 5000 and 4000 B.C., falling steadily to a minimum
about 2200 B.C. The small maximum at 1275 B * G - * s
dated by evidence of migration from Europe to Asia ; in
Europe k is supported by other evidence which refers to about
this period but cannot be dated accurately. The great
maximum of rainfall about 500 B.C. in Europe appears to
324 CLIMATE THROUGH THE AGES
have come somewhat earlier in Western Asia, and after this
there is some conflict of detail which may be due to lack or
misinterpretation of data, or to errors of dating. The
oscillations in Western Asia appear to have been more pro-
nounced than in Europe, but the scales of the diagrams are
only relative, and arid areas are much more sensitive to small
variations of rainfall than more humid regions. The dry
period about A.D. 700 on which Huntington lays so much
stress is also found in Europe, and a general excess of rainfall
about A.D. 1300 is common to both. Considering that the
two curves are based on entirely different and independent
data, the measure of agreement seems to prove that at least
the major climatic oscillations are real and widespread.
It should be noted, however, that R. G. F. Schomberg (n)
does not admit the reality of these variations of rainfall in
Central Asia, considering that the apparent evidence is
due to other causes, especially changes in river courses,
which easily erode the soft sandy soil. In the discussion
of the last of these papers several speakers questioned this
verdict. It is, of course, against all climatological experience
that rainfall could have maintained a dead level for several
thousand years, but it is not unlikely that Huntington has
magnified the range of the oscillations.
There is some evidence of former moister conditions in
Northern India. E. J. H. Mackay (12), describing excavations
at Mohenjo-Daru, near the west bank of the Indus about 270
miles above Karachi in a very dry region, states that about
2750 B.C. culverts were specially constructed to carry away
storm water, and between 2750 and 2500 the site was partially
abandoned because of serious flooding by the Indus. This
agrees with the paucity of migrations about this time.
Reference may also be made to some speculations by V.
Unakar (13) on the interpretation of the RG-VEDA. He
finds evidence of three types of climate : first a period of
cool weather with rains fairly uniformly distributed through
the year and few thunderstorms ; second a stormy period
when winter depressions gave copious rains ; and finally a
period of increasing drought. The date is uncertain, but
Unakar provisionally places the sequence as possibly Covering
the period 5000 to 2000 B.C.
A few references given by G. W. Bishop (14) to events
ASIA 325
in China throw some light on the climatic changes in that
country. Thus we have :
1766 B.C. First dynasty overthrown by a popular revolt
following seven years of drought. Fig. 33 shows a secondary
minimum of rainfall in Western Asia about this date, but
not comparable with the minimum of 2200 B.C.
1 122 B.C. Second dynasty overthrown by popular revolt
and invasion from the west. This disturbance clearly
coincides with a minimum in Western Asia.
842-771 B.C. Period of turmoil and invasion from the
west. Great drought, accompanied by disturbances, recorded
for about 822 B.C. This is in remarkable agreement with
Erdtman's " dry heat-wave " in the closing centuries of the
Sub-boreal in Europe.
Four hundred years of anarchy and confusion began in
the third century A.D., and this again coincides with a dry
period in Europe and especially in Central Asia, and a
shorter period of disintegration in the tenth century also
falls in a period of drought.
Co-Ching Chu (15) published an analysis of the Chinese
archives since A.D. 100 on the same lines as that for Europe
described in the preceding chapter. His results for all China,
tabulated by centuries, are as follows :
Century.
Floods.
Droughts.
A.D.
2
18
35
3
15
24
4
5
4i
5
18
37
6
10
41
7
13
43
8
3i
41
9
24
43
Raini-
Century.
Floods.
Droughts.
"Raini
ness."
A.D.
ness."
34
IO
36
6 4
36
38
1 1
41
69
37
ii
12
56
58
49
33
3
43
77
36
20
14
57
60
49
23
'5
24
54
3i
43
16
43
84
34
36
17
67
82
43
Table 27. Floods and droughts in China.
There is a general tendency for the number of floods to
increase relatively to the number of droughts in the later
centuries ; when this is allowed for, the fourth, sixth, and
seventh centuries and, later, the fifteenth and sixteenth
centuries stand out as predominantly dry ; the second and
third, eighth, twelfth, and fourteenth centuries as wet. The
general agreement with the results of the similar tabulation
326 CLIMATE THROUGH THE AGES
for Europe is very good. The curve of raininess in China,
based on these data, is shown below the curve for Western
Asia in Fig. 33.
Co-Ching Chu also remarks that " In a recent bulletin
published by the U.S. Department of Labour, Ta Chen has
found that Chinese migration can be grouped into three
periods : those of the seventh, fifteenth, and nineteenth
centuries. During the first period, Chinese migrated to the
Pescadores and Formosa ; in the second period, to Malaysia ;
and in the third, about 1860 . . . with destinations in
Hawaii, North America, and South Africa. Mr Chen found
that the most significant causes of emigration are pressure of
population and droughts and famines ; while during the last
century Chinese emigration was much accelerated by the ease
of communication and by the demand for labour to open up
new lands. Such, however, cannot be said of the seventh
or fifteenth centuries." The dryness of these two periods
shown in Table 27 is confirmed by these results.
Co-Ching Chu also gives some records bearing on the
variations of temperature. The number of severe winters
per century during the sixth to sixteenth centuries falls to a
minimum between A.D. 600 and 800, rises to a maximum
between A.D. noo and 1400, with a well-marked crest in the
fourteenth century, and subsequently decreases again. The
variations of frequency, like those of raininess, run closely
parallel with the variations in Europe. The author also
examines the dates of the latest spring snowfall in each decade
at Hangchow during the period 1131-1260, and finds that the
average date, gth April, is nearly a month later than the date
of the latest spring snowfall during the period 1905-1914,
suggesting that the climate was colder and stormier in the
twelfth and thirteenth centuries than at present, and con-
firming the evidence of the severe winters. It is also interesting
to remark that the author finds a parallelism between the
occurrence of a severe climate and the frequency of sunspots.
K. A. Wittfogel (16) finds some slight evidence that in
North China about 1600 to noo B.C. the winter was warmer
than at present, interest in crops and agricultural rainfall
starting very early in the year. He thinks that the** summer
rainfall may have been slightly greater than now.
* Finally, some reference is necessary to the ruins of Angkor
ASIA 327
(17), a lost city, formerly the centre of the powerful and
highly civilised Khmer Empire, which developed in the
steaming jungle of French Cambodia, between the Mekong
River and the frontier of Siam, in about 14 N. Angkor
flourished between A.D. 600 and 1200, and the empire seems
to have reached its highest point about A.D. 1000. Climatically,
the region is very similar to Yucatan, which also had a high
civilisation, the relics of which are now buried in thick tropical
jungle (see Chapter XXII. ), though it lies nearer to the
equator than does Yucatan, and belongs definitely to an ex-
tension of the equatorial rain-forest belt. The climatic
conditions of Angkor are very unfavourable for the develop-
ment of a high civilisation at the present day, and the founding
of a great city (the population is estimated at a million) in
such a site suggests a much drier climate about A.D. 600.
The causes of the break up of the empire and the abandon-
ment of the city are not known, but it is an interesting
possibility that about A.D. 1000 or 1050 the climate became
moister in conformity with the changes in other parts of
the world, and that the inhabitants fought a losing fight
against the advancing tide of tropical vegetation for nearly
two centuries before they finally gave up the struggle and
migrated to more open country.
REFERENCES
(1) PEAKE, H. J. E. " The Bronze Age and the Celtic world." London, 1922.
(2) HUNTINGTON, E. " Mainsprings of civilisation." New York and London,
1945-
(3) GRANT, C. P. " The Syrian desert." New York, 1938.
(4) TOYNBEE, A. J. " A study of history." Vol. III. Oxford Univ. Press,
1934-
(5) " THE CAMBRIDGE ANCIENT HISTORY," vol. i. Cambridge, 1923.
(6) HADDON, A. C. " The wanderings of peoples." Cambridge, 1919.
(7) BRUCKNER, E. " KHmaschwankungen seit 1700 . . ." Vienna, 1890.
(8) HUNTINGTON, E. " The pulse of Asia." Boston and New York, 1907.
(9) BERG, L. " Das Problem der Klimaanderung in geschichtlicher Zeit."
Geogr, Abh. hrsg. von. A. Penck in Berlin, 10, H. '2, 1914.
(10) GREGORY, J. W. "Is the earth drying up?" London, Geogr. J., 43,
1914, p. 154.
(n) SCHOMBERG, R. C. F. " The aridity of the Turfan area." London, Geogr.
J., 72, 1928, p. 357.
. " The climatic conditions of the Tarim Basin." idem, 75, 1930,
P- 3 r 3-
. " Alleged changes in the climate of southern Turkestan." idem,
80, 1932, p. 132.
(12) MACKAY, E. J. H. "Further excavations at Mohenjo-Daro." London,
J. R. Soc. Arts, 82, 1934, p. 206.
CLIMATE THROUGH THE AGES
(13) UNAKAR, V. " Meteorology in the RG-VEDA." J. Asiat. Soc., Bombay
Branch, 9, 1933, P- 53 J io> '934* P- 38-
(14) BISHOP, C. W. " The geographical factor in the development of Chinese
civilisation." New York, N.Y., Geogr. Rev., 12, 1922, p. 31.
(15) CHU, CO-CHING. " Climate pulsations during historic time in China."
New York, N.Y., Geogr. Rev., 16, 1926, p. 274.
(16) WITTFOGEL, K. A. " Meteorological records from the divination inscrip-
tions of Shang." New York, N.Y., Geogr. Rev., 30, 1940, p. no.
(17) CANDEE, H. CHURCHILL. "Angkor the magnificent." ' (Witherby), 1925.
CHAPTER XX
AFRICA
THE most important source of information as to the
variations of rainfall in Africa is provided by the
levels of the River Nile. As is well known, the Nile
commences in Lake Victoria, in Central Africa, and flows
to Lake Albert as the Victoria Nile. From here it continues
as the Bahr-el-Jebel, becoming known as the White Nile
after the junction of the Sobat River. At Khartoum it
receives the Blue Nile, and near Berber the Atbara River,
both of which originate in the mountains of Abyssinia. From
the junction of the Blue Nile to the Mediterranean, a distance
of i, 800 miles, it receives no appreciable accession of water.
The level of the Nile passes through an extremely regular
annual variation ; the water is at its lowest in April and May,
it rises slowly and irregularly in June and the first half of
July, but rapidly and steadily in the latter half of July and the
first half of August, remaining high during September and
commencing to fall rapidly in October. The regular annual
flood is the source of the fertility of Egypt ; without it the
whole land would be a barren desert, and hence the levels of
the flood have been recorded annually, probably for some
thousands of years. The lowest level reached at the stage of
low water has been recorded less regularly. Many of these
records have been lost, but enough remain to form a very
valuable series, which has been collected and published by
Prince Omar Toussoun (i). The values of maxima and
minima at the Roda gauge, Cairo, smoothed by forming
fifty-year means commencing at successive intervals of ten
years, are shown in Fig. 35.
It is necessary to understand exactly the significance of
both the high and low levels. The White Nile drains a large
area of equatorial Africa which has a considerable annual
rainfall ^distributed fairly evenly throughout the year ; more-
over, it passes through two large lakes, Victoria and Albert,
which further regulate the flow. Hence the White Nile
3*9
330
CLIMATE THROUGH THE AGES
above its junction with the Sobat River discharges an almost
constant volume of water throughout the year (2). The Blue
Nile, the Atbara, and the Sobat River, on the other hand,
originate in Abyssinia, which receives the greater part of its
rainfall in the summer months. At Addis Ababa the mean
annual rainfall is 46 inches, and of this amount 35 inches fall
in June, July, August, and September. Hence it is these
rivers, and especially the Blue Nile, which supply the waters
of the annual flood, in which the White Nile plays very little
part. The Abyssinian rainfall is monsoonal ; Sir Henry
Lyons (3) showed that it is closely related to the pressure in
AD 200
400
Fig. 35. Levels of Nile. Flood stage and low-level stage.
the neighbourhood of Cairo, high pressure preceding a low
Nile flood, and low pressure a high flood. Variations of
pressure at Cairo are representative of those found over a
wide area, extending from Beirut to Mauritius, and from
Cairo to Hong Kong. Pressure variations in this area are
generally opposite to those in South America and adjoining
regions, and the fluctuations of the Nile flood therefore
represent the " see-saw " of pressure between the old and the
new worlds. The minimum level, on the other hand, depends
chiefly on the rainfall in the equatorial belt of low pressure,
which is very closely connected with the intensity of the general
circulation of the atmosphere. For this reason the level of
the Nile during the stage of low water is the better guide to
AFRICA 331
the general rainfall of equatorial Africa, while the flood levels
represent the monsoon rainfall of the eastern highlands.
We should expect the former to show a closer relation to the
rainfall of Europe than the latter.
It will be noticed that both maxima and minima show a
general upward trend. This is due to the deposit of silt,
which has been steadily raising the level of the Nile bed for
thousands of years, at the rate of about o i metre per century.
This is represented by the straight sloping lines in Fig. 35,
but it is probable that the rate has varied from time to time.
The steep rise of the minima in the latest years is due to
artificial control of the water, especially by the Delta Barrage,
and the data for these years are useless for our purpose.
The records are made up as follows : there is an almost
continuous series of records of both high and low levels
extending from A.D. 641 to 1480. From 1480 to 1830 there
is a broken record of the flood levels, and the data of low
levels are very scanty. From 1830 onwards the series are again
complete, but their value is greatly lessened by the irrigation
works. There are no connected series of records known
earlier than A.D. 641, but Prince Omar Toussoun gives
determinations of the levels which constituted " weak floods,"
" good floods," and " strong floods " in the fifth century
B.C., and in the first, second, and fourth centuries A.D., as well
as in the later centuries. From the seventh to the nineteenth
centuries the mean of these three levels averages o 48 metre
below the mean annual level for the century, but this difference
is very variable, with some tendency for a secular increase.
Hence the means obtained from these weak, good, and strong
floods can be regarded merely as indications, which point to
generally good floods in the fifth century B.C., and in the first
and fourth centuries A.D., and rather poorer floods in the
second century A.D.
The maximum about 500 B.C. finds some support in
Herodotus' "History" (4). Prince Omar Toussoun quotes
a passage to the effect that unless the flood rose to a level of
15 or 16 coudees ("cubits," 8-0 or 8-5 metres) it did not
overflow the fields ; he therefore takes this as representing an
average ^ood flood. He thinks that these figures refer to the
" effective flood," that is, the height to which the flood rose
above the level of low water, for he points out that " the levels
332 CLIMATE THROUGH THE AGES
given by Herodotus, four centuries B.C., are the same (actually
they are rather higher) as those cited by Ammien Marcellin
(Ammianus Marcellinus), four centuries A.D. Now it is
impossible, with the continual elevation of the soil, that
after an interval of eight centuries these levels should have
remained the same, if they had for base the same zero* Hence
it is necessary to consider all the levels mentioned by these
authors as being effective levels. 55
I do not agree with this argument, for the irrigation value
of a flood depends on its gauge level, and not the range
from low water, and I think the gauge level would be more
likely to be reported. The figures quoted would agree
equally well with the assumption that in the fifth century
B.C. the floods were generally good, while in the fourth
century A.D. they were generally poor. Even if the figures
do refer to " effective floods, 55 however, they still point to
relatively high floods in the fifth century B.C. From the
seventh to the nineteenth century A.D. the average " effective
height " of a good flood is 7-4 metres, and the figures quoted by
Herodotus exceed this by half a metre or more. This piece of
evidence cannot, however, be regarded as more than an
indication.
Herodotus has another passage (Book II., Chapter 97)
which may be interpreted in the same sense ; " When the Nile
overflows, the country is converted into a sea, and nothing
appears but the cities, which look like islands in the Aegean. 55
This happens now only when the flood is very high.
A passage from Pliny, quoted by Prince Omar Toussoun,
states that the regular flood of the Nile in the first century A.D.
was 1 6 coud^es (8-5 metres). Whether this refers to the
actual or " effective 55 flood, it represents a rather good supply
of flood water, and consequently points to good rains in
Abyssinia.
During the period of complete records from the seventh to
the fifteenth centuries there is a fairly good agreement between
the flood levels and the low-water stage, although the fluctua-
tions of the latter are the more violent. Both show a
minimum about 775, a maximum about 870, a minimum about
960, a maximum at r 1 10, and a double minimum at 1 220 and
1300. After 1370 the curves become divergent and generally
opposed ; it seems as if the figures recorded were mainly the
AFRICA 333
highest maxima of the floods and lowest minima of the low-
level stage. The true rainfall during this period is probably
to be obtained by a judicious blend of both sets of data.
There are no other sources of information for Africa which
can compare in detail with these Nile flood records. The
climate of the Mediterranean provinces of Africa has been
exhaustively examined by H. Leiter (5) on the basis of the
literary references, mainly in the Roman writings. Leiter
finds no evidence that there has been any appreciable variation
of rainfall since Roman times, but he thinks there may have
been a slight rise of temperature during the historic period.
G. Negri (6) similarly examined the evidence for changes of
climate in Gyrenaica, and concluded that there had been no
appreciable change of climate. Both these authors were
concerned more with the question of progressive desiccation
than with fluctuations of climate ; they prove fairly conclu-
sively that the rainfall of this northernmost belt of Africa was
not much greater during the early centuries of the Christian
era than it is at present, but they pay comparatively little
attention to the possibility of prolonged wet or dry periods in
the intervening centuries. Nevertheless, their careful work
does seem to show that in this part of the earth's surface the
climatic fluctuations were probably of smaller amplitude than
in the regions to the north and to the south, i.e., in Europe
and at the sources of the Nile. On the other hand, a meteoro-
logical register kept at Alexandria by Claudius Ptolemaius in
the first century of the Christian era, and described by G.
Hellmann (7), strongly suggests a considerable change of the
summer climate in Northern Egypt since that date. Hellmann
examines the register in detail, and concludes that there
is no direct evidence that the observations were not actually
made in or near Alexandria. The record appears to state
quite clearly the name of the Observer and the place where
the observations were made. Fortunately, there is a long
series of good recent observations in Alexandria with which
these early observations can be compared.
This register was re-examined for me by Miss L. D. Sawyer
(8), who concluded that the disagreement between Ptolemaius'
register alid recent observations is not so great as is represented
by Hellmann. The following table is based on her figures,
converted to percentages :
334
CLIMATE THROUGH THE AGES
Winds from
N. NE. E.
SE. S. SW.
First Century
Hellmann
702
2 28 IO
Sawyer . .
II O 2
2 30 8
W. NW. Variable.
20
25 14 8
Calm.
Present . . . 35 n 7 4 5 5 5 25 3
Table 28. Frequency of winds in Alexandria.
Miss Sawyer points out that northerly winds are most
frequently referred to in winter, when they are now least
regular ; in summer there are references to the beginning and
end of the Etesian winds in Egypt over 40 days apart, during
which period other winds are mentioned occasionally but no
reference is made to actual north winds on any of the days in
that period. This indicates that it is the unusual rather than
the more commonplace weather conditions that are referred to.
Hence, while the register indicates that the winds in the first
century differed appreciably from the present, the difference
is not so impossibly great as Hellmann makes out.
Miss Sawyer did not discuss the descriptions of weather
phenomena, but the same conclusion seems to hold.
The number of storms is rather high, but such observations
are of a relative nature. Observations of rainfall occur under
various designations, most of which are clear. The signifi-
cance of one term (takas} is not quite clear, but it probably
means " fine rain." The frequency of rain, as shown in
Table 29, does not differ greatly from the present frequency
(though, if " fine rain " is included, the number is rather high),
but the annual variation, with its entries in summer, is quite
Days
Jan.
Fcb
. Mar.
Apr.
May June
July Aug. Sep. Oct.
Nov. Dec.
Rain
ist Century
4
3
5
3
i
2
3
4
3
2
ist Century,
" Fine rain "
i
o
i
3
4
5
2
o
2
2
Present
1 1
6
5
i
i
o
O
i
7
10
Thunder
ist Century
i
i
i
2
2
i
I
I
o
O
Present
0-7
'
\ 0-3
O-2
3 o-i
0-4
0*
7 i'5
Great Heat
ist Century
o
o
O
O
3
8
6
I
o
o
Present
o
2
6
12
i
3
2
" Weather
Changes "
*
ist Century
4
o
3
2
4
O
2
i
4
I
I
Table 29. Frequencies of meteorological phenomena in Egypt.
AFRICA 335
different from the present. Summer in Alexandria is now
completely rainless. The distribution of thunder also differs
from the present, and the annual total is high. The occur-
rences of " great heat " in the old register also fail to agree
with the present climate. The constant strong winds from
north or north-west in summer temper the heat and give this
season fewer very hot days than the seasons immediately
before and after the midsummer months. The column for
" Present " under hot days shows the number of occasions
during the period 1873 to 1896 on which the hottest day of
the year occurred in the month in question.
Very remarkable also are the frequencies of " weather
changes." At present the period from May to September
is one of almost uninterrupted fine dry weather. The
Calendar of Antiochus, dated about A.D. 200, which also
refers to Egypt, is still more remarkable in this respect, since
out of fifty-one records of " weather changes," nineteen
occur in the period May to September.
These observations point to a climate very different from
the present summer climate of Alexandria, and resembling
much more that of Northern Greece. The divergence is,
in fact, so striking that in spite of the apparent trustworthiness
of the record, Hellmann considers that there must be some-
thing wrong, since a climatic change of this degree would imply
that meteorological conditions differed from the present,
not alone over Northern Egypt, but over a very wide region,
if not over the whole earth. The records agree, however,
with our somewhat scanty records of the Nile flood for that
period, and with the evidence from Kharga Oasis described
in the next paragraph, while in Chapter XXII. we shall see
that the climatic changes did in fact extend over a large part
of the Northern Hemisphere in a manner agreeing with the
requirements of meteorology.
H. J. L. Beadnell (9) has given us an interesting study of
the probable changes in the water supply of the oasis of
Kharga, which lies 100 miles west of the Nile valley and 400
miles south of the Mediterranean, and further details have
been added by Miss Caton-Thompson and Miss E. W.
Gardner* ( i o). The water in this oasis is provided by wells,
the rainfall being extremely small and erratic.
In Pleistocene times the Kharga depression was supplied
336 CLIMATE THROUGH THE AGES
with water by springs which formed mounds, but it is not now
accepted that the floor of the depression was ever occupied by
an extensive lake. In the Neolithic (7000-5000 B.C.) these
springs ceased to flow, and holes were dug in the tops of the
mounds to obtain water. The dead or dying springs were
covered by dune sands. From Early Egyptian to Persian
times (5000-525 B.C.) the oasis was practically uninhabitable
owing to lack of water. The Persians replenished the supply
of water by sinking deep artesian wells. In Kharga itself
the oldest ruins belong to the time of Darius, about 500 B.C.,
which was a period of great prosperity.
The oasis continued to be of importance until the
beginning of the seventh century A.D., but there was a
temporary decline in the third and early fourth centuries.
In the seventh century the oasis decayed. No further
evidence is available until the twelfth century, when the
oasis was found to be almost depopulated. In A.D. 1225
Kharga appears to have been more prosperous than it was
about 1150, and by A.D. 1300 there appears to have been a
still further improvement. After this we have no information.
At first sight, this history of the Kharga Oasis seems to be
very complete evidence of changes of climate in this part of
Africa, but Beadnell expresses a doubt. The water supply
is entirely derived from wells in two layers of water-bearing
sandstone, and the greater part of the supply is artesian,
rising to the surface under considerable pressure and forming
flowing wells. These water-bearing sandstones underlie a
great area in this part of Africa, but it is only in depressions
that they are at a small enough depth to be tapped. The
origin of the water is doubtful ; there are three possible sources
the Nubian reaches of the Nile, the great swamp regions of
the Sudan, and the rains of Abyssinia or Darfur. Beadnell
thinks that the Nile is the most probable source. The total
amount of water in the sandstone is very large, and may
represent the accumulation of hundreds or even thousands
of years. During Roman times a large number of wells were
put down, and these gradually drained the water-bearing
sandstones, so that the water supply fell off. After the
Romans left, wells which became choked were not cleaned
out, and the water supply decreased still further.
This suggested explanation shows that the variations of
AFkICA
337
water supply in Kharga Oasis do not necessarily represent
synchronous variations of rainfall ; at least the evidence is
not so convincing as it appears at first sight. Nevertheless,
the variations in the water supply agree so well with the
evidence from other parts of Northern Africa that they must
be due in part to climatic causes, if not in the region of the
oasis, at least in the region where the water originates. The
evidence may be set out in parallel columns as shown in
Table 30, the last column being in part an anticipation of
later paragraphs.
Date.
Kharga Oasis,
Nile.
Other localities.
500 B.C.
A.D. 200
A.D. 4OO
A.D. 7OO
A.D. II5O
A.D. 1225
Prosperous.
Extensive well boring.
Temporary decline of
prosperity.
Improvement.
Floods high.
Floods good. | Alexandria rainier in summer.
Floods poorer.
Floods good.
Great decline.
Almost depopulated.
Mandingan Empire, A.D. 320-
680 (see below).
Minimum level, Mandingan Empire broke up.
A.D. 700-1000. j
Rise of low- 1
level stage ca. j
A.D. 1 100.
More prosperous. j Level low.
A.D. 1300 ! Still further improve- j Level low.
! ment. !
Traffic in now waterless
Eastern Desert.
Prosperous Sudanese States.
Table 30. Variations of climate in Northern Africa.
The chief disagreement is between the Kharga Oasis and
the levels of the Nile from A.D. noo to 1300. If the evidence
derived from the Kharga Oasis for this period stood alone,
one would have no hesitation in rejecting it. It has, however,
a certain amount of support from other regions. The figures
of raininess in Europe, after a temporary maximum about
A.D. 1075 fell to a minimum in 1175, rising again to a maxi-
mum about 1325. Similarly, in Asia the Caspian (Fig. 34)
appears to have reached a low level about 1150, rising again
to a maximum in 1300. Mr G. W. Murray informs me that
i 22
338 CLIMATE THROUGH THE AGES
there is evidence of considerable pilgrim traffic across the
Red Sea to Jeddah in the thirteenth century from a port
afterwards abandoned for lack of drinking water. At about
the same time we have the prosperous Mossi States of the
Sudan (see below). All this seems to confirm the Kharga
evidence. It is possible that at this time the fluctuations in
the equatorial regions were following a different regime from
those in the north temperate rainfall belt. The levels of the
Nile in Fig. 35 show that while the fluctuation^ of the Nile
flood from A.D. 800 to 1300 were generally similar to those of
the low-water stage, they were on a very much smaller scale.
It has been pointed out that the low-level stage represents
mainly the fluctuations of the equatorial rainfall, while the
flood level represents the rainfall of Abyssinia, and this
difference of scale may possibly imply that the equatorial
fluctuations at that time died out northwards, and that the
northern part of the continent came under the influence of
the north temperate fluctuations. It is evident, however,
that further data will be required before the climatic fluctua-
tions in this part of the world can be set out in reliable detail.
The Sahara itself is at present occupied only by a few
wandering tribes. There is, however, some historical evidence
that during at least one, and possibly two historical periods
the level of civilisation in the desert was appreciably higher
than at present (u). The first period is that of the
Mandingan Empire. The Mandinke are Sudanese negroes,
who, according to native tradition, maintained a Saharan
Empire from about A.D. 320 to 680. After this date the empire
broke up, and it was not until the thirteenth century that the
second and more authentic cultural period began. Early
in the fourteenth century " the greatest Sudanese State of
which there was any authentic record " was centred at Mali,
in French Guinea ; about the same time the Mossi formed a
powerful state in the great bend of the Niger, and the Housa
State was developed at Kano in Nigeria. The Moslem
Empire of the Central Sudan spread over a large part of
the Sahara in the thirteenth century, and increased rapidly
in importance until it reached its highest pitch about
1526-1545.
The shore-lines of the enclosed Central African Lake
basins of Nakuru-Elementeita and Naivasha, according to
AFRICA 339
E. Nilsson (12) point to a number of post-glacial fluctuations.
At the end of the last main pluviation (Lake III.) the lakes
dried completely. The first post-pluvial lake (IV.) reached
almost to the depth of Lake III. but persisted for a shorter
time. The lakes again dried completely ; after they re-
formed (Lake V.) there were minor oscillations, superposed
on a gradual fall, but no complete drying out. Lake V. is
associated with Leakey's " Gumban A and B " cultures
(Nakuran) (13). A Gumban A site has been found in the
deposits of Lake V. The Gumban B burial was near the
lake and contains fish bones, and Leakey considers that it
was of about the same date. It yielded a bead which cannot
be earlier than 3000 B.C. and was probably later. In 1931 I
thought the Nakuran lake must represent the Sub-Atlantic,
gooo
Egypf
&ot AFrica
Fig. 36. Variations of rainfall in Africa.
(In the curve for East Africa the dates of maxima are conjectural.)
which was then placed about 850 B.C. (but is now dated about
500 B.C.) and this correlation was accepted by the archae-
ologists. Working backwards, the dry stage between lakes
IV. and V. then represents the " Climatic Optimum."
Nilsson, however, correlates Lake IV. with the Neolithic
of Egypt, about 6000 to 5000 B.C., i.e., roughly the Atlantic
stage of Europe. The Gumban A and B are not certainly
contemporaneous and the latter may possibly represent a
high-level stage of the lake subsequent to the highest level.
If we date the Nakuran as late as 500 B.C. there is a long
interval after the Late-glacial which is not accounted for.
In the lower curve of Fig. 36 I have adopted Nilsson's cor-
relation but extended the rainy period to about 1000 B.C.,
and indicated the main arid period as ending about 5500 B.C.
This leaves the Sub-boreal and Sub-Atlantic of Europe
340 CLIMATE THROUGH THE AGES
represented only by minor oscillations of the lake levels.
This is quite possible, since the Sub-boreal is not now regarded
as very dry and the Sub-Atlantic was not associated with a
marked advance of the glaciers and ice-sheets. The dating
of this curve is to be regarded as conjectural.
On this interpretation. Lake VI., which appears to have
been quite important, represents the Sub-Atlantic, and Lake
VII. may date from the thirteenth and fourteenth centuries.
There is some evidence (14) that Lake Tanganyika was at a
much lower level less than 1,300 years ago, when it consisted of
two lakes separated by an isthmus. The natives have a legend
of the submergence of this isthmus and the joining of the lakes.
G. W. Hobley (15) has collected a certain amount of
evidence as to climatic changes in East Africa. Much of
this is purely geological, and no dated historical evidence
is given. Most interesting is the reference to Jubaland,
where there are large numbers of artificial mounds, some
30 feet high, believed to be funeral mounds of an extinct
race. fc In addition the large number of well-excavated
wells, often over 40 feet deep, and the traces of artificial
dams, all go to prove that this area, which is now practically
a desert, once carried a large and organised population."
The coast of East Africa from Mombasa northward is
studded with ruined towns of the Mahommedan period,
but their climatic significance is not obvious.
Farther south we have the ruined cities of Mashonaland,
of which the best known is Zimbabwe. These are now
attributed to native construction in the fourteenth century,
but they may be a reflection of outside influences rather than
a product of high local culture due to favourable climatic
conditions.
REFERENCES
(1) TOUSSOUN, Prince OMAR. " Mmoire sur Thistoire du Nil." Le Cairc,
Mem. Inst. Egypt, vol. ix.
(2) LYONS, H. G. " The physiography of the River Nile and its basin." Cairo,
1906.
(3) LYONS, H. G. " On the relation between variations of atmospheric pressure
in North-east Africa and the Nile flood.** London, Proc. R. Soc., A. 76,
1905, p. 66.
(4) HFRODOTUS, The History of. Transl. by GEORGE RAWLINSON, 2 Vols.,
Everyman.
(5) LEFTER, H. " Die Frage der Klimaanderung wfthrend geschichtlicher Zeit
in Nordafrika." Wien, Abh. K. K. Geogr. Gesellsch., 8, 1909, p. i.
AFRICA 341
(6) NEGRI, C. " Sul clima della Libia attraverso i tempi storici." Roma,
Mem. Ace. Nuovi Lincei, ser. 2, vol. i.
(7) HELLMANN, G. " Uber die Agyptischen Witterungsangaben im Kalendar
von Claudius Ptolemaeus." Berlin, Sitzungsber. preuss. Akad. Wiss., 13,
1916, p. 332.
(8) SAWYER, L. D. " Note on Egyptian winds in Ptolemy's * Prognostics '
and Hellmann's criticism of them." London, Q,. J- R* Meteor. Soc.,
57, 1931, p. 26.
(9) BEADNELL, H. J. LLEWELLYN. " An Egyptian oasis. An account of the
oasis of Kharga in the Libyan desert, with special reference to its history,
physical geography, and water supply. " London, 1909.
(10) CATON-THOMPSON, G., and E. W. GARDNER. " The prehistoric geography
of Kharga Oasis." London, Geogr. J., 80, 1932, p. 371.
(i i) KEANE, A. H. " Man, past and present." Rev. ed. Cambridge, 1920.
(12) NILSSON, E. " Quaternary glaciations and pluvial lakes in British East
Africa." Geogr. Ann., Stockholm, 13, 1931, p. 249.
(13) LEAKEY, L. S. B. " The stone age cultures of Kenya Colony." Cambridge,
Univ. Press, 1931.
(14) THEEUWS, R. " Le lac Tanganyika." Mouvement Gt'ogr., 33, 1920, col.
625 ; 34, 1921, col. 49.
(15) HOBLEY, C. W. "The alleged desiccation of East Africa." London,
Geogr. J., 44, 1914, p. 467.
CHAPTER XXI
AMERICA AND GREENLAND
OUR chief source of information about the climatic
changes in North America is the rate of growth of
the " Big Trees " or Sequoias of California. Some
of these trees are of astounding age, and carry our records
back long before the Christian era. There have been
difficulties, of course, and the close comparison and averaging
of the records of a large number of trees have been required
to give a reliable record of the rate of growth. Douglass has
used these records very effectively in investigating rainfall
periodicities in California, but for the discussion of long-period
climatic fluctuations certain corrections are necessary, and
these are difficult to determine. Huntington (i) believes that
the course of climatic variation is the same in California as in
Central Asia, and he accordingly employs the levels of the
Caspian for the final calibration of his curve of tree growth.
This method, however, is fraught with danger ; if one wishes
to compare variations of climate in America with those in
Asia, the American curve should, if possible, be derived
entirely from local evidence. The material for such a
discussion on American evidence only is provided by a
valuable Carnegie Institution publication entitled " Quaternary
Climates " (2), a collection of papers by J. Claude Jones,
Ernst Antevs, and Ellsworth Huntington. In this volume
Antevs gives the results of a reinvestigation of the tree-growth
data, based on all Huntington's measurements (451 trees)
corrected for age by intrinsic evidence only.
Huntington's curve, even after correction for age, longevity
and " flaring " (see p. 345) shows a much greater variability
in the earlier years than in the later, which is most probably
due to the much smaller number of trees. I accordingly
applied a scale correction which decreased from the Beginning
to the end of the curve and was designed to reduce the
variability about the mean to approximately the same value
342
AMERICA AND GREENLAND
This corrected curve is the
343
uppermost in
throughout.
Fig. 37-
The middle curve is that given by Antevs (2) for trees in
dry situations, slightly smoothed to bring out the more lasting
variations.
Antevs first divided Huntington's material into two groups,
trees growing in dry situations and trees growing in wet
situations. The width of the rings in each decade was plotted
separately for each tree or, in some instances, for small groups
1000
500 B.C.
A.O 5OO
IOOO
I5OO
Fig. 37. Variations of rainfall in U.S.A.
of trees of the same age. A smooth " middle line " was then
drawn through the graph, while the maxima and minima
were connected by drawing smooth " tangents " on either
side of this middle line. The fluctuations were then reduced
to the same basis by dividing the distance of any point on the
graph from the middle line by the distance between the tangents
at that point. Since it proved difficult to draw tangents for
the earlier portions of the curve, when the trees were young
and growing rapidly, only the middle line was drawn for these
parts of the curves, the distances from this line being used
without correction. For this part of the curves, before 200
B.C., the fluctuations therefore appear to be greater in
magnitude than for the later part.
The results for the different trees or groups of trees were
then added together, a correction being applied to allow for
the incoming of successive groups.
344 CLIMATE THROUGH THE AGES
The curves based on trees growing in dry and moist
situations show good agreement after A.D. 800, when they
are based on a large amount of material. The chief difference
is the retardation of the maxima on the " dry " curve.
Previous to A.D. 800 the agreement is not so good, presumably
owing to the smaller amount of material. The differences
are largely in the minor fluctuations, and when the curves
are smoothed a better agreement is obtained.
These curves cannot be regarded as measures of the rainfall
only ; they must include other factors of tree growth such as
temperature and sunshine. These factors themselves, however,
are presumably related in some way to rainfall ; for example,
when sunshine is abundant, temperature is high and rainfall
small. Huntington (2, p. 162) gives the results of correlating
the rate of growth of 112 Sequoias with the rainfall of
Sacramento, 1863 to 1910, the rainfall season being taken as
July to June. The correlation between the annual tree
growth and the rainfall of the immediately preceding season
is small (+0-13), but when the two preceding seasons are
added together the coefficient becomes +'22. The re-
lationship becomes closer the longer the period preceding the
tree growth over which the rainfall is summed, and the
correlation between the annual growth and the total rainfall
of the ten preceding years is +0-58. Thus the curves of tree
growth evidently reproduce with a fair degree of accuracy
the variations of rainfall from one decade to another. In
plotting the corrected curves of tree growth in Fig. 37, the
values have been assigned to a date five years before the year
in which the growth ring was formed, in order to allow for
this lag.
The method by which Antevs corrected his curves obviously
tends to eliminate all fluctuations of long period, and the
curves cannot be expected to show the major fluctuations of
rainfall similar to that between the seventh and eleventh
centuries in Europe. A better measure of the long-period
variations is probably given by the data corrected by
Huntington's method (i), but omitting the " Caspian correction
factor." Two corrections are applied, for age and for
longevity. Trees grow at different rates according to their age,
young trees usually growing rapidly and old trees slowly.
Trees which are destined to have a long life usually grow more
AMERICA AND GREENLAND 345
slowly at first than their neighbours which are likely to die
much sooner.
The " corrective factor for age " was readily obtained by
averaging the rates of growth of a number of trees in the
corresponding years of their lives. In this way the climatic
and other peculiarities of the individual years (which come
at different times in the lives of different trees) are eliminated ;
it is found, for example, that the average growth of trees i year
old is o- 10 inch ; 10 years old, o- 15 inch ; 40 years old, 0-20
inch ; 100 years old, o- 10 inch ; 200 years old, 0-05 inch ;
and so on. The growth curves of the individual trees are then
corrected for age by dividing the first year's growth by o- 1, the
tenth year's growth by 0-15, and so on.
The " correction for longevity " was obtained in a similar
manner, the average rate of growth of trees which when felled
had lived for 100 years, 200 years, 300 years, and so on, being
plotted against the number of years of growth and a smooth
curve drawn through the points.
There is another source of error, namely, that due to
c< flaring " and " buttressing " at the base of the trees.
" Flaring " means the spreading out of the base of the tree
so that instead of descending nearly vertically, the trunk
meets the ground at an angle, like a cone. The correct
width of the growth-ring would be that measured at right
angles to the surface of the trunk ; since the measurements
were actually made on a horizontal surface, the widths found
are too great. " Buttressing " means that the cross section
of the trunk of an old tree is not circular, but develops pro-
tuberances and furrows which add to its strength. The
measurements were more easily made across the buttresses
than across the furrows, and this again tends to increase the
apparent growth in the later portions of the curve.
The later portions of Huntington's curve can be checked
by two incidental facts mentioned by him. He states (i)
that in moist places there are plenty of young trees of all ages,
but on dry mountain slopes, while there are plenty of mature
trees 500 or more years old, there are no young trees except
an occasional seedling or tree of three or four years' growth.
This suggests that the climate has been drier than that of
to-day since about A.D. 1400, and that previous to that date
it was considerably moister. The other fact is that farther
346 CLIMATE THROUGH THE AGES
north, on the shores of Mono Lake, the rings of growth of a
tree killed by the rising salt water show that it had been
growing since 1775. This proves that the level of the lake has
not been as high as it is at present since at least 1775- Both
these facts fit in with the later portion of curve and confirm
the correction for " flaring."
J. Claude Jones (2) has discussed the variations of level of
the remnants of Lake Lahontan in the Great Basin of Western
U.S.A. This was one of the great lakes formed in Western
America during the Quaternary glaciation ; at its maximum
it covered a continuous area of about 8,500 square miles.
As the level fell it split up into a number of separate lakes,
and at present the old basin of Lake Lahontan is occupied by
large desert " playas " with several small lakes in depressions
Humboldt, North and South Carson, Pyramid, Winnemuc^a,
Walker, and Honey Lakes. The discussion centres mainly
on Pyramid and Winnemucca Lakes. Jones gives a variety
of observations of the deposits of calcareous tufa, etc., but his
interpretation of them is obviously erroneous, since they lead
him to the conclusion that the mastodon and the camel lived
on in North America into historic times. He has confused
recent phenomena with those belonging to the Quaternary
pluvial period, and it is necessary to go over his work and sort
out the data from the two periods.
Calcareous deposits or tufas are widely distributed over
a large part of the old basin of Lake Lahontan. The tufa is
in three forms, lithoid (stony), dendritic, and crystalline.
The lithoid and dendritic forms are found at all levels in the
basin, and appear to have been formed by the activities of
algae ; the crystalline form occurs as a mineral (thinolite),
which seems to have been altered from crystals of aragonite
deposited from a saturated solution of calcium carbonate ;
it occurs only in the lowest levels of the basin. Evidently,
it was not until Lake Lahontan had dwindled almost to its
present small remnants that the water became sufficiently
saturated for calcium carbonate to be deposited directly
without the agency of plant life. There seems no doubt, as
E. Antevs points out in the second memoir of the collection,
that the mass of the calcareous tufa, including the ithinolite,
was formed during the shrinkage of Lake Lahontan after the
Quaternary expansion ; when the thinolite was deposited, the
AMERICA AND GREENLAND 347
lake must have been intensely salt, but at present Pyramid and
Winnemucca Lakes are only slightly salt, the salinity in 1882
being 0-35 per cent, in Pyramid and 0-36 per cent, in
Winnemucca.
If we know the total amount of salt in a lake and the average
amount carried in by the rivers in the course of a year, we can
calculate the period in years since the lake was fresh. As
Pyramid and Winnemucca Lakes are separated only by a low
divide, and both receive branches of the same river, the
Truckee, a very small expansion would suffice to unite them
in a single lake, so that for the purposes of the discussion they
can be treated as one. Jones gives four separate determina-
tions of the age of the system. The first two depend on the
method described above, of dividing the total amount of
saline matter present in the lakes by the annual contribution
of the river, but calculating the age from the chlorine and
sodium separately (2, pp. 28-29). Jones writes :
Using a detailed map of Pyramid and Winnemucca Lakes, it is
possible to obtain the volumes of the lakes by determining the areas
of the sub-lacustrine contours by means of a planimeter and cal-
culating the volumes of the respective sections. Such a determina-
tion indicated the amount of water present in Pyramid Lake as
7-787 cubic miles and 1-142 cubic miles in Winnemucca Lake.
The Truckee River has an average flow based on measurements at
Vista, a station in the Truckee Canyon below all the larger tribu-
taries, made during the years 1899 to 191 1 inclusive, of 0-274 cubic
mile per year. It would take Truckee River 28-42 years to supply
the water at present in Pyramid Lake, and 4-17 years additional to
fill Winnemucca Lake. But Pyramid Lake contains 1,455 P ar ts
per million chlorine, Winnemucca Lake 2,184 parts per million,
and the Truckee only 13 parts per million. ... It would there-
fore take the Truckee 3,180 years to supply the chlorine in Pyramid
Lake and 701 years additional to furnish that of Winnemucca Lake,
or 3,881 years for both. A similar calculation, using sodium instead
of chlorine, gave 2,447 years necessary, and the other substances gave
still lower results. Of these calculations the first is probably more
nearly the truth, as chlorine is the least likely to be removed from
solution. No great degree of accuracy can J3e claimed, for many
factors may have influenced the result. While the Truckee River
is the only stream of considerable volume that flows into the lakes,
yet a considerable amount of water is supplied by the intermittent
streams and springs about the borders. . . . The amount of salts
carried probably varied somewhat with the increase in flow of the
river . . . although . . . the present data indicate no very great
change. Of the factors mentioned, only one, the last, would tend
348 CLIMATE THROUGH THE AGES
to make the period greater, while the others would cause the actual
period to be less than the calculated duration. . . .
The question may be approached from an entirely different angle.
As it happened in 1913, the level of Pyramid Lake was but 5 inches
below the level at the time of Russell's visit. A sample was col-
lected near the locality where he obtained his southern sample, and
the chlorine determined. The gain during the 31 years that had
elapsed between the two visits was 23 parts per million. As the
lake had essentially the same volume in both instances and the
samples were taken at the same locality, the variable factors are
eliminated as far as possible. Dividing the total chlorine found in
1913 by the gain and multiplying the result by 31, the years that had
elapsed, gave i ,956 years as the time necessary for the chlorine to
accumulate, providing the present conditions had not been
materially changed.
This method is open to the criticism that it depends on but two
analyses, and while it is of value as corroborative evidence, yet it
cannot be considered as conclusive.
Still another method, one used by Russell, may be employed.
Knowing the total amount of chlorine in the two lakes and the rate
of evaporation, the length of time necessary to evaporate enough
water to supply the chlorine may be determined. Using the recent
analyses, Pyramid Lake contains 1,440 parts per million of chlorine.
Assuming that the water carried into the lake was as fresh as in the
Truckee River, the water before evaporation contained 13 parts per
million of chlorine. This would make it necessary to evaporate
1 1 1 cubic miles of river water to concentrate i cubic mile of
Pyramid Lake water, or, since the lake contains 7 787 cubic miles,
864-36 cubic miles have been evaporated since the beginning of
Pyramid Lake. Similarly, 180-43 cu bic miles additional would be
required to furnish the chlorine in Winnemucca Lake.
The loss of water from the surface of a lake by evaporation
depends on two factors, the rate of evaporation and the area
of the lake. With regard to the rate of evaporation, no actual
measurements are available for Pyramid or Winnemucca,
but Jones gives determinations obtained in three different
ways : the evaporation from open pans on land at Fallen
averages 65-14 inches a year, and Bigelow, from observations
near Reno, concluded that the evaporation over open water is
about five-eighths of that from a pan on land, giving the
evaporation from the lakes as 40-7 inches a year. Salton
Sea, formed by a break of the Colorado River, but now
receiving no appreciable supplies of water, has been falling
at the average rate of 55*6 inches a year. Finally, the
average inflow of the Truckee River into Pyramid and
AMERICA AND GREENLAND 349
Winnemucca, divided by the present areas of these lakes,
gives an annual evaporation of 52 inches. From these three
determinations, Jones estimates the average evaporation as
about 50 inches a year. If the lake had always maintained
its present area of about 370 square miles, the concentration
of the chlorine would require 4,300 years. Jones thinks,
however, that the average level of the lake has been higher
than the present level, and since there is a shelf cut in the rock
1 1 o feet above the present surface of the lake, showing that
for a long period the lake stood at that level, he adopts no
feet as the average level. This gives an average area of 550
square miles, and a duration of 2,400 years. It seems certain,
however, that the rock shelf dates from an earlier period,
probably the close of the Quaternary pluvial period. Antevs
(3, p. 102) quotes Gale and Huntington to the effect that there
is a well-marked outflow channel through Emerson Pass,
70 feet above Pyramid Lake. Jones (2, p. 40) gives the maxi-
mum level of this pass as 78 feet, and states that there is no
evidence of overflow, the summit of the pass having a floor of
fine clays and silts. Jones realised the importance of this
point, and appears to have examined the ground thoroughly.
In either event, however, it is obviously impossible for
Winnemucca and Pyramid Lakes to have stood more than
78 feet above their present level without overflowing and
becoming fresh. Hence the average level of 1 1 o feet above
the present and the average area of 550 square miles are too
great, and the age determination of 2,400 years too short.
This method, therefore, gives the age of the present lakes as
probably something less than 4,300 years.
Thus we have four determinations, which are not, however,
quite independent of each other. These give respectively
3,880, 2,447, J >956, and 4,300 years. From these figures it
seems probable that Pyramid and Winnemucca Lakes were
fresh some time between 2,000 and 4,000 years ago.
A lake may become fresh in one of two ways, either by
overflowing into another basin, so that there is a flow of
water through it which sweeps away the accumulated salt,
or by becoming dry for a period long enough to allow the
salts to* be buried below subaerial deposits. A short dry
period will not suffice ; the overlying deposits must be so
thick that when the lake forms again the salts are protected
35O CLIMATE THROUGH THE AGES
from the water and are not redissolved. The answer to the
question in which of these two ways Pyramid and Winnemucca
Lakes became dry, depends on the interpretation of the
phenomena in Emerson Pass. If Jones is correct, the forma-
tion of the present lakes was preceded by a long dry period
which ended 2,000 to 4,000 years ago, and since then they have
never overflowed. If Gale is correct, the lakes probably
overflowed 2,000 to 4,000 years ago, and so became fresh, and
there is no evidence of a preceding dry period. Since Jones
examined the pass with Gale's work in mind, he is the more
likely to be correct.
This conclusion is supported by an investigation of W. van
Winkle into the salt contents of Abert and Summer Lakes, in
Oregon, which are remnants of the old Quaternary Lake
Ghewaucan, north of Lake Lahontan, also without outlet.
Van Winkle writes (3, p. 123) : " A conservative estimate of
the age of Summer and Abert Lakes, based on their concen-
tration and area, the composition of the influent waters, and
the rate of evaporation heretofore assumed, is 4,000 years.
It is quite possible that the lakes are recent pools, and that
the salt and soda deposits of Early Quaternary Chewaucan
Lake lie buried beneath them."
It is known from the work of Antevs and de Geer that
the major variations in the rate of recession of the ice-sheets
at the close of the Quaternary glaciation in North America
ran closely parallel with the variations in Scandinavia. In
the post-glacial period, the peat-bogs show a succession of
wet and dry periods which closely resembles the Scandinavian
succession. H. P. Hansen (4, 5) states that in Eastern North
America the climatic succession shown by the peat-bogs
closely resembles that of North-west Europe :
Eastern N. America. North-west Europe.
Ice-retreat (Hudsonian) Late-glacial.
Spruce, fir (cool, moist) Pre-boreal.
Pine (warmer but still cool) Boreal.
Oak and hemlock (warm, moist) Atlantic.
Oak and hickory (warm, dry) Sub-boreal.
Oak, chestnut, spruce (cooler, moister) Sub-atlantic.
4.
The pollen profiles reveal consistent and definite evidence
for a dry period, which is best developed in east Washington
AMERICA AND GREENLAND 351
and Oregon. In the North-west Pacific states of U.S.A.
the dry period was less developed owing to the proximity of
the Pacific. Early in the dry period there was a great
explosive eruption of Mount Mazama ; the distribution of the
pumice shows that at the time the winds blew from south and
west. The climate was cooler and moister in the early
post-glacial than at any time since, but the earliest forests
differed little from the present. There is evidence of a rather
wet period in the Puget Sound region some time alter 7000
B.C. when the moisture-loving hemlock expanded. There
was another abrupt spread of hemlock about 2000 B.C. (The
dates appear to be estimates from the thickness of sedimentary
deposits.)
K. Bryan (6) states that the bogs of Eastern Canada
generally originated in ponds and lakes, bordered by a
forest richer than the present, indicating a warm period.
A cooler moister climate was followed by a warm dry period
which again changed to the present cool moist climate.
The changes in North America cannot be dated by archaeo-
logical evidence, but there seems no reason to doubt the
approximate synchronism of corresponding stages in North
America and Scandinavia, especially as the two areas seem
to be linked up to some extent by the deposits in Iceland
and Greenland. This would give a long dry period during
the third and second millennia B.C. in Eastern North America,
which fits in excellently with the lake records.
In this connexion there is some interest in a note in Nature,
28th November 1925, p. 796, to the effect that the first
settlement of the arid coast of Southern California is now
dated about 3,000 years ago. The concordance with the lake
evidence may be accidental, but it may mean that before that
date the supply of fresh water was insufficient for settlement.
The result of this discussion, therefore, seems to be that
about 3,000 years ago there was either the end of a long dry
period or a period of relatively heavy rainfall, probably the
former, possibly both, and in any event an increase of rainfall.
Jones' discussion of the levels indicated by the lake terraces
is useless for the climates of the historical period, because
they are all above the level of the Emerson Pass, and therefore
belong to an earlier period in the history of Lake Lahontan,
more than 3,000 years ago.
352 CLIMATE THROUGH THE AGES
Further evidence is supplied by the levels of Owens Lake,
in Southern California (2, p. 200). There seems to be no
doubt that the freshening of this lake occurred through the
level rising so high during a wet period that the lake overflowed
its basin. This lake is supplied by the Owens River, and,
according to H. S. Gale, analyses show that the river, at the
point where it was tested, would require 4,200 years to supply
the chlorine and 3,500 years to supply the sodium now in
Owens Lake. This gives 4,000 years as the maximum period
since the freshening, but there are several factors which make
this estimate too high. The analyses were taken at a point
some way up the valley, and omit the lower third of the basin,
which contains old saline clays, and across which the river
flows slowly ; moreover, no allowance is made for greater
rainfall in the past. Hence, Huntington concludes that the
most probable length of the period which has elapsed since
Owens Lake was fresh is between 2,000 and 2,500 years.
Finally, we have the evidence of Walker Lake (2, p. 46).
This is fed by Walker River, and there is no possibility that
it was ever freshened by overflowing. Its salt content is
only 0-25 per cent., and Jones calculates that the present
rate of supply would accumulate this amount in about 1,160
years or possibly less. Jones' theory of the origin of the lake
is that it was formed by a change in the course of Walker
River at the time of maximum level of Lake Lahontan, but
this change, if it took place, must have occurred in Quaternary
times. All that the lake shows us is that a minor dry period
ended about 1,100 years ago.
Thus we may sum up the evidence afforded by the lakes
as follows : Some time after the great expansion of the lakes
associated with the Quaternary glaciation of America, there
occurred a long period of desiccation, in which Abert and
Summer Lakes, and probably also Pyramid and Winnemucca
Lakes, dried up completely. About 3,000 years ago this period
of desiccation was brought to an end by an increase of rainfall
to a value above its present amount, refilling the basins of
Abert and Summer Lakes, but without causing them to
overflow, filling the basins of Pyramid and Winnemucca
Lakes, and perhaps causing them to overflow, and &lso filling
Owens Lake and causing it to overflow. This was the time
of greatest rainfall in Western North America during the
AMERICA AND GREENLAND 353
historical period. Then followed a period of decreased rainfall,
not enough to cause the complete disappearance of Abert,
Summer, Pyramid, and Winnemucca Lakes, but enough to
dry up Walker Lake for a period long enough to bury the salt
accumulations. This secondary dry period ended about 1,100
years ago, or A.D. 800. The lakes do not give any indication
of the happenings after A.D. 800. There is some historical
evidence ; thus Huntington (i) states that when the Aztecs
founded the city of Mexico, about A.D. 1325, the level of the
lake of Mexico was high, and that another period of high water
occurred about 1550.
Let us now see to what extent these lacustrine fluctuations
fit in with the curve of tree growth. Unfortunately, the
tree-growth curve helps us little during the early most crucial
period of the change from dry to moist conditions. Antevs 5
curves show a maximum at 840 B.C., a minimum at 740 B.C.,
and a second maximum at 660 B.C. Huntington's data
show a maximum at 960 B.C., a minimum at 780 B.C., and a
second maximum at 660 B.C. ; the maximum at 840 B.C. on
Antevs' curves is barely indicated. These early growth
curves are based on relatively few trees, and the corrections
are uncertain, so that we cannot say more than that the wet
period had definitely begun by 660 B.C., but may have begun
two centuries or more earlier. It is interesting to note that
the Chinese records also indicate a dry period from 842 to 771
B.C. The rainfall maximum indicated by the overflow of
Owens Lake presumably corresponds with the rapid growth
of the trees from 480 to 250 B.C., shown on all the curves ;
during this interval the rainfall reached its absolute maximum
for the whole period since 1000 B.C. The period of drought
during which Walker Lake dried up seems to extend from
about A.D. 400 to 850 ; it is shown much more definitely
on Huntington's than on Antevs' curve, owing to the different
methods of correction adopted. The deep minimum shown
by all the curves in the fifteenth century was apparently of
too brief duration to cause even Walker Lake to dry up
completely.
The alternation of dry and wet periods has also been traced
by Hunlington (i) in the archaeological remains of Arizona
and New Mexico. In these dry regions, whose crying need
is water for agricultural purposes, he distinguishes three
354 CLIMATE THROUGH THE AGES
periods of maximum occupation or prosperity. In the oldest
of these the people, whom he terms the Hohokam, were not
limited to the neighbourhood of the water-courses but lived
on the open plateau, and apparently depended on rainfall
instead of on irrigation. The second people, the Pajaritans,
lived partly on the irrigable land, but partly on the Pajaritan
plateau. The latest pre-Columbian race was the Pueblo,
who depended on irrigation, but lived in valleys where there
is not now sufficient water for that purpose. The Pueblo
village of Gran Quivera was still populous at the coming of the
Spaniards. These three periods evidently correspond with
three wet periods ; there is no evidence of continuity, and in
some places, e.g., Ghaco Valley, the deposits containing
remains of the different periods are separated by silts without
human remains, formed during dry periods. The last of tJie
three evidently represents the rainfall maximum from about
A.D. 1 200 to 1400 ; it was the least important of the rainfall
maxima. No dates can be assigned to the earlier periods,
but the Pajaritan occupation presumably includes the period
from 750 B.C. to A.D. 400, when the rainfall was much heavier.
The first occupation, by the Hohokam, appears to be much
older, and may have occurred during a very early rainfall
maximum older than the oldest of the trees, but represented
in Eastern North America by the period of peat-formation
corresponding with the Atlantic stage in Europe.
The lowest curve in Fig. 37 has been reconstructed from a
paper by E. Schulman (7) in the Colorado Plateau, based not
only on living trees but also on specimens of timber from ruins
of Indian buildings. Apart from the trough and peak in the
thirteenth to fourteenth centuries it tends to vary oppositely
to Huntington's curve. This is probably due to its low
latitude (35-40 N.) and is of interest in connexion with
Huntington's theory of the shift of the climatic belts. With
regard to the scale of the curves, it may be remarked that
the maximum area of Pyramid and Winnemucca Lakes was
less than one and one-half times the present area, so that the
maximum of rainfall indicated by the curve is less than 50
per cent., and probably not more than 25 per cent., greater
than the present rainfall.
Farther south we have the remarkable ancient Mayan
civilisation of Yucatan (8, 9). This country is at present
AMERICA AND GREENLAND 355
covered by almost impenetrable forests, the climate is hot,
moist, and enervating, while the inhabitants are idle and
uncultured. Buried in the forests are the ruins of great
cities, decorated by elaborate carving, and indicating a greater
and more progressive population and a high level of civilisation,
one of the features of which, as is well known, was the con-
struction of an elaborate calendar. The problems of Mayan
history and chronology have not yet been completely solved,
but it seems probable that before 400 B.C. there was little
forest, the winters being dry and cool. Between 400 B.C. and
100 B.C. the climate became somewhat moister and more
uniform ; this is the time of the earliest carvings. The
highest level of culture was reached in the period 100 B.C. to
A.D. 300, first in the south, later in the north. By A.D. 300
climate had become less favourable. The deterioration
continued in A.D. 300-450 ; the forest advanced and civilisation
declined in the south. From A.D. 450 to 900 the forest spread
over the whole country, especially north Yucatan, and
civilisation fell to a low ebb. There was a marked improve-
ment in A.D. 900-1 100 accompanied by a great deal of building,
but climate deteriorated again in 1100-1300. From 1300 to
1450 there was a climatic improvement but culture did not
respond to any extent. From 1450 onwards climate has been
continually unfavourable. Sapper (9) thinks that the decline
of civilisation was due partly to climatic changes and partly
to the introduction of malaria.
Since it takes time for both forests and civilisation to respond
to the effects of climatic changes, we may date the latter about
50 years earlier than the changes in the level of civilisation.
This gives us the following comparison with Huntington's
curve :
Western U.S.A. Yucatan.
Wet 500-250 B.C., IQO B.C.-A.D. 200. Dry 500 B.C.-A.D. 250.
Dry A.D. 300- 800. Wet A.D. 400- 850.
Wet A.D. 900-1100. Dry A.D. 850-1050.
Dry A.D. 1100-1300. Wet A.D. 1050-1250.
Wet A,D. 1300-1400. Dry A.D. 1250-1400.
Dry A.D. 1450-1550. Wet A.D. 1400-
In tha dry regions of Asia and Arizona the periods of high
culture were attributed to an increase of rainfall, but Yucatan
now suffers from too much rain, and any increase would make
356 CLIMATE THROUGH THE AGES
the conditions even less favourable than at present. Hence
Huntington (8) infers that the great periods of Mayan history
were times of decreased rainfall in Yucatan. Since they
coincide with rainy periods farther north, we are evidently
dealing here with a redistribution of rainfall. The way in
which this was probably brought about will be discussed in the
next chapter.
The supposed climatic changes in Greenland have been
a matter of controversy for many years, but excavations,
described by Hovgaard (10), appear to establish their existence
beyond doubt. Icelanders settled in Greenland in the tenth
century A.D., and two colonies were established, the Eastern
Settlement, just west of Gape Farewell, and the Western
Settlement, 1 70 miles up the west coast. The settlers brought
with them cattle and sheep, which were successfully reared ^at
first, and they even attempted to grow grain, but before very
long the colonies became dependent on supplies from Norway.
Norway itself was passing through a time of stress, however,
and the visits of ships became fewer and fewer, until some
time in the fifteenth century they ceased altogether, and the
colonies were lost sight of. For many centuries their fate was
unknown, but the history of the Eastern Settlement has now
been made out by the excavations of a Danish archaeological
expedition at Herjolfsnes, near Cape Farewell. The most
important evidence is derived from the excavation of the
churchyard, in soil which is now frozen solid throughout the
year, but which, when the bodies were buried, must have
thawed for a time in summer, because the coffins, shrouds,
and even the bodies were penetrated by the roots of plants.
At first the ground thawed to a considerable depth, for the
early coffins were buried comparatively deeply. After a time
these early remains were permanently frozen in, and later
burials lie nearer and nearer to the surface. Wood became
too precious to use for coffins, and the bodies were wrapped
in shrouds and laid directly in the soil. Finally, at least
five hundred years ago, the ground became permanently
frozen, and has remained in that condition ever since, thus
preserving the bodies. The remains show a gradual deteriora-
tion in the physique of the colonists ; their teeth specially
are much worn, indicating that they lived mainly on hard
and poorly nourishing vegetable food.
AMERICA AND GREENLAND 357
The change of climate indicated by these facts is borne
out by the evidence as to the ice conditions. When the
colonies were first settled, there were traces of the former
existence of the Eskimos, but none then lived so far south.
The Eskimos follow the seals, which frequent the edge of
the ice, and this indicates that in the tenth century the ice-
edge in Baffin Bay lay far to the north. In the thirteenth
century the Eskimos reappeared and advanced persistently
southward, until by the middle of the fourteenth century
they had occupied the Western Settlement, which apparently
they destroyed.
The accounts of the early Norse voyages to Greenland
are remarkably free from references to ice conditions, and,
in fact, as O. Pettersson (n) points out, it is difficult to
uaderstand how their protracted explorations could have
been carried out if the ice conditions had been anything
like those of the present day. Pettersson's chart of the old
Norse sailing routes shows a track direct from Iceland to the
east coast of Greenland in latitude 66 N., then down the
coast to Cape Farewell, and up the west coast. According
to the documentary evidence which he adduces, this route-
at present almost impossible was followed until about
A.D. 1 200, when it was abandoned for a more southerly route.
On the other hand, as early as A.D. 998 a shipwrecked party
was ice-bound on the east coast of Greenland, probably near
or north of Angmagsalik. It is to be noticed that the ship was
wrecked on the coast and not on the ice.
The early climatic history of Greenland, therefore, appears
to have been somewhat as follows : When the country was
colonised in the tenth century its climate was much more
favourable than at present, for herds of sheep and cattle
thrived. There was less ice than at present in the East
Greenland Current, and it is even possible that at first there
was no ice at all ; Baffin Bay seems to have been largely free
of ice. But in the second half of this century the climate was
already deteriorating, and about A.D. 1000 there came a
foretaste of the coming ice. After this, conditions apparently
improved slightly, and the colony appears to have prospered
during oaost of the eleventh and twelfth centuries. Towards
the close of the twelfth century deterioration again set in,
and the ice conditions rapidly became very bad. The
358 CLIMATE THROUGH THE AGES
summer thaw became shorter and shorter, and about A.D.
1400 the ground became permanently frozen. Communica-
tion with the mother-country was broken, life became too
hard to bear, and the colonies finally perished.
For South America the only evidence I can find of climatic
changes in the historical period is given by E. Taulis (12)
who estimated the rainfall of each year from 1535 to 1931 in
Chile on a scale of 1-5. From his figures it appears that there
were wet periods about 1550 and in 1684 to 1700, and a great
drought from 1770 to 1783. This agrees with Antevs' curve
of tree growth.
REFERENCES
(1) HUNTINGTON, E. " The climatic factor, as illustrated in arid America."
Washington, 1914.
(2) WASHINGTON, CARNEGIE INSTITUTION. Publication No. 352. " Quaternary
climates." Papers by J. CLAUDE JONES, ERNST ANTEVS, and ELLSWORTH
HUNTINGTON. Washington, July 1925.
(3) WASHINGTON, U.S. GEOLOGICAL SURVEY. Water Supply Paper 363.
" Quality of the surface waters of Oregon." By WALTON VAN WINKLE.
1914.
(4) HANSEN, H. P. " Postglacial forest succession and climate in the Oregon
Cascades." Amer. J. Sci., 244, 1946, p. 710.
(efi > " Postglacial forest succession, climate and chronology in the
Pacific North-west." Philadelphia, Trans. Amer. Phil. Soc., 37, Pt. i, 1947.
(6) BRYAN, K. ** Palaeoclirnatology in North America as a result of the study
of peat bogs." <X Gletscherk., 20, 1932, p. 76.
(7) SCHULMAN, E. " Nineteen centuries of rainfall history in the Southwest."
Milton, Mass., Bull. Amer. meteor. Soc^ 19, 1938, p. 311.
(8) HUNTINGTON, E. "Civilisation and climate." 3rd ed. New Haven, 1924.
(9) SAPPER, K. " Klimaanderungen und das alte Mayareich." Beitr. Geoph.,
Leipzig, 34 (Koppenbd 3), 1931, p. 333.
(10) HOVGAARD, W. " The Norsemen in Greenland. Recent discoveries at
Herjolfsnes." New York, N.Y., Geogr. Reu., 15, 1925, p. 605.
(u) PETTERSSON, O. "Climatic variations in historic and prehistoric time."
Svenska Hydrogr.-Biol. Komrn. Skr., 5. Goteborg, 1914.
(12) TAULIS, E. " De la distribution des pluies au Chili. La periodicit6 des
pluies depuis quatre cents ans." Geneve, Mat. e'tude calam., 9, 1934, p. 3.
CHAPTER XXII
THE INTERPRETATION OF CLIMATIC FLUCTUATIONS
IN THE HISTORICAL PERIOD
FROM the preceding four chapters we see that during
the historical period there have been several climatic
fluctuations of quite appreciable magnitude ; in some
parts of the Northern Hemisphere the fluctuations were closely
similar over wide areas, while in other parts they were in
distinct opposition. We must now try to discover the causes
of these variations. Fig. 38 gives general curves of estimated
6QOO
IQOO
&C, Q A.CX
*\
0>-
Fig. 38. Variations of rainfall, world.
rainfall for Europe, Asia, North America and for East Africa
from the equator northwards. The curve for Europe was
constructed by superposing the first three curves of Fig. 31
and drawing a mean curve through them ; that for U.S.A. was
drawn in the same way from Huntington's and Antevs' curves.
The first three curves of Fig. 38 show a good deal of resemblance
with some discrepancies which may be due to difficulties of
precise dating. The discrepancies in the curve for Africa are
greater but, as previously stated, the dating of this curve is
only conjectural ; the maximum shown at 1250 B.C. may
easily be pushed back to 2000 B.C.
359
360 CLIMATE THROUGH THE AGES
The first three curves all refer to the region between about
35 and 65 N., in which the rainfall is mainly brought by
barometric depressions (Chapter II.), while the curve for
Africa aims at showing the variations in the equatorial belt
of low pressure. Between these two lies the sub-tropical high
pressure belt where the rainfall is mainly monsoonal, and is
greatest when the general circulation of the atmosphere is
weakest. Our information about past climates in this region
is scanty, but such as it is, it suggests that the variations were
in the opposite direction to those farther north. First we have
the rainfall maximum at Mohenjo-Daru in India about
2750 to 2500 B.C., which comes in the middle of the long dry
period in Europe and Asia. Then the variations in Yucatan
are directly opposed to those in the Western United States,
and there are indications that in the later stages at least the
variations of rainfall in Cambodia agreed with those in
Yucatan. This zonal distribution strongly suggests that the
variations of rainfall are related to changes in the zonal
circulation of the atmosphere.
Since the total amount of air is fixed, the average barometric
pressure over the whole surface of the planet must be always
the same, and an excess in one region must be compensated
by a deficit in some other region. Now it has been found,
especially by Sir Gilbert Walker (i), that this process of
compensation is not haphazard ; it follows a clearly marked,
though not inviolable rule. When, in a region where pressure
is normally high, such as one of the sub-tropical anticyclones,
it rises even higher than usual, there is a tendency for pressure
to be higher than usual in all those parts of the world where it is
normally high, and lower than usual in all those parts of the
world where it is normally low. That is to say, if pressure
is above normal in, for example, the Azores anticyclone, it
will tend to be above normal over a belt stretching more or
less completely round the globe from Hawaii to the north of
Mexico, and across Bermuda and the Azores to North Africa.
On the other hand, pressure will tend to be below normal
over the belt of storminess which runs from Kamchatka and
the Aleutian Islands across Southern Canada and New-
foundland to Iceland, the British Isles, Norway, a*nd the
North of Asia. At the same time there will also be a tendency
for pressure to be below normal near the equator.
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 361
362 CLIMATE THROUGH THE AGES
Some possible causes of these variations of post-glacial
climate are shown in Fig. 39. This is divided at 500 B.C. into
two parts, the time-scale on the right being two and a half
times that on the left. First we have variations of sea level.
These are known accurately only in Scandinavia, but this
region is important because of its proximity to the only broad
gateway to the Arctic. A little before 5000 B.C. the Ancylus
emergence gave place to the Litorina subsidence, which
reached its maximum between 4500 and 4000 B.C., after which
the sea gradually receded. By 500 B.C. it had reached nearly
its present level, and since then the changes have been un-
important. It was shown in Chapter VIII. that the decrease
of continentality at the maximum of the Litorina Sea must
have raised the winter temperature by about 5 F., and that
this agreed closely with the observed rise of temperature <dn
Scandinavia. The subsidence of the land, and the readier
access of southerly winds, would also affect conditions in the
Arctic Ocean, decreasing the area of floating ice, and so
probably cause a general amelioration of temperature over
all the higher latitudes of the Northern Hemisphere.
Evidence of a post-glacial Climatic Optimum has been
found in Franz Josef Land, Spitsbergen, Norway, the Baltic
Shores, Iceland, Greenland, Ireland, Eastern and Central
North America, Patagonia and Tierra del Fucgo, New Zealand,
Southern and Eastern Australia, South Africa and the Ant-
arctic. We do not know that all these are of the same date,
but there is one feature common to a large proportion of the
deposits which points strongly in this direction the bulk of
the evidence for the post-glacial Climatic Optimum is
derived from or associated with beaches raised a few feet
above the present sea-level. In the Baltic the raised beaches
are higher and are definitely associated with a subsidence of
the land, but in most parts of the world the change of level
was remarkably uniform. A uniform change of level at many
far distant points is almost certainly due to a rise of the sea
and not to a subsidence of the land. A rise of sea-level may
be due to one of three causes :
(a) A decrease in depth of part of the sea floor, com-
pensated by a decrease in the elevation of part of
the land area.
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 363
(b) An increase in the volume of sea water without
change of mass, owing to a decrease in density.
(c) The actual addition of water to the oceans.
The general rise of sea-level during the Climatic Optimum
is hard to estimate precisely, owing to the difficulty of obtaining
the exact levels of old sea beaches, but it seems to have been
of the order of 10 feet. Of the three possible causes of this
rise, (a) can be dismissed very shortly. The principal land
areas in which there was extensive post-glacial subsidence
are Scandinavia and North America north of the Great
Lakes, but both these subsidences were largely compensated
by elevation of the land to the southward ; moreover, the
period of maximum depression was probably over before the
height of the Climatic. Optimum. Causes (b) and (c) are
both possible, but are difficult to estimate.
The mean depth of the oceans is approximately 12,000 feet.
Taking the coefficient of expansion of water as -00015 f r
one centigrade degree, we find that an increase of temperature
by i C. or 1-8 F. would raise the mean level of the surface
by i 8 feet. Thus a rise of the mean temperature of the whole
mass of the oceans by 5 F. would raise the general level by
5 feet. The increase in the surface temperature of the
northern North Atlantic approached 5 F. and in the Arctic
and Baffin Bay the increase was probably even greater. The
temperature of the lower oceanic layers is determined by that
of the polar oceans, but the warming of the whole ocean mass
would be very slow and the general rise of temperature is not
likely to have been nearly so much.
The only ways in which water can be added to the ocean
are by a decrease in the level of enclosed lakes unconnected
with the sea, and a decrease in the volume of the ice-sheets
and glaciers. The volume of water in enclosed lakes without
outlet is so small in comparison with the area of the oceans
that it can be neglected. The ice-sheets, however, are on a
different scale. The ice-covered area in Greenland and the
Antarctic is about six million square miles, and the average
thickness of the ice is nearly 5,000 feet. If all this ice were
melted^ it would raise the general level of the oceans by from
140 to 190 feet. The area occupied by the oceans is about
140 million square miles, or 23 times the area occupied by
364 CLIMATE THROUGH THE AGES
ice, so that in order to raise the level by ten feet, it would be
necessary to melt off 230 feet of ice. Because of the difference
of density between glacier ice and water we may put the figure
at 250 feet. Now we know that even in the much less intense
warm period of the early Middle Ages the boundaries of the
Greenland ice-sheet retreated appreciably, which implies a
corresponding diminution of thickness, so that in the pro-
longed warm period of the Climatic Optimum a lowering
of the average level of the ice-sheets by 250 feet is quite possible.
These two factors, increase of ocean temperature and increase
in the mass of water, appear to be quite competent between
them to raise the general level of the oceans by 10 feet, the
greater part of this being due to the melting of ice.
The second factor to be considered in our climatic re-
construction is the annual range of temperature. As described
in Chapter V., the obliquity of the ecliptic appears to have
reached a maximum about 8150 B.C., and to have decreased
steadily since that date. Also, about 8500 B.C. the earth
was farthest from the sun (aphelion) in the northern winter,
whereas it is now farthest from the sun in the northern summer.
Both these factors would cause an appreciably greater seasonal
range of radiation in the ninth millennium B.C. than at present.
This change is shown by the second full curve on the left of
Fig- 39-
Over most of the world these " astronomical " changes do
not affect the total supply of solar radiation appreciably,
but in the Arctic, which receives little or no solar radiation
in winter, the effect of increased seasonal contrast is to increase
the total solar radiation considerably.
The broken curve between these two full curves represents
the variation of temperature in North-west Europe, copied
from Fig. 32. It is seen that the left-hand part of this curve
is a mean between the curve of sea-level and that of summer
radiation. The " Climatic Optimum " occurred about 5000
B.C., after which temperature fell gradually until about
3000 B.C. The fluctuations in the years between 3000 B.C.
and 500 B.C. will be discussed later.
The climate of the Boreal phase, about 6000 B.C., appears
to have been definitely more continental than at g/iy sub-
sequent time, with cold winters and hot dry summers in many
parts of the Northern Hemisphere, This is probably due to the
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 365
combination of high obliquity with winter in aphelion ; in
North-west Europe the larger land area and the shutting off
of the Baltic was also a factor. The present situation,
decreased obliquity and winter in perihelion, gives the
opposite effect of mild winters and cool summers. The
ice-sheets retreated rapidly between 8000 and 7000 B.C.,
while since the beginning of the Christian era there does not
seem to have been any general retreat, only long-period
oscillations about a mean position. To this extent the long-
period changes of climate since 8000 B.C. support the
astronomical theory.
The third factor is the variation of solar activity. Sir
Gilbert Walker (2) has pointed out that the contrast between
the zones of low and high pressure is apparently controlled
tQ some extent by variations of solar activity. When sunspots
become more numerous, pressure increases in the areas where
it is already high, and decreases in those where it is already
low. In the temperate storm belts, a high sunspot number
tends to be associated with low pressure, great storminess, and
heavy rainfall. According to Huntington and Visher (3),
the belt of storminess in the Northern Hemisphere moves
southward and increases in intensity at times of many sun-
spots, but moves northward and decreases in intensity at
times of few sunspots. C. E. P. Brooks (4) found that the
annual frequency of thunderstorms shows a fairly close
relation to the sunspot number. In many parts of the world,
including Siberia, Sweden, Norway and Scotland in the
north and the West Indies, South-eastern U.S.A., Southern
Asia and the Tropical Pacific in the south, the frequency of
thunderstorms is greatest when sunspots are most numerous.
Between these two belts is a region including England and
Wales, Holland, Germany and the Northern and Western
U.S.A., in which the relation is small, but still generally
positive. Since in the interior of the continents and in
tropical regions a good deal of rain is associated with thunder-
storms, this suggests that rainfall maxima should coincide
with maxima of sunspots, and there is other evidence that on
the whole the total rainfall over the land areas is greatest
when sunspots are most numerous. Our next step must,
therefore, be to construct a curve which will represent the
variations of solar activity over as long a period as possible.
366 CLIMATE THROUGH THE AGES
For our knowledge of sunspot frequencies since 1749 we
are mainly indebted to the researches of R. Wolf, who has
compiled a complete table beginning with that year (5).
The earlier data are based mainly on a long but rather
fragmentary series of records from China. The first record
of a sunspot occurs in the Chinese archives in A.D. 188, and
the first aurora in A.D. 194, but records only become frequent
in the fourth century, apparently reaching a maximum about
374. Another maximum, both of spots and aurora, occurs in
535-540, the first half of the sixth century giving us 20 records
of spots and 13 of aurorae. In the seventh and eighth centuries
the number of records is very small, rising to another maximum
about 840, in which year there are records of 90 sunspots,
while brilliant aurorae occurred in 839 and 840. The records
of sunspots again become very few between A.D. 850 and 107,0,
though there is a secondary maximum of aurorae about 993.
There is a great outburst of sunspots in the years 1077 to 1079,
and the frequency of both spots and aurorae remains very
high until about 1250, with probably a secondary maximum
about 1 20 1. The last half of the thirteenth and the first
half of the fourteenth centuries again show a falling off in
the records, but about 1370-1375 they become very numerous,
and WoLer considers that the absolute maximum of solar
activity during the Christian era occurred in 1372. If so,
this maximum was of very brief duration, tor there are no
records of sunspots between 1383 and 1511, while the frequency
of aurorae also decreases. From about 1676 to 1725 there
was an extraordinary dearth of sunspots, followed by maxima
about 1778 and 1837.
From these figures of sunspots and aurorae an attempt
has been made to construct a curve of solar activity since
the occurrence of the first spot in A.D. 188. The early portion
of this curve is not reliable, probably depending more on the
accidental circumstances which led to the making and pre-
serving of records than on the variations of the phenomena
observed, but it seems probable that the maxima of the
eleventh and fourteenth centuries, and perhaps also that
of the ninth century, are real. From 1750 to 1940, the curve
is based on xo-year means of the relative numbers. This
curve of solar activity is shown at the top right-hand side of
39-
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 367
The construction of a curve of solar activity during the
past few thousand years would be facilitated if we had any
definite knowledge as to the cause of the sunspot cycle.
Various hypotheses have been put forward, the favourite
being the disturbance of the sun's surface by the influence
of the planets, especially Jupiter. Jupiter completes his
journey round the sun in 1 1 -86 years, but when the influence
of the other planets is added to that of Jupiter the result is an
irregular recurrence with an average length of slightly less than
1 1 8 years, which bears some resemblance to the sunspot
curve. The divergences are, however, too great for the com-
plete acceptance of this theory. H. H. Turner (6) has
devised an interesting alternative, which supposes that sun-
spots are due to the impact of meteors belonging to a swarm
(tjie " Sunspot Swarm ") which pursues an elliptical orbit
round the sun with a period which averages slightly over
eleven years, but varies in length owing to interference with
the Leonid Swarm. The latter has a period of 33^ years.
According to Turner, the " Sunspot Swarm " originated in
A.D. 271 owing to the Leonid Swarm coming into conflict
with the rings of Saturn, but so far as I am aware there is
very little if any positive evidence for the existence of the
Sunspot Swarm of meteorites, and in Chapter IV. we found
some evidence for the existence of an eleven-year cycle in
meteorological phenomena long before the beginning of the
Christian era.
The two curves on the right below the sunspot curve show
the variations in the thickness of the annual layers of Lake
Saki, South Russia, and the thicknesses of growth rings of
trees in Western U.S.A. according to Antevs, slightly smoothed.
Previous to about A.D. 800 these do not show much relation-
ship to the curve of solar activity, but as previously stated,
the latter curve is not reliable for this period. The minimum
about A.D. 250, for example, may be due entirely to a gap in
the records. The long minimum of solar activity between
600 and 750 is reflected in the Russian curve, as is the peak
about 850. The sunspot maximum of 1077-1079 is reflected
in the highest peak of Antevs' curve, though the latter appears
to come^ about 30 years earlier, and the general shape of the
Russian curve from noo to 1300 is very similar to the solar
curve. The great peak about 1372 also appears in both
368 CLIMATE THROUGH THE AGES
rainfall curves, though again somewhat early in the tree rings.
Finally, the long dearth of sunspots from 1676 to 1725 is faith-
fully reflected in the annual layers ; it is also shown in the
actual rainfall observations in Western Europe (Chapter
XVIIL).
The short record of the low-level stage of the Nile (Fig. 35)
can only be relied upon between A.D. 640 and 1400, but
during this period it presents considerable similarity to the
unspot curve. Thus we have :
Sunspot maxima A.D. 620 840 1077 I2O I 37
Nile, low-level stage . 645 880 uoo 1225 X 375
The maxima of level in the low-water stage of the Nile
apparently follow sunspot maxima by intervals of from five
to forty years, but the order of importance of the maxin>a
differs greatly in the two curves. In the Nile, the great
crest at uoo completely dominates all the later variations,,
and the peak at 1375 is insignificant. In this connexion it
must be remembered that the Nile curves have been corrected
on the assumption that the deposition of alluvium raises the
level of the whole valley, including the low-level channel,
at a uniform rate. The alluvium is deposited by the flood ;
the low stage of the Nile is supplied by water which has been
filtered by its passage through a series of lakes. Hence a
consistently high level at the time of low water, as happened
about uoo, might perform a good deal of erosion, and by
cutting out a deep if narrow channel, result in a long series
of very low minimum levels in subsequent years. This
would be facilitated by the series of weak floods, which
apparently occurred at about the same time, and which
would bring little alluvium. For these reasons it is possible
that the depression between about A.D. 1150 and 1400 may
not indicate the true level of equatorial rainfall.
Finally we come to a hypothesis due to O. Pettersson (7)
that variations of climate in the historical period have been
caused by long-period variations in the circulation of the
oceans caused by changes in the " tide-generating force. 55
The latter varies with the declination and proximity of the
sun and moon to the earth and, in addition to shorter variations,
reaches maxima at variable intervals which average about
1,700 years. Pettersson gives the dates of these maxima as
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 369
about 3500 B.C., 1900 B.C., 250 B.C., and A.D. 1433, and of
the minima as about 2800 B.C., 1200 B.C., and A.D. 550. These
variations are shown in the middle curve of Fig, 39.
Pettersson points out that in addition to the surface tides,
which would have a greater range at maxima than at minima
of the tidal force, there are also internal tides, formed where
a relatively light less saline layer rests on a heavier more
saline layer, and these submarine tides have actually been
measured in the entrances to the Baltic, attaining a range
of 80 to 90 feet. Submarine waves also enter the Arctic basin,
where they were first traced by Nansen. At periods of
maximum tide they are stronger and are able to break up the
ice, leading to an increase in the amount of drift ice carried
out into the North Atlantic by the polar currents. At tidal
minima, on the other hand, the ice is broken up to a much
less extent, and so there is little drift ice in the Greenland and
Iceland seas.
Drift ice is an important factor in increasing the storminess
and consequently the rainfall of temperate latitudes, and in
deflecting the storm tracks into lower latitudes, and we should
accordingly expect the maxima of tidal force to be maxima
of rainfall also. This appears to be the case with the tidal
maxima of 1900 B.C., 250 B.C., and A.D. 1433, while the minima
in 2800 B.C., 1 200 B.C., and A.D. 550 are also clearly shown.
Further, with increased drift ice in the Atlantic we should
expect lower temperatures in the coastal regions of Western
Europe, and the temperature curve shows that on the whole
this was so. It seems that there is good support for
Pettersson's theory as well as for that of solar activity, and that
the actual variations of climate since about 3000 B.C. may
have been to a large extent the result of these two agents.
To facilitate comparison, the rainfall curves for Europe,
Asia, and the U.S.A. in Fig. 38 have been combined in the
lowest full curve of Fig. 39. This was constructed by first
superposing the three curves and sketching in a general
average. The positions of maxima and minima on all the
individual curves of Figs. 31, 33, 35, and 36 were then marked
on the curve, which was adjusted to bring the peaks and troughs
into accord with the " majority verdict." This was done to
eliminate as far as possible the uncertainties of dating. The
broken curve is Pettersson's curve, extended backwards by
24
370 CLIMATE THROUGH THE AGES
assuming a periodicity of 1,750 years, and modified, after
A.D. 100, by superposing on it the curve of solar activity.
The results are of great interest. From A.D. 100 onwards the
fit is quite as good as can be expected from the nature of the
data, both in the broad swing and in the peaks at intervals of
two or three hundred years. From 3000 B.C. to A.D. o the long-
period variations of rainfall also fit very well. From 2000
B.C. onwards, shorter oscillations of rainfall are superposed on
these long waves, and it is a reasonable supposition that these
may also be related to variations of solar activity. Before
3000 B.C., however, the two curves are in direct opposition.
There are three possible reasons for this :
1. The dating of the rainfall curve is incorrect. Although the
curves for Europe and Asia were constructed independently,
their dating before 2500 B.C. ultimately depends mainly
on the dating of the cultures of Egypt and the Euphrates
valley, which may not yet be quite established.
2. The extrapolation backwards of the tidal curve may be
incorrect. The error is unlikely to be sufficiently great
to invalidate the apparent opposition.
3. The climatic effect of tidal variations was reversed about
3000 B.C. The possibility of this depends on the ice
conditions in the Arctic. The apparent opposition may
be accidental, due to the fact that the Litorina subsidence
happened to coincide with a minimum of the tidal force.
It is of interest, however, to attempt a reconstruction of the
history of the Arctic on the assumption that the opposition
is real.
During the Climatic Optimum the mean temperature of
the Arctic was many degrees higher than now, as shown, for
example, by the growth of peat-bogs in Spitsbergen. Owing
to the high obliquity and winter in aphelion the winters were
cold and the summers very mild, the net result being a gain
of solar heat. The Arctic Ocean may have been in a stage
at which ice floes tended to form in winter but to break up
and melt in summer without much ice finding its way into the
polar currents. In such conditions a slight excess or deficit
of heat would make a large difference to the development of the
ice.
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 371
In periods of minimum tidal force the amount of warm
water finding its way into the Arctic basin was also a minimum,
and an ice-sheet would be likely to form in winter, which,
when it broke up in summer, would supply some ice for the
polar currents to carry into the Atlantic, though less than at
present. These were the rainy periods. At maximum tidal
force on the other hand the Arctic received a large amount of
warm saline water. Both the high temperature and salinity
would act against the freezing of the ocean surface, so that
in these periods there may have been either no ice at all or so
little that it broke up and melted away in summer without
any ice reaching the North Atlantic. In such circumstances
depressions would tend to follow northerly tracks into the
Arctic Ocean instead of across Europe, giving a dry period in
Western Europe. This is not entirely speculative ; much the
same happens under present conditions when a long spell of
southerly winds between Iceland and Novaya Zemlya drives
the ice edge unusually far north, and this is almost invariably
followed by a drought in Western Europe (8). Tidal force
alone, however, cannot account for the relatively heavy
rainfall of the Atlantic period compared with the present.
In Europe this might be put down to the larger and warmer
Baltic of the Litorina Sea, but if as seems probable the rainy
period occurred also in Asia and North America some more
general cause must be sought. This may be either the
generally higher temperature of the oceans due to the smaller
amount of sea and glacier ice, or possibly a period of increased
solar activity.
By about 2500 B.C. these favourable conditions had largely
passed. Scandinavia had risen almost to its present level,
and the contrast between winter and summer had greatly
decreased. Hence the Arctic Ocean became cooler and more
liable to freeze. The main characteristic of the Sub-boreal
in Western Europe seems to have been the instability of its
climate, periods of drought and heat alternating with periods
when the climate resembled the present, at intervals of perhaps
a few hundred years. It is not unlikely that the Arctic ice-cap
had now reached the critical stage between non-persistence
and persistence it was difficult for it to become established,
but once firmly formed it was difficult to destroy. In such
circumstances wide oscillations of climate would be expected.
372 CLIMATE THROUGH THE AGES
I think that, paradoxically, persistence would be aided by
increased tidal force, which would cool the Gulf Stream Drift
by the ice carried into it by the polar currents. It must be
remembered that a firm unbroken ice-cap grows more slowly
than broken sea ice which allows the sea to freeze between
the ice-floes.
The final stage came about 500 B.C. when for some reason
the Arctic ice-cap at last became firmly established, apparently
very extensively, after a few centuries of heat and drought.
The reason for this change is not clear ; it may have been
due to a change of solar activity or possibly to explosive
volcanic activity. This period of an established Arctic ice-cap
and stormy weather apparently lasted for about a thousand
years, but the favourable climate of Iceland and Greenland,
and the absence of ice in the accounts of the early Nopse
voyages, suggest that some time after A.D. 500 the ice-cap
reverted to the semi-permanent stage, and remained so unti]
nearly 1200.
The latest maximum of tide-generating force is dated by
Pettersson as A.D. 1433. Although this force has been repre-
sented in Fig. 39 by a smooth curve, this is far from being
the real condition. Superposed on the long period are shorter
ones of about 90 and 9 years. From Pettersson's diagram
the actual maxima apparently occurred in three sharp peaks
about 1340, 1430, and 1520. Now the main peak on Antevs'
curve of tree-growth comes in the decade 1331-1340, with a
secondary peak about 1431-1440. In the Lake Saki varves the
peak is about 1450 with secondary peaks at 1390 and 1530.
These dates fit in rather better with Pettersson's curve than with
the curve of solar activity. We note also that about 1340
the westerly winds in England were more persistent than at
present, indicating a more stable Icelandic low.
On the other hand the great "storm floods' 5 of the twelfth to
fourteenth centuries, on which Pettersson sets great store, come
on the whole before the absolute maximum of Pettersson's
curve. The maximum damage occurred in 1170-1178, 1240-
1253, 1267-1292, 1374-1377, and 1393-1404, the middle one
being the most prolonged and severe ; there was also a great
flood in 1421. These fit in better with the sunspot cprve than
with Pettersson's. Marine inundations require a combination
of violent storms and high tides and these disasters of the
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 373
twelfth to fourteenth centuries may represent maximum stormi-
ness in the North Sea associated with great solar activity, at a
time when the tidal range was approaching its maximum.
There may also have been a slight subsidence of the land about
this time, especially in the fen districts where drainage opera-
tions had been carried on. In any case the main period of
great marine inundations would not be likely to continue after
the tidal maximum, for by that time the most vulnerable areas
would have been overflowed and protective measures taken.
The remarkable agreement between Pettersson's rather
obscure " tide-generating force " and the major variations
of climate since 3000 B.C. is surprising, and seems to show that,
under favourable conditions, comparatively small causes may
have disproportionately large effects. The favourable con-
ditions are the stratification of the upper layers of the North
Atlantic, the existence of the submarine Wyville Thomson
ridge at just the right depth between Atlantic and Arctic,
the critical stage of the Arctic ice, just over the border between
non-glacial and glacial, and the maximum range of the tidal
force itself. Such a coincidence is not likely to have recurred
often in geological time, and in spite of the apparent effective-
ness of Pettersson's tidal force in recent millennia we may
safely discount it as a permanent agent in climatic changes.
We may conclude this summary of post-glacial history with
brief references to four more recent climatic chapters :
1. The dry period of the sixteenth century.
2. The great outburst of glaciers about 1600.
3. The dry period of 1701-1750 in Western Europe.
4. The rise of winter temperature in 1850-1940.
Comparatively rapid variations of climate, of the order of a
century, have presumably always occurred, and are shown
in the thicknesses of the glacial varves, lake deposits and tree
rings, but they can be properly examined only when we can
assemble sufficient facts, especially about the prevailing winds,
to enable us to reconstruct the probable pressure distribution,
and this is not possible before the sixteenth century. Here we
have to add another factor to our list of causes, namely, the
variations of the atmospheric circulation referred to on
p. 66. The atmosphere, like the sea, is in a state of perpetual
374 CLIMATE THROUGH THE AGES
oscillation, the " waves " varying in length from a few hours
to many years, the result being highly complex changes in the
distribution of pressure from day to day, month to month and
year to year. These changes can be predicted for a day or
two, and are the basis of modern weather forecasting. Much
effort has been devoted to their analysis in the hope of fore-
casting for longer periods ahead, but they are only periodic
to a slight extent and so far the attempts have been un-
successful. It is highly probable, however, that the longer
oscillations are due partly to variations of solar activity and
partly to interactions between the circulations of the atmosphere
and the oceans. They take the form of an alternate weakening
and strengthening of the whole circulation of the atmosphere.
In the periods of weak circulation the low pressure centres
near Iceland and the Aleutians are smaller, shallower a$d
less stable, and anticyclones readily develop over the western
margins of the continents. The winds are variable and thq
climate is " continental," with cold dry winters and hot
summers. In the periods of strong circulation the low
pressure areas are enlarged and intensified and powerful
south-westerly air streams invade the western parts of the
continents. The climate becomes " oceanic " with mild
rainy winters and cool summers. Even in the middle of an
" oceanic " period, however, there are occasional " con-
tinental " years, such as 1921, and vice versa, and the change
from one type to the other often seems to be abrupt.
i . The latter half of the sixteenth century appears to have
been mainly " continental," rather dry on the whole in the
north temperate zone. In Western Europe the winds were
probably more easterly than now, and the winters were cold.
This seems to have been a time of minimum solar activity.
There are few records of storms, and a reasonable inference is
that the floating ice-cap suffered little disturbance and was able
to grow in extent and solidity. The result would be more
frequent incursions of cold Arctic air over Russia and extensions
of the Siberian anticyclone across Northern Europe. This
would give Northern and Western Europe frequent easterly
winds, cold in winter, hot in summer. Similar conditions
probably occurred in North America. The mayi storm
tracks were deflected southwards, and this period seems to
have been rainy both in South-east Europe and in Yucatan.
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 375
2. The great outburst of mountain glaciation which began
at the end of the sixteenth or early in the seventeenth century
was so remarkable that this period has been termed the " Little
Ice- Age." In the Alps and Iceland it began about 1600
and reached a maximum about 1643. In both countries the
advances exceeded those at any other period since late-glacial
times. There was a retreat in the first half of the eighteenth
century, followed by a readvance in the first half of the
nineteenth century, which gave place to a rapid retreat after
1850. In Southern Norway and Alaska, on the other hand,
the maximum advance did not occur until about 1750.
It is now generally agreed that the most favourable con-
ditions for the growth of glaciers are snowy winters and cool
damp summers. The snowfall, however, takes time to
accumulate, and the maximum extension of a glacier lags
behind the greatest accumulation of snow by a number of
years, depending on the size and length of the glacier. Hence
the " glacial period " which began about 1600 probably
reflects the snowfall of the latter part of the sixteenth century.
We have seen that this period was probably mainly anti-
cyclonic over Northern Europe, and that depressions
followed southerly tracks. The greatest snowfall occurs in
the northern halves of depressions, consequently this was a
time of heavy snowfall in the Alps, Pyrenees and Iceland.
During this period the snowfall accumulated at high levels,
but at first the glaciers were unable to extend into the valleys
because of the low mean annual temperature, which decreased
their viscosity and kept them frozen to the ground. As soon
as this limitation was removed, the glaciers grew rapidly in
extent. At this time, however, Norway and Alaska still
remained under the influence of the northern anticyclone,
and the glacial advance did not extend to those regions until
later.
3. The first half of the eighteenth century is the first period
for which instrumental observations are available. This period
was discussed by G. E. P. Brooks (9). The greater frequency
of north-easterly winds and the light rainfall over Western
Europe (see p. 309) point to a decreased intensity of the
Icelandic low, and there is some suggestion that the Aleutian
low was also weak. In summer, anticyclones tended to
develop over Western Europe. The lack of snowfall and
376 CLIMATE THROUGH THE AGES
probable high summer temperatures caused a recession of
the Alpine glaciers, but in Norway snowfall increased and the
glaciers advanced. The weather type of 1921 was probably
the norm instead of the exception.
Shortly after 1750 this continental type changed to a more
oceanic type, with milder winters and cooler, rainier summers.
In England the change seems to have taken place rather
abruptly in 1752 and was attributed by the commonalty to
the change of the calendar in that year. This oceanic type
continued for about a century, culminating in 1850 with
another maximum advance of the glaciers. There was a
brief return of the continental type from 1794 to 1810, analysed
by G. E. P. Brooks (10), which gave a famous period of severe
winters in Western Europe.
4. Since 1850 winter temperatures have tended to rise
over all the north temperate and Arctic regions and probably
in corresponding latitudes of the Southern Hemisphere.*
The change was slow and irregular at first, but became very
rapid after 1900. The rise in the mean temperature of the
three winter months, from 1851-1900 to 1901-1930, amounted
to 5 F. or more in Western and Central Europe. This
change was associated with a marked strengthening of the
atmospheric circulation and steady west-south-west winds in
Western Europe. There was little change of summer tem-
perature. Glaciers and ice-sheets receded very rapidly, and
after 1918 little or no drift ice reached the shores of Iceland.
The rise of winter temperature progressed from south to
north, and Central Europe may have passed the crest as early
as 1920 when the rise in the Arctic was in full swing. The
magnitude of the change in the Arctic is shown by the mean
winter temperatures of Spitsbergen, which rose by 16 F.
between 1911-1920 and 1931-1935. The edge of the main area
of Arctic ice also receded towards the pole by some hundreds
of miles. Since January 1940 the winter climate of Europe
has reverted abruptly to greater severity, but it is too soon
to say whether this is the beginning of another long period of
continental climate or only a temporary fluctuation.
This concludes the examination of historical changes of
climate, and also the analysis of the causes of climatic variations.
The problem has proved to be one of great complexity, but
throughout the book I have tried to examine each suggested
CLIMATIC FLUCTUATIONS IN THE HISTORICAL PERIOD 377
cause impartially. The results seem to me to point very
strongly to the following conclusions :
1 . The major climatic oscillations, lasting millions of years,
are due to the major cycles of mountain-building and
degradation, and their geographical effects in the widest sense,
which possibly include variations in the amount of carbon
dioxide and volcanic dust in the atmosphere.
2. Climatic oscillations of the second order, lasting
thousands or tens of thousands of years, are due to two or
possibly three causes :
(a) Minor changes in the land and sea distribution, caused
partly by the shifting of the load on the earth's crust by
erosion and partly by the isostatic effects of the growth
and decay of the ice-sheets themselves. These were
mainly effective during periods of high orography.
(b) Astronomical changes eccentricity of the earth's
orbit, obliquity of the ecliptic, precession of the
equinoxes and possibly other causes. These are con-
tinuously effective and can be traced in some of the
warm periods. They may have caused the succession
of glacial and interglacial periods.
(c) Possiblylong period variations of solar activity. Clima-
tic oscillations of a few hundred years appear to be
related to solar changes and it is a reasonable inference
that the range of solar activity in the course of tens of
thousands of years has been greater than the range
during the Christian era. If such changes did occur,
they must have caused considerable changes of
precipitation.
3. Climatic oscillations lasting a few hundred years. So
far as the evidence goes, these seem to be due mainly to
variations of solar activity.
4. Climatic oscillations lasting for shorter periods, up to a
hundred years or so. These may be due in part to variations
of solar activity but there is evidence that they are often due
to changes in the general circulation of the atmosphere which
may have no external cause. They result from the inter-
action ($ the winds, ocean currents and floating ice-fields
which we know to occur, but which, in the present state of
our knowledge, is incalculable. These changes must always
378 CLIMATE THROUGH THE AGES
have occurred, but were probably on a smaller scale during
the warm periods, when there were no polar ice-caps, than
during the glacial periods.
Other factors, hitherto unsuspected, may be discovered and
prove to be important, but as a result of this analysis there
seems to be no necessity to introduce hypothetical agents such
as clouds of cosmic dust, strange stars or great disturbances
of the earth's axis of rotation. The known causes set out in
this book suffice to account for the variations of climate during
geological and historical time.
REFERENCES
(1) WALKER, SIR GILBERT. " Correlations in seasonal variation of weather.
VIII. A preliminary study of world weather." Calcutta, Indian Met.
Mem., 24, Pt. 4, 1923. *
(2) . idem. " VI. Sunspots and pressure.'' idem., 21, Pt. 12, 1915.
(3) HUNTINGTON, E., and S. S. VISHER. " Climatic changes, their nature and
causes." New Haven, 1922.
(4) BROOKS, C. E. P. " The variation of the annual frequency of thunderstorms
in relation to sunspots." London, Q.J.R. Meteor. Soc., 60, 1934, P- J 53-
(5) Zurich, Vierteljahrschrif*, 38, 1893, p. 77 ; and Washington, D.C., Monthly
Weather Rev., 30, 1902, p. 173.
(6) TURNER, H. H. " On a simple method of detecting discontinuities in a
series of recorded observations, with an application to sunspots." London
Mon. Not. R. astr. Soc., 74, 1913, p. 82.
(7) PETTERSSON, O. " Climatic variations in historic and prehistoric time."
Svtnska Hydrogr.-Biol. Komm. Skriften, 5, 1914.
(8) BROOKS, C. E. P., and J. GLASSPOOLE. " The drought of 1921." London,
Q. J. R. Meteor. Soc., 48, 1922, p. 139.
(9) BROOKS, C. E. P. " The climate of the first half ol the eighteenth century."
London, Q. J. R. Meteor. Soc., 56, 1930, p. 389.
'10) BROOKS, C. E. P. " Winds in London during the early i9th Century."
London, Meteor. Mag., 67, 1932, p. 56.
APPENDIX I
THE GEOLOGICAL TIME-SCALE
The Age of the Earth. Various methods have been suggested
from time to time by which we can determine the approximate
period which has elapsed since the formation of the first solid
crust of the earth ( i ) . The older methods were based on the
thickness of sedimentary rocks or the amount of salt in the
ocean, divided by the present rate of accumulation ; they
assumed that the present rate of geological processes is a fair
average of their rate throughout geological times. This
assumption we now know to be false ; the present is a period
of abnormally high relief, and in addition the unconsolidated
deposits of the last ice-age facilitate denudation. The
maximum age of the oldest rocks calculated from the rate
of denudation is 350 million (3'5Xio 8 ) years, and this is
probably only about one-fourth of their real age.
Presumably the sun is older than the earth, and calculations
of the age of the sun based on the supply of energy by the
aggregation of hydrogen atoms gives 140,000 million (i*4X
io ix ) years as the age of the sun. The sun apparently existed
alone for a long period before a passing star disrupted it to
form the solar system, for a calculation of the time since
Mercury first took shape as a planet indicates that the age of the
solar system is probably not greater than 10,000 million
(io 10 ) years.
The most reliable method of calculating the age of any
particular portion of the earth's crust is based on the
phenomena of radio-activity. As is now well known, the
elements uranium and thorium are continually breaking up
and passing through a series of changes, the end-products of
which are lead and helium. So far as we know, in the natural
state the rate at which each of these elements disintegrates
is a peculiarity of the element itself, and is entirely independent
of the physical changes, such as variations of pressure and
temperature, which it undergoes. If a sample of rock contains
a certain amount of uranium, at the end of about 5,000
379
380 CLIMATE THROUGH THE AGES
million years it will contain half this amount, at the end of
another 5,000 million years one-quarter, and so on, the amount
remaining continually halving in 5,000 million years. The
original mass of uranium will never quite disappear. Suppose,
now, that a rock when it solidified contained a certain amount
of uranium, but no lead or helium. To-day it contains
uranium, lead, and helium, and from the ratio of the amount
of uranium to the amount of lead, or to the amount of helium,
we can calculate the number of millions of years which have
elapsed since that particular rock was formed. The uranium-
lead ratio gives the more reliable estimates, for helium, being
a gas, is liable to escape, and the uranium-helium estimates
are systematiceilly too low. In this way the following ages
(in millions of years) have been calculated for different
geological periods :
Oligocene ... 26
Eocene .... 60
Carboniferous . . 260-300
Devonian . . 310-340
Archaean . . . 560-1,340
In 1947, A. Holmes (2) concluded that " on the evidence
at present available, the most probable age of the earth is
about 3,350 million years."
J. Joly (3) developed an interpretation of the geological
history of the earth, which leads him to much smaller values
for the ages of the rocks. Owing to the universal presence
of radio-active material in the earth's crust, both sial and
sima, there is a perpetual generation of heat. In the conti-
nental masses of sial this heat is able to escape, but in the
deeper layer of sima it is unable to escape and so goes on
accumulating until the sima reaches its melting point. Melting
begins at a considerable depth and proceeds gradually upwards.
Finally, melting extends to such a height that the accumulated
heat is able to escape into the oceans, and the substratum
again becomes solid. Owing to the changes of density and
volume involved, the period of melting is one of continental
subsidence, while the period of solidification is a time of
mountain-building. Melting escape of heat solidification
constitute a cycle, and according to Joly's calculations a
cycle requires from forty to sixty million years to consummate
itself. The number of complete cycles recognised is quite
small four or five, according to different authors. The
APPENDIX 381
periods of mountain-building closing the cycles occur as
follows :- Laurentian and Algoman revolutions in the
Archaean, Huronian or Killarney closing the Archaean, Appa-
lachian or Hercynian closing the Palaeozoic, and Alpine in the
Miocene or Pliocene. Other revolutions which are recognised
by some geologists but not by others are the Caledonian,
occurring in the Silurian, and the Laramide in the Cretaceous.
Hence the total duration of time since the Laurentian can have
been only 200 to 300 million years. This estimate is quite
incompatible with the usually accepted data of radio-activity
given by the uranium-lead ratio, and Joly seeks to explain
the discrepancy by supposing that the speed of radio-active
processes has not in fact been constant during geological time,
but that part of the lead contained in the early rocks was
formed from isotopes of uranium which disintegrated at a
greater rate than the only form known at present. There are
minute peculiarities in some of uranium effects in the early
rocks which support this view, but it does not seem probable,
since the longer periods fit in better with the mass of geophysical
data. A. Holmes (4), reviewing Joly's work, states that the
generally accepted ages of the rocks are not likely to be in
error by more than 10 per cent. He accepts the period of
about forty million years for one of the cycles, but thinks that
there have been about five times as many cycles as Joly
supposes, and suggests that the main revolutions recognised
by Joly arc the concluding stages of major cycles in which
the melting and consolidation of the deeper seated magma
is added to that of the more superficial magma which by
itself formed only minor revolutions. Inspection of the later
part of the geological record seems to support Holmes' view ;
for instance, minor periods of orogenesis occurred at the end
of the Jurassic, in the Upper Cretaceous, in the Oligocene-
Miocene, and in the Quaternary. The age of the Eocene is
given by the uranium-lead ratio as 60 million years, so that
we may take 80 million years for the interval between the
Upper Cretaceous mountain-building and the Quaternary,
or a period of two minor cycles.
If we take the total thickness of the stratified rocks and
calculate the time which would be required for their formation
on the assumption that denudation has always proceeded at
its present rate, we obtain for the age of the oldest rocks only
382
CLIMATE THROUGH THE AGES
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APPENDIX 383
about 150 million years, which is much too low. A much
better agreement is obtained if we assume that in each period
the rate of denudation has been proportional to the average
elevation. If for any geological period we divide the total
thickness of the stratified rocks by the average elevation during
that period (Table 31), we get a number which we may
term the " duration-ratio " of that period. The sum of all
the duration-ratios from the beginning of the Keweenawan
series (Upper Proterozoic) to the close of the Pliocene is
approximately 156. The age of some probably Upper
Proterozoic uraninite from Morogoro, East Africa, is given by
the uranium-lead ratio as 560 million years. Hence a
" duration-ratio " of unity corresponds with an actual duration
of 3-6 million years. Similarly, the sum of the duration-
ratios since the beginning of the Devonian is 107, while the
greatest age determined from the uranium-lead ratio for a
Devonian rock is 340 million years, giving a value of 3-2
million years for a duration-ratio of unity. The corresponding
value deduced from the Carboniferous radio-active rocks
(duration-ratio 87, age 300 million years) is 3-4, and from the
Eocene (duration-ratio 18, age 60 million years) is 3-3.
These four values are in sufficiently good agreement, and we
can adopt as the time-equivalent of a duration-ratio of unity
the mean period of 3 4 million years. This gives us the ages
and durations of the various geological periods shown in Table
31, which are in sufficiently good agreement with the
determinations by radio-active ratios.
REFERENCES
("i) JEFFREYS, H. " The earth, its origin, history, and physical constitution."
2 ed. Cambridge, 1929.
(2) HOLMES, A. " A revised estimate of the age of the earth." London, Nature,
'59> 1947, P- 127.
(3) JoxYj J- " The surface-history of the earth." Oxford, 1925.
(4) HOLMES, A. " Radio-activity and geology." Nature, 116, 1925, p. 891.
APPENDIX II
THEORIES OF CLIMATIC CHANGE
The various theories of the causes of climatic change are set out
below, classified into types. The intention is not to provide
a complete bibliography, but to give some indication as to
where a description of the theory can be most readily found.
I. COSMIGAL.
CALVERWELL, E. P. Geol. Mag., 32, 1895, p. 64. [Gas-filled
regions in space.]
GUILLEMIN, . Arch. sci. phys., (3) 22, p. 585. [Cosmical
dust in space.]
HOYLE, F., and R. A. LYTTLETON. See Chapter IV.
IVES, R. See Chapter IV.
NOELKE, F. " Das Problem der Entwicklungsgeschichte
unseres Plane tensystems." Berlin, 1908. [Cosmical dust
in space.]
II. SOLAR RADIATION.
DUBOIS, E. See Chapter IV.
FAYE, . " Concordance des epoques geologiques avec les
epoques cosmogoniques." Paris, C. R. Acad. 6a., 100,
1885, P- 9 26 -
FISCHER, E. " Eiszeittheorie." Heidelberg, 1902. [The sun
moves in an elliptical orbit ; when far from focus velocity
falls off and temperature decreases, giving an ice-age.]
HUNTINGTON, E., and S. S. VISHER. See Chapter IV.
JAEKEL, O. %s. D. Geol. Ges., 57, 1905, Monatsber., p. 223
[The separation of each of the inner planets from the sun
was accompanied by a decrease of radiation and an ice-age
on the earth ; obsolete.]
SIMPSON, SIR GEORGE. See Chapter IV.
III. ASTRONOMICAL.
ADHEMAR, J. " Les revolutions de la mer. Deluges perio-
diques." Paris, 1842. [Glaciation with winter in
aphelion at maximum eccentricity of earth's orbit.]
BALL, Sir R. "The cause of an ice-age." London, 1891.
[Precession of the equinoxes.]
384
APPENDIX 385
CROLL, J. " Climate and time in their geological relations."
London, 1875. [Eccentricity, glaciation with winter in
aphelion.]
EKHOLM, N. London, Q. J. R. Meteor. Soc., 27, 1901, p. 27.
[Obliquity of ecliptic.]
HILDEBRANDT, M. " Die Eiszeiten der Erde." Berlin, 1901.
[Glaciation with small eccentricity.]
MILANKOVITCH, M. See Chapter V.
MURPHY, J. J. London, Q,. J. Geol. Soc., 32, 1876, p. 400.
[Glaciation with summer in aphelion at maximum
eccentricity.]
PETTERSSON, O. (Tidal variations.) See Chapter XXII.
SPITALER, R. See Chapter V.
IV. EARTH HEAT.
HOFFMANN, J. F. Beitr. Geophys., 9, 1908, p. 405. [Warm
periods due to heat set free by decomposition of organisms
in strata.]
MANSON, MARSDEN. See Chapter VII.
PROBST, J. " Klima und Gestaltung der Erde in ihren
Wechselwirkungen." Stuttgart, 1887. [Warm springs.]
WAGNER, A. (Radio-active heat.) See Chapter X.
We may include here :
FRANKLIN, A. V. Toronto, J. R. Astr. Soc. Canada, 12, 1918,
p. 450. [Radiation from a warm moon.]
V. POLE- MOVEMENTS AND DRIFT OF CRUST.
KREIGHGAUER, D. " Die Aquatorfrage in der Geologic.'*
Steyl, 1902. [Polar movements.]
SIMROTH, H. " Die Pendulationstheorie." Leipzig, 1907.
OLDHAM, R. D. Geol. Mag., 23, 1886, p. 300.
WEGENER, A. Sec Chapter XIII.
VI. ELEVATION.
ENQUIST, F. Bull. Geol. Inst. Upsala, 12, 1915, p. 35. [Deep-
ening of ocean basins.]
GREGOIRE, A. Bull. Soc. Beige Geol., 23, 1909, p. 154. [The
sea floor is colder than the land, hence when there is a
reversal the sea is warmed, increasing evaporation, which
causes greater snowfall on cold land.]
LE CONTE, J. " The Ozarkian and its significance." J. Geol. y
Chicago, 7, 1899, P- 5.25-
RAMSAY, W. Ofversigt afFinska Vetenskaps Soc. Forh., 52, 1910,
Afd. H. See also Chapter X.
SCHUCHERT, CH. Carnegie Inst., Washington, Publ. 192,
.,1914, p. 263.
UPHAM, W. Amer. Geol. y 6, 1890, p. 327.
25A
386 CLIMATE THROUGH THE AGES
VII. LAND AND SEA DISTRIBUTION.
BROOKS, C. E. P. See Chapter VIII.
HARMER, the late F. W. London, Q,. J. R. Meteor. Soc., 51,
!925> P- 247.
KERNER, F. v. Wien, Sitzungsber. K. Akad. Wiss., Math.-nat.
Kl., 122, Abt. 20, 1913, p. 233.
LYELL, CH. "Principles of geology." nth ed. London,
1892.
SEMPER, M. s. D. Geol. Ges., 48, 1896, p. 261.
VIII. OCEAN CURRENTS.
CHAMBERLIN, T. C. See Chapter III. [Reversal of deep sea
circulation.]
HULL, E. London, Q. J. Geol. Soc., 53, 1897, p. 107.
[Deflection of Gulf Stream by Antillean continent.]
KLEIN, H. J. Gaea, 41, 1905, p. 449. [Deflection of Gulf
Stream by land projection from Newfoundland towards
Cape Verde Islands.]
IX. CHANGES IN COMPOSITION OF ATMOSPHERE.
ARRHENIUS, S. Phil. Mag., 41, 1896, p. 237. [Carbon
dioxide.]
CALLENDAR, G. S. (Carbon dioxide.) See Chapter VI.
CHAMBERLIN, T. C. J. Geol., Chicago, 7, 1899, pp. 545, 667,
752. [Carbon dioxide.]
FREGH, F. ^s. Ges. Erdk., Berlin, 37, 1902, p. 611. [Carbon
dioxide.]
HARBOE, E. G. %js. D. Geol. Ges., 50, 1898, p. 441. [Water
vapour from volcanoes.]
HARL, E. and A. Bull. Soc. Geol. France, n, 1911, p. 118.
[Probable greater pressure of air in geological times. See
Chapter II.]
MANSON, MARSDEN. See Chapter VII.
X. VOLCANIC DUST.
HUMPHREYS, W. H. See Chapter VI.
SARASIN, P. and F. Basle, Verh. Naturf. Ges., 13, 1901, p. 603.
XL CHANGES OF ATMOSPHERIC CIRCULATION.
ABBE, C. Washington, U.S. Weather Bureau. Monthly
Weather Review, 34, 1906, p. 559. [Slight changes of
circulation.]
DEELEY, R. M. Geol. Mag., (6) 2, 1915, p. 450. [Effect of
great polar water areas on stratosphere.]
DINES, W. H. See Chapter II.
HARMER, F. W. See Chapter II.
INDEX
Abbe, C., 386
Abbot, C. G., ioj, 1 1 8, 120
Abert Lake, 350
Abyssinia, rainfall, 330, 338
Adhemar, J., 384
Africa,
Climate in historical period,
.
pluvial periods, 276
post-pluvial, 339
Aftonian, 242
Alaska,
glaciation, 242
Tertiary climate, 242
Aldrich, L. B., 112, 1 20
Alexandria, climate in first
century, 333
Algoman revolution, 381
Algonkian
climate, 226
mountain-building, 205
Alpine
folding, 178
revolution, 381
Alps, traffic over, 143, 301
America, climate in historical
period, 3420.
Amorite migration, 292, 320
Anau, 318
Angaraland, 247, 248, 257
Angkor, ruins of, 326
o
Angstrom, A., 112, 120, 123, 126,
128
Antarctic,
Carboniferous climate, 259
climate in geological times, 243
glaciation of, 239
pressure and winds, 64
Antevs, E., 1 13, 120, 181, 191, 236,
243, 246, 263, 273, 277,
278, 342, 358
Anticyclones,
continental winter, 46, 60
glacial, 641!.
sub-tropical, 46, 55, 61
Antillean Continent, 8 1 , 386
Antiochus, Calendar of, 335
Aphelion, 102, 364
Appalachian revolution, 38 1
Arabian migration, 292
Arago, 286, 294
Aral, Sea of, 322
Aramaean migration, 292, 320
Arch<ocyathin<e, 23, 226, 245
Arctic Ocean, ice in, 416., 1391!.,
268, 371, 376.
Arctowski, H., 119, 121
Arizona, archaeology, 353
Arkell, W. J., 194, 197
Arldt, T., 203, 205, 220, 247, 262
Arrhenius, S., 386
Aryans, 291, 320
Asama, Mt., 1 18
Asia, climate in historical period,
3 1 8ft"., 359
Astronomical theories, iO2ff., 384
Atlantic period, 142, 147, 173,
296^-, 339
Atlantis, 248, 257
Atmosphere, composition of,
ii3lT., 386
Atmospheric
circulation, 49!!.
changes of, 273, 373, 386
zonal, 54
Aurora and sunspots, 366
Australia, climatic history, 244
Bacon, Sir F., 313, 317
Ball, Sir R., 384
Baltic stadium, 106
Beadnell, H. J. L., 335, 341
Belgium, Meteorological Chron-
icle of, 287
Benguela Currents, 76, 132
Berg, L., 322, 327
Berry, E. W., 24, 27, 228, 241,
246
" Big Trees," see Sequoias.
Bishop, C. W., 324, 328
Bjerknes, J., 67
Boden See, variations of level,
30off.
3*7
25U
388
INDEX
Boreal
climate, 173
period, 2g6fF., 364
zone (faunal), 23
Bradley, W. H., 108, 109, 310
Brahe, Tycho, 284, 294, 312
Britton, C. E., 289, 294, 303, 304,
316
Bronze Age, 296, 298
Brooks, C. E. P., 33, 45, 67, 99,
101, 119, 121, 157, 164,
i?5 312, 317, 3 6 5> 375-
376, 378
Bruckner, E., 93, 101, 154, 290,
294> 32i, 327
Bryan, K., 264, 273, 277, 278,
35 1 > 358
" Burial of Olympia," see Hunt-
ington, E.
Caledonian
folding, 178
revolution, 381
California Current, 132
Callendar, G. S. } 117, 120
Calverwell, E. P., 384
Gandee, H. C., 328
Carbon dioxide, 1 16, 265, 386
Carboniferous,
climate of, 108, 247fF.
deserts, 237
geography, 248
gl-aciation, see Permot-arboni-
ferous
mountain-building, 178, 205,
249
ocean currents, 248, 250
volcanic action, 211, 250
winds, 252
Carruthers, N., 309
Caspian, variations of rainfall, 321
Caton-Thompson, G., 335, 341
Ceratodus, 26, 172, 194, 226
Chamberlin, T. C., 79, 88, 116,
203, 220, 386
Chandler, M. E. J., 96, 101
Chancy, R. W., 196, 241, 246
Chewaucan Lake, 350
Childe, V. G., 316
Chile, variations of rainfall, 358
China, climate in historical period,
322, 3.25
Chinese archives, 325, 366
Chronology,
geological, 379
of ice age, 106, 107
post-glacial, 296
Chu, Co-Ching, 325, 328
Gimbrian flood, 302
Circulation,
atmospheric, see Atmospheric
circulation
oceanic, see Oceanic circulation
" Civilisation and climate," see
Huntington, E.
Climatic
change, theories of (listed), 384
factors, astronomical, loiff.
geographical, 20 iff.
solar, 89ff.
Optimum, 105, 142, 297, 362,
zones, 23, 174
Close, Sir C., 222, 230
Cloudiness, I22if.
and elevation, 183
and temperature, I22ff., 183
Clouds, reflection of radiation by,
I 12
Coal Measures, 171, 236, 257
Coleman, A. P., 231, 233, 245, 246
Golima, eruption of, 113
Constance, Lake, 3OofT.
Continental drift, 22 iff.
Continentality
and glaciation, 154
and temperature, I29ff., 203,
212
variations of, 108, 204
Coral reefs, 194, 225
Gordilleran ice-sheets, 273
Correlation coefficients, 210, 214
Cosmical
dust, 384
theories of climatic change, 384
Cretaceous, 108
climatic zones, 23, 239
deserts, 25
geography, 240
glaciation, 177, 216, 228
ocean currents, 240
Croll,J., 103, 109, 385
Crowther, E. M., 168, 176
Cycles, see Periodicities
Cyclone tracks, 98, 160, 273
Cyrenaica, climate ir/ historical
period, 333
INDKX
389
Dacque, E., 108, 109, 205, 220
Ball, W. H., 242, 246
Date palm in Palestine, 286
David, Sir T. W. E., 233, 235, 244,
245, 250
Dead. Sea, level of, 322
Deeley, R. M., 386
Defant, A., 119, 121
de Geer, G., 84, 103, 243
Deluge, Noachian, 289
Depressions, see Cyclones
Deserts, 24, 126, 167, 172, 237,
238
Desiccation, 169, 286
Devonian, 24
Dines, W. H., 66, 67, noff., 120,
*25
Doldrums, 49, 174, 276
Dorians, 320
Dorsey, H. G., 55, 67, 264, 277
Drayson, Col., 102
Drought Commission, South
Africa, 176
Droughts, 304!!.
Dry period, sixteenth century, 373
Dubois, E., 99, 101
Duration-ratio, 382
Durst, C. S., 88
Dwyka tillite, 235
Earth,
age of, 379
heat, 22, 127, 179, 385
East Greenland Current, 75, 143
Easton, C., 310, 317
Eccentricity of earth's orbit, 102,
1 08, 384
Ecliptic, obliquity of, 102, 364
Ekholm, N., 385
Elamites, 320
Elevation
and glaciation, 1 78
effect on temperature, 184, 210,
.385
variations, 203
England, rainfall records, 307!!.
Enquist, F., 385
Eocene, 108
climate, 96
flora, 24
glaciation, 177
temperature, 139!!.
Equinoxes, precession of, 103
Erdtman, G., 296, 297, 316
Eskimos, 357
Europe,
climate in historical period,
.
variations of rainfall, 299, 305,
359
of temperature, 310, 361
Eurydesma cot 'datum , 234
Evaporation, 165, 185, 297
Equatorial Current, 71, 253
, -., 384
Fenno-Scandian moraine, 295
Ficker, H. v., 276, 278
Fischer, E., 384
Fleure, H. J^, 84, 88
Flint, R. Fl, 55, 67, 264, 274, 277,
278
Flood legend, 289
Floods, 306
Forbes, J. D., 132, 157
Fowle, F. E., 118, 120
Franklin,
A. V., 385
B, 117
Freeh, F., 386
Frost in Miocene, 26, 196
Gale, H. S., 349, 352
Gamblian pluvial, 276
Gams, H., 106, 143, 157, 299, 316
Gangamopteris, 234
Gardner, E. W., 335, 34 1
Gas-filled regions in space, 384
Geer, G. de, see de Geer, G.
Geikie,J., 154
Geographical factors of climate,
20 iff., 386
Geological periods, ages of, 382
Glacial
anticyclones, 64
deposits, correlation of, 228,
242, 264
periods, see Ice ages
Glaciers, expansion of, historical,
375
Glasspoole,J., 308, 378.
Glossopteris, 234, 236
Godwin, H., 298, 316
Gondwanaland, 247ff.
Granlund, E., 298, 316
Grant, C. P., 319, 327
Greece, variations of climaiu, 31^
39
INDEX
Greenland,
climate in historical period, 143,
356
inland ice, 52
weather, 63
Gregoire, A., 385
Gregory, J. W., 322, 327
" Grenzhorizont," 298
Guiana Current, 81
Guillemin, -., 384
Gulf Stream, 66, 72, 81, i34 ff - 3 86
Gumban culture, 339
Gunz glaciation, 84, 93, 106, 107,
242, 269
Gunz-Mindel interglacial, 93, 242
Gypsum, deposits of, 25, 172, 237,
238
Haddon, A. C,, 327
Hail in Tertiary, 163
Hallstatt period, 300
Handlirsch, A., 250, 262
Hansen, H. P., 350, 358
Harboe, E. G., 386
Hardy, E. M., 298, 316
Harle, E. and A., 120, 121, 386
Harmer, Sir F., 66, 67, 84, 88,
258, 262, 386
Hasluck, M., 302, 316
Haurwitz, B., 123, 128, 255
Hedley, G., 153, 157
Heer, O., 136, 157
Helium, 379
Hellmann, G., 284, 294, 333, 341
Hennig, R., 317
Hepworth, M. W. G., 88
Hercynian
folding, 178, 233
revolution, 381
Herjolfsnes, excavations at, 356
Herodotus, 322, 331, 340
Higgins, L. S., 311, 31?
Hildebrandsson, H. H., 285, 294
Hildebrandt, M., 385
Himpel, K., 96, 101
Historical period, climates of,
Hobbs, W. H., 63, 67, 163, 175
Hobley, C. W., 340, 341
Hoffmann, J. F., 385
Hohokam, 354
Holland, rainfall, 309
Holmes, A., 380, 383
Housa State, 338
Hovgaard, \V., 356,
Hoyle, F., 94, 101
Hull, E., 386
Humboldt Current, 76, 132, 271
Humphreys, W. J., 117, 120
Hunt, T. M., 119, 121, 164, 175,
3 I2 > 3 1 ?
Huntington, E.,
" Burial of Olympia," 294, 315,
3 1 ?* 322
" Civilisation and climate,"
292, 294, 356, 358
" Climatic changes, their nature
and cause," 86, 88, 98,
" Climatic factor," 342, 353,
358
" Mainsprings of civilisation,"
319, 321, 327
4< Pulse of Asia," 321, 327
" Pulse of progress," 314, 317
" World power and evolution,"
294
Huronian revolution, 381
Hyksos, 291, 320
Ice ages,
causes of, 265
astronomical, 103(1'.
carbon dioxide, 116
interstellar matter, 94, 384
mountain-building, 178
Nova outbreaks, 96
ocean currents, 84
sea temperature, 266
solar radiation, 91
sunspots, 98
Permo-Carboniferous, icu,
23iff., 257
Pre-Cambrian, 180
Quaternary, 177, 215, 263!!'.
Ice, cooling effect of, 32rT., 184
Iceland, climate in historical
period, 303
Ice-sheets,
area of, 113, 263
nourishment of, 163
weather over, 63
winds over, 62
Ihering, H. v., 196, 197
Illinoian glaciation, 242
India, climate in historical period,
3*4 . f
Insects, holometabolism, 250
INDEX
39 *
Instrumental records of weather,
238
Interglacial periods, 94, 242, 273
Interstellar matter, 94, 384
Ireland, climate in historical
g period, 302
Islarmtic expansion, 292
Isoflors, Eocene, 241
Istakhri, 322
Ives, R. L., 100, 10 1
Jaekel, O, 384
Jafnites, 320
Jana Sea, 138
Jeffreys, H., 53, 67, 179, 191, 204,
220, 224, 230, 383
Jerseyan glaciation, 242
Joly,J., 380, 383
Jones, J. C., 346, 358
Jwlien, M., 233
Jurassic,
climate, 194
deserts, 25
faunal zones, 23
temperature, 141
Kafuan, pluvial, 276
Kamasia, Lake, 95, 276
Kansan glaciation, 242
Kashmir, glaciation, 265
Kassites, 320
Katmai, eruption of, 113, 118
Keane, A. H., 341
Keewatin ice-sheet, 273
Kenriard, A. S., 299
Kerner, F., 41, 45, 80, 95, 101,
134, i37fF., 157, 176, 386
Kessler, P., 274, 278
Khanikof, -., 32:;
Kharga oasis, 335
i&llarney revolution, 381
Klein, H.J., 386
Koch, L., 144, 157
Koppen, W., 104, 109, 169, 176,
225, 230, 231
Krakatoa, eruption of, 113, 118
Kreichgauer, D., 385
Krige, L.J., 128
Kubierschky, -., 25, 195
Labrador Current, 74ff., 137, 268
Labradorean ice-sheet, 274
Labrijn, /\., 311, 317
La Cour, P., 284, 294, 312,
317
Lagrange, J. L., 102
Lahontan, Lake, 346ff.
Lake-dwellings, 299
Lakes, fluctuations of, 276, 290,
297> 346ff.
Lang, R., 167, 176
Laramide revolution, 381
Lasareff, P., 78, 88, 208, 251
La Tene stage, 301
Laterite, 167
Laurentian revolution, 381
Leaching factor, soils, 168
Leakey, L. S. B., 341
Le Conte, J., 180, 191, 385
Leiter, H., 333, 340
Leverett, F., 242, 246
Lias, 216
Litorina
period, i47fF.
submergence, 362
" Little Ice-Age," 301, 375
Loess, 275
Loewc, F., 265, 277
Lop Nor, 323
Lowe, E. j., 304, 317
Lung-fish, see Ceratodus
Lyell, C., 386
Lyons, Sir H., 330, 340
Lyttleton, R. A., 94, 101
Mackay, E. J. H., 324, 327
Makalian pluvial, 277
Malaria, 314, 355
Mandingaii Empire, 338
Mauley, G., 180, 191, 311, 317
Manson, M., 126, 128, 161, 175
Mariolopoulos, E. G., 316, 317
Markham, S. F., 293, 294
Mar tonne, E. de, 191
Matthes, F. E., 63, 67, 301, 316
Mayan civilisation, 355
" Mediterranean " climate, 59,
172
Meinardus, W., 64, 67
Merle, W., 284, 294, 312
Mesozoic, 120, 138, i92ff., 227
deserts, 24, 238
Meteorites, 1 19
Meyer, G. M., 303, 312, 316, 317
Mexico, Lake of, 353
Migrations, 291, 320
from China, 326
Milan rainfall, 309
Milankovitch, M., 102, 104, 109
INDEX
Mindel glaciation, 93, 106, 107,
264
Mindel-Riss interglacial, 94, 242,
264, 270, 273
Miocene,
frost, 196
see also Tertiary
Mohenjo-Daru, 324, 360
Mono, Lake, 346
Monsoon rainfall, 171
Monsoons, 61, 254
Moreau, R. E., 277, 278
Mortensen, H., 271, 277
Moslem civilisation, 315
Mossi State, 338
Mountain-building, 205, 381
and climate, 128, 177*?., 385
Miihlbergian glaciation, 106
Murphy, J. J., 104, 109, 385
Murray, G. W., 337
Nakuran pluvial, 339
Nearctis, 248, 257
Nebraskan ice-sheet, 242
Negri, C., 333, 341
Neolithic, 296
Neumayr, M,, 23, 27
New Mexico, archaeology, 353
Nile,
hydrography of, 329
variations of level, 329ff., 368
Nilsson, E., 95, 101, 276, 278, 339,
34i
Noelke, F., 384
" Non-glacial "
circulation, 53
periods, 321!., i39ff.
temperatures, 4 iff.
Nordhagen, R., 106, 143, 157, 299,
316
Norfolkian, 242
Norse voyages, 144, 303, 357
" North water," 72, 79
Nova outbreaks, 96
Obic Sea, 138
Obliquity of ecliptic, 102, 108, 364
Ocean
currents, 68fl'.
variation of, 204(1., 386
depth of, 85, 362
Oceanic
circulation, 68fT.
reversal of, 79
Oceans as regulators, 85, 267
Oldham, R. D., 385
Old Red Sandstone, 24, 108, 226
Oligocenc, 242. See also Tertiary
Orogenesis, see Mountain-building
Osborn, H. F., 242, 246 \
Ostrovo, Lake, 302
Owens Lake, 352
Oxus, change of course, 323
Ozarkian, 385
Palceocrystic ice, 39, 75
Pala'Ogeography, 20 iff., 247
Palestine, climate in historical
period, 286
Pajaritans, 354
Pangaea, 221
Paris, rainfall, 309
Paschinger, V., 271, 277
Peake, H. J. E., 84, 88, 291, 294,
327
Peat-bogs, 173, 296
in North America, 350
tropical, 171
Pelee, eruption of, 113
Penck, A., 93, 101, 154
Pendulation theory, 385
Periglacial climate, 274
Perihelion, 102
Periodicities, 97, 108, 310
Permian, 25, 195, 234
deserts, ^37
geography and ocean currents,
252
Permo-Carboniferous, 100, 231 ft*.,
247ff., 257
Petterssen, O., 143, 157, 357, 358.
368, 378
Phrygians, 300
Planetary circulation, 50
Plato, 169, 316
Pleistocene, see Quaternary
Pliocene, see Tertiary
Pluvial periods, 95, 271, 276
Polar
climate, 173
east winds, 53, 1 74
front, 54, 59
whirls, 52
Pole, movements of, 229, 244, 385
Pollen spectra, 296, 350
Potonie, H., 171, 176 /
Precession of equinoxes, 103
INDEX
393
Precipitation, 158!?.
convectional or instability, 162
cyclonic, 160
distribution, 164, 169
in Quaternary, 59, 166, 275
in warm periods, 185, i93ff.
le^l of maximum, 1 59
orographic, 159
over ice-sheets, 162
Pressure
distribution, 46^.
in warm periods, 62
systematic variation, 360
Priestley, R. E., 239, 243, 246,
I 259, 262
Probst, J., 385
Proterozoic, 177, 215, 226
Ptolemaius, Claudius, 333
Pueblos, 354
pyramid Lake, 346, 352
" Quaternary Climates," 342, 358
Quaternary,
atmospheric circulation, 55
chronology, 106, 107
climate, 263!!'.
outside ice-sheets, 274
elevation in, 183, 269
ice-age, 177, 215, 2631!
ice-sheets, area, 263
correlation, 242, 264
retreat of, 277, 295
isotherms, 135, 154
land and sea, 152
precipitation, 59, 166, 275
snow-line, 264
winds, 59
Radiation,
absorption by atmosphere, i loff.
by volcanic dust, 1 13, 1 1 7, 255
by water vapour, 1 14
reflection from clouds,
255
from snow, 1 12, 256
solar, 89
measurement of, 89
and temperature, 361
variations, 90, 96, 384
terrestrial, i loif.
Radio-activity, 179, 379
Rain, see precipitation
" Rain-faltor," 167
Rainfall,
fluctuation*, 361
Africa, 329^, 359
America, 342fT., 359
Asia, 3i8fT., 359
Europe, 297, 305, 359
records, 3076*.
zones, 164, 174
Rain-forests, tropical, 170
" Raininess," 306, 325
Raised beaches, 362
Ramsay, Sir A., 233
Ramsay, W., 180. 191, 385
Ray, L.L., 277
Reeds, C. A., 242, 246
Regression coefficients, 2 1 1
Reid, C., 154
Reid, E. M., 96, 101
RG-VEDA, 324
Riss glaciation, 94, 106, 107, 242,
264
Riss-Wurm interglacial, 94, 242,
26 4.. .
Rome, civilisation and climate,
314
Rossby, C. G., 54, 67
Rudistes, 225, 239, 240
Russell, I. C., 348
Russia, South, variations of rain-
fall, 361
Sabine Island, 222
Sahara, climatic fluctuations, 338
Saki, Lake, 301, 367, 372
St Wilfrid's drought, 289
Salinity of oceans, 86
Salt deposits, 25, 172, 234, 237,
238
Salton Sea, 348
Sand ford, K. S., 277, 278
Sangamon, interglacial, 242
Sapper, K., 358
Sarasiii, P. and F., 386
Sawyer, L. D., 333, 341
Sayles, R. W., 245
Scandinavia, variations of sea
level, 362
Scanian glaciation, 242
Scherhag, R., 66, 67
Schomberg, R. C. F., 324, 327
Schostakowitsch, W. B., 301, 316
Schuchert, C., 261, 262, 385
Schulman, E., 354, 358
394 INDEX
Sea-level,
lowering by glacir.tion, 181, 263
variations of, 361, 373
Sea temperature and glaciation,
266
Seistan Lake, 323
Semite migration, 320
Semper, M., 386
Sequoias, 293, 3421!., 361
Shaw, Sir N., 51, 67
Sial, 222, 380
Siberia, glaciation, 264, 265
Sicilian beach, 85
Silurian, 178, 205
Sima, 222, 380
Simpson, Sir G., 64, 67, 91, 101,
114, 163, 175, 262
Simroth, H., 385
Smith, E. H., 277
Smithsonian Institution, 89, 101
Snowfall,
level of maximum, 159, 271
over ice-sheets, 163, 273
Snow-line, 159, 264
Soil colour and climate, 167
Solar-cyclonic hypothesis, 98
Solar
activity and rainfall, 366
variations of, 361
see also Sunspots
constant, 90
radiation, see Radiation, solar
Solberg, H., 67
Speerschneider, C. I. H., 303, 316
Spitaler, R., 86, 88, 103, 109, 132,
I56> 157
Squantum tillite, 232
Stein, Sir A., 322
Stockwell, J. N., 102, 108
" Storm floods," 303, 372
Stormmess and sunspots, 98, 365
Storms, 306
Storm tracks, 98, 160, 273, 369
Strabo, 322
Stratosphere, 51, 386
Sub- Atlantic period, 173, 296ff.,
34
Sub-boreal period, 296ff., 340
Sumerians, 320
Summer Lake, 350, 352
Sun, age of, 379
Sunspot
cycle, cause of, 367
length, 97, 108
Sunspots, 97, 366
and meteors, 367
and Nile levels, 368
and rainfall, 98, 365
and storminess, 98, 365
and temperature, 98 i
and thunderstorms, 98, 365
relative number, 97
variations of, 97, 366
Supan, A., 174
Susa, 319
Siissmilch, C. A., 233, 244, 245,
246
Sverdrup, H. U., 88
Symons, G. J., 304, 307, 31';
Syria, caravan travel, 319
TarT,J. A., 231, 245
Tanganyika Lake, 340
Tansley, A. G., 298, 316
Taulis, E., 358
Temperature,
and continentality, I29ff., 203,
212
decrease with height, 211, 212
distribution, 1 29!!.
over ice-sheets, 33fT., 154
variations, 2041!.
*' Terra Nova,*' 246, 262
Tertiary
climate, 26, 83, 108, 163, 173,
igafT., 228, 237, 242
flora, 24, 136, 241
isotherms, 135
Tethys Sea, 239, 247, 248, 251
Theeuws, R., 341
Thunderstorms and sunspots, 98,
365
Tidal friction, 225
" Tide-generating force," 3^Y
Tomboro, eruption, 1 1 9
Torridon Sandstone, 226
Toussoun, Prince Omar, 329, 340
Toynbee, A. J. 5 319, 327
Trade winds, 55, 83
Quaternary, 83
Tree growth, 171, 236, 293, 3421!.,
372
Triassic, 25, 195
Tripolje, 319
Tropopause, 51
Truckee River, 347
INDEX
395
Turner, H. H., 367, 378
" Twilight of the Gods/' 289
Uhlig, V., 23, 27, 138
Umbgrove, J. H. F., 191
Unafcar, V., 324, 328
Upilam, W., 385
Upwelling water, 73, 76, 267
Uranium-lead ratio, 380
U.S.A., rainfall of western, 343,
359, 361
Vanderlinden, E., 287, 294
" Varve " clays, 84, 232, 293, 301
Versregan, R., 312
Vine in Palestine, 286
Visher, S. S., 86, 88, 98, 101, 365,
378
Volcanic
* action, variations of, 204, 206,
208
dust, 113, 117, 255, 260, 270,
386
Volga Sea, 248, 251, 258
Waagen, L., 234
Wagner, A., 179, 191
Walker Lake, 352
Walker, Sir G., 360, 365, 378
Warming, E., 170, 176
Warm periods,
aridity, 24
atmospheric circulation, 6 1 ,
62
oceanic circulation, 77^.
since 1850, 376
weather of, i92ff.
Water mills, 303
Water vapour, effect on climate,
1 14, 386
Wayland, E. J., 276, 278
Weather ^
of glacial periods, 274
of warm periods, 192!!
Wegener, A., 104, 109, 22 iff., 230,
231
Weidmann, S., 231, 245
White, D., 171, 176
Willis, Bailey, 261, 262
Winds, 46ff.
in historical period, 285, 31 iff.
in ice-age, 59
in warm periods, 55
over ice-sheets, 62
Winkle, W. van, 350, 358
Winnemucca Lake, 346, 352
Winters, severe, 376
Wisconsin glaciation, 242, 264, 274
Wittfogel, K. A., 326, 328
Wolf, R., 366
Woodworm, J. B., 232, 245
Wright, C. S., 239, 243, 246, 259,
262
Wundt, W., 113, 1 20
Wurm glaciation, 94, 106, 107,
242, 265, 270
Wyville Thomson ridge, 84, 266
Yarmouth interglacial, 243, 264
Yucatan, climatic variations, 354
Zeiller, R., 246
Zeuner, F. E., 105, 109, 219, 264
270, 277
Zimbabwe, 340
Zones,
climatic, 174
of rainfall, 164, 174
of temperature, 1 74
warm periods, 23, 241
wind, 54, 174
