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G78'^9.S^
HARVARD COLLEGE
LIBRARY
GUT OF THE
HARVARD UNIVERSmr
PRESS
CLIMATIC CHANGES
THEIR NATURE AND CAUSES
CLIMATIC CHANGES
THEm NATUBE AND CAC8E8
I /
t h
r
i
PUBLISHED ON THE FOUNDATION
ESTABLISHED IN MEMOBY OF
THEODOBE L. GLASGOW
OTHER BOOKS BY THE SAME AUTHORS
ELLSWOETH HUNTINGTON
A. Four hooks showing the development of knowledge as to Historical PulsO'
tions of Clitnaie,
The Pnlse of Asia. Boston, 1907.
Explorations in Turkestan. Expedition of 1903. Washington, 1905.
Palestine and Its Transformation. Boston, 1911.
The Climatic Factor, as lUostrated in Arid America. Washington, 1914.
B. Two books Uhtstrating the effect of climate on man.
Civilization and Climate. New Haven, 1915.
World Power and Evolution. New Haven, 1919.
C. Four hooks illustrating the general principles of Geography,
Asia: A Geography Beader. Chicago, 1912.
The Bed Man's Continent New Haven, 1919.
Principles of Human Geography (with 8. W. Gushing). New York, 1920.
Business Geography (with ¥. E. Williams). New York, 1922.
D. A companion to the present volume.
Earth and Sun: An Hypothesis of Weather and Sunspots. New Haven.
In press.
STEPHEN SABGENT VI8HEB
(Geography, Geology and Biology of Southern Dakota. Vermilion, 1912.
The Biology of Northwestern South Dakota. Vermilion, 1914.
The Geography of South Dakota. Vermilion, 1918.
Handbook of the Geology of Indiana (with others). Indianapolis, 1922.
Hurricanes of Australia and the South Pacific. Melbourne, 1922.
/
CLIMATIC CHANGES
THEIB NATURE AND CAUSES
BY
ELLSWORTH HUNTINGTON
Beaeareh Aasoeiate in G«ographj' in Tale Univeraity
AND
STEPHEN SABGENT VISHEB
Associate Professor of Qeologj
in Indiana TJniTersitj
F^i^^**9
NEW HAVEN
YALE UNIVEESITY PBESS
LONDON: HUMPHHET MILFORD: OXFOBD UNIVEBSITT PRESS
MDOCOCXXn
\ V ' " i^v ^ ' y '- ^ ^ ' "■ ■ .
S S<^7^. g^.S"
v^
V^'T
/
''.-. f\
\ •
\,
COPYBIGHT 1922 BT
YALE UNIVERSITY PBESS
Published 1922.
THE THEODORE L. GLASGOW MEMORIAL
PUBLICATION FUND
The present volume is the fifth work published by the Tale
University Press on the Theodore L. Glasgow Memorial Publica-
tion Fund. This foundation was established September 17, 1918,
by an anon3rmous gift to Yale University in memory of Flight
Sub-Lieutenant Theodore L. Glasgow, R.N. He was bom in
Montreal, Canada, and was educated at the University of Toronto
Schools and at the Royal Military College, Kingston. In August,
1916, he entered the Royal Naval Air Service and in July, 1917,
went to France with the Tenth Squadron attached to the Twenty-
second Wing of the Royal Flying Corps. A month later, August
19, 1917, he was killed in action on the Ypres front.
TO
THOMAS CHEOWDER CHAMBERLIN
OF THE UNIVERSITY OF CmCAOO
WHOSE CLEAB AND MASTERLY DISCUSSION
OF THE GREAT PROBLEMS OF TERRESTRIAL EVOLUTION
HAS BEEN ONE OF THE MOST INSPIRINO FACTORS
IN THE WRITINQ OF THIS BOOK
Thebb is a toy, which I have heard, and I would not have
it given over, hut waited upon a little. They say it is ob-
served in the Low Countries (I know not in what part),
that every five and thirty years the same kind and suit
of years and weathers comes about again; as great frosts,
great wet, great droughts, warm winters, summers unth
little heat, and the like, and they call it the prime; it is a
thing I do the rather mention, because, computing back-
4
wards, I have found some concurrence.
rEANCIS BACON
PREFACE
T T"NITY is perhaps the keynote of modem science.
I This means tmity in time, for the present is but
\^^ the outgrowth of the past, and the future of the
present. It means unity of process, for there seems to be
no sharp dividing line between organic and inorganic,
physical and mental, mental and spiritual. And the unity
of modem science means also a growing tendency toward
cooperation, so that by working together scientists dis-
cover much that would else have remained hid.
This book illustrates the modern trend toward unity in
all of these ways. First, it is a companion volume to
Earth and Sun. That volume is a discussion of the causes
of weather, but a consideration of the weather of the
present almost inevitably leads to a study of the climate
of the past. Hence the two books were written originally
as one, and were only separated from considerations of
convenience. Second, the unity of nature is so great that
when a subject such as climatic changes is considered, it
is almost impossible to avoid other subjects, such as the
movements of the earth's crust. Hence this book not only
discusses climatic changes, but considers the causes of
earthquakes and attempts to show how climatic changes
may be related to great geological revolutions in the
form, location, and altitude of the lands. Thus the book
has a direct bearing on all the main physical factors
which have molded the evolution of organic life, includ-
ing man.
xii PREFACE
In the third place, this volume illustrates the unity of
modem science because it is preeminently a cooperative
product. Not only have the two authors shared in its
production, but several of the Yale Faculty have also
cooperated. From the geological standpoint, Professor
Charles Schuchert has read the entire manuscript in its
final form as well as parts at various stages. He has
helped not only by criticisms, suggestioiis, and facts, but
by paragraphs ready for the printer. In the same way
in the domain of physics. Professor Leigh Page has re-
peatedly taken time to assist, and either in writing or by
word of mouth has contributed many pages. In astron-
omy, the same cordial cooperation has come with equal
readiness from Professor Frank Schlesinger. Professors
Schuchert, Schlesinger, and Page have contributed so
materially that they are almost co-authors of the volume.
In mathematics. Professor Ernest W. Brown has been
similarly helpful, having read and criticised the entire
book. In certain chemical problems. Professor Harry W.
Foote has been our main reliance. The advice and sugges-
tions of these men have frequently prevented errors, and
have again and again started new and profitable lines of
thought. If we have made mistakes, it has been because
we have not profited sufficiently by their cooperation. If
the main hypothesis of this book proves sound, it is
largely because it has been built up in constant consulta-
tion with men who look at the problem from different
points of vision. Our appreciation of their generous and
unstinted cooperation is much deeper than would appear
from this brief paragraph.
Outside the Yale Faculty we have received equally
cordial assistance. Professor T. C. Chamberlin of the Uni-
versity of Chicago, to whom, with his permission, we take
great pleasure in dedicating this volume, has read the
PREFACE xiii
entire proof and has made many helpful suggestions.
We cannot speak too warmly of our appreciation not
only of this, but of the way his work has served for years
as an inspiration in the preliminary work of gathering
data for this volume. Professor Harlow Shapley of Har-
vard University has contributed materially to the chap-
ter on the sun and its journey through space ; Professor
Andrew E. Douglass of the University of Arizona has
put at our disposal some of his unpublished results;
Professors S. B. Woodworth and Reginald A. Daly, and
Mr. Robert W. Sayles of Harvard, and Professor Henry
F. Reid of Johns Hopkins have suggested new facts and
sources of information; Professor E. R. Cumings of
Indiana University has critically read the entire proof;
conversations with Professor John P. Buwalda of the
Umversity of CaUfornia whUe he was teaching at YaJe
make him another real contributor; and Mr. Wayland
Williams has contributed the interesting quotation from
Bacon on page x of this book. Miss Edith S. Russell has
taken great pains in preparing the manuscript and in
suggesting many changes that make for clearness. Many
others have also helped, but it is impossible to make due
acknowledgment because such contributions have become
so thoroughly a part of the mental background of the
book that their source is no longer distinct in the minds
of the authors.
The division of labor between the two authors has not
followed any set rules. Both have had a hand in all parts
of the book. The main draft of Chapters VII, VIII, IX,
XI, and Xin was written by the junior author ; his con-
tributions are also especially numerous in Chapters X
and XV; the rest of the book was written originally by
tiie senior author.
CONTENTS
PAGE
L The Uniformity of Climate 1
n. The VariabiUty of Climate 16
IIL Hypotheses of Climatic Change .... 33
rv. The Solar Cyclonic Hypothesis .... 51
V. The CUmate of History 64
VI. The Climatic Stress of the Fourteenth Cen-
tury 98
VII. Glaciation According to the Solar Cyclonic
Hypothesis 110
VTH. Some Problems of Glacial Periods . . . 130
IX. The Origin of Loess 155
X. Causes of Mild Geological Climates . . . 166
XL Terrestrial Causes of Climatic Changes . . 188
Xn. Post-Glacial Crustal Movements and Cli-
matic Changes 215
XJH. The Changing Composition of Oceans and
Atmosphere 223
XIV. The Effect of Other Bodies on the Sun . . 242
XV. The Sun's Journey through Space . . . 264
XVI. The Earth's Crust and the Sun .... 285
LIST OF ILLUSTRATIONS
PAGE
Fig. 1. Climatic changes and mountain building 25
Fig. 2. Storminess at sunspot maxima vs.
minima 54
Fig. 3. Relative rainfall at times of increasing
and decreasing sunspots . . . . 58, 59
Fig. 4. Changes of climate in California and in
western and central Asia .... 75
Fig. 5. Changes in California climate for 2000
years, as measured by growth of Se-
quoia trees 77
Fig. 6. Distribution of Pleistocene ice sheets . 123
Fig. 7. Permian geography and glaciation . . 145
¥ig. 8. Effect of diminution of storms on move-
ment of water 175
Fig. 9. Cretaceous Paleogeography .... 201
Fig. 10. Climatic changes of 140,000 years as in-
ferred from the stars* 279
Fig. 11. Sunspot curve showing cycles, 1750 to
1920 283
Fig. 12. Seasonal distribution of earthquakes . 299
Fig. 13. Wandering of the pole from 1890 to 1898 303
TABLES
PAGE
1. The Geological Time Table 5
2. Types of Climatic Sequence 16
3. Correlation Coefficients between Bainfall
and Growth of Sequoias in California . 80
4. Correlation Coefficients between Rainfall
Records in California and Jerusalem . 84
5. Theoretical Probability of Stellar Ap-
proaches 260
6. Thirty-Eight Stars Having Largest Known
Parallaxes 276,277
7. Destructive Earthquakes from 1800 to 1899
Compared with Sunspots 289
8. Seasonal March of Earthquakes 295
9. Deflection of Path of Pole Compared with
Earthquakes . 305
10. Earthquakes in 1903 to 1908 Compared with
Departures of the Projected Curve of the
Earth's Axis from the Eulerian Position 306
CHAPTER I
THE UNIFORMITY OF CLIMATE
THE role of climate in the life of today suggests its
importance in the past and in the future. No hu-
man being can escape from the fact that his food,
clothing, shelter, recreation, occupation, health, and
energy are all profoundly influenced by his climatic sur-
roundings. A change of season brings in its train some
alteration in practically every phase of human activity.
Animals are influenced by climate even more than man,
for they have not developed artificial means of protect-
ing themselves. Even so hardy a creature as the dog
becomes notably different with a change of climate. The
thick-haired ** husky *' of the Eskimos has outwardly
little in common with the small and almost hairless
canines that grovel under foot in Mexico. Plants are even
more sensitive than animals and men. Scarcely a single
species can flourish permanently in regions which differ
more than 20° C. in average yearly temperature, and for
most the limit of successful growth is 10°.^ So far as we
yet know every living species of plant and animal, includ-
ing man, thrives best under definite and limited conditions
of temperature, humidity, and sunshine, and of the com-
position and movement of the atmosphere or water in
which it lives. Any departure beyond the limits means
lessened efficiency, and in the long run a lower rate of
1 W. A. Setchell : The Temperature Interval in the Geographical Distribu-
tion of Marine Algs&; Science^ VoL 52, 1920, p. 187.
2 CLIMATIC CHANGES
reproduction and a tendency toward changes in specific
characteristics. Any great departure means suffering or
death for the individual and destruction for the species.
Since climate has so profound an influence on life
today, it has presumably been equally potent at other
times. Therefore few scientific questions are more im-
portant than how and why the earth 's climate has varied
in the past, and what changes it is likely to undergo in
the future. This book sets forth what appear to be the
chief reasons for climatic variations during historic and
geologic times. It assumes that causes which can now be
observed in operation, as explained in a companion
volume entitled Earth and Sun, and in such books as
Humphreys' Physics of the Air, should be carefully
studied before less obvious causes are appealed to. It
also assumes that these same causes will continue to
operate, and are the basis of all valid predictions as to
the weather or climate of the future.
In our analysis of climatic variations, we may well
begin by inquiring how the earth's climate has varied
during geological history. Such an inquiry discloses three
great tendencies, which to the superficial view seem con-
tradictory. All, however, have a similar effect in provid-
ing conditions under which organic evolution is able to
make progress. The first tendency is toward uniformity,
a uniformity so pronounced and of such vast duration
as to stagger the imagination. Superposed upon this
there seems to be a tendency toward complexity. During
the greater part of geological history the earth 's climate
appears to have been relatively monotonous, both from
place to place and from season to season; but since the
Miocene the rule has been diversity and complexity, a
condition highly favorable to organic evolution. Finally,
the uniformity of the vast eons of the past and the
THE UNIFORMITY OF CLIMATE 8
tendency toward complexity are broken by pulsatory
changes, first in one direction and then in another. To
our limited human vision some of the changes, such as
glacial periods, seem to be waves of enormous propor-
tions, but compared with the possibilities of the universe
they are merely as the ripples made by a summer zephyr.
The uniformity of the earth's climate throughout the
vast stretches of geological time can best be realized by
comparing the range of temperature on the earth during
that period with the possible range as shown in the entire
solar system. As may be seen in Table 1, the geological
record opens with the Archeozoic era, or **Age of Uni-
cellular Life, ' ' as it is sometimes called, for the preceding
cosmic time has left no record that can yet be read.
Practically no geologists now believe that the beginning
of the Archeozoic was less than one hundred million
years ago ; and since the discovery of the peculiar proper-
ties of radium many of the best students do not hesitate
to say a billion or a billion and a half.^ Even in the
Archeozoic the rocks testify to a climate seemingly not
greatly different from that of the average of geologic
time. The earth's surface was then apparently cool
enough so that it was covered with oceans and warm
enough so that the water teemed with microscopic life.
The air must have been charged with water vapor and
with carbon dioxide, for otherwise there seems to be no
possible way of explaining the formation of mudstones
and sandstones, limestones of vast thickness, carbona-
ceous shales, graphites, and iron ores.* Although the
Archeozoic has yielded no generally admitted fossils, yet
what seem to be massive algae and sponges have been
sj. Barrell: Bhjtlims and the MeasarementB of Geologic Time; Bull.
Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.
8 Pirason and Schuehert: Textbook of Geology, 1915, pp. 538-550.
4 CLIMATIC CHANGES
found in Canada. On the other hand, abundant life it
believed to have been present in the oceans, for by nc
other known means would it be possible to take from thi
air the vast quantities of carbon that now form carbona
ceous shales and graphite.
In the next geologic era, the Proterozoic, the re-
searches of Walcott have shown that besides the marine
algSB there must have been many other kinds of life. The
Proterozoic fossils thus far discovered include not onlj
microscopic radiolarians such as still form the red ooze
of the deepest ocean floors, but the much more signifi-
cant tubes of annelids or worms. The presence of the
annelids, which are relatively high in the scale of organi-
zation, is generally taken to mean that more lowly fomai
of animals such as coelenterates and probably even tb
moUusca and primitive arthropods must already havi
been evolved. That there were many kinds of marin
invertebrates living in the later Proterozoic is indicate
by the highly varied life and more especially the trilo
bites found in the oldest Cambrian strata of the nex
succeeding period. In fact the Cambrian has spongeg
primitive corals, a great variety of brachiopods, th
beginnings of gastropods, a wonderful array of trilobites
and other lowly forms of arthropods. Since, under th
postulate of evolution, the life of that time forms an ub
broken sequence with that of the present, and since man;
of the early forms differ only in minor details from tho
of today, we infer that the climate then was not ve
different from that of today. The same line of reasoni
leads to the conclusion that even in the middle of t
Proterozoic, when multicellular marine animals mxa
already have been common, the climate of the earth h{
already for an enormous period been such that all tl
lower types of oceanic invertebrates had already evolve
THE UNIFORMITY OF CLIMATE
6
TABLE 1
THE GEOLOGICAL TIME TABLE*
COSMIC TIME
Formative Era. Birth and growth of the earth. BeginningB of
the atmosphere, hydrosphere, continental platforms, oceanic
basins, and possibly of life. No known geological record.
GEOLOGIC TIME
Archeozoic Era. Origin of simplest life.
Proterozoio Era. Age of invertebrate origins. An early and a late
ice age, with one or more additional ones indicated.
Paleozoic Era. Age of primitive vertebrate dominance.
Cambrian Period. First abundance of marine animals and domi-
nance of trilobites.
Ordovidan Period. First known fresh-water fishes.
Silurian Period, First known land plants.
Devonian Period. First known amphibians. ''Table Mountain"
ice age.
Mississippian Period. Bise of marine fishes (sharks) .
Pennsylvanian Period, Bise of insects and first period of marked
coal accumulation.
Permian Period. Bise of reptiles. Another great ice age.
Mssozoic Era. Age of reptile dominance.
Triaseic Period. Bise of dinosaurs. The period closes with a cool
climate.
Jurassic Period. Bise of birds and flying reptiles.
Comanchean Period. Bise of flowering plants and higher insects.
Cretaceous Period. Bise of archaic or primitive mammalia.
CsNOZOic Era. Age of mammal dominance.
Early Cenoeoio or Eocene and Oligocene time. Bise of higher
mammals. Glaciers in early Eocene of the Laramide Moun-
tains.
Late Cenoeoic or Miocene and Pliocene time. Transformation of
ape-like animals into man.
Glacial or Pleistocene time. Last great ice age.
PBESENT TIME
PsTCHOZOic Era. Age of man or age of reason. Includes the
present or ' ' Becent time, ' ' estimated to be probably less than
30,000 years.
4 From Charles Schuchert in The Evolution of the Earth and Its In-
habitants: Edited by B. S. Lull, New Haven, 1918, but with revisions by
Professor Schuchert.
6 CLIMATIC CHANGES
Moreover, they could live in most latitudes, for the in
direct evidences of life in the Archeozoic and Protero
zoic rocks are widely distributed. Thus it appears tha
at an almost incredibly early period, perhaps many hun
dred million years ago, the earth's climate diflFered onl|
a little from that of the present.
The extreme limits of temperature beyond which t
climate of geological times cannot have departed can
approximately determined. Today the warmest parts
the ocean have an average temperature of about 30**G
on the surface. Only a few forms of life live where thi
average temperature is much higher than this. In desert
to be sure, some highly organized plants and animals ca
for a short time endure a temperature as high as 75
(167°F.). In certain hot springs, some of the lowest un
cellular plant forms exist in water which is only a litt
below the boiling point. More complex forms, howeve
such as sponges, worms, and all the higher plants an
animals, seem to be unable to live either in water or a
where the temperature averages above 45°C. (113**P
for any great length of time and it is doubtful wheth
they can thrive permanently even at that temperatur
The obvious unity of life for hundreds of millions
years and its presence at all times in middle latitudes at
far as we can tell seem to indicate that since the bo-
ginning of marine life the temperature of the ocean
cannot have averaged much above 50° C. even in the
warmest portions. This is putting the limit too high
rather than too low, but even so the warmest parts o
the earth can scarcely have averaged much more thai
20° warmer than at present.
Turning to the other extreme, we may inquire hoM
much colder than now the earth 's surface may have beei
since life first appeared. Proterozoic fossils have been
THE UNIFORMITY OF CLIMATE 7
found in places where the present average temperature
approaches 0**C. If those places should be colder than
now by 30° C, or more, the drop in temperature at the
equator would almost certainly be still greater, and the
seas everywhere would be permanently frozen. Thus
life would be impossible. Since the contrasts between
summer and winter, and between the poles and the
equator seem generally to have been less in the past than
at present, the range through which the mean tempera-
ture of the earth as a whole could vary without utterly
destroying life was apparently less than would now be
the case.
These considerations make it fairly certain that for at
least several hundred million years the average tempera-
ture of the earth's surface has never varied more than
perhaps 30** C. above or below the present level. Even this
range of 60°C. (108°F.) may be double or triple the range
that has actually occurred. That the temperature has not
passed beyond certain narrow limits, whatever their
exact degree, is clear from the fact that if it had done so,
all the higher forms of life would have been destroyed.
Certain of the lowest unicellular forms might indeed have
persisted, for when dormant they can stand great ex-
tremes of dry heat and of cold for a long time. Even
so, evolution would have had to begin almost anew. The
supposition that such a thing has happened is untenable,
for there is no hint of any complete break in the record
of life during geological times, — ^no sudden disappear-
ance of the higher organisms followed by a long period
with no signs of life other than indirect evidence such as
occurs in the Archeozoic.
A change of 60** C. or even of 20** in the average tem-
perature of the earth's surface may seem large when
viewed from the limited standpoint of terrestrial ex-
8 CLIMATIC CHANGES
perience. Viewed, however, from the standpoint of
cosmic evolution, or even of the solar system, it seems
a mere trifle. Consider the possibilities. The tempera-
ture of empty space is the absolute zero, or — 273 °C.
To this temperature all matter must fall, provided it
exists long enough and is not appreciably heated by colli-
sions or by radiation. At the other extreme lies the
temperature of the stars. As stars go, our sun is only
moderately hot, but the temperature of its surface is
calculated to be nearly 7000** C, while thousands of miles
in the interior it may rise to 20,000'' or lOOjOOO"* or some
other equally unknowable and incomprehensible figure.
Between the limits of the absolute zero on the one hand,
and the interior of a sun or star on the other, there is
almost every conceivable possibility of temperature.
Today the earth *s surface averages not far from 14** C,
or 287** above the absolute zero. Toward the interior,
the temperature in mines and deep wells rises about l^'C.
for every 100 meters. At this rate it would be over 500° C.
at a depth of ten miles, and over 5000° at 100 miles.
Let us confine ourselves to surface temperatures,
which are all that concern us in discussing climate. It
has been calculated by Poynting" that if a small sphere
absorbed and re-radiated all the heat that fell upon it,
its temperature at the distance of Mercury from the sun
would average about 210 **C.; at the distance of Venus,
85^ ; the earth 27° ; Mars —30° ; Neptune —219°. A planet
much nearer the sun than is Mercury might be heated to
a temperature of a thousand, or even several thousand,
degrees, while one beyond Neptune would remain almost
at absolute zero. It is well within the range of possibility
that the temperature of a planet's surface should be
» J. H. Poynting: Kadiation in the Solar System; Phil. Trans. A, 1903,
202, p. 525.
THE UNIFORMITY OP CLIMATE 9
anywhere from near — 273''C. up to perhaps 5000°C. or
more, although the probability of low temperature is
much greater than of high. Thus throughout the whole
vast range of possibilities extending to perhaps 10,000**,
the earth claims only 60® at most, or less than 1 per cent.
This may be remarkable, but what is far more remark-
able is that the earth's range of 60° includes what seem
to be the two most critical of all possible temperatures,
namely, the freezing point of water, 0**C., and the tem-
perature where water can dissolve an amount of carbon
dioxide equal to its own volume. The most remarkable
fact of all is that the earth has preserved its temperature
within these narrow limits for a hundred million years,
or perchance a thousand million.
To appreciate the extraordinary significance of this
last fact, it is necessary to realize how extremely critical
are the temperatures from about 0** to 40° C, and how
difficult it is to find any good reason for a relatively
uniform temperature through hundreds of millions of
years. Since the dawn of geological time the earth's
temperature has apparently always included the range
from about the freezing point of water up to about the
point where protoplasm begins to disintegrate. Hender-
son, in The Fitness of the Environment, rightly says that
water is *'the most familiar and the most important of
all things." In many respects water and carbon dioxide
form the most unique pair of substances in the whole
realm of chemistry. Water has a greater tendency than
any other known substance to remain within certain
narrowly defined limits of temperature. Not only does it
have a high specific heat, so that much heat is needed to
raise its temperature, but on freezing it gives up more
heat than any substance except ammonia, while none of
the couMnon liquids approach it in the amount of addi-
10 CLIMATIC CHANGES
tional heat required for conversion into vapor after the
temperature of vaporization has been reached. Again,
water substance, as the physicists call all forms of H2O,
is unique in that it not only contracts on melting, but
continues to contract until a temperature several degrees
above its melting point is reached. That fact has a vast
importance in helping to keep the earth's surface at a
uniform temperature. If water were like most liquids,
the bottoms of all the oceans and even the entire body of
water in most cases would be permanently frozen.
Again, as a solvent there is literally nothing to com-
pare with water. As Henderson* puts it: '* Nearly the
whole science of chemistry has been built up around
water and aqueous solution. ' ' One of the most significant
evidences of this is the variety of elements whose pres-
ence can be detected in sea water. According to Hender
son they include hydrogen, oxygen, nitrogen, carbon,
chlorine, sodimn, magnesium, sulphur, phosphorus, whicl
are easily detected; and also arsenic, csBsium, gold
lithium, rubidium, barium, lead, boron, fluorine, iron.
iodine, bromine, potassium, cobalt, copper, manganese
nickel, silver, silicon, zinc, aluminium, calcium, an^
strontium. Yet in spite of its marvelous power of solu
tion, water is chemically rather inert and relatively
stable. It dissolves all these elements and thousands
their compounds, but still remains water and can easi
be separated and purified. Another unique property 0:
water is its power of ionizing dissolved substances,
property which makes it possible to produce electri
currents in batteries. This leads to an almost infini
array of electro-chemical reactions which play an almos
dominant role in the processes of life. Finally, nc
common liquid except mercury equals water in its powei
• Lw J. Henderson: The Fitness of the Environment^ 1913. '
THE UNIFORMITY OF CLIMATE 11
of capillarity. This fact is of enormous moment in
biology, most obviously in respect to the soil.
Although carbon dioxide is far less familiar than
water, it is almost as important. ^ ^ These two simple sub-
stances, '* says Henderson, **are the common source of
every one of the complicated substances which are pro-
duced by living beings, and they are the common end
products of the wearing away of all the constituents of
protoplasm, and of the destruction of those materials
which yield energy to the body. ' * One of the remarkable
physical properties of carbon dioxide is its degree of
solubility in water. This quality varies enormously in
different substances. For example, at ordinary pressures
and temperatures, water can absorb only about 5 per
cent of its own volume of oxygen, while it can take up
about 1300 times its own volume of ammonia. Now for
carbon dioxide, unlike most gases, the volume that can
be absorbed by water is nearly the same as the volume
of the water. The volumes vary, however, according to
temperature, being absolutely the same at a temperature
of about 15°C. or 59^F., which is close to the ideal tem-
perature for man's physical health and practically the
same as the mean temperature of the earth's surface
when all seasons are averaged together. ^^ Hence, when
water is in contact with air, and equilibrium has been
established, the amount of free carbonic acid in a given
volume of water is ahnost exactly equal to the amount
in the adjacent air. Unlike oxygen, hydrogen, and nitro-
gen, carbonic acid enters water freely ; unlike sulphurous
oxide and ammonia, it escapes freely from water. Thus
the waters can never wash carbonic acid completely out
of the air, nor can the air keep it from the waters. It is
the one substance which thus, in considerable quantities
relative to its total amount, everywhere accompanies
12 CLIMATIC CHANGES
water. In earth, air, fire, and water alike these two sub-
stances are always associated.
** Accordingly, if water be the first primary con-
stituent of the environment, carbonic acid is inevitably
the second, — ^because of its solubility possessing an
equal mobility with water, because of the reservoir of the
atmosphere never to be depleted by chemical action in
the oceans, lakes, and streams. In truth, so close is the
association between these two substances that it is
scarcely correct logically to separate them at all; to-
gether they make up the real environment and they never
part company. ' "
The complementary qualities of carbon dioxide and
water are of supreme importance because these two are
the only known substances which are able to form a vast
series of complex compounds with highly varying chemi-
cal formulae. No other known compounds can give oflf
or take on atoms without being resolved back into their
elements. No others can thus change their form freely
without losing their identity. This power of change with-
out destruction is the fundamental chemical character-
istic of life, for life demands complexity, change, and
growth.
In order that water and carbon dioxide may combine
to form the compounds on which life is based, the water
must be in the liquid form, it must be able to dissolve
carbon dioxide freely, and the temperature must not be
high enough to break up the highly complex and delicate
compounds as soon as they are formed. In other words,
the temperature must be above freezing, while it must
not rise higher than some rather indefinite point between
50° C. and the boiling point, where all water finally turns
into vapor. In the whole range of temperature, so far as
7 Henderson : loc, cit,, p. 138.
THE UNIFORMITY OF CLIMATE 18
we know, there is no other interval where any such com-
plex reactions take place. The temperature of the earth
for hundreds of millions of years has remained firmly
fixed within these limits.
The astonishing quality of the earth *s uniformity of
temperature becomes still more apparent when we con-
sider the origin of the sun's heat. What that origin is
still remains a question of dispute. The old ideas of a
burning sun, or of one that is simply losing an original
supply of heat derived from some accident, such as colli-
sion with another body, were long ago abandoned. The
impact of a constant supply of meteors affords an ahnost
equally unsatisfactory explanation. Moulton' states that
if the sun were struck by enough meteorites to keep up
its heat, the earth would almost certainly be struck by
enough so that it would receive about half of 1 per cent
as much heat from them as from the sun. This is millions
of times more heat than is now received from meteors.
If the sun owes its heat to the impact of larger bodies at
longer intervals, the geological record should show a
series of interruptions far more drastic than is actually
the case.
It has also been supposed that the sun owes its heat
to contraction. If a gaseous body contracts it becomes
warmer. Finally, however, it must become so dense that
its rate of contraction diminishes and the process ceases.
Under the sun's present condition of size and density a
radial contraction of 120 feet per year would be enough
to supply all the energy now radiated by that body. This
seems like a hopeful source of energy, but Kelvin cal-
culated that twenty million years ago it was ineffective
and ten million years hence it will be equally so. More-
over, if this is the source of heat, the amount of radia-
8 F. B. Monlton : Introduction to Astronomj^ 1916.
14 CLIMATIC CHANGES
tion from the sun would have to vary enormously.
Twenty million years ago the sun would have extended
nearly to the earth *s orbit and would have been so tenu-
ous that it would have emitted no more heat than some
of the nebulae in space. Some millions of years later^
when the sun's radius was twice as great as at present,
that body would have emitted only one-fourth as much
heat as now, which would mean that on the earth's sur-
face the theoretical temperature would have been 200**
below the present level. This is utterly out of accord with
the uniformity of climate shown by the geological record.
In the future, if the sun 's contraction is the only source
of heat, the sun can supply the present amount for only
ten million years, which would mean a change utterly
unlike anything of which the geological record holds
even the faintest hint.'
Altogether the problem of how the sun can have re-
mained so uniform and how the earth's atmosphere and
other conditions can also have remained so uniform
throughout hundreds of millions of years is one of the
most puzzling in the whole realm of nature. If appeal is
taken to radioactivity and the breaking up of uranium
into radium and helium, conditions can be postulated
which will give the required amount of energy. Such is
also the case if it be supposed that there is some unknown
process which may induce an atomic change like radio-
activity in bodies which are now supposed to be stable
elements. In either case, however, there is as yet no
satisfactory explanation of the uniformity of the earth's
climate. A hundred million or a thousand million years
ago the temperature of the earth's surface was very
much the same as now. The earth had then presumably
ceased to emit any great amount of heat, if we may judge
• Moulton: loc. cit.
I
THE UNIFORMITY OF CLIMATE 16
from the fact that its surface was cool enough so that
^eat ice sheets could accumulate on low lands within 40"^
of the equator. The atmosphere was apparently almost
like that of today, and was almost certainly not different
enough to make up for any great divergence of the sun
from its present condition. We cannot escape the stu-
pendous fact that in those remote times the sun must
have been essentially the same as now, or else that some
utterly imknown factor is at work.
CHAPTER II
THE VARIABILITY OF CLIMATE
THE variability of the earth's climate is almost as
extraordinary as its uniformity. This variabilit
is made up partly of a long, slow tendency in on
direction and partly of innumerable cycles of every con|
ceivable duration from days, or even hours, up to million
of years. Perhaps the easiest way to grasp the full co
plexity of the matter is to put the chief types of climati
sequence in the form of a table.
TABLE 2
TYPES OF CLIMATIC SEQUENCE
1. Cosmic uniformity. 7. Bruckner periods.
2. Secular progression. 8. Sunspot cycles.
3. Geologic oscillations. 9. Seasonal alternations.
4. Glacial fluctuations. 10. Pleionian migrations.
6. Orbital precessions. 11. Cyclonic vacillations.
6. Historical pulsations. 12. Daily vibrations.
In assigning names to the various types an attempj
has been made to indicate something of the nature of thi
sequence so far as duration, periodicity, and general
tendencies are concerned. Not even the rich English
language of the twentieth century, however, fumisheg
words with enough shades of meaning to express all tha
THE VARIABILITY OF CLIMATE 17
is desired. Moreover, except in degree, there is no sharp
distinction between some of the related types, such as
glacial fluctuations and historic pulsations. Yet, taken as
a whole, the table brings out the great contrast between
two absolutely diverse extremes. At the one end lies well-
nigh eternal uniformity, or an extremely slow progress in
one direction throughout countless ages; at the other,
rapid and regular vibrations from day to day, or else
irregular and seemingly unsystematic vacillations due to
cyclonic storms, both of which types are repeated mil-
lions of times during even a single glacial fluctuation.
The meaning of cosmic uniformity has been explained
in the preceding chapter. Its relation to the other types
of climatic sequences seems to be that it sets sharply
defined limits beyond which no changes of any kind have
ever gone since life, as we know it, first began. Secular
progression, on the other hand, means that in spite of all
manner of variations, now this way and then the other,
the normal climate of the earth, if there is such a thing,
has on the whole probably changed a little, perhaps be-
coming more complex. After each period of continental
uplift and glaciation — for such are preeminently the
times of complexity— it is doubtful whether the earth has
ever returned to quite its former degree of monotony.
Today the earth has swung away from the great diversity
of the glacial period. Yet we still have contrasts of what
seem to us great magnitude. In low depressions, such as .
Turfan in the central deserts of Eurasia, the thermom-
eter sometimes ranges from 0**F. in the morning to 60**
in the shade at noon. On a cloudy day in the Amazon
forest close to the seashore, on the contrary, the tempera-
ture for months may rise to 85** by day and sink no lower
than 75** at night.
The reasons for the secular progression of the earth *s
18 CLIMATIC CHANGES
climate appear to be intimately connected with those
which have caused the next, and, in many respects, more
important type of climatic sequence, which consists of
geological oscillations. Both the progression and the
oscillations seem to depend largely on three purely ter-
restrial factors: first, the condition of the earth's in-
terior, including both internal heat and contraction;
second, the salinity and movement of the ocean; and
third, the composition and amount of the atmosphere^
To begin with the earth's interior — ^its loss of heat ap-
pears to be an almost negligible factor in explaining
either secular progression or geologic oscillation. Accord-
ing to both the nebular and the planetesimal hypotheses,
the earth's crust appears to be colder now than it was
hundreds or thousands of millions of years ago. Th«
emission of internal heat, however, had probably ceased
to be of much climatic significance near the beginning or
the geological record, for in southern Canada glaciatioi
occurred very early in the Proterozoic era. On the other
hand, the contraction of the earth has produced remark-
able eflfects throughout the whole of geological time. l!
has lessened the earth's circumference by a thousand
miles or more, as appears from the way in which th»
rocks have been folded and thrust bodily over ovk
another. According to the laws of dynamics this muak
have increased the speed of the earth's rotation, thui
shortening the day, and also having the more importani
effect of increasing the bulge at the equator. On the othei
hand, recent investigations indicate that tidal retardatioi
has probably diminished the earth's rate of rotatioi
more than seemed probable a few years ago, thus length-
ening the day and diminishing the bulge at the equator
Thus two opposing forces have been at work, one caus-
ing acceleration and one retardation. Their combined
THE VARIABILITY OF CLIMATE 19
jffect may have been a factor in causing secular progres-
lion of climate. It almost certainly was of much im-
)ortance in causing pronounced oscillations first one way
ind then the other. This matter, together with most of
hose touched in these first chapters, will be expanded in
ater parts of the book. On the whole the tendency ap-
pears to have been to create climatic diversity in place
tf uniformity.
The increasing salinity of the oceans may have been
jQother factor in producing secular progression, al-
hough of slight importance in respect to oscillations.
Vhile the oceans were still growing in volume, it is gen-
rally assumed that they must have been almost fresh
or a vast period, although Chamberlin thinks that the
hange in salinity has been much less than is usually
apposed. So far as the early oceans were fresher than
dose of today, their deep-sea circulation must have been
»ss hampered than now by the heavy saline water which
J produced by evaporation in warm regions. Although
tiis saline water is warm, its weight causes it to descend,
istead of moving poleward in a surface current; this
escent slows up the rise of the cold water which has
loved along in the depths of the ocean from high lati-
ides, and thus checks the general oceanic circulation,
f the ancient oceans were fresher and hence had a freer
irculation than now, a more rapid interchange of polar
od equatorial water presumably tended to equalize the
[imate of all latitudes.
Again, although the earth's atmosphere has probably
[langed far less during geological times than was
>rmerly supposed, its composition has doubtless varied,
'he total volume of nitrogen has probably increased, for
xat gas is so inert that when it once becomes a part of
le air it is almost sure to stay there. On the other hand.
20 CLIMATIC CHANGES
the proportions of oxygen, carbon dioxide, and wfi
vapor must have fluctuated. Oxygen is taken out c
stantly by animals and by all the processes of i
weathering, but on the other hand the supply is increa
when plants break up new carbon dioxide derived fi
volcanoes. As for the carbon dioxide, it appears pi
able that in spite of the increased supply fumishec
volcanoes the great amounts of carbon which have gr^
ally been locked up in coal and limestone have apj
ciably depleted the atmosphere. Water vapor also I
be less abundant now than in the past, for the presi
of carbon dioxide raises the temperature a little
thereby enables the air to hold more moisture. When
area of the oceans has diminished, and this has recu
very often, this likewise would tend to reduce the w
vapor. Moreover, even a very slight diminution in
amount of heat given off by the earth, or a decrea
evaporation because of higher salinity in the oc
would tend in the same direction. Now carbon dioxide
water vapor both have a strong blanketing effect whe
heat is prevented from leaving the earth. Therefore
probable reduction in the carbon dioxide and w
vapor of the earth's atmosphere has apparently te
to reduce the climatic monotony and create diversit
complexity. Hence, in spite of many reversals, the
eral tendency of changes, not only in the earth 's intej
and in the oceans, but also in the atmosphere, appean
be a secular progression from a relatively monotoi
climate in which the evolution of higher organic fc
would scarcely be rapid to an extremely diverse
complex climate highly favorable to progressive e\
tion. The importance of these purely terrestrial ager
must not be lost sight of when we come to discuss o
agencies outside the earth.
THE VARIABILITY OF CLIMATE 21
In Table 2 the next type of climatic sequence is geo-
^gie oscillation. This means slow swings that last
lillions of years. At one extreme of such an oscillation
le climate all over the world is relatively monotonous ;
. returns, as it were, toward the primeval conditions at
le beginning of the secular progression. At such times
lagnoUas, sequoias, figs, tree ferns, and many other
npes of s;btropical plants grew far north in plaL Uke
reenland, as is well known from their fossil remains of
dddle Cenozoic time, for example. At these same times,
ad also at many others before such high types of plants
ad evolved, reef-making corals throve in great abun-
ance in seas which covered what is now Wisconsin,
[ichigan, Ontario, and other equally cool regions. Today
lese regions have an average temperature of only about
0**F. in the warmest month, and average well below
reezing in winter. No reef -making corals can now live
^here the temperature averages below 68° F. The re-
^mblance of the ancient corals to those of today makes
; highly probable that they were equally sensitive to low
jmperature. Thus, in the mild portions of a geologic
Bcillation the climate seems to have been so equable and
niform that many plants and animals could live 1500
nd at other times even 4000 miles farther from the
quator than now.
At such times the lands in middle and high latitudes
rere low and small, and the oceans extended widely over
lie continental platforms. Thus unhampered ocean cur-
ents had an opportunity to carry the heat of low lati-
ades far toward the poles. Under such conditions, es-
pecially if the conception of the great subequatorial
ontinent of Gondwana land is correct, the trade winds
nd the westerlies must have been stronger and steadier
ban now. This would not only enable the westerlies.
22 CLIMATIC CHANGES
which are really southwesterlies, to carry more heat th<
now to high latitudes, but would still further strength
the ocean currents. At the same time, the air presuma
contained an abundance of water vapor derived fr<
the broad oceans, and an abundance of atmosph
carbon dioxide inherited from a preceding time w
volcanoes contributed much carbon dioxide to the
These two constituents of the atmosphere may h
exercised a pronounced blanketing effect whereby k
heat of the earth with its long wave lengths was kepti
although the energy of the sun with its shorter wn
lengths was not markedly kept out. Thus everything rif
have combined to produce mild conditions in high li
tudes, and to diminish the contrast between equator d
pole, and between summer and winter.
Such conditions perhaps carry in themselves the SGBi
of decay. At any rate while the lands lie quiet durinji
period of mild climate great strains must accumulate!
the crust because of the earth's contraction and tii
retardation. At the same time the great abundance'
plants upon the lowlying plains with their mild climai
and the marine creatures upon the broad contineiji
platforms, deplete the atmospheric carbon dioxide. Rs
of this is locked up as coal and part as limestone derini
from marine plants as well as animals. Then somethj{
happens so that the strains and stresses of the crust ^
released. The sea floors sink; the continents becol
relatively high and large ; mountain ranges are tormi
and the former plains and emergent portions of t
continental platforms are eroded into hills and valle^
The large size of the continents tends to create deseH
and other types of climatic diversity; the presence ^
mountain ranges checks the free flow of winds and
creates diversity; the ocean currents are like
THE VARIABILITY OF CLIMATE 28
checked^ altered, and diverted so that the flow of heat
from low to high latitudes is diminished. At the same
time evaporation from the ocean diminishes so that a
decrease in water vapor combines with the previous de-
pletion of carbon dioxide to reduce the blanketing effect
of the atmosphere. Thus upon periods of mild monotony
there supervene periods of complexity, diversity, and
severity. Turn to Table 1 and see how a glacial climate
again and again succeeds a time when relative mildness
prevailed almost everywhere. Or examine Fig. 1 and
notice how the lines representing temperatures go up and
down. In the figure Schuchert makes it clear that when
the lands have been large and mountain-making has been
important, as shown by the high parts of the lower shaded
area, the cUmate has been severe, as shown by the descent
of the snow line, the upper shaded area. In the diagram
the climatic oscillations appear short, but this is merely
because they have been crowded together, especially in
the left hand or early part. There an inch in length may
represent a hundred million years. Even at the right-
liand end an inch is equivalent to several million years.
The severe part of a climatic oscillation, as well as the
mild part, will be shown in later chapters to bear in itself
certain probable seeds of decay. While the lands are
being uplifted, volcanic activity is likely to be vigorous
and to add carbon dioxide to the air. Later, as the moun-
tains are worn down by the many agencies of water,
wind, ice, and chemical decay, although much carbon
'dioxide is locked up by the carbonation of the rocks, the
carbon locked up in the coal is set free and increases the
carbon dioxide of the air. At the same time the continents
settle slowly downward, for the earth's crust though
rigid as steel is nevertheless slightly viscous and will
flow if subjected to sufficiently great and enduring pres-
24 CLIMATIC CHANGES
sure. The area from which evaporation can take place
is thereby increased because of the spread of the oceans
over the continents, and water vapor joins with the car-
bon dioxide to blanket the earth and thus tends to keep it
uniformly warm. Moreover, the diminution of the lands
frees the ocean currents from restraint and permits them
to flow more freely from low latitudes to high. Thus in
the course of millions of years there is a return toward
monotony. Ultimately, however, new stresses accumulate
in the earth's crust, and the way is prepared for another
great oscillation. Perhaps the setting free of the stresses
takes place simply because the strain at last becomes
irresistible. It is also possible, as we shall see, that an
external agency sometimes adds to the strain and thereby
determines the time at which a new oscillation shall
begin.
In Table 2 the types of climatic sequences which fol-
low ** geologic oscillations'' are *' glacial fluctuations,"
*' orbital precessions" and ** historical pulsations."
Glacial fluctuations and historical pulsations appear to
be of the same type, except as to severity and duration,
and hence may be considered together. They will be
treated briefly here because the theories as to their
causes are outlined in the next two chapters. Oddly
enough, although the historic pulsations lie much closer
to us than do the glacial fluctuations, ihej were not
discovered until two or three generations later, and are
still much less known. The most important feature of
both sequences is the swing from a glacial to an inter-
glacial epoch or from the arsis or accentuated part of an
historical pulsation to the thesis or unaccented part. In a
glacial epoch or in the arsis of an historic pulsation,
storms are usually abundant and severe, the mean tem-
perature is lower than usual, snow accumulates in high
5
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26 CLIMATIC CHANGES
latitudes or upon lofty mountains. For example, in the
last such period during the fourteenth century, great
floods and droughts occurred alternately around the
North Sea; it was several times possible to cross th«
Baltic Sea from Germany to Sweden on the ice, and th*
ice of Greenland advanced so much that shore ice caused
the Norsemen to change their sailing route between Ice-
land and the Norse colonies in southern Greenland. At
the same time in low latitudes and in parts of the con-
tinental interior there is a tendency toward diminishd
rainfall and even toward aridity and the formation d
deserts. In Yucatan, for example, a diminution in tropi-
cal rainfall in the fourteenth century seems to have give
the Mayas a last opportunity for a revival of their decaj
ing civilization.
Among the climatic sequences, glacial fluctuations aie
perhaps of the most vital import from the standpoint i
organic evolution ; from the standpoint of human histoij
the same is true of climatic pulsations. Glacial epock
have repeatedly wiped out thousands upon thousands i
species and played a part in the origin of entirely net
types of plants and animals. This is best seen when tic
life of the Pennsylvanian is contrasted with that of tk
Permian. An historic pulsation may wipe out an entiit
civilization and permit a new one to grow up with a radh
cally different character. Hence it is not strange that tk
causes of such climatic phenomena have been discussel
with extraordinary vigor. In few realms of science h*
there been a more imposing or more interesting array d
theories. In this book we shall consider the more impor-
tant of these theories. A new solar or cyclonic hypothesi
and the hypothesis of changes in the form and altitude d
the land will receive the most attention, but the othei
THE VARIABILITY OF CLIMATE 27
chief hypotheses are outlined in the next chapter, and are
frequently referred to throughout the volume.
Between glacial fluctuations and historical pulsations
in duration, but probably less severe than either, come
orbital precessions. These stand in a group by them-
selves and are more akin to seasonal alternations than
to any other type of climatic sequence. They must have
occurred with absolute regularity ever since the earth
began to revolve around the sun in its present elliptical
orbit. Since the orbit is elliptical and since the sun is in
one of the two foci of the ellipse, the earth's distance
from the sun varies. At present the earth is nearest the
sun in the northern winter. Hence the rigor of winter in
the northern hemisphere is mitigated, while that of the
southern hemisphere is increased. In about ten thousand
years this condition will be reversed, and in another ten
thousand the present conditions will return once more.
Such climatic precessions, as we may here call them,
must have occurred unnumbered times in the past, but
they do not appear to have been large enough to leave in
the fossils of the rocks any traces that can be distin-
guished from those of other climatic sequences.
We come now to Bruckner periods and sunspot cycles.
The Bruckner periods have a length of about thirty-three
years. Their existence was suggested at least as long ago
as the days of Sir Francis Bacon, whose statement about
them is quoted on the flyleaf of this book. They have
since been detected by a careful study of the records of
the time of harvest, vintage, the opening of rivers to
navigation, and the rise or fall of lakes like the Caspian
Sea. In his book on Klimaschwankungen seit 1700,
Briickner has collected an uncommonly interesting assort-
ment of facts as to the climate of Europe for more than
two centuries. More recently, by a study of the rate of
28 CLIMATIC CHANGES
growth of trees, Douglass, in his book on Climatic Cycles
and Tree Growth, has carried the subject still further.
In general the nature of the 33-year periods seems to be
identical with that of the 11- or 12- year sunspot cycle,
on the one hand, and of historic pulsations on the other.
For a century observers have noted that the variations
in the weather which everyone notices from year to year
seem to have some relation to sunspots. For generations,
however, the relationship was discussed without leading
to any definite conclusion. The trouble was that the same
change was supposed to take place in all parts of the
world. Hence, when every sort of change was found
somewhere at any given sunspot stage, it seemed as
though there could not be a relationship. Of late years,
however, the matter has become fairly clear. The chief
conclusions are, first, that when sunspots are numerous
the average temperature of the earth ^s surface is lower
than normal. This does not mean that all parts are cooler,
for while certain large areas grow cool, others of less
extent become warm at times of many sunspots. Second,
at times of many sunspots storms are more abundant
than usual, but are also confined somewhat closely to
certain limited tracks so that elsewhere a diminution of
storminess may be noted. This whole question is dis-
cussed so fully in Earth and Sun that it need not detain
us further in this preliminary view of the whole problem
of climate. Suffice it to say that a study of the sunspot
cycle leads to the conclusion that it furnishes a clue to
many of the unsolved problems of the climate of the
past, as well as a key to prediction of the future.
Passing by the seasonal alternations which are fully
explained as the result of the revolution of the earth
around the sun, we may merely point out that, like the
daily vibrations which bring Table 2 to a close, they
THE VARIABILITY OF CLIMATE 29
emphasize the outstanding fact that the main control of
terrestrial climate is the amount of energy received from
the sun. This same principle is illustrated by pleionian
migrations. The term **pleion^' comes from a Greek word
meaning **more.'' It was taken by Arctowski to desig-
nate areas or periods where there is an excess of some
climatic element, such as atmospheric pressure^ rainfall,
or temperature. Even if the effect of the seasons is elimi-
nated, it appears that the course of these various ele-
ments does not run smoothly. As everyone knows, a period
like the autumn of 1920 in the eastern United States may
be unusually warm, while a succeeding period may be
unseasonably cool. These departures from the normal
show a certain rough periodicity. For example, there is
evidence of a period of about twenty-seven days, corre-
sponding to the sun's rotation and formerly supposed to
be due to the moon's revolution which occupies almost
the same length of time. Still other periods appear to
have an average duration of about three months and of
between two and three years. Two remarkable discoveries
have recently been made in respect to such pleions. One
is that a given type of change usually occurs simulta-
neously in a number of well-defined but widely separated
centers, while a change of an opposite character arises
in another equally well-defined, but quite different, set
of centers. In general, areas of high pressure have one
type of change and areas of low pressure the other type.
So systematic are these relationships and so completely
do they harmonize in widely separated parts of the earth,
that it seems certain that they must be due to some out-
side cause, which in all probability can be only the sun.
The second discovery is that pleions, when once formed,
travel irregularly along the earth 's surface. Their paths
have not yet been worked out in detail, but a general
80 CLIMATIC CHANGES
migration seems well established. Because of this, it is
probable that if unusually warm weather prevails in one
part of a continent at a given time, the " thermo-pleion, * '
or excess of heat, will not vanish but will gradually move
away in some particular direction. If we knew the path
that it would follow we might predict the general tem-
perature along its course for some months in advance.
The paths are often irregular, and the pleions frequently
show a tendency to break up or suddenly revive. Prob-
ably this tendency is due to variations in the sun. When
the sun is highly variable, the pleions are numerous and
strong, and extremes of weather are frequent. Taken as
a whole the pleions offer one of the most interesting and
hopeful fields not only for the student of the causes of
climatic variations, but for the man who is interested in
the practical question of long-range weather forecasts.
Like many other climatic phenomena they seem to repre-
sent the combined effect of conditions in the sun and
upon the earth itself.
The last of the climatic sequences which require ex-
planation is the cyclonic vacillations. These are familiar
to everyone, for they are the changes of weather which
occur at intervals of a few days, or a week or two, at all
seasons, in large parts of the United States, Europe,
Japan, and some of the other progressive parts of the
earth. They do not, however, occur with great frequency
in equatorial regions, deserts, and many other regions.
Up to the end of the last century, it was generally sup-
posed that cyclonic storms were purely terrestrial in
origin. Without any adequate investigation it was as-
sumed that all irregularities in the planetary circulation
of the winds arise from an irregular distribution of heat
due to conditions within or upon the earth itself. These
irregularities were supposed to produce cyclonic storms
THE VARIABILITY OF CLIMATE 81
in certain limited belts, but not in most parts of the
world. Today this view is being rapidly modified. Un-
doubtedly, the irregularities due to purely terrestrial
conditions are one of the chief contributory causes of
storms, but it begins to appear that solar variations also
play a pari It has been found, for example, that not
only the mean temperature of the earth 's surface varies
in harmony with the sunspot cycle, but that the frequency
and severity of storms vary in the same way. Moreover,
it has been demonstrated that the sun's radiation is not
constant, but is subject to innumerable variations. This
does not mean that the sun 's general temperature varies,
but merely that at some times heated gases are ejected
rapidly to high levels so that a sudden wave of energy
strikes the earth. Thus, the present tendency is to believe
that the cyclonic variations, the changes of weather
which come and go in such a haphazard, irresponsible
way, are partly due to causes pertaining to the earth
itself and partly to the sun.
From this rapid survey of the types of climatic se-
qnences, it is evident that they may be divided into four
^eat groups. First comes cosmic uniformity, one of the
most marvelous and incomprehensible of all known facts.
We simply have no explanation which is in any respect
adequate. Next come secular progression and geologic
oscillations, two types of change which seem to be due
mainly to purely terrestrial causes, that is, to changes in
the lands, the oceans, and the air. The general tendency
of these changes is toward complexity and diversity, thus
producing progression, but they are subject to frequent
reversals which give rise to oscillations lasting millions
of years. The processes by which the oscillations take
place are fully discussed in this book. Nevertheless, be-
cause they are fairly well understood, they are deferred
82 CLIMATIC CHANGES
until after the third group of sequences has been dis-
cussed. This group includes glacial fluctuations, historic
pulsations, Briickner periods, sunspot cycles, pleionian
migrations, and cyclonic vacillations. The outstanding
fact in regard to all of these is that while they are greaily
modified by purely terrestrial conditions, they seem to
owe their origin to variations in the sun. They form the
chief subject of Earth and Sun and in their larger phases
are the most important topic of this book also. The last
group of sequences includes orbital precessions, seasoiial
alternations, and daily variations. These may be re-
garded as purely solar in origin. Yet their influence, like
that of each of the other groups, is much modified by the
earth *s own conditions. Our main problem is to separate
and explain the two great elements in climatic changes,
— the effects of the sun, on the one hand, and of the earth
on the other.
CHAPTER ni
HYPOTHESES OF CLIMATIC CHANGE
THE next step in onr study of climate is to review
the main hypotheses as to the causes of glada-
tion. These hypotheses apply also to other types
of climatic changes. We shall concentrate on glacial
periods, however, not only because they are the most
dramatic and well-known types of change, but because
they have been more discussed than any other and have
also had great influence on evolution. Moreover, they
stand near the middle of the types of climatic sequences,
and an imderstanding of them does much to explain the
others. In reviewing the various theories we shaU not
attempt to cover all the ground, but shall merely state
the main ideas of the few theories which have had an
important influence upon scientific thought.
The conditions which any satisfactory climatic hy-
pothesis must satisfy are briefly as follows :
(1) Due weight must be given to the fact that changes
of climate are almost certainly due to the combined effect
of a variety of causes, both terrestrial and solar or
cosmic.
(2) Attention must also be paid to both sides in the
long controversy as to whether glaciation is due pri-
marily to a diminution in the earth 's supply of heat or to
a redistribution of the heat through changes in atmos-
pheric and oceanic circulation. At present the great
84 CLIMATIC CHANGES
majority of authorities are on the side of a diminution of
heat, but the other view also deserves study.
(3) A satisfactory hypothesis must explain the fiie-
quent synchronism between two great types of phe-
nomena; first, movements of the earth's crust whereby
continents are uplifted and mountains upheaved; and,
second, great changes of climate which are usually
marked by relatively rapid oscillations from one extreme
to another.
(4) No hypothesis can find acceptance unless it satis-
fies the somewhat exacting requirements of the geolo^cal
record, with its frequent but irregular repetition of long,
mild periods, relatively cool or intermediate periods like
the present, and glacial periods of more or less severity
and perhaps accompanying the more or less widespr4»d
uplifting of continents. At least during the later glacial
periods the hypothesis must explain numerous cUmatic
epochs and stages superposed upon a single general
period of continental upheaval. Moreover, although lis-
torical geology demands cycles of varied duration and
magnitude, it does not furnish evidence of any rij^d
periodicity causing the cycles to be uniform in length or
intensity.
(5) Most important of all, a satisfactory explanation
of climatic changes and crustal deformation must take
account of all the agencies which are now causing similar
phenomena. Whether any other agencies should be con-
sidered is open to question, although the relative im-
portance of existing agencies may have varied.
I. CrolVs Eccentricity Theory. One of the most in-
genious and most carefully elaborated scientific hy-
potheses is CroU's* precessional hypothesis as to ihe
effect of the earth's own motions. So well was this worhed
1 James Croll: Climate and Time, 1876.
HYPOTHESES OF CLIMATIC CHANGES 85
^ out that it was widely accepted for a time and still finds a
place in popular but unscientific books, such as Wells'
Outline of History, and even in scientific works like
Wright's Quaternary Ice Age. The gist of the hypothe-
sis has already been given in connection with the type of
climatic sequence known as orbital precessions. The earth
is 93 million miles away from the sun in January and 97
million in July. The earth's axis **precesses," however,
just as does that of a spi^ming top. Hence arises what is
known as the precession of the equinoxes, that is, a
steady change in the season at which the earth is in peri-
helion, or nearest to the sun. In the course of 21,000 years
the time of perihelion varies from early in January
through the entire twelve months and back to January.
Moreover, the earth's orbit is slightly more elliptical at
certain periods than at others, for the planets sometimes
become bunched so that they all pull the earth in one
direction. Hence, once in about one hundred thousand
years the effect of the elliptical shape of the earth 's orbit
is at a maximum.
GroU argued that these astronomical changes must
alter the earth's climate, especially by their effect on
winds and ocean currents. His elaborate argument con-
tains a vast amount of valuable material. Later investi-
gation, however, seems to have proven the inadequacy of
his hypothesis. In the first place, the supposed cause does
not seem nearly sufficient to produce the observed results.
Second, CroU 's hypothesis demands that glaciation in the
northern and southern hemisphere take place alternately.
A constantly growing collection of facts, however, indi-
cates that glaciation does not occur in the two hemi-
spheres alternately, but at the same time. Third, the
hypothesis calls for the constant and frequent repetition
of glaciation at absolutely regular intervals. The geo-
86 CLIMATIC CHANGES
logical record shows no such regularity, for sometimeB
several glacial epochs follow in relatively close succe^
sion at irregular intervals of perhaps fifty to two hun-
dred thousand years, and thus form a glacial period ; and
then for millions of years there are none. Fourth, the
eccentricity hypothesis provides no adequate explanation
for the glacial stages or subepochs, the historic pulsa-
tions, and the other smaller climatic variations which are
superposed upon glacial epochs and upon one another in
bewildering confusion. In spite of these objections, tkere
can be little question that the eccentricity of the earth's
orbit and the precession of the equinoxes with the result-
ing change in the season of perihelion must have some
climatic effect. Hence CroU's theory deserves a perma-
nent though minor place in any full discussion of the
causes of climatic changes.
II. The Carbon Dioxide Theory. At about the time
that the eccentricity theory was being relegated to a
minor niche, a new theory was being developed which
soon exerted a profound influence upon geological
thought. Chamberlin,^ adopting an idea suggested by
ST. C. Ghamberlin: An attempt to frame a working hypothesis of the
eause of glaeial periods on an atmospheric basis; Jour. Geol., VoL Til,
1899, pp. 545-584, 667-686, 757-787.
T. C. Ghamberlin and B. D. Salisbury: Geology, Vol. II, 1906, pp. 98-
106, 655-677, and VoL III, pp. 432-446.
S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out Bome
investigations on carbon dioxide which have had a pronounced effect on
later conclusions.
F. Freeh adopted Arrhenius' idea and developed it in a paper entitled
Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte
Bendu, Tenth (Mexico) Ck>ngr. GeoL Intern., 1907 (=1908), pp. 299-325.
The exact origin of the carbon dioxide theory has been stated so variously
that it seems worth while to give the exact facts. Prompted by the tag-
geetion of Tyndall that glaciation might be due to depletion of atmospheric
carbon dioxide, Ghamberlin worked up the essentials of his early vien
before he saw any publication from Arrhenius, to* whom the idea has ofUn
been attributed. In 1895 or earlier Ghamberlin began to give the carta
dioxide hypothesis to his students and to discuss it before local scientiie
f
HYPOTHESES OF CLIMATIC CHANGES 87
Tyndall, fired the imagination of geologists by his skill-
ftd exposition of the part played by carbon dioxide in
causing climatic changes. Today this theory is probably
more widely accepted than any other. We have already
seen that the amount of carbon dioxide gas in the at-
mosphere has a decided climatic importance. Moreover,
there can be little doubt that the amount of that gas in
the atmosphere varies from age to age in response to the
extent to which it is set free by volcanoes, consumed by
plants, combined with rocks in the process of weathering,
dissolved in the ocean or locked up in the form of coal
and limestone. The main question is whether such varia-
tions can produce changes so rapid as glacial epochs and
historical pulsations.
Abundant evidence seems to show that the degree to
which the air can be warmed by carbon dioxide is sharply
limited. Humphreys, in his excellent book on the Physics
of the Air, calculates that a layer of carbon dioxide forty
centimeters thick has practically as much blanketing
effect as a layer indefinitely thicker. In other words, forty
centimeters of carbon dioxide, while having no appreci-
bodies. In 1897 he prepared a paper on ''A Group of Hypotheses Bearing
on Climatic Changes," Jour. Geol., Vol. V (1897), to be read at the meeting
of the British Association at Toronto, basing his conclusions on Tyndall's
determination of the eompetencj of carbon dioxide as an absorber of heat
radiated from the earth. He had essentially completed this when a paper by
Arrhenius "On the influence of carbonic acid in the air upon the tem-
perature of the ground," PhiL Mag., 1896, pp. 237-276, first came to his
attention. Chamberlin then changed his conservative, tentative statement of
the functions of carbon dioxide to a more sweeping one based on Arrhenius '
very definite quantitative deductions from Langley's experiments. Both
Langley and Arrhenius were then in the ascendancy of their reputations
and seemingly higher authorities could scarcely have been chosen, nor a
finer combination than experiment and physico-mathematical development.
Arrhenius' deductions were later proved to have been overstrained, while
Langley 's interpretation and even his observations were challenged. Cham-
berlin 's latest views are more like his earlier and more conservative state-
ment.
88 CLIMATIC CHANGES
able effect on sunlight coming toward the earth, would
filter out and thus retain in the atmosphere all the oat-
going terrestrial heat that carbon dioxide is capable of
absorbing. Adding more would be like adding another
filter when the one in* operation has already done all that
that particular kind of filter is capable of doing. Accord-
ing to Humphreys ' calculations, a doubling of the carbon
dioxide in the air would in itself raise the average tem-
perature about 1.3°C. and further carbon dioxide would
have practically no effect. Reducing the present supply
by half would reduce the temperature by essentially the
same amount.
The effect must be greater, however, than would ap-
pear from the figures given above, for any change in
temperature has an effect on the amount of water vapor,
which in turn causes further changes of temperature.
Moreover, as Chamberlin points out, it is not clear
whether Humphreys allows for the fact that when the
40 centimeters of CO2 nearest the earth has been heated
by terrestrial radiation, it in turn radiates half its heat
outward and half inward. The outward half is all ab-
sorbed in the next layer of carbon dioxide, and so on.
The process is much more complex than this, but the end
result is that even the last increment of CO2, that is, the
outermost portions in the upper atmosphere, must ap-
parently absorb an infinitesimally small amount of heat.
This fact, plus the effect of water vapor, would seem to
indicate that a doubling or halving of the amount of COs
would have an effect of more than 1.3 °C. A change of
even 2**C. above or below the present level of the earth's
mean temperature would be of very appreciable climatic
significance, for it is commonly believed that during the
height of the glacial period the mean temperature was
only 5° to 8°C. lower than now.
HYPOTHESES OF CLIMATIC CHANGES 89
Nevertheless, variations in atmospheric carbon dioxide
do not necessarily seem competent to produce the rela-
tively rapid climatic fluctuations of glacial epochs and
historic pulsations as distinguished from the longer
swings of glacial periods and geological eras. In Cham-
berlin's view, as in ours, the elevation of the land, the
modification of the currents of the air and of the ocean,
and all that goes with elevation as a topographic agency
constitute a primary cause of climatic changes. A special
effect of this is the removal of carbon dioxide from the
air by the enhanced processes of weathering. This, as he
carefully states, is a very slow process, and cannot of
itself lead to anything so sudden as the oncoming of
glaciation. But here comes Chamberlin 's most distinctive
contribution to the subject, namely, the hypothesis that
changes in atmospheric temperature arising from varia-
tions in atmospheric carbon dioxide are able to cause a
reversal of the deep-sea oceanic circulation.
According to Chamberlin 's view, the ordinary oceanic
circulation of the greater part of geological time was
the reverse of the present circulation. Warm water de-
scended to the ocean depths in low latitudes, kept its heat
while creeping slowly poleward, and rose in high lati-
tudes producing the warm climate which enabled corals,
for example, to grow in high latitudes. Chamberlin holds
this opinion largely because there seems to him to be no
other reasonable way to accoxmt for the enormously long
warm periods when heat-loving forms of life lived in
what are now polar regions of ice and snow. He explains
this reversed circulation by supposing that an abundance
of atmospheric carbon dioxide, together with a broad
distribution of the oceans, made the atmosphere so warm
that the evaporation in low latitudes was far more rapid
than now. Hence the surface water of the ocean became
40 CLIMATIC CHANGES
a relatively concentrated brine. Such a brine is heavy
and tends to sink, thereby setting up an oceanic circula-
tion the reverse of that which now prevails. At present
the polar waters sink because they are cold and henee
contract. Moreover, when they freeze a certain amount
of salt leaves the ice and thereby increases the salinity
of the surrounding water. Thus the polar water sinks
to the depths of the ocean, its place is taken by wanner
and lighter water from low latitudes which moves pole-
ward along the surface, and at the same time the cold
water of the ocean depths is forced equatorward below
the surface. But if the equatorial waters were so concen-
trated that a steady supply of highly saline water kept
descending to low levels, the direction of the circulation
would have to be reversed. The time when this would
occur would depend upon the delicate balance between
the downward tendencies of the cold polar water and of
the warm saline equatorial water.
Suppose that while such a reversed circulation pre-
vailed, the atmospheric CO2 should be depleted, and the
air cooled so much that the concentration of the equa-
torial waters by evaporation was no longer sufficient to
cause them to sink. A reversal would take place, the
present type of circulation would be inaugurated, and
the whole earth would suffer a chill because the sur-
face of the ocean would become cool. The cool surface-
water would absorb carbon dioxide faster than the pre-
vious warm water had done, for heat drives off gases
from water. This would hasten the cooling of the at-
mosphere still more, not only directly but by diminishing
the supply of atmospheric moisture. The result would be
glaciation. But ultimately the cold waters of the higher
latitudes would absorb all the carbon dioxide they could
hold, the slow equatorward creep would at length permit
HYPOTHESES OF CLIMATIC CHANGES 41
the cold water to rise to the surface in low latitudes.
There the warmth of the equatorial sun and the depleted
supply of carbon dioxide in the air would combine to
cause the water to give up its carbon dioxide once more.
If the atmosphere had been sufficiently depleted by that
time, the rising waters in low latitudes might give up
more carbon dioxide than the cold polar waters absorbed.
Thus the atmospheric supply would increase, the air
would again grow warm, and a tendency toward de-
gladation, or toward an inter-glacial condition would
arise. At such times the oceanic circulation is not sup-
posed to have been reversed, but merely to have been
checked and made slower by the increasing warmth.
Thus inter-glacial conditions like those of today, or even
considerably warmer, are supposed to have been pro-
duced with the present type of circulation.
The emission of carbon dioxide in low latitudes could
not permanently exceed the absorption in high latitudes.
After the present type of circulation was finally estab-
lished, which might take tens of thousands of years, the
two would gradually become equal. Then the conditions
which originally caused the oceanic circulation to be
reversed would again destroy the balance; the atmos-
pheric carbon dioxide would be depleted ; the air would
grow cooler; and the cycle of glaciation would be re-
peated. Each cycle would be shorter than the last, for not
only would the swings diminish Uke those of a pendulum,
but the agencies that were causing the main depletion of
the atmospheric carbon dioxide would diminish in inten-
sity. Finally as the lands became lower through erosion
and submergence, and as the processes of weathering
became correspondingly slow, the air would gradually be
able to accumulate carbon dioxide ; the temperature would
increase ; and at length the oceanic circulation would be
42 CLIMATIC CHANGES
reversed again. When the warm saline waters of low lati-
tudes finally began to sink and to set up a flow of warm
water poleward in the depths of the ocean, a glacial
period would definitely come to an end.
This hypothesis has been so skillfully elaborated, and
contains so many important elements that one can
scarcely study it without profound admiration. We be-
lieve that it is of the utmost value as a step toward tiie
truth, and especially because it emphasizes the great
function of oceanic circulation. Nevertheless, we are
unable to accept it in full for several reasons, which
may here be stated very briefly. Most of them will be dis-
cussed fully in later pages.
(1) While a reversal of the deep-sea circulation would
undoubtedly be of great climatic importance and would
produce a warm climate in high latitudes, we see no
direct evidence of such a reversal. It is equally true that
there is no conclusive evidence against it, and the possi-
bility of a reversal must not be overlooked. There seem,
however, to be other modifications of atmospheric and
oceanic circulation which are able to produce the ob-
served results.
(2) There is much, and we believe conclusive, evidence
that a mere lowering of temperature would not produce
glaciation. What seems to be needed is changes in atmos-
pheric circulation and in precipitation. The carbon
dioxide hypothesis has not been nearly so fully developed
on the meteorological side as in other respects.
(3) The carbon dioxide hypothesis seems to demand
that the oceans should have been almost as saline as now
in the Proterozoic era at the time of the first known
glaciation. Ohamberlin holds that such was the case, but
the constant supply of saline material brought to the
ocean by rivers and the relatively small deposition of
HYPOTHESES OF CLIMATIC CHANGES 48
such material on the sea floor seem to indicate that the
eariy oceans must have been much fresher than those of
today.
(4) The carbon dioxide hypothesis does not attempt
to explain minor climatic fluctuations such as post-glacial
stages and historic pulsations, but these appear to be of
the same nature as glacial epochs, differing only in
degree.
(5) Another reason for hesitation in accepting the
carbon dioxide hypothesis as a full explanation of glacial
fluctuations is the highly complex and non-observational
character of the explanation of the alternation of glacial
and inter-glacial epochs and of their constantly decreas-
ing length.
(6) Most important of all, a study of the variations of
weather and of climate as they are disclosed by present
records and by the historic past suggests that there are
now in action certain other causes which are competent
to explain glaciation without recourse to a process whose
action is beyond the realm of observation.
These considerations lead to the conclusion that the
carbon dioxide hypothesis and the reversal of the oceanic
circulation should be regarded as a tentative rather than
a final explanation of glaciation. Nevertheless, the action
of carbon dioxide seems to be an important factor in pro-
ducing the longer oscillations of climate from one geo-
logical era to another. It probably plays a considerable
part in preparing the way for glacial periods and in
making it possible for other factors to produce the more
rapid changes which have so deeply influenced organic
evolution.
III. The Form of the Land. Another great cause of
climatic change consists of a group of connected phe-
nomena dependent upon movements of the earth 's crust.
44 CLIMATIC CHANGES
As to the climatic potency of changes in the lands there
is practical agreement among students of climatology
and glaciation. That the height and extent of the conti-
nents, the location, size, and orientation of mountain
ranges, and the opening and closing of oceanic gateways
at places like Panama, and the consequent diversion of
oceanic currents, exert a profound effect upon cUniate
can scarcely be questioned. Such changes may be intro-
duced rapidly, but their disappearance is usually slow
compared with the rapid pulsations to which climate has
been subject during historic times and during stages of
glacial retreat and advance, or even in comparison with
the epochs into which the Pleistocene^ Permian, and
perhaps earlier glacial periods have been divided. Hence,
while crustal movements appear to be more important
than the eccentricity of the earth's orbit or the amount of
carbon dioxide in the air, they do not satisfactorily ex-
plain glacial fluctuations, historic pulsations, and espe-
cially the present little cycles of climatic change. All
these changes involve a relatively rapid swing from one
extreme to another, while an upheaval of a continent,
which is at best a slow geologic process, apparently
cannot be undone for a long, long time. Hence such an
upheaval, if acting alone, would lead to a relatively long-
lived climate of a somewhat extreme type. It would help
to explain the long swings, or geologic oscillations be-
tween a mild and uniform climate at one extreme, and a
complex and varied climate at the other, but it would not
explain the rapid climatic pulsations which are closely
associated with great movements of the earth's crust. It
might prepare the way for them, but could not cause
them. That this conclusion is true is borne out by the fact
that vast mountain ranges, like those at the close of the
Jurassic and Cretaceous, are upheaved without bringing
HYPOTHESES OF CLIMATIC CHANGES 46
on glacial climates. Moreover, the marked Permian ice
age follows long after the birth of the Hercynian Moun-
tians and before the rise of others of later Permian
origin.
IV. The Volcanic Hypothesis. In the search for some
cause of climatic change which is highly eflScient and yet
able to vary rapidly and independently, Abbot, Fowle,
Humphreys, and others,' have concluded that volcanic
eruptions are the missing agency. In Physics of the Air,
Humphreys gives a careful study of the effect of vol-
canic dust upon terrestrial temperature. He begins with
a mathematical investigation of the size of dust particles,
and their quantity after certain eruptions. He demon-
strates that the power of such particles to defliect light of
short wave-lengths coming from the sun is perhaps thirty
times more than their power to retain the heat radiated
in long waves from the earth. Hence it is estimated that
if a Krakatoa were to belch forth dust every year or
two, the dust veil might cause a reduction of about 6°C.
in the earth ^s surface temperature. As in every such com-
plicated problem, some of the author's assumptions are
open to question, but this touches their quantitative and
not their qualitative value. It seems certain that if vol-
canic explosions were frequent enough and violent
enough, the temperature of the earth 's surface would be
considerably lowered.
Actual observation supports this theoretical conclu-
sion. Humphreys gathers together and amplifies all that
he and Abbot and Fowle have previously said as to obser-
vations of the sun's thermal radiation by means of the
• C. G. Abbot and P. E. Fowle: Voleanoes and Climate; Smiths. Mise.
GoU., VoL 60, 1913, 24 pp.
W. J. HmnphreyB: Voleanie dost and other factors in the production of
climatic and their possible relation to ice ages; Bull. Mount Weather
Obeervatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air, 1920.
46 CLIMATIC CHANGES
pyrheliometer. This summing np of the relations between
the heat received from the sun, and the occurrence of
explosive volcanic eruptions leaves little room for doubt
that at frequent intervals during the last century and a
half a slight lowering of terrestrial temperature has
actually occurred after great eruptions. Nevertheless, it
does not justify Humphreys' final conclusion that ** phe-
nomena within the earth itself suffice to modify its own
climate, . . . that these and these alone have actually
caused great changes time and again in the geologic
past/' Humphreys sees so clearly the importance of the
purely terrestrial point of view that he unconsciously
slights the cosmic standpoint and ignores the important
solar facts which he himself adduces elsewhere at con-
siderable length.
In addition to this the degree to which the temperature
of the earth as a whole is influenced by volcanic eruptions
is by no means so clear as is the fact that there is some
influence. Arctowski,* for example, has prepared numer-
ous curves showing the march of temperature month
after month for many years. During the period from
1909 to 1913, which includes the great eruption of Katmai
in Alaska, low temperature is found to have prevailed at
the time of the eruption, but, as Arctowski puts it, on the
basis of the curves for 150 stations in all parts of the
world : ' * The supposition that these abnormally low tem-
peratures were due to the veil of volcanic dust produced
by the Katmai eruption of June 6, 1912, is completely out
of the question. If that had been the case, temperature
would have decreased from that date on, whereas it was
decreasing for more than a year before that date. ' '
^H. Arctowski: The Pleionian Cycle of Climatic Fluctuations; Awi .
Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New York
Academy of Sciences, Vol. 24, 1914.
HYPOTHESES OF CLIMATIC CHANGES 47
Koppen,* in his comprehensive study of temperature
for a hundred years, also presents a strong argument
against the idea that volcanic eruptions have an im-
portant place in determining the present temperature of
the earth. A volcanic eruption is a sudden occurrence.
Whatever effect is produced by dust thrown into the air
must occur within a few months, or as soon as the dust
has had an opportunity to be wafted to the region in
question. When the dust arrives, there will be a rapid
drop through the few degrees of temperature which the
dust is supposed to be able to account for, and thereafter
a slow rise of temperature. If volcanic eruptions actually
caused a frequent lowering of terrestrial temperature in
the hundred years studied by Koppen, there should be
more cases where the annual temperature is decidedly
below the normal than where it shows a large departure
in the opposite direction. The contrary is actually the
ease.
A still more important argument is the fact that the
earth is now in an intermediate condition of climate.
Throughout most of geologic time, as we shall see again
and again, the climate of the earth has been milder than
now. Regions like Greenland have not been the seat of
glaciers, but have been the home of types of plants which
now thrive in relatively low latitudes. In other words, the
earth is today only part way from a glacial epoch to what
may be called the normal, mild climate of the earth — a
climate in which the contrast from zone to zone was much
less than now, and the lower air averaged warmer. Hence
it seems impossible to avoid the conclusion that the
cause of glaciation is still operating with considerable
5W. Koppen: t^ber mehrjahrige Perioden der Witterung ins besondere
ozer die Il-jahrige Periode der Temperatur. Also, Lufttemperaturen
Sonnenflecke und Vulcanausbruche; Meteorologische Zeitsehrift, Vol. 7,
1914, pp. 305-328.
y^
48 CLIMATIC CHANGES
although diminished efficiency. But volcanic dust is
obviously not operating to any appreciable extent at
present, for the upper air is almost free from dust a large
part of the time.
Again, as Chamberlin suggests, let it be supposed that
a Exakatoan eruption every two years would produce a
glacial period. Unless the most experienced field workers
on the glacial formations are quite in error, the various
glacial epochs of the Pleistocene glacial period had a
joint duration of at least 150,000 years and perhaps twice
as much. That would require 75,000 Exakatoan eruptions.
But where are the pits and cones of such eruptions?
There has not been time to erode them away since the
Pleistocene glaciation. Their beds of volcanic ash would
presumably be as voluminous as the glacial beds, but
there do not seem to be accumulations df any such size.
Even though the same volcano suffered repeated explo-
sions, it seems impossible to find sufficient fresh volcanic
debris. Moreover, the volcanic hypothesis has not yet
offered any mechanism for systematic glacial variations.
Hence, while the hypothesis is important, we must search
further for the full explanation of glacial fluctuations,
historic pulsations, and the earth's present quasi-gladal
climate.
V. The Hypothesis of Polar Wandering. Another hy-
pothesis, which has some adherents, especially among
geologists, holds that the position of the earth 's axis has
shifted repeatedly during geological times, thus causing
glaciation in regions which are not now polar. Astrophys-
icists, however, are quite sure that no agency could
radically change the relation between the earth and its
axis without likewise altering the orbits of the planets to
a degree that would be easily recognized. Moreover, the
distribution of the centers of glaciation both in the Per-
HYPOTHESES OF CLIMATIC CHANGES 49
mian and Pleistocene periods does not seem to conform
to this hypothesis.
VI. The Thermal Solar Hypothesis. The only other
explanations of the climatic changes of glacial and his-
toric times which now seem to have much standing are
two distinct and almost antagonistic solar hypotheses.
One is the idea that changes in the earth's climate are
dne to variations in the heat emitted by the snn and
hence in the temperature of the earth. The other is the
entirely different idea that climatic changes arise from
solar conditions which cause a redistribution of the
earth* s atmospheric pressure and hence produce changes
in winds, ocean currents, and especially storms. This
second, or *' cyclonic,'' hypothesis is the subject of a book
entitled Earth and Sun, which is to be published as a
companion to the present volume. It will be outlined in
the next chapter. The other, or thermal, hypothesis may
be dismissed briefly. Unquestionably a permanent change
in the amount of heat emitted by the sun would perma-
nently alter the earth's climate. There is absolutely no
evidence, however, of any such change during geologic
time. The evidence as to the earth's cosmic uniformity
and as to secular progression is all against it. Suppose
that for thirty or forty thousand years the sun cooled off
enough so that the earth was as cool as during a glacial
epoch. As gladation is soon succeeded by a mild climate,
some agency would then be needed to raise the sun's
temperature. The impact of a shower of meteorites might
accomplish this, but that would mean a very sudden heat-
ing, such as there is no evidence of in geological history.
In fact, there is far more evidence of sudden cooUng than
of sudden heating. Moreover, it is far beyond the bounds
of probability that such an impact should be repeated
again and again with just such force as to bring the cli-
60 CLIMATIC CHANGES
mate back almost to where it started and yet to allow for
the slight changes which cause secular progression.
Another and equally cogent objection to the thennal form
of solar hypothesis is stated by Humphreys as follows:
**A change of the solar constant obviously alters all sur-
face temperatures by a roughly constant percentage.
Hence a decrease of the heat from the sun would in gen-
eral cause a decrease of the interzonal temperature
gradients ; and this in turn a less vigorous atmospheric
circulation, and a less copious rain or snowfall — exactly
the reverse of the condition, namely, abundant precipita-
tion, most favorable to extensive glaciation. ' '
This brings us to the end of the main hypotheses as to
climatic changes, aside from the solar cyclonic hypothesis
which will be discussed in the next chapter. It appears
that variations in the position of the earth at perihelion
have a real though slight influence in causing cycles with
a length of about 21,000 years. Changes in the carbon
dioxide of the air probably have a more important but
extremely slow influence upon geologic oscillations.
Variations in the size, shape, and height of the continents
are constantly causing all manner of climatic complica-
tions, but do not cause rapid fluctuations and pulsations.
The eruption of volcanic dust appears occasionally to
lower the temperature, but its potency to explain the
complex climatic changes recorded in the rocks has prob-
ably been exaggerated. Finally, although minor changes
in the amount of heat given out by the sun occur con-
stantly and have been demonstrated to have a climatic
effect, there is no evidence that such changes are the main
cause of the climatic phenomena which we are trying to
explain. Nevertheless, in connection with other solar
changes they may be of high importance.
CHAPTER IV
THE SOLAR CYCLONIC HYPOTHESIS
THE progress of science is made up of a vast suc-
cession of hypotheses. The majority die in early
infancy. A few live and are for a time widely
accepted. Then some new hypothesis either destroys them
completely or shows that, while they contain elements of
truth, they are not the whole truth. Li the previous chap-
ter we have discussed a group of hypotheses of this kind,
and have tried to point out fairly their degree of truth so
far as it can yet be determined. Li this chapter we shall
outline still another hypothesis, the relation of which to
present climatic conditions has been fuUy developed in
Earth and Sun; while its relation to the past will be ex-
plained in the present volume. This hypothesis is not
supposed to supersede the others, for so far as they are
true they cannot be superseded. It merely seems to ex-
plain some of the many conditions which the other
hypotheses apparently fail to explain. To suppose that
it will suffer a fate more glorious than its predecessors
would be presumptuous. The best that can be hoped is
that after it has been pruned, enriched, and modified, it
may take its place among the steps which finally lead to
the goal of truth.
In this chapter the new hypothesis will be sketched in
broad outline in order that in the rest of this book the
reader may appreciate the bearing of all that is said.
Details of proof and methods of work will be omitted.
62 CLIMATIC CHANGES
since they are given in Earth and Sun. For the sake of
brevity and clearness the main conclusions will be stated
without the qualifications and exceptions which are fully
explained in that volume. Here it will be necessary to
pass quickly over points which depart radically from ac-
cepted ideas, and which therefore must arouse serious
question in the minds of thoughtful readers. That, how-
ever, is a necessary consequence of the attempt which
this book makes to put the problem of climate in such
form that the argument can be followed by thoughtful
students in any branch of knowledge and not merely by
specialists. Therefore, the specialist can merely be asked
to withhold judgment until he has read all the evidence
as given in Earth and Sun, and then to condemn only
those parts that are wrong and not the whole argument
Without further explanation let us turn to our main
problem. In the realm of climatology the most important
discovery of the last generation is that variations in the
weather depend on variations in the activity of the sun's
atmosphere. The work of the great astronomer. New-
comb, and that of the great climatologist, Koppen, have
shown beyond question that the temperature of the
earth *s surface varies in harmony with variations in the
number and area of sunspots.^ The work of Abbot has
shown that the amount of heat radiated from the sun also
varies, and that in general the variations correspond with
those of the sunspots, although there are exceptions,
especially when the spots are fewest. Here, however,
there at once arises a puzzling paradox. The earth cer-
1 The so-called sunspot numbers to which reference is made again and
again in this book are based on a system devised by Wolf and revised bj
A. Wolf er. The nmnber and size of the spots are both taken into account.
The numbers from 1749 to 1900 may be found in the Monthly Weather
Beyiew for April, 1902, and from 1901 to 1918 in the same journal for
1920.
THE SOLAR CYCLONIC HYPOTHESIS 68
tainly owes its warmth to the sun. Yet when the sun emits
the most energy, that is, when sunspots are most numer-
ous, the earth's surface is coolest. Doubtless the earth
receives more heat than usual at such times, and the
upper air may be warmer than usual. Here we refer only
to the air at the earth's surface.
Another large group of investigators have shown thaij
atmospheric pressure also varies in harmony with the
number of sunspots. Some parts of the earth's surface
have one kind of variation at times of many sunspots and
other parts the reverse. These differences are systematic
and depend largely on whether the region in question
happens to have high atmospheric pressure or low. The
net result is that when sunspots are numerous the
earth's storminess increases, and the atmosphere is
thrown into commotion. This interferes with the stable /
planetary winds, such as the trades of low latitudes and/
the prevailing westerlies of higher latitudes. Instead of
these regular winds and the fair weather which they
biing, there is a tendency toward frequent tropical hurri-
canes in the lower latitudes and toward more frequent
and severe storms of the ordinary type in the latitudes
where the world's most progressive nations now live.
With the change in storminess there naturally goes a
change in rainfall. Not all parts of the world, however, \
have increased storminess and more abundant rainfall \
when sunspots are numerous. Some parts change in the I
opposite way. Thus when the sun's atmosphere is par- I
ticularly disturbed, the contrasts between different parts /
of the earth's surface are increased. For example, the /
northern United States and southern Canada become
more stormy and rainy, as appears in Fig. 2, and the
same is true of the Southwest and along the south Atlan-
tic coast. In a crescent-shaped central area, however.
54 CLIMATIC CHANGES
extending from Wyoming through Missouri to Nova
Scotia, the namber of storms and the amotmt of rainfall
decrease.
The two controlling factors of any climate are the
temperature and the atmospheric pressure, for they de-
termine the winds, the storms, and thus the rainfall. A
study of the temperature seems to show that the peculiar
paradox of a hot sun and a cool earth is due largely to
the increased storminess dnring times of many sunspots.
The earth's surface is heated by the rays of the sun, but
Fig. 2. Storminess at sunspot maodma vs. minima.
(After Kitamer.-}
Based on nine gears' Dearest sunapot minima and nine jears' nearest sun-
spot maxima in tlie three annspot cycles from 1888 to 1918. Heavy shading
indicates excess of stormineas when ennspots are Dumerous. FJgnree indieat*
averase yearly number of atorms by which years of nuLXimiun aonspota
exceed those of minimam sunspots.
THE SOLAR CYCLONIC HYPOTHESIS 65
most of the rays do not in themselves heat the air as
they pass through it. The air gets its heat largely from
the heat absorbed by the water vapor which is intimately
mingled with its lower portions, or from the long heat
waves sent out by the earth after it has been warmed by
the sun. The faster the air moves along the earth's sur-
face the less it becomes heated, and the more heat it takes
away. This sounds like a contradiction, but not to anyone
who has tried to heat a stove in the open air. If the air
is still, the stove rapidly becomes warm and so does the
air around it. If the wind is blowing, the cool air delays
the heating of the stove and prevents the surface from
ever becoming as hot as it would otherwise. That seems
to be what happens on a large scale when sunspots are
numerous. The sun actually sends to the earth more
energy than usual, but the air moves with such unusual
rapidity that it actually cools the earth 's surface a trifle
by carrying the extra heat to high levels where it is lost
into space.
There has been much discussion as to why storms are
numerous when the sun's atmosphere is disturbed. Many
investigators have supposed it was due entirely and
directly to the heating of the earth's surface by the sun.
This, however, needs modification for several reasons.
In the first place, recent investigations show that in a
great many cases changes in barometric pressure precede
changes in temperature and apparently cause them by
altering the winds and producing storms. This is the
opposite of what would happen if the effect of solar heat
upon the earth's surface were the only agency. In the
second place, if storms were due exclusively to variations
in the ordinary solar radiation which comes to the earth
as light and is converted into heat, the solar effect ought
56 CLIMATIC CHANGES
to be most pronounced when the center of the sun's
visible disk is most disturbed. As a matter of fact the
storminess is notably greatest when the edges of the
solar disk are most disturbed. These facts and others lead
to the conclusion that some agency other than heat must
also play some part in producing storminess.
The search for this auxiliary agency raises many diffi-
cult questions which cannot yet be answered. On the
whole the weight of evidence suggests that electrical
phenomena of some kind are involved, although varia-
tions in the amount of ultra-violet light may also be
important. Many investigators have shown that the sun
emits electrons. Hale has proved that the sun, like the
earth, is magnetized. Sunspots also have magnetic fields
the strength of which is often fifty times as great as that
of the sun as a whole. If electrons are sent to the earth,
they must move in curved paths, for they are deflected
by the sun's magnetic field and again by the earth's
magnetic field. The solar deflection may cause their
effects to be greatest when the spots are near the sun's
margin; the terrestrial deflection may cause concentra-
tion in bands roughly concentric with the magnetic poles
of the earth. These conditions correspond with the known
facts.
Farther than this we cannot yet go. The calculations of
Humphreys seem to indicate that the direct electrical
effect of the sun's electrons upon atmospheric pressure
is too small to be of appreciable significance in intensify-
ing storms. On the other hand the peculiar way in which
activity upon the margins of the sun appears to be corre-
lated not only with atmospheric! electricity, but with
barometric pressure, seems to be equally strong evidence
in the other direction. Possibly the sun's electrons and
its electrical waves produce indirect effects by being
THE SOLAR CYCLONIC HYPOTHESIS 67
converted into heat^ or by causing the formation of ozone
and the condensation of water vapor in the upper air.
Any one of these processes would raise the temperature
of the upper air, for the ozone and the water vapor would
be formed there and would tend to act as a blanket to
hold in the earth's heat. But any such change in the tem-
perature of the upper air would influence the lower air
through changes in barometric pressure. These con-
siderations are given here because the thoughtful reader
is likely to inquire how solar activity can influence
storminess. Moreover, at the end of this book we shall
take up certain speculative questions in which an elec-
trical hypothesis will be employed. For the main por-
tions of this book it makes no difference how the sun's
variations influence the earth's atmosphere. The only
essential point is that when the solar atmosphere is active
the storminess of the earth increases, and that is a matter
of direct observation.
Let us now inquire into the relation between the small
cyclonic vacillations of the weather and the types of
climatic changes known as historic pulsations and glacial
fluctuations. One of the most interesting results of recent
investigations is the evidence that sunspot cycles on a
small scale present almost the same phenomena as do
historic pulsations and glacial fluctuations. For instance,
when sunspots are numerous, storminess increases
markedly in a belt near the northern border of the area
of greatest storminess, that is, in southern Canada and
thence across the Atlantic to the North Sea and Scandi-
navia. (See Figs. 2 and 3.) Corresponding with this is the
fact that the evidence as to climatic pulsations in historic
times indicates that regions along this path, for instance
Greenland, the North Sea region, and southern Scandi-
Fig. 3. Relative rainfall at times of increasing and decreasing
sunspots.
HesT? shading, more lain with increaaing spots. Light shading, mors rain with d*-
creasing spots. No data for unshaded areas.
Figures indicate percentagee of the average rainfall by which the rainfall during
periods of increasing spots exceeds or falls short of rainfeJl during periods of decreas-
ing spots. The excess or deficiency is stated in percentagee of the average. Tfaiinfali
data from Walker : Snnspots and Bain fall.
. Relative rainfaU at times of increasing and decreasing
sunspots.
Heavy shading, more rain with increasiiig spots. Light shading, moie rain with de-
creasing spota. No data for nnshaded areas.
FigDrea indicate percentagss of the average rainfall by which the rainfall during
periods of increasing spots exceeds or falls short of rainf all during periods of decreas-
ing spots. The excess or deflcienej is stated in percentages of the average. Bainfall
data from Walker: Bonspots and BaJnfall.
60 CLIMATIC CHANGES
navia, were visited by especially frequent and severe
storms at the climax of each pulsation. Moreover, the
greatest accumulations of ice in the glacial period were
on the poleward border of the general regions where now
the storms appear to increase most at times of solar
activity.
Even more clear is the evidence from other regions
where storms increase at times of many sunspots. One
such region includes the southwestern United States,
while another is the Mediterranean region and the semi-
arid or desert parts of Asia farther east. In these regions
innumerable ruins and other lines of evidence show that
at the climax of each climatic pulsation there was more
storminess and rainfall than at present, just as there
now is when the sun is most active. In still earlier times,
while ice was accumulating farther north, the basins of
these semi-arid regions were filled with lakes whose
strands still remain to tell the tale of much-increased
rainfall and presumable storminess. If we go back still
further in geological times to the Permian glaciation, the
areas where ice accumulated most abundantly appear to
be the regions where tropical hurricanes produce the
greatest rainfall and the greatest lowering of tempera-
ture at times of many sunspots. From these and many
other lines of evidence it seems probable that historic
pulsations and glacial fluctuations are nothing more than
sunspot cycles on a large scale. It is one of the funda-
mental rules of science to reason from the known to the
unknown, from the near to the far, from the present to
the past. Hence it seems advisable to investigate whether
any of the climatic phenomena of the past may have
arisen from an intensification of the solar conditions
which now appear to give rise to similar phenomena on
a small scale.
THE SOLAR CYCLONIC HYPOTHESIS 61
The rest ef this chapter will be devoted to a resume
of certain tentative conclusions which have no bearing
on the main part of this book, but which apply to the
closing chapters. There we shall inquire into the perio-
dicity of the climatic phenomena of geological times, and
shall ask whether there is any reason to suppose that the
sun's activity has exhibited similar periodicity. This
leads to an investigation of the possible causes of dis-
turbances in the sun's atmosphere. It is generally as-
sumed that sunspots, solar prominences, the bright clouds
known as faculae, and other phenomena denoting a per-
turbed state of the solar atmosphere, are due to some
cause within the sun. Yet the limitation of these phe-
nomena, especially the sunspots, to restricted latitudes,
as has been shown in Earth and Sun, does not seem to be
in harmony with an internal solar origin, even though
a banded arrangement may be normal for a rotating
globe. The fairly regular periodicity of the sunspots
seems equally out of harmony with an internal origin.
Again, the solar atmosphere has two kinds of circula-
tion, one the so-called **rice grains,'' and the other
the spots and their attendant phenomena. Now the rice
grains present the appearance that would be expected in
an atmospheric circulation arising from the loss of heat
by the outer part of a gaseous body like the sun. For
these reasons and others numerous good thinkers from
Wolf to Schuster have held that sunspots owe their
periodicity to causes outside the sun. The only possible
cause seems to be the planets, acting either through
gravitation, through forces of an electrical origin, or
through some other agency. Various new investigations
which are describied in Earth and Sun support this con-
clusion. The chief difficulty in accepting it hitherto has
been that although Jupiter, because of its size, would be
62 CLIMATIC CHANGES ^
expected to dominate the sunspot cycle, its period of
11.86 years has not been detected. The sunspot cycle has
appeared to average 11.2 years in length, and has been
called the 11-year cycle. Nevertheless, a new analysis of
the sunspot data shows that when attention is concen-
trated upon the major maxima, which are least subject to
retardation or acceleration by other causes, a periodicity
closely approaching that of Jupiter is evident. Moreover,
when the effects of Jupiter, Saturn, and the other planets
are combined, they produce a highly variable curve which
has an extraordinary resemblance to the sunspot curve.
The method by which the planets influence the sun's
atmosphere is still open to question. It may be through
tides, through the direct effect of gravitation, through
electro-magnetic forces, or in some other way. Whichever
it may be, the result may perhaps be slight differences of
atmospheric pressure upon the sun. Such differences
may set in motion slight whirling movements analogous
to terrestrial storms, and these presumably gather mo-
mentum from the sun's own energy. Since the planet-
ary influences vary in strength because of the continuous
change in the relative distances and positions of the
planets, the sun's atmosphere appears to be swayed by
cyclonic disturbances of varying degrees of severity. The
cyclonic disturbances known as sunspots have been
proved by Hale to become more highly electrified as they
increase in intensity. At the same time hot gases pre-
sumably well up from the lower parts of the solar atmos-
phere and thereby cause the sim to emit more heat. Thus
by one means or another, the earth's atmosphere appears
to be set in commotion and cycles of climate are in-
augurated.
If the preceding reasoning is correct, any disturbance
of the solar atmosphere must have an effect upon the
THE SOLAR CYCLONIC HYPOTHESIS 68
earth's climate. If the disturbance were great enough and
of the right nature it might produce a glacial epoch. The
planets are by no means the only bodies which act upon
the sun, for that body sustains a constantly changing
relation to millions of other celestial bodies of all sizes
up to vast universes, and at all sorts of distances. If the
sun and another star should approach near enough to one
another, it is certain that the solar atmosphere would be
disturbed much more than at present.
Here we must leave the cyclonic hypothesis of climate
and must refer the reader once more to Earth and Sun
for fuller details. In the rest of this book we shall discuss
the nature of the climatic changes of past times and shall
inquire into their relation to the various climatic hypothe-
ses mentioned in the last two chapters. Then we shall
inquire into the possibility that the solar system has ever
been near enough to any of the stars to cause appreciable
disturbances of the solar atmosphere. We shall complete
our study by investigating the vexed question of why
movements of the earth's crust, such as the uplifting of
continents and moimtain chains, have generally occurred
at the same time as great climatic fluctuations. This
would not be so surprising were it not that the climatic
phenomena appear to have consisted of highly complex
cycles while the uplift has been a relatively steady move-
ment in one direction. We shall find some evidence that
the solar disturbances which seem to cause climatic
changes also have a relation to movements of the crust.
CHAPTER V
THE CLIMATE OF HISTORY*
WE are now prepared to consider the climate of
the past. The first period to claim attention is
the few thousand years covered by written
history. Strangely enough, the conditions during this
time are known with less accuracy than are those of
geological periods hundreds of times more remote. Yet
if pronounced changes have occurred since the days of
the ancient Babylonians and since the last of the post-
glacial stages, they are of great importance not only
because of their possible historic effects, but because they
bridge the gap* between the little variations of climate
which are observable during a single lifetime and the
great changes known as glacial epochs. Only by bridging
the gap can we determine whether there is any genetic
relation between the great changes and the small. A full
discussion of the climate of historic times is not here
advisable, for it has been considered in detail in numer-
ous other publications.^ Our most profitable course would
seem to be to consider first the general trend of opinion
and then to take up the chief objections to each of the
main hypotheses.
In the hot debate over this problem during recent
iMuch of this chapter is taken from The Solar Hypothesis of dimatie
Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.
> Ellsworth Huntington: Explorations in Turkestan, 1905; The Pulse of
Asia, 1907; Palestine and Its Transformation, 1911; The Climatic Factor,
1915; World Power and Evolution, 1919.
THE CLIMATE OF HISTORY 65
decades the ideas of geographers seem to have gone
through much the same metamorphosis as have those of
geologists in regard to the climate of far earlier times.
As every geologist well knows, at the dawn of geology
people believed in climatic uniformity — that is, it was
supposed that since the completion of an original creative
act there had been no important changes. This view
quickly disappeared and was superseded by the hypothe-
sis of progressive cooling and drying, an hypothesis
which had much to do with the development of the nebu-
lar hypothesis, and which has in turn been greatly
strengthened by that hypothesis. The discovery of evi-
dence of widespread continental glaciation, however,
necessitated a modification of this view, and succeeding
years have brought to light a constantly increasing num-
ber of glacial, or at least cool, periods distributed
throughout almost the whole of geological time. More-
over, each year, almost, brings new evidence of the great
complexity of glacial periods, epochs, and stages. Thus,
for many decades, geologists have more and more been
led to believe that in spite of surprising uniformity, when
viewed in comparison with the cosmic possibilities, the
climate of the past has been highly unstable from the
viewpoint of organic evolution, and its changes have been
of all degrees of intensity.
Geographers have lately been debating the reality of
historic changes of climate in the same way in which
geologists debated the reality of glacial epochs and
stages. Several hypotheses present themselves but these
may all be grouped under three headings; namely, the
hypotheses of (1) progressive desiccation, (2) climatic
uniformity, and (3) pulsations. The hypothesis of pro-
gressive desiccation has been widely advocated. In many
of the drier portions of the world, especially between 30°
66 CLIMATIC CHANGES
and 40° from the equator, and preeminently in western
and central Asia and in the southwestern United States,
almost innumerable facts seem to indicate that two or
three thousand years ago the climate was distinctly
moister than at present. The evidence includes old lake
strands, the traces of desiccated springs, roads in places
now too dry for caravans, other roads which make de-
tours around dry lake beds where no lakes now exist, and
fragments of dead forests extending over hundreds of
square miles where trees cannot now grow for lack of
water. Still stronger evidence is furnished by ancient
ruins, hundreds of which are located in places which are
now so dry that only the merest fraction of the former
inhabitants could find water. The ruins of Pahnyra, in
the Syrian Desert, show that it must once have been a
city like modem Damascus, with one or two hundred
thousand inhabitants, but its water supply now suffices
for only one or two thousand. All attempts to increase the
water supply have had only a slight effect and the water
is notoriously sulphurous, whereas in the former days,
when it was abundant, it was renowned for its excellence.
Hundreds of pages might be devoted to describing simi-
lar ruins. Some of them are even more remarkable for
their dryness than is Niya, a site in the Tarim Desert of
Chinese Turkestan. Yet there the evidence of desiccation
within 2000 years is so strong that even so careful and
conservative a man as Hann,' pronounces it **uber-
zeugend. ' '
A single quotation from scores that might be used will
iUustratfe the conclusions of some of the most careful
archaeologists.^
s J. Hann: Klimatologie, Vol. 1, 1908, p. 352.
« H. C. Butler: Desert S^ia, the Land of a Lost GiTOization; Qeographi-
cal Review, Feb., 1920, pp. 77-108.
THE CLIMATE OF HISTORY 67
Among the regions which were once popnious and highly
civilized, bnt which are now desert and deserted, there are few
which were more closely connected with the beginnings of our
own civilization than the desert parts of Syria and northern
Arabia. It is only of recent years that the vast extent and great
importance of this lost civilization has been f nlly recognized and
that attempts have been made to reduce the extent of the unex-
plored area and to discover how much of the territory which has
long been known as desert was formerly habitable and inhabited.
The results of the explorations of the last twenty years have been
most astonishing in this regard. It has been found that practi-
cally all of the wide area lying between the coast range of the
eastern Mediterranean and the Euphrates, appearing upon the
maps as the Syrian Desert, an area embracing somewhat more
than 20,000 square miles, was more thickly populated than any
area of similar dimensions in England or in the United States
is today if one excludes the immediate vicinity of the large
modem cities. It has also been discovered that an enormous
desert tract lying to the east of Palestine, stretching eastward
and southward into the country which we know as Arabia, was
also a densely populated country. How far these settled regions
extended in antiquity is still unknown, but the most distant
explorations in these directions have failed to reach the end of
ruins and other signs of former occupation.
The traveler who has crossed the settled, and more or less
populous, coast range of northern Syria and descended into the
narrow fertile valley of the Orontes, encounters in any farther
journey toward the east an irregular range of limestone hills
lying north and south and stretching to the northeast almost
halfway to the Euphrates. These hills are about 2,500 feet high,
rising in occasional peaks from 3,000 to 3,500 feet above sea level.
They are gray and unrelieved by any visible vegetation. On
ascending into the hills the traveler is astonished to find at every
turn remnants of the work of men's hands, paved roads, walls
which divided fields, terrace walls of massive structure. Pres-
ently he comes upon a small deserted and partly ruined town
68 CLIMATIC CHANGES
composed of buildings large and small constructed of beauti-
fully wrought blocks of limestone, all rising out of the barren
rock which forms the ribs of the hills. If he mounts an eminence
in the vicinity, he will be still further astonished to behold
similar ruins lying in all directions. He may count ten or fifteen
or twenty, according to the commanding position of his lookout.
From a distance it is often difficult to belieTC that these are not
inhabited places; but closer inspection reveals that the gentle
hand of time or the rude touch of earthquake has been laid upon
every building. Some of the towns are better preserved than
others; some buildings are quite perfect but for their wooden
roofs which time has removed, others stand in picturesque ruins,
while others still are level with the ground. On a far-off hilltop
stands the ruin of a pagan temple, and crowning some lofty ridge
lie the ruins of a great Christian monastery. Mile after mile of
this barren gray country may be traversed without encountering
a single human being. Day after day may be spent in traveling
from one ruined town to another without seeing any g^reen
thing save a terebinth tree or two standing among the ruins,
which have sent their roots down into earth still preserved in
the foundations of some ancient building. No soil is visible
anywhere except in a few pockets in the rock from which it
could not be washed by the torrential rains of the wet season;
yet every ruin is surrounded with the remains of presses for the
making of oil and wine. Only one oasis has been discovered in
these high plateaus.
Passing eastward from this range of hills, one descends into a
gently rolling country that stretches miles away toward the
Euphrates. At the eastern foot of the hills one finds oneself in a
totally different country, at first quite fertile and dotted with
frequent villages of flat-roofed houses. Here practically all the
remains of ancient times have been destroyed through ages of
building and rebuilding. Beyond this narrow fertile strip the
soil grows drier and more barren, until presently another kind
of desert is reached, an undulating waste of dead soil. Few walls
or towers or arches rise to break the monotony of the unbroken
THE CLIMATE OF HISTORY 69
landscape ; but the earef ul explorer will find on closer examina-
tion that this region was more thickly populated in antiquity
even than the hill country to the west. Every unevenness of the
surface marks the site of a town^ some of them cities of con-
siderable extent.
We may draw certain very definite conclusions as to the
former conditions of the country itself. There was soil upon the
northern hills where none now exists, for the buildings now show
unfinished foundation courses which were not intended to be
seen; the soil in depressions without outlets is deeper than it
formerly was; there are hundreds of olive and wine presses in
localities where no tree or vine could now find footing; and
there are hillsides with ruined terrace walls rising one above
the other with no sign of earth near them. There was also a large
natural water supply. In the north as well as in the south we
find the dry beds of rivers^ streams, and brooks with sand and
pebbles and well-worn rocks but no water in them from one
year's end to the other. We find bridges over these dry streams
and crudely made washing boards along their banks directly
below deserted towns. Many of the bridges span the beds of
streams that seldom or never have water in them and give
clear evidence of the great climatic changes that have taken
place. There are well heads and well houses, and inscriptions
referring to springs; but neither wells nor springs exist today
except in the rarest instances. Many of the houses had their
rock-hewn cisterns, never large enough to have supplied water
for more than a brief period, and corresponding to the cisterns
which most of our recent forefathers had which were for con-
venience rather than for dependence. Some of the towns in
southern Syria were provided with large public reservoirs, but
these are not large enough to have supplied water to their
original populations. The high plateaus were of course without
irrigation; but there are no signs, even in the lower flatter
country, that irrigation was ever practiced ; and canals for this
purpose could not have completely disappeared. There were
forests in the immediate vicinity, forests producing timbers of
great length and thickness ; for in the north and northeast prac-
70 CLIMATIC CHANGES
tically all the buildings had wooden roofs, wooden intermediate
floors, and other features of wood. Costly buildings, such as
temples and churches, employed large wooden beams; but wood
was used in much larger quantities in private dwellings, shops,
stables, and barns. If wood had not been plentiful and cheap —
which means grown near by — ^the builders would have adopted
the building methods of their neighbors in the south, who used
very little wood and developed the most perfect type of lithic
architecture the world has ever seen. And here there exists a
strange anomaly: Northern Syria, where so much wood was
employed in antiquity, is absolutely treeless now; while in the
mountains of southern Syria, where wood must have been
scarce in antiquity to have forced upon the inhabitants an almost
exclusive use of stone, there are still groves of scrub oak and
pine, and travelers of half a century ago reported large forests
of chestnut trees." It is perfectly apparent that large parts of
Syria once had soil and forests and springs and rivers, while it
has none of these now, and that it had a much larger and better
distributed rainfall in ancient times than it has now.
Professor Butler *s careful work is especially interest-
ing because of its contrast to the loose statements of
those who believe in climatic uniformity. So far as I am
aware, no opponent of the hypothesis of climatic changes
has ever even attempted to show by careful statistical
analysis that the ancient water supply of such ruins w^as
no greater than that of the present. The most that has
been done is to suggest that there may have been sources
of water which are now unknown. Of course, this might
be true in a single instance, but it could scarcely be the
case in many hundreds or thousands of ruins.
B This is due to the fact that where these forests occur, in Gilead for
example, the mountains to the west break down, so that the west winds with
water from the Mediterranean are able to reach the inner range without
having lost all their water. It is one of the misfortunes of Syria that its
mountains generally rise so close to the sea that they shut off rainfall from
the interior and cause the rain to fall on slopes too steep for easy cultivation.
THE CLIMATE OF HISTORY 71
Although the arguments in favor of a change of cli-
mate during the last two thousand years seem too strong
to be ignored, their very strength seems to have been a
source of error. A large number of people have jumped
to the conclusion that the change which appears to have
occurred in certain regions occurred everywhere, and
that it consisted of a gradual desiccation.
Many observers, quite as careful as those who believe
in progressive desiccation, point to evidences of aridity
in past times in the very regions where the others find
proof of moisture. Lakes such as the Caspian Sea fell to
such a low level that parts of their present floors were
exposed and were used as sites for buildings whose ruins
are still extant. Elsewhere, for instance in the Tian-Shan
Mountains, irrigation ditches are found in places where
irrigation never seems to be necessary at present. In
SyiS and North Africa during the early centuries of the
Christian era the Eomans showed unparalleled activity
in building great aqueducts and in watering land which
then apparently needed water almost as much as it does
today. Evidence of this sort is abundant and is as con-
vincing as is the evidence of moister conditions in the
past It is admirably set forth, for example, in the com-
prehensive and ably written monograph of Leiter on the
climate of North Africa.* The evidence cited there and
elsewhere has led many authors strongly to advocate the
hypothesis of climatic uniformity. They have done ex-
actly as have the advocates of progressive change, and
have extended their conclusions over the whole world and
over the whole of historic times.
The hypotheses of climatic uniformity and of progres-
• U. Leiter: Die Frage der Klimaanderang waherend geBchiehtlicher Zeit
in Nordafrika. Abhandl. K. K. Geographischen G^esellschaft, Wien, 1909^
p. 143.
72 CLIMATIC CHANGES
sive change both seem to be based on reliable evidence.
They may seem to be diametrically opposed to one
another, but this is only when there is a failure to group
the various lines of evidence according to their dates, and
according to the types of climate in which they happen
to be located. When the facts are properly grouped in
both time and space, it appears that evidence of moist
conditions in the historic Mediterranean lands is found
during certain periods ; for instance, four or five hundred
years before Christ, at the time of Christ, and 1000 A. D-
The other kind of evidence, on the contrary, culminates
at other epochs, such as about 1200 B. C. and in the
seventh and thirteenth centuries after Christ. It is also
found during the interval from the culmination of a moist
epoch to the culmination of a dry one, for at such times
the climate was growing drier and the people were under
stress. This was seemingly the case during the period
from the second to the fourth centuries of our era. North
Africa and Syria must then have been distinctly better
watered than at present, as appears from Butler's vivid
description ; but they were gradually becoming drier, and
the natural effect on a vigorous, competent people like the
Bomans was to cause them to construct numerous engi-
neering works to provide the necessary water.
The considerations which have just been set forth have
led to a third hypothesis, that of pulsatory climatic
changes. According to this, the earth's climate is not
stable, nor does it change uniformly in one direction. It
appears to fluctuate back and forth not only in the little
waves which we see from year to year or decade to
decade, but in much larger waves, which take hundreds of
years or even a thousand. These in turn seem to merge
into and be imposed on the greater waves which form
glacial stages, glacial epochs, and glacial periods. At the
THE CLIMATE OF HISTORY 78
present time there seems to be no way of determining
whether the general tendency is toward aridity or toward
glaciation. The seventh century of our era was appar-
ently the driest time during the historic period — distinctly
drier than the present — ^but the thiri;eenth century was
almost equally dry, and the twelfth or thirteenth before
Christ may have been very dry.
The best test of an hypothesis is actual measurements.
In the case of the pulsatory hypothesis we are fortu-
nately able to apply this test by means of trees. The
growth of vegetation depends on many factors — soil, ex-
posure, wind, sun, temperature, rain, and so forth. In a
dry region the most critical factor in determining how a
tree ^s growth shall vary from year to year is the supply
of moisture during the few months of most rapid
growth.^ The work of Douglass® and others has shown
that in Arizona and California the thickness of the
annual rings affords a reliable indication of the amount
of moisture available during the period of growth. This
is especially true when the growth of several years is
taken as the unit and is compared with the growth of
a similar number of years before or after. Where a long
series of years is used, it is necessary to make corrections
to eliminate the effects of age, but this can be done by
mathematical methods of considerable accuracy. It is
difficult to determine whether the climate at the beginning
tA moet earefal and conyincing study of this problem is embodied in
an article hj J. W. Smith : The Effects of Weather upon the Yield of Com ;
Monthly Weather Beview, Vol. 42, 1914, pp. 78-92. On the basis of the
yield of com in Ohio for 60 years and in other states for shorter periods,
he shoivs that the rainfall of July has almost as much influence on the crop
as has the rainfall of all other months combined. See his Agricultural
Meteorology, New York, 1920.
8 See chapter by A. E. Douglass in The Climatic Factor ; and his book on
Climatic Cycles and Tree-Growth; Carnegie Inst., 1919. Also article by
M. N. Stewart: The Belation of Precipitation to Tree Growth, in the
Monthly Weather Review, Vol. 41, 1913.
74 CLIMATIC CHANGES
and end of a tree 's life was the same, but it is easily pos-
sible to determine whether there have been pulsations
while the tree was making its growth. If a large number
of trees from various parts of a given district all formed
thick rings at a certain period and then formed thin ones
for a hundred years, after which the rings again become
thick, we seem to be safe in concluding that the trees have
lived through a long, dry period. The full reasons for this
belief and details as to the methods of estimating climate
from tree growth are given in The Climatic Factor.
The results set forth in that volume may be summa-
rized as follows : During the years 1911 and 1912, under
the auspices of the Carnegie Institution of Washington,
measurements were made of the thickness of the rings of
growth on the stumps of about 450 sequoia trees in Cali-
fornia. These trees varied in age from 250 to nearly
3250 years. The great majority were over 1000 years of
age, seventy-nine were over 2000 years, and three over
3000. Even where only a few trees are available the
record is surprisingly reliable, except where occasional
accidents occur. Where the number approximates 100,
accidental variations are largely eliminated and we may
accept the record with considerable confidence. Accord-
ingly, we may say that in California we have a fairly
accurate record of the climate for 2000 years and an
approximate record for 1000 years more. The final re-
sults of the measurements of the California trees are
shown in Fig. 4, where the climatic variations for 3000
years in California are indicated by the soUd line. The
high parts of the line indicate rainy conditions, the low
parts, dry. Aji examimation of this curve shows that
during 3000 years there have apparently been climatic
variations more important than any which have taken
place during the past century. In order to bring out the
§
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76 CLIMATIC CHANGES
details more clearly, the more reliable part of the Cali-
fornia curve, from 100 B. C. to the present time, has been
reproduced in Fig. 5. This is identical with the corre-
sponding part of Fig. 4, except that the vertical scale is
three times as great.
The curve of tree growth in California seems to be a
true representation of the general features of climatic
pulsations in the Mediterranean region. This conclusion
was originally based on the resemblance between the
solid line of Fig. 4, representing tree growth, and the
dotted line representing changes of climate in the eastern
Mediterranean region as inferred from the study of ruins
and of history before any work on this subject had been
done in America.* The dotted line is here reproduced for
its historical significance as a stage in the study of cli-
matic changes. If it were to be redrawn today on the
basis of the knowledge acquired in the last twelve years,
it would be much more like the tree curve. For example,
the period of aridity suggested by the dip of the dotted
line about 300 A. D. was based largely on Professor
Butler's data as to the paucity of inscriptions and ruins
dating from that period in Syria. In the recent article,
from which a long quotation has been given, he shows
that later work proves that there is no such paucity. On
the other hand, it has accentuated the marked and sudden
decay in civilization and population which occurred
shortly after 600 A. D. He reached the same conclusion
to which the present authors had come on wholly different
grounds, namely, that the dip in the dotted line about 300
A. D. is not warranted, whereas the dip about 630 A. D.
is extremely important. In similar fashion the work of
9 The dotted line is taken from Palestine and Its Transformation, pp.
327 and 403.
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78 CLIMATIC CHANGES
Stein^^ in central Asia makes it clear that the contrast
between the water supply about 200 B. C. and in the pre-
ceding and following centuries was greater than was
supposed on the basis of the scanty evidence available
when the dotted line of Pig. 4 was drawn in 1910.
Since the curve^ of the California trees is the only con-
tinuous and detailed record yet available for the climate
of the last three thousand years, it deserves most careful
study. It is especially necessary to determine the degree
of accuracy with which the growth of the trees repre-
sents (1) the local rainfall and (2) the rainfall of remote
regions such as Palestine. Perhaps the best way to deter-
mine these matters is the standard mathematical method
of correlation coefficients. If two phenomena vary in
perfect unison, as in the case of the turning of the wheels
and the progress of an automobile when the brakes are
not applied, the correlation coefficient is 1.00, being posi-
tive when the automobile goes forward and negative
when it goes backward. If there is no relation between
two phenomena, as in the case of the number of miles run
by a given automobile each year and the number of
chickens hatched in the same period, the coefficient is
zero. A partial relationship where other factors enter
into the matter is represented by a coefficient between
zero and one, as in the case of the movement of the auto-
mobile and the consumption of gasoline. In this case the
relation is very obvious, but is modified by other factors,
including the roughness and grade of the road, the
amount of traffic, the number of stops, the skill of the
driver, the condition and load of the automobile, and the
state of the weather. Such partial relationships are the
kind for which correlation coefficients are most useful,
for the size of the coefficients shows the relative im-
to M. A. Stein : Bains of Desert Cathajr, London, 1912.
THE CLIMATE OF HISTORY 79
portance of the various factors. A correlation coefiScient
four times the probable error, which can always be deter-
mined by a formula well known to mathematicians, is
generally considered to afford evidence of some kind of
relation between two phenomena. When the ratio between
coefficient and error rises to six, the relationship is re-
garded as strong.
Few people would question that there is a connection
between tree growth and rainfall, especially in a climate
with a long summer dry season like that of OaUfomia.
But the growth of the trees also depends on their posi-
tiojuJ^e-wnount of shading, the temperature, insect pests,
blights, the wind with its tendency to break the branches,
and a number of other factors. Moreover, while rain
commonly favors growth, great extremes are relatively
less helpful than more moderate amounts. Again, the
roots of a tree may tap such deep sources of water that
neither drought nor excessive rain produces much effect
for several years. Hence in comparing the growth of the
huge sequoias with the rainfall we should expect a corre-
lation coefficient high enough to be convincing, but de-
cidedly below 1.00. Unfortunately there is no record of
the rainfall where the sequoias grow, the nearest long
record being that of Sacramento, nearly 200 miles to the
northwest and close to sea level instead of at an altitude
of about 6000 feet.
Applying the method of correlation coefficients to the
annual rainfall of Sacramento and the growth of the
sequoias from 1863 to 1910, we obtain the results shown
in Table 3. The trees of Section A of the table grew in
moderately dry locations although the soil was fairly
deep, a condition which seems to be essential to sequoias.
In this case, as in all the others, the rainfall is reckoned
from July to June, which practically means from October
TABLE 3
CORRELATION COEFFICIENTS BETWEEN
RAINFALL AND GROWTH OF SEQUOIAS
IN CALIFORNIA"
A. Sacbamkmto Rainfall and Obowth ot 18 Skquoias in Dbt
Locations, 1861-1910
(r) («) W
1 year of rainfall —0.059 ±0.096 0.6
2 years of rainfall -|-0.288 ±0.090 3.2
3 years of rainfall +0.570 ±0.066 8.7
4 years of rainfall +0.470 ±0.076 6.2
B. Sacramento Baixpall and Growth of 112 Sequoias Mostly in
Moist Locations, 1861-1910
3 years of rainfall +0.340 ±0.087 3.9
4 years of rainfall +0.371 ±0.084 4.6
5 years of rainfall +0.398 ±0.082 4.9
6 years of rainfall +^-*18 ±0.079 5.3
7 years of rainfall +0.471 ±0.076 6.2
8 years of rainfall (+0.520) ±0.071 7.3
9 years of rainfall +0.575 ±0.065 8.8
10 years of rainfall +0.577 ±0.065 8.8
C. Sacramento Rainfall and Growth of 80 Sequoias in Moist
Locations, 1861-1910
10 years of rainfall +0.605 ±0.062 9.8
D. Annual Sequoia Growth and Bainfall of Precsdino 5 Years
AT Stations on Southern Pacific Railroad
2 -2^ >^ 111^ ell lie ^il
(r) (e) W
Sacramento, 1861-1910 70 19.40 200 +0.398 ±0.081 4.9
Colfax, rl871-1909 2400 48.94 200 +0.122 ±0.113 1.1
Summit, 1871-1909 7000 48.07 200 +0.148 ±0.113 1.3
Truckee, 1871-1909 5800 27.12 200 +0.300 ±0.105 2.9
Boca, 1871-1909 5500 20.34 200 +0,604. ±0.076 8.0
Winnemncca, 1871-1909 4300 8.65 300 +0.492 ±0.089 5.5
11 In the preparation and interpretation of this table the help of Mr.
G. B. Cressey is gratefully acknowledged.
THE CLIMATE OF HISTORY 81
to May, since there is almost no summer rain. Thus the
tree growth in 1861 is compared with the rainfall of the
preceding rainy season, 1860-1861, or of several preced-
ing rainy seasons as the table indicates.
In the first line of Section A a correlation coefficient
of only — 0.056, which is scarcely six>tenths of the prob-
able error, means that there is no appreciable relation
between the rainfall of a given season and the growth
dnring the following spring and summer. The roots of
the sequoias probably penetrate so deeply that the rain
and melted snow of the spring months do not sink down
rapidly enough to influence the trees before the growing
season comes to an end. The precipitation of two pre-
ceding seasons, however, has some effect on the trees, as
appears in the second line of Section A, where the corre-
lation coefficient is +0.288, or 3.2 times the probable
error. When the rainfall of three seasons is taken into
account the coefficient rises to +0.570, or 8.7 times the
probable error, while with four years of rainfall the coeffi-
cient begins to fall off. Thus the growth of these eighteen
sequoias on relatively dry slopes appears to have de-
pended chiefly on the rainfall of the second and third
preceding rainy seasons. The growth in 1900, for
example, depended largely on the rainfall in the rainy
seasons of 1897-1898 and 1898-1899.
Section B of the table shows that with 112 trees, grow-
ing chiefly in moist depressions where the water supply
is at a maximum, the correlation between growth and
rainfall, +0.577 for ten years' rainfall, is even higher
than with the dry trees. The seepage of the underground
water is so slow that not until four years' rainfall is
taken into account is the correlation coefficient more than
four times the probable error. When only the trees grow-
ing in moist locations are employed, the coefficient be-
82 CLIMATIC CHANGES
tween tree growth and the rainfall for ten years rises to
the high figure of +0.605, or 9.8 times the probable error,
as appears in Section C. These figures, as weU as many
others not here published, make it clear that the curve of
sequoia growth from 1861 to 1910 affords a fairly close
indication of the rainfall at Sacramento, provided allow-
ance be made for a delay of three to ten years due to the
fact that the moisture in the soil gradually seeps down
the mountain-sides and only reaches the sequoias after a
considerable interval.
If a rainfall record were available for the place where
the trees actually grow, the relationship would probably
be still closer.
The record at Fresno, for example, bears out this con-
clusion so far as it goes. But as Fresno lies at a low alti-
tude and its rainfall is of essentially the Sacramento
type, its short record is of less value than that of Sacra-
mento. The only rainfall records among the Sierras at
high levels, where the rainfall and temperature are ap-
proximately like those of the sequoia region, are found
along the main line of the Southern Pacific railroad. This
runs from Oakland northeastward seventy miles across
the open plain to Sacramento, then another seventy miles,
as the crow flies, through Colfax and over a high pass
in the Sierras at Summit, next twenty miles or so down
through Truckee to Boca, on the edge of the inland basin
of Nevada, and on northeastward another 160 miles to
Winnemucca, where it turns east toward Ogden and Salt
Lake City. Section D of Table 3 shows the correlation
coefScients between the rainfall along the railroad and
the growth of the sequoias. At Sacramento, which lies
fairly open to winds from the Pacific and thus represents
the general climate of central California, the coefficient
is nearly five times the probable error, thus indicating a
THE CLIMATE OF HISTORY 88
real relation to sequoia growth. Then among the foothills
of the Sierras at Colfax, the coefficient drops till it is
scarcely larger than the probable error. It rises rapidly,
however, as one advances among the mountains, until at
Boca it attains the high figure of +0.604 or eight times
the probable error, and continues high in the dry area
farther east. In other words the growth of the sequoias
is a good indication of the rainfall where the trees grow
and in the dry region farther east
In order to determine the degree to which the sequoia
record represents the rainfall of other regions, let us
select Jerusalem for comparison. The reasons for this
selection are that Jerusalem furnishes the only available
record that satisfies the following necessary conditions :
(1) its record is long enough to be important; (2) it is
located fairly near the latitude of the sequoias, 32''N
versus ST^'N; (3) it is located in a similar type of climate
with winter rains and a long dry summer; (4) it lies well
above sea level (2500 feet) and somewhat back from the
seacoast, thus approximating although by no means
duplicating the condition of the sequoias; and (5) it lies
in a region where the evidence of cUmatic changes during
historic times is strongest. The ideal place for comparison
would be the valley in which grow the cedars of Lebanon.
Those trees resemble the sequoias to an extraordinary
degree, not only in their location, but in their great age.
Some day it will be most interesting to compare the
growth of these two famous groups of old trees.
The correlation coefficients for the sequoia growth
and the rainfall at Jerusalem are given in Section A,
Table 4. They are so high and so consistent that they
scarcely leave room for doubt that where a hundred or
more sequoias are employed, as in Fig. 5, their curve of
growth affords a good indication of the fluctuations of
J- ^
TABLE 4
CORRELATION COEFFICIENTS BETWEEN
RAINFALL RECORDS IN CALI-
FORNIA AND JERUSALEM
A. Jkbusalbm Bainfall roB 3 Yiabs and Yasious (teoups of
Se<)U0U8"
If 1 1 IIP
OS Alt B3SS,«
(r) (e) W
11 trees measured by Douglass 4-0.453 ±0.078 5.8
.80 trees, moist locations, Qroups lA,
IIA, IIIA, VA +0.500 ±0.073 6.8
101 trees, 69 in moist locations, 32 in
dry, I, II, m +0.616 ±0.061 10.1
112 trees, 80 in moist locations, 32 in
dry, I, II, III, V +0.675 ±0.053 12.7
B. Baintall at Jeeusalbm and at Stations in Galipoknu and
Nevada
4 S ytaT9 ^ 4 6 ytan »
^ "tsS "S^ tiS
3 • -sl xl -^l I
II S 68 sill II silt
(r) («) (r) W
Sacramento, 70 1861-1910 +0.386 4.7 +0.352 4.2
Colfax, 2400 1871-1909 +0.311 3.1 +0.308 3.0
Summit, 7000 1871-1909 +0.099 0.9 +0.248 2.3
Truckee, 5800 1871-1909 +0.229 2.2 +0.337 3.3
•Boca, 5500 1871-1909 +0.482 6.4 +0.617 8.6
Winnemucca, 4300 1871-1909 +^.235 2.2 \^0.2tQ 2.4
San Bernardino, 1050 1871-1909 +0.275 2.7 +0.177 1.8
C. Rainfall fob 3 Ysabs at Califoknia and Nevada Stations,
1871-1909
Sl S85l
(r) W
Sacramento and San Bernardino +0.663 10.7
San Bernardino and Winnemucca +^-^^^ ^-^
IS For the tree data used in these comparisons, see The Climatic Factor,
p. 328, and A. E. Douglass: Climatic Cycles and Tree Growth, p. 123.
• One year interx>olated.
THE CLIMATE OF HISTORY 86
climate in western Asia. The high coefficient for the
eleven trees measured by Douglass suggests that where
the number of trees falls as low as ten, as in the part of
Fig. 4 from 710 to 840 B. C, the relation between tree
growth and rainfall is still close even when only one
year 's growth is considered. Where the unit is ten years
of growth, as in Figs. 4 and 5, the accuracy of the tree
curve as a measure of rainfall is much greater than when
a single year is used as in Table 4. When the unit is
raised to thirty years, as in the smoothed part of Fig. 4
previous to 240 B. C, even four trees, as from 960 to
1070, probably give a fair approximation to the general
changes in rainfall, while a single tree prior to 1110 B. C.
gives a rough indication.
Table 4 shows a peculiar feature in the fact that the
correlations of Section A between tree growth and the
rainfall of Jerusalem are decidedly higher than those
between the rainfall in the two regions. Only at Sacra-
mento and Boca are the rainfall coefficients high enough
to be conclusive. This, however, is not surprising, for
even between Sacramento and San Bernardino, only 400
miles apart, the correlation coefficient for the rainfall
by three-year periods is only 10.7 times the probable
error, as appears in Section C of Table 4, while between
San Bernardino and Winnemucca 500 miles away, the
corresponding figure drops to 2.8. It must be remem-
bered that in some respects the growth of the sequoias is
a much better record of rainfall than are the records kept
by man. The human record is based on the amount of
water caught by a little gauge a few inches in diameter.
Every gust of wind detracts from the accuracy of the
record ; a mile away the rainfall may be double what it
is at the gauge. Each sequoia, on the other hand, draws
its moisture from an area thousands of times as large as
86 CLIMATIC CHANGES
a rain gauge. Moreover, the trees on which Figs. 4 and 5
are based were scattered over an area fifty miles long
and several hundred square miles in extent. Hence they
represent the summation of the rainfall over an area
millions of times as large as that of a rain gauge. This
fact and the large correlation coefficients between sequoia
growth and Jerusalem rainfall should be considered in
connection with the fact that all the coefficients between
the rainfall of California and Nevada and that of Jeru-
salem are positive. K full records of the complete rainfall
of California and Nevada on the one hand and of the
eastern Mediterranean region on the other were available
for a long period, they would probably agree closely.
Just how widely the sequoias can be used as a measure
of the climate of the past is not yet certain. In some
regions, as will shortly be explained, the climatic changes
seem to have been of an opposite character from those
of California. In others the Californian or eastern Medi-
terranean type of change seems sometimes to prevail but
is not always evident. For example, at Malta the rainfall
today shows a distinct relation to that of Jerusalem and
to the growth of the sequoias. But the correlation coeffi-
cient between the rainfall of eight-year periods at Naples,
a little farther north, and the growth of the sequoias at
the end of the periods is — 0.132, or only 1.4 times the
probable error and much too small to be significant. This
is in harmony with the fact that although Naples has
summer droughts, they are not so pronounced as in Cali-
fornia and Palestine, and the prevalence of storms is
much greater. Jerusalem receives only 8 per cent of its
rain in the seven months from April to October, and
Sacramento 13, while Malta receives 31 per cent and
Naples 43. Nevertheless, there is some evidence that in
the past the climatic fluctuations of southern Italy fol-
THE CLIMATE OF HISTORY 87
lowed nearly the same course as those of California and
Palestine. This apparent discrepancy seems to be ex-
plained by our previous conclusion that changes of cli-
mate are due largely to a shifting of storm tracks. When
sunspots are numerous the storms which now prevail in
northern Italy seem to be shifted southward and traverse
the Mediterranean to Palestine just as similar storms
are shifted southward in the United States. This perhaps
accounts for the agreement between the sequoia curve
and the agricultural and social history of Rome from
about 400 B. C. to 100 A. D., as explained in World Power
and Evolution. For our present purposes, however, the
main point is that since rainfall records have been kept
the fluctuations of climate indicated by the growth of the
sequoias have agreed closely with fluctuations in the
rainfall of the eastern Mediterranean region. Presumably
the same was true in the past. In that case, the sequoia
curve not only is a good indication of climatic changes or
pulsations in regions of similar climate, but may serve
as a guide to coincident but different changes in regions
of other types.
An enormous body of other evidence points to the same
conclusion. It indicates that while the average climate
of the present is drier than that of the past in regions
having the Mediterranean type of winter rains and
summer droughts, there have been pronounced pulsations
during historic times so that at certain times there has
actually been greater aridity than at present. This, con-
clusion is so important that it seems advisable to examine
the only important arguments that have been raised
against it, especially against the idea that the general
rainfall of the eastern Mediterranean was greater in the
historic past than at present. The first objection is the
unquestionable fact that droughts and famines have
88 CLIMATIC CHANGES
occurred at periods which seem on other evidence to have
been moister than the present. This argoment has been
much used, but it seems to have little force. If the rain-
fall of a given region averages thirty inches and varies
from fifteen to forty-five, a famine will ensue if the rain-
fall drops for a few years to the lower limit and does not
rise much above twenty for a few years. If the climate of
the place changes during the course of centuries, so that
the rainfall averages only twenty inches, and ranges
from seven to thirty-five, famine will again ensue if the
rainfall remains near ten inches for a few years. The
ravages of the first famine might be as bad as those of
the second. They might even be worse, because when the
rainfall is larger the population is likely to be greater
and the distress due to scarcity of food would affect a
larger number of people. Hence historic records of
famines and droughts do not indicate that the climate
was either drier or moister than at present. They merely
show that at the time in question the climate was drier
than the normal for that particular period.
The second objection is that deserts existed in the past
much as at present. This is not a real objection, however,
for, as we shall see more fully, some parts of the world
suffer one kind of change and others quite the opposite.
Moreover, deserts have always existed, and when we talk
of a change in their climate we merely mean that their
boundaries have shifted. A concrete example of the mis-
taken use of ancient dryness as proof of climatic uni-
formity is illustrated by the march of Alexander from
India to Mesopotamia. Hedin gives an excellent presen-
tation of the case in the second volume of his Overland
to India. He shows conclusively that Alexander's army
suffered terribly from lack of water and provisions. This
certainly proves that the climate was dry, but it by no
THE CLIMATE OF HISTORY
89
means indicates that there has been no change from the
past to the present. We do not know whether Alexander's
march took place during an especially dry or an espe-
cially wet year. In a desert region like Makran, in
sonthem Persia and Beluchistan^ where the chief diffi-
culties occurred, the rainfall varies greatly from year to
year. We have no records from Makran, but the condi-
tions there are closely similar to those of southern
Arizona and New Mexico. In 1885 and 1905 the rainfall
for five stations in that region was as follows :
Mean rainfaU dur-
1885
1906
ino period gince
obiervatione
began
Tuma, Arizona,
2.72
11.41
3.13
Phoenix, Arizona,
3.77
19.73
7.27
Tucson, Arizona,
5.26
24.17
11.66
Lordsburg, New Meadco,
3.99
19.50
8.62
El Paso, Texas (on New
Mexico border).
Average,
7.31
4.61
17.80
9.06
7.95
18.52
These stations are distributed over an area nearly 500
miles east and west. Manifestly a traveler who spent the
year 1885 in that region would have had much more diffi-
culty in finding water and forage than one who traveled
in the same places in 1905. During 1885 the rainfall was
42 per cent less than the average, and during 1905 it was
134 per cent more than the average. Let us suppose, for
the sake of argument, that the average rainfall of south-
eastern Persia is six inches today and was ten inches in
the days of Alexander. If the rainfall from year to year
varied as much in the past in Persia as it does now in
New Mexico and Arizona, the rainfall during an ancient
90 CLIMATIC CHANGES
dry year, corresponding in character to 1885, would have
been about 5.75 inches. On the other hand, if we suppose
that the rainfall then averaged less than at present, — ^let
us say four inches, — a wet year corresponding to 1905 in
the American deserts might have had a rainfall of about
ten inches. This being the case, it is clear that our esti-
mate of what Alexander's march shows as to climate
must depend largely on whether 325 B. C. was a wet year
or a dry year. Inasmuch as we know nothing about this,
we must fall back on the fact that a large army accom-
plished a journey in a place where today even a small
caravan usually finds great difficulty in procuring forage
and water. Moreover, elephants were taken 180 miles
across what is now an ahnost waterless desert, and yet
the old historians make no comment on such a feat which
today would be practically impossible. These things seem
more in harmony with a change of climate than with
uniformity. Nevertheless, it is not safe to place much
reliance on them except when they are taken in con-
junction with other evidence, such as the numerous ruins,
which show that Makran was once far more densely
populated than now seems possible. Taken by itself, such
incidents as AJexander's march cannot safely be used
either as an argument for or against changes of climate.
The third and strongest objection to any hypothesis
of climatic changes during historic times is based on
vegetation. The whole question is admirably set forth by
J. W. Gregory," who gives not only his own results, but
those of the ablest scholars who have preceded him. His
conclusions are important because they represent one of
the few cases where a definite statistical attempt has been
made to prove the exact condition of the climate of the
IS J. W. Gregory: Is the Earth Drying XTpf Geog. Jour., Vol. 43, 1914,
pp. 148-172 and 293-318.
THE CLIMATE OF HISTORY 91
past. After stating various less important reasons for
believing that the climate of Palestine has not changed,
he discusses vegetation. The following quotation indi-
cates his line of thought. A sentence near the beginning
is italicized in order to call attention to the importance
which Gregory and others lay on this particular kind of
evidence :
Some more certain test is necessary than the general con-
clusions which can be based upon the historical and geographical
evidence of the Bible. In the absence of rain gauge and thermo-
metric records, the most precise test of climate is given by the
vegetation; and fortunately the palm affords a very delicate test
of the past climate of Palestine and the eastern Mediterranean.
• . . The date palm has three limits of growth which are deter-
mined by temperature; thus it does not reach full maturity or
produce ripe fruit of good quality below the mean annual tem-
perature of 69°F. The isothermal of 69^ crosses southern Algeria
near Biskra; it touches the northern coasts of Cyrenaica near
Dema and passes Egypt near the mouth of the Nile, and then
bends northward along the coast lands of Palestine.
To the north of this line the date palm grows and produces
fruit, which only ripens occasionally, and its quality deteriorates
as the temperature falls below 69^. Between the isotherms of
68^ and 64^, limits which include northern Algeria, most of
Sicily, Malta, the southern parts of Greece and northern Syria,
the dates produced are so unripe that they are not edible. In the
next cooler zone, north of the isotherm of 62"", which enters
Europe in southwestern Portugal, passes through Sardinia,
enters Italy near Naples, crosses northern Greece and Asia
Minor to the east of Smyrna, the date palm is grown only for
its foliage, since it does not fruit.
Hence at Benghazi, on the north African coast, the date palm
is fertile, but produces fruit of poor quality. In Sicily and at
Algiers the fruit ripens occasionally and at Rome and Nice the
palm is grown only as an ornamental tree.
92 CLIMATIC CHANGES
The date palm therefore affords a test of variations in mean
annual temperature of three grades between 62'' and 69^.
This test shows that the mean annual temperature of Palestine
has not altered since Old Testament times. The palm tree now
grows dates on the coast of Palestine and in the deep depression
around the Dead Sea^ but it does not produce fruit on the high-
lands of Judea. Its distribution in ancient times, as far as we
can judge from the Bible, was exactly the same. It grew at
''Jericho, the city of palm trees" (Deut. xxxiv: 3 and 2 Chron.
xxviii: 15), and at Engedi, on the western shore of the Dead
Sea (2 Chron. xx:2; Sirach xxiy:14); and though the palm
does not still live at Jericho — ^the last apparently died in 1838 —
its disappearance must be due to neglect, for the only climatic
change that would explain it would be an increase in cold or
moisture. In olden times the date palm certainly grew on the
highlands of Palestine; but apparently it never produced fruit
there, for the Bible references to the palm are to its beauty and
erect growth: "The righteous shall flourish like the palm" (Ps.
xcii: 12) ; ''They are upright as the palm tree" (Jer. x: 5) ;
"Thy stature is like to a palm tree" (Cant, vii: 7). It is used as
a symbol of victory (Rev. vii: 9), but never praised as a source
of food.
Dates are not once referred to in the text of the Bible, but
according to the marginal notes the word translated "honey" in
2 Chron. xxxi : 5 may mean dates. . . .
It appears, therefore, that the date palm had essentially the
same distribution in Palestine in Old Testament times as it has
now; and hence we may infer that the mean temperature was
then the same as now. If the climate had been moister and cooler,
the date could not have flourished at Jericho. If it had been
warmer, the palms would have grown freely at higher levels and
Jericho would not have held its distinction as the city of palm
trees.**
In the main Gregory's conclusions seem to be well
grounded, although even according to his data a change
1* Geog. Jour., Vol. 43, pp. 159161.
THE CLIMATE OF HISTORY 98
of 2"* or 3° in mean temperature would be perfectly
feasible. It will be noticed, however, that they apply to
te mpera ture and not tojaiaf all. They merely prove that
two thousand years ago the mean temperature of Pales-
tine and the neighboring regions was not appreciably dif-
ferent from what it is today. This, however, is in no sense
out of harmony with the hypothesis of climatic pulsa-
tions. Students of glaciation believe that during the last
glacial epoch the mean temperature of the earth as a
whole was only 5° or 6°C. lower than at present. If the
difference between the climate of today and of the time of
Christ is a tenth as great as the difference between the
climate of today and that which prevailed at the culmina-
tion of the last glacial epoch, the change in two thousand
years has been of large dimensions. Yet this would re-
quire a rise of only half a degree Centigrade in the mean
temperature of Palestine. Manifestly, so slight a change
would scarcely be detectable in the vegetation.
The slightness of changes in mean temperature as com-
pared with changes in rainfall may be judged from a
comparison of wet and dry years in various regions. For
example, at Berlin between 1866 and 1905 the ten most
rainy years had an average precipitation of 670 mm. and
a mean temperature of 9.15"* C. On the other hand, the ten
years of least rainfall had an average of 483 mm. and a
mean temperature of 9.35°. In other words, a difference
of 137 mm., or 39 per cent, in rainfall was accompanied
by a difference of only 0.2° C. in temperature. Such con-
trasts between the variability of mean rainfall and mean
temperature are observable not only when individual
years are selected, but when much longer periods are
taken. For instance, in the western Gulf region of the
United States the two inland stations of Vicksburg, Mis-
sissippi, and Shreveport, Louisiana, and the two mari-
94 CLIMATIC CHANGES
time stations of New Orleans, Louisiana, and Galveston,
Texas, lie at the margins of an area about 400 miles long.
During the ten years from 1875 to 1884 their rainfall
averaged 59.4 inches," while during the ten years from
1890 to 1899 it averaged only 42.4 inches. Even in a
region so well watered as the Gulf States, such a change
— 40 per cent more in the first decade than in the second
— ^is important, and in drier regions it would have a great
effect on habitability. Yet in spite of the magnitude of
the change the mean temperature was not appreciably
different, the average for the four stations being 67.36^F.
during the more rainy decade and 66.94° F. during the
less rainy decade — a difference of only 0.42**F. It is worth
noticing that in this case the wetter period was also the
warmer, whereas in Berlin it was the cooler. This is
probably because a large part of the moisture of the Gulf
States is brought by winds having a southerly com-
ponent. Similar relationships are apparent in other
places. We select Jerusalem because we have been dis-
cussing Palestine. At the time of writing, the data avail-
able in the Quarterly Journal of the Palestine Explora-
tion Fund cover the years from 1882-1899 and 1903-1909.
Among these twenty-five years the thirteen which had
most rain had an average of 34.1 inches and a tempera-
ture of 62.04°F. The twelve with least rain had 24.4 inches
and a temperature of 62.44**. A difference of 40 per cent
in rainfall was accompanied by a difference of only
0.4'^F. in temperature.
The facts set forth in the preceding paragraphs seem
to show that extensive changes in precipitation and
storminess can take place without appreciable changes of
mean temperature. If such changed conditions can per-
is See A. J. Henry : Secular Variation of Precipitation in the Unit«d
States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.
THE CLIMATE OF HISTORY 95
sist for ten years, as in one of our examples, there is no
logical reason why they cannot persist for a hundred or
a thousand. The evidence of changes in climate during the
historic period seems to suggest changes in precipitation
much more than in temperature. Hence the strongest of
all the arguments against historic changes of climate
seems to be of relatively little weight, and the pulsatory
hypothesis seems to be in accord with all the known facts.
Before the true nature of climatic changes, whether
historic or geologic, can be rightly understood, another
point needs emphasis. When the pulsatory hypothesis
was first framed, it fell into the same error as the hy-
pothese, of nmfonnity and of progressive ehange-ehTt
is, the assumption was made that the whole world is
either growing drier or moister with each pulsation. A
study of the ruins of Yucatan, in 1912, and of Guatemala,
in 1913, as is explained in The Climatic Factor, has led to
the conclusion that the climate of those regions has
changed in the opposite way from the changes which
appear to have taken place in the desert regions farther
south. These Maya ruins in Central America are in many
cases located in regions of such heavy rainfall, such dense
forests, and such malignant fevers that habitation is now
practically impossible. The land cannot be cultivated
except in especially favorable places. The people are
terribly weakened by disease and are among the lowest
in Central America. Only a hundred miles from the un-
healthful forests we find healthful areas, such as the
coasts of Yucatan and the plateau of Guatemala. Here
the vast majority of the population is gathered, the large
towns are located, and the only progressive people are
found. Nevertheless, in the past the region of the forests
was the home of by far the most progressive people who
are ever known to have lived in America previous to the
96 CLIMATIC CHANGES
days of Columbus. They alone brought to high perfection
the art of sculpture ; they were the only American people
who invented the art of writing. It seems scarcely credi-
ble that such a people would have lived in the worst pos-
sible habitat when far more favored regions were close
at hand. Therefore it seems as if the climate of eastern
Guatemala and Yucatan must have been relatively dry
at some past time. The Maya chronology and traditions
indicate that this was probably at the same time when
moister conditions apparently prevailed in the subarid
or desert portions of the United States and Asia. Fig. 3
shows that today at times of many sunspots there is
a similar opposition between a tendency toward stormi-
ness and rain in subtropical regions and toward aridity
in low latitudes near the heat equator.
Thus our final conclusion is that during historic times
there have been pulsatory changes of climate. These
changes have been of the same type in regions having
similar kinds of dimate, but of different and sometimes
opposite types in places having diverse climates. As to
the cause of the pulsations, they cannot have been due to
the precession of the equinoxes nor apparently to any
allied astronomical cause, for the time intervals are too
short and too irregular. They cannot have been due to
changes in the percentage of carbon dioxide in the atmos-
phere, for not even the strongest believers in the climatic
efficacy of that gas hold that its amount could fluctuate in
any such violent way as would be necessary to explain
the pulsations shown in the California curve of tree
growth. Volcanic activity seems more probable as at least
a partial cause, and it would be worth while to investigate
the matter more fully. Nevertheless, it can apparently
be only a minor cause. In the first place, the main effect
of a cloud of dust is to alter the temperature, but
THE CLIMATE OF HISTORY 97
Gregory 's summary of the palm and the vine shows that
variations in temperature are apparently of very slight
importance during historic times. Again, ruins on the
bottoms of enclosed salt lakes, old beaches now under the
water, and signs of irrigation ditches where none are now
needed indicate a climate drier than the present. Vol-
canic dust, however, cannot account for such a condi-
tion, for at present the air seems to be practically free
from such dust for long periods. Thus we now experience
the greatest extreme which the volcanic hypothesis per-
mits in one direction, but there have been greater ex-
tremes in the same direction. The thermal solar hypothe-
sis is likewise unable to explain the observed phenomena,
for neither it nor the volcanic hypothesis offers any expla-
nation of why the climate varies in one way in Medi-
terranean climates and in an opposite way in regions
near the heat equator.
This leaves the cyclonic hypothesis. It seems to fit the
facts, for variations in cyclonic storms cause some
regions to be moister and others drier than usual. At the
same time the variations in temperature are slight, and
are apparently different in different regions, some places
growing warm when others grow cool. In the next chap-
ter we shall study this matter more fully, for it can best
be appreciated by examining the course of events in a
sp«Z century.
CHAPTER VI
THE CLIMATIC STRESS OF THE FOURTEENTH
CENTURY
IN order to give concreteness to our picture of the
cUmatic pulsations of historic times let us take a
specific period and see how its changes of climate
were distributed over the globe and how they are related
to the little changes which now take place in the sunspot
cycle. We will take the fourteenth century of the Chris-
tian era, especially the first half. This period is chosen
because it is the last and hence the best known of the
times when the climate of the earth seems to have taken
a considerable swing toward the conditions which now
prevail when the sun is most active, and which, if inten-
sified, would apparently lead to glaciation. It has already
been discussed in World Power and Evolution, but its
importance and the fact that new evidence is constantly
coming to light warrant a fuller discussion.
To begin with Europe ; according to the careful account
of Pettersson* the fourteenth century shows
a record of extreme climatic variations. In the cold winters the
rivers Rhine, Danube, Thames, and Po were frozen for weeks
and months. On these cold winters there followed violent floods,
so that the rivers mentioned inundated their valleys. Such floods
are recorded in 55 summers in the 14th century. There is, of
1 0. Pettersson : The connection between hydrographical and meteorologi-
cal phenomena; Quarterly Journal of the Boyal Meteorological Society, VoL
38, pp. 174-175.
STRESS OF FOURTEENTH CENTURY 99
course, nothing astonishing in the fact that the inundations of
the great rivers of Europe were more devastating 600 to 700
years ago than in our days, when the flow of the rivers has been
regulated by canals, locks, etc. ; but still the inundations in the
13th and 14th centuries must have surpassed everything of that
kind which has occurred since then. In 1342 the waters of the
Rhine rose so high that they inundated the city of Mayence and
the Cathedral ''usque ad cingulum hominis." The walls of
Cologne were flooded so that they could be passed by boats in
July. This occurred also in 1374 in the midst of the month of
February, which is of course an unusual season for disasters of
the kind. Again in other years the drought was so intense that
the same rivers, the Danube, Rhine, and others, nearly dried up,
and the Rhine could be forded at Cologne. This happened at least
twice in the same century. There is one exceptional summer of
such evil record that centuries afterwards it was spoken of as
''the old hot summer of 1357."
Pettersson goes on to speak of two oceanic phenomena
on which the old chronicles lay greater stress than on
all others :
The first [is] the great storm-floods on the coast of the North
Sea and the Baltic, which occurred so frequently that not less
than nineteen floods of a destructiveness unparalleled in later
times are recorded from the 14th century. The coastline of the
North Sea was completely altered by these floods. Thus on
January 16, 1300, half of the island Heligoland and many other
islands were engulfed by the sea. The same fate overtook the
island of Borkum, torn into several islands by the storm-flood of
January 16, which remoulded the Frisian Islands into their
present shape, when also Wendingstadt, on the island of Sylt,
and Thiryu parishes were engulfed. This flood is known under
the name of "the great man-drowning." The coasts of the Baltic
also were exposed to storm-floods of unparalleled violence. On
November 1, 1304, the island of Ruden was torn asunder from
Rugen by the force of the waves. Time does not allow me to
dwell upon individual disasters of this kind, but it will be well
100 CLIMATIC CHANGES
to note that of the nineteen great floods on record eighteen
occurred in the cold season between the antunmal and yemal
equinoxes.
The second remarkable phenomenon mentioned by the chron-
icles is the freezing of the entire Baltic, which occurred many
times during the cold winters of these centuries. On such occsr
sions it was possible to travel with carriages over the ice from
Sweden to Bomholm and from Denmark to the Gterman coast
(Lubeck), and in some cases even from Gotland to the coast of
Estland.
Norlind* says that **the only authentic accounts** of
the complete freezing of the Baltic in the neighborhood
of the Kattegat are in the years 1296, 1306, 1323, and
1408. Of these 1296 is *'much the most uncertain,** while
1323 was the coldest year ever recorded, as appears from
the fact that horses and sleighs crossed regularly from
Sweden to Gtermany on the ice.
Not only central Europe and the shores of the North
Sea were marked by climatic stress during the four-
teenth century, but Scandinavia also suffered. As Petters-
son puts it :
On examining the historic (data) from the last centuries of
the Middle Ages, Dr. Bull of Christiania has come to the con-
clusion that the decay of the Norwegian kingdom was not so
much a consequence of the political conditions at that time, as
of the frequent failures of the harvest so that com [wheat] for
bread had to be imported from Liibeck^ Rostock, Wismar and so
forth. The Hansa Union undertook the importation and ob-
tained political power by its economic influence. The Norwegian
land-owners were forced to lower their rents. The population
decreased and became impoverished. The revenue sank 60 to 70
per cent. Even the income from Church property decreased.
s A. Norlind : Einige Bemerkungen fiber das Klima der historischen Zeit
nebst einem Yerzeichnis mittelaltlieher WittenmgB erBcheimmgen; Lands
Univ. Araskrift, N. P., Vol. 10, 19U, 63 pp.
STRESS OF FOURTEENTH CENTURY 101
In 1367 com was imported from Liibeck to a valne of one-
half million kroner. The trade balance inclined to the disad-
vantage of Norway whose sole article of export at that time was
dried Ssh. (The production of fish increased enormously in the
Baltic regions off south Sweden because of the same changes
which were influencing the lands, but this did not benefit Nor-
way.) Dr. BuU draws a comparison with the conditions described
in the Sagas when Nordland [at the Arctic Circle] produced
enough com to feed the inhabitants of the country. At the time
of Asbjom Selsbane the chieftains in Trondhenas [still farther
north in latitude 69^] grew so much com that they did not need
to go southward to buy com unless three successive years of
dearth had occurred. The province of Trondheim exported wheat
to Iceland and so forth. Probably the turbulent political state
of Scandinavia at the end of the Middle Ages was in a great
measure due to unfavorable climatic conditions, which lowered
the standard of life, and not entirely to misgovemment and
political strife as has hitherto been taken for granted.
During this same unfortunate first half of the four-
teenth century England also suffered from conditions
whichy if sufficiently intensified^ might be those of a gla-
cial period. According to Thorwald Eogers* the severest
famine ever experienced in England was that of 1315-
1316^ and the next worst was in 1321. In f act, from 1308
to 1322 great scarcity of food prevailed most of the time.
Other famines of less severity occurred in 1351 and 1369.
**The same cause was at work in all these cases/' says
Bogers, '^incessant rain, and cold, stormy summers. It
is said that the inclemency of the seasons affected the
cattle, and that numbers perished from disease and
want." After the bad harvest of 1315 the price of wheat,
vrhich was already high, rose rapidly, and in May, 1316,
was about five times the average. For a year or more
thereafter it remained at three or four times the ordinary
s Thorwald Bogers : A History of Agrieultnre and Prices in England.
102 CLIMATIC CHANGES
level. The severity of the famine may be jndged from the
fact that previous to the Great War the most notable
scarcity of wheat in modem England and the highest
relative price was in December, 1800. At that time wheat
cost neariy three times the usual amount, instead of five
as in 1316. During the famine of the early fourteenth cen-
tury * 4t is said that people were reduced to subsist upon
roots, upon horses and dogs, and stories are told of even
more terrible acts by reason of the extreme famine. '* The
number of deaths was so great that the price of labor
suffered a permanent rise of at least 10 per cent. There
simply were not people enough left among the peasants
to do the work demanded by the more prosperous class
who had not suffered so much.
After the famine came drought. The year 1325 appears
to have been peculiarly dry, and 1331, 1344, 1362, 1374,
and 1377 were also dry. In general these conditions do
little harm in England. They are of interest chiefly as
showing how excessive rain and drought are apt to
succeed one another.
These facts regarding northern and central Europe
during the fourteenth century are particularly significant
when compared with the conclusions which we have
drawn in Earth and Sun from the growth of trees in
Germany and from the distribution of storms. A careful
study of all the facts shows that we are dealing with two
distinct types of phenomena. In the first place, the climate
of central Europe seems to have been peculiarly conti-
nental during the fourteenth century. The winters were
so cold that the rivers froze, and the summers were so
wet that there were floods every other year or oftener.
This seems to be merely an intensification of the condi-
tions which prevail at the present time during periods of
many sunspots, as indicated by the growth of trees at
STRESS OF FOURTEENTH CENTURY 108
Eberswalde in Germany and by the number of storms in
winter as compared with summer. The prevalence of
droughts, especiaUy in the spring, is also not inconsistent
with the existence of floods at other seasons, for one of
the chief characteristics of a continental climate is that
the variations from one season to another are more
marked than in oceanic climates. Even the summer
droughts are typically continental, for when continental
conditions prevail, the difference between the same sea-
son in different years is extreme, as is well illustrated in
Kansas. It must always be remembered that what causes
famine is not so much absolute dryness as a temporary
diminution of the rainfall.
The second type of phenomena is peculiarly oceanic in
character. It consists of two parts, both of which are
precisely what would be expected if a highly continental
climate prevailed over the land. In the first place, at cer-
tain times the cold area of high pressure, which is the
predominating characteristic of a continent during the
winter, apparently spread out over the neighboring
oceans. Under such conditions an inland sea, such as the
Baltic, would be frozen, so that horses could cross the ice
even in the Far West. In the second place, because of the
unusually high pressure over the continent, the baro-
metric gradients apparently became intensified. Hence at
the margin of the continental high-pressure area the
winds were unusually strong and the storms of corre-
sponding severity. Some of these storms may have
passed entirely along oceanic tracks, while others in-
vaded the borders of the land, and gave rise to the floods
and to the wearing away of the coast described by
Pettersson.
Turning now to the east of Europe, Bruckner 's* study
^E. Briiekner: Elimaficliwanlnuigen seit 1700, Vienna, 1891.
104 CLIMATIC CHANGES
of the Caspian Sea shows that that region as well as
western Europe was subject to great climatic vicissitudes
in the first half of the fourteenth century. In 1306-1307
the Caspian Sea, after rising rapidly for several years,
stood thirty-seven feet above the present level and it
probably rose still higher during the succeeding decades.
At least it remained at a high level, for Hamdulla, the
Persian, tells us that in 1325 a place called Aboskun was
under water.**
Still further east the inland lake of Lop Nor also rose
at about this time. According to a Chinese account the
Dragon Town on the shore of Lop Nor was destroyed by
a flood. From Himley's translation it appears that the
level of the lake rose so as to overwhelm the city com-
pletely. This would necessitate the expansion of the lake
to a point eighty miles east of Lulan, and fully fifty from
the present eastern end of the Kara Koshun marsh. The
water would have to rise nearly, or quite, to a strand
which is now clearly visible at a height of twelve feet
above the modem lake or marsh.
In India the fourteenth century was characterized by
what appears to have been the most disastrous drought
in all history. Apparently the decrease in rainfall here
was as string as the Lrease in other parte of the
world. No statistics are available but we are told that in
the great famine which began in 1344 even the Mogul
emperor was unable to obtain the necessaries of life for
his household. No rain worth mentioning fell for years.
In some places the famine lasted three or four years, and
in some twelve, and entire cities were left without an in-
habitant. In a later famine, 1769-1770, which occurred in
Bengal shortly after the foundation of British rule in
5 For a full discussion of the changes in the Caspian Sea see The Pulse
of Asia, pp. 329-358.
STRESS OF FOURTEENTH CENTURY 106
India, but while the native officials were still in power,
a third of the population, or ten out of thirty millions,
X>erished. The famine in the first half of the fourteenth
century seems to have been far worse. These Indian
famines were apparently due to weak summer monsoons
caused presumably by the failure of central Asia to warm
up as much as usual. The heavier snowfall, and the
greater cloudiness of the siunmer there, which probably
accompanied increased storminess, may have been the
reason.
The New World as well as the Old appears to have
been in a state of climatic stress during the first half of
the fourteenth century. According to Pettersson, Green-
land furnishes an example of this. At first the inhabitants
of that northland were fairly prosperous and were able
to approach from Iceland without much hindrance from
the ice. Today the North Atlantic Ocean northeast of
Iceland is full of drift ice much of the time. The border
of the ice varies from season to season, but in general it
extends westward from Iceland not far from the Arctic
circle and then follows the coast of Greenland south-
ward to Cape Farewell at the southern tip and around to
the western side for fifty miles or more. Except under
exceptional circumstances a ship cannot approach the
coast until well northward on the comparatively ice-free
west coast. In the old Sagas, however, nothing is said of
ice in this region. The route from Iceland to Greenland
is carefully described. In the earliest times it went from
Iceland a trifle north of west so as to approach the coast
of Greenland after as short an ocean passage as possible.
Then it went down the coast in a region where approach
is now practically impossible because of the ice. At that
time this coast was icy close to the shore, but there is no
sign that navigation was rendered difficult as is now the
106 CLIMATIC CHANGES
case. Today no navigator would think of keeping close
inland. The old route also went north of the island on
which Cape Farewell is located, although the narrow
channel between the island and the mainland is now so
blocked with ice that no modem vessel has ever pene-
trated it. By the thirteenth century, however, there ap-
pears to have been a change. In the Kungaspegel or
Kings' Mirror, written at that time, navigators are
warned not to make the east coast too soon on accoxmt
of ice, but no new route is recommended in the neighbor-
hood of Cape Farewell or elsewhere. Finally, however,
at the end of the fourteenth century, nearly 150 years
after the Kungaspegel, the old sailing route was aban-
doned, and ships from Iceland sailed directly southwest
to avoid the ice. As Pettersson says :
... At the end of the thirteenth and the beginning of the
fourteenth century the European civilization in Greenland was
wiped out by an invasion of the aboriginal population. The col-
onists in the Vesterbygd were driven from their homes and
probably migrated to America leaving behind their cattle in the
fields. So they were found by Ivar Bardsson, steward to the
Bishop of Gardar, in his official journey thither in 1342.
The Eskimo invasion must not be regarded as a common raid.
It was the transmigration of a people, and like other big move-
ments of this kind [was] impelled by altered conditions of
nature, in this case the alterations of climate caused by [or
which caused f] the advance of the ice. For their hunting and
fishing the Eskimos require an at least partially open arctic
sea. The seal, their principal prey, cannot live where the surface
of the sea is entirely frozen over. The cause of the favorable
conditions in the Viking-age was, according to my hypothesis,
that the ice then melted at a higher latitude in the arctic seas.
The Eskimos then lived further north in Greenland and
North America. When the climate deteriorated and the sea which
gave them their living was closed by ice the Eskimos had to find
STRESS OF FOURTEENTH CENTURY 107
a more suitable neighborhood. This they found in the land
colonized by the Norsemen whom they attacked and finally
annihilated.
Finally, far to the south in Yucatan the ancient Maya
civilization made its last flickering effort at about this
time. Not much is known of this but in earlier periods
the history of the Mayas seems to have agreed quite
closely with the fluctuations in climate.* Among the
Mayas, as we have seen, relatively dry periods were the
times of greatest progress.
Let us turn now to Fig. 3 once more and compare the
climatic conditions of the fourteenth century with those
of periods of increasing rainfall. Southern England,
Ireland, and Scandinavia, where the crops were ruined
by extensive rain and storms in summer, are places
where storminess and rainfall now increase when sun-
spots are numerous. Central Europe and the coasts of the
North Sea, where flood and drought alternated, are re-
gions which now have relatively less rain when sunspots
increase than when they diminish. However, as appears
from the trees measured by Douglass, the winters become
more continental and hence cooler, thus corresponding to
the cold winters of the fourteenth century when people
walked on the ice from Scandinavia to Denmark. When
such high pressure prevails in the winter, the total rain-
fall is diminished, but nevertheless the storms are more
severe than usual, especially in the spring. In south-
eastern Europe, the part of the area whence the Caspian
derives its water, appears to have less rainfall during
times of increasing sunspots than when sunspots are few,
but in an equally large area to the south, where the moun-
• S. Q. Morley: The Inscriptions at Cop&n; Carnegie Inst, of Wash., No.
219, 1920.
Ellsworth Huntington: The Bed Man's Continent, 1919.
108 CLIMATIC CHANGES
tains are higher and the run-off of the rain is more rapid,
the reverse is the case. This seems to mean that a slight
diminution in the water poured in by the Volga would
be more than compensated by the water derived from
Persia and from the Oxus and Jaxartes rivers, which in
the fourteenth century appear to have filled the Sea of
Aral and overflowed in a large stream to the Caspian.
Still farther east in central Asia, so far as the records go,
most of the country receives more rain when sunspots
are many than when they are few, which would agree
with what happened when the Dragon Town was inun-
dated. In India, on the contrary, there is a large area
where the rainfall diminishes at times of many sunsi)ot8,
thus agreeing with the terrible famine from which the
Moguls suffered so severely. In the western hemisphere,
Greenland, Arizona, and California are all parts of the
area where the rain increases* with many sunspots, while
Yucatan seems to lie in an area of the opposite type. Thus
all the evidence seems to show that at times of climatic
stress, such as the fourteenth century, the conditions
are essentially the same as those which now prevail at
times of increasing sunspots.
As to the number of sunspots, there is little evidence
previous to about 1750. Yet that Uttle is both interesting
and important. Although sunspots have been observed
with care in Europe only a little more than three cen-
turies, the Chinese have records which go back nearly to
the beginning of the Christian era. Of course the records
are far from perfect, for the work was done by indi-
viduals and not by any great organization which con-
tinued the same methods from generation to generation.
The mere fact that a good observer happened to use his
smoked glass to advantage may cause a particular period
to appear to have an unusual number of spots. On the
STRESS OF FOURTEENTH CENTURY 109
other handy the fact that such an observer finds spots
at some times and not at others tends to give a valuable
check on his results, as does the comparison of one
observer's work with that of another. Hence, in spite of
many and obvious defects, most students of the problem
agree that the Chinese record possesses much value, and
that for a thousand years or more it gives a fairly true
idea of the general aspect of the sun. In the Chinese
records the years with many spots fall in groups, as
would be expected, and are sometimes separated by long
intervals. Certain centuries appear to have been marked
by unusual spottedness. The most conspicuous of these
is the fourteenth, when the years 1370 to 1385 were par-
ticularly noteworthy, for spots large enough to be visible
to the naked eye covered the sun much of the time. Hence
Wolf ,^ who has made an exhaustive study of the matter,
concludes that there was an absolute maximum of spots
about 1372. While this date is avowedly open to question,
the great abundance of sunspots at that time makes it
probable that it cannot be far wrong. If this is so, it
seems that the great climatic disturbances of which we
have seen evidence in the fourteenth century occurred at
a time when sunspots were increasing, or at least when
solar activity was under some profoundly disturbing in-
fluence. Thus the evidence seems to show not merely that
the climate of historic times has been subject to im-
portant pulsations, but that those pulsations were mag-
nifications of the little climatic changes which now take
place in sunspot cycles. The past and the present are
apparently a unit except as to the intensity of the
Ranges.
7 Bee BnmniaTy of Wolf's work with additional information bj H. Fritz;
Zurich Vierteljahrschrift, Vol. 38, 1893, pp. 77-107.
CHAPTER VII
GLACIATION ACCORDING TO THE SOLAR-
CYCLONIC HYPOTHESIS^
THE remarkable phenomena of glacial periods
afford perhaps the best available test to which
any climatic hypothesis can be subjected. In this
chapter and the two that follow, we shall apply this test.
Since much more is known about the recent Great Ice
Age, or Pleistocene glaciation, than about the more
ancient glaciations, the problems of the Pleistocene wUl
receive especial attention. In the present chapter the
oncoming of glaciation and the subsequent disappear-
ance of the ice will be outlined in the light of what would
be expected according to the solar-cyclonic hypothesis.
Then in the next chapter several problems of especial
climatic significance will be considered, such as the locali-
zation of ice sheets, the succession of severe glacial and
mild inter-glacial epochs, the sudden commencement of
glaciation and the peculiar variations in the height of the
snow line. Other topics to be considered are the occur-
rence of pluvial or rainy climates in non-glaciated re-
gions, and glaciation near sea level in subtropical
latitudes during the Permian and Proterozoic. Then in
Chapter IX we shall consider the development and dis-
tribution of the remarkable deposits of wind-blown ma-
terial known as loess.
Facts not considered at the time of framing an hypothe-
1 This chapter is an amplification and revision of the sketch of the glacial
period contained in The Solar Hypothesis of Climatic Changes; Boll. GeoL
Soc. Am., Vol. 25, 1914.
THE GLACIAL PERIOD 111
sis are especially significant in testing it. In this particu-
lar case, the cyclonic hypothesis was framed to explain
the historic changes of climate revealed by a study of
ruins, tree rings, and the terraces of streams and lakes,
without special thought of glaciation or other geologic
changes. Indeed, the hypothesis had reached nearly its
present form before much attention was given to geo-
logical phases of the problem. Nevertheless, it appears
to meet even this severe test.
According to the solar-cyclonic hypothesis, the Pleisto-
cene glacial period was inaugurated at a time when cer-
tain terrestrial conditions tended to make the earth
especially favorable for glaciation. How these conditions
arose will be considered later. Here it is enough to state
what they were. Chief among them was the fact that the
continents stood unusually high and were unusually
large. This, however, was not the primary cause of gla-
ciation, for many of the areas which were soon to be
glaciated were little above sea level. For example, it
seems clear that New England stood less than a thousand
feet higher tiian now. Indeed, Salisbury^ estimates that
eastern North America in general stood not more than
a few hundred feet higher than now, and W. B. Wright*
reaches the same conclusion in respect to the British
Isles. Nevertheless, widespread lands, even if they are
not all high, lead to climatic conditions which favor
glaciation. For example, enlarged continents cause low
temperature in high latitudes because they interfere with
the ocean currents that carry heat polewards. Such con-
tinents also cause relatively cold winters, for lands cool
much sooner than does the ocean. Another result is a
>B. D. Salisbury: Physical Geography of the PleistoeenOi in Outlines of
Geologic History, by Willis, Salisbury, and others, 1910, p. 265.
s The Quaternary Ice Age, 1914, p. 364.
112 CLIMATIC CHANGES
diminntion of water vapor, not only because cold air
cannot hold much vapor, but also because the oceanic
area from which evaporation takes place is reduced by
the emergence of the continents. Again, when the conti-
nents are extensive the amount of carbonic add gas in
the atmosphere probably decreases, for the augmented
erosion due to uplift exposes much igneous rock to the
air, and weathering consiunes the atmospheric carbon
dioxide. When the supply of water vapor and of atmos-
pheric carbon dioxide is smaU, an extreme type of climate
usually prevails. The combined result of all these condi-
tions is that continental emergence causes the climate to
be somewhat cool and to be marked by relatively great
contrasts from season to season and from latitude to
latitude.
When the terrestrial conditions thus permitted glada-
I - * ^ V tion, unusual solar activity is supposed to have greatly
increased the number and severity of storms and to have
altered their location, just as now happens at times of
many sunspots. If such a change in storminess had oc-
curred when terrestrial conditions were unfavorable for
glaciation, as, for example, when the lands were low and
there were widespread epicontinental seas in middle and
high latitudes, glaciation might not have resulted. In the
Pleistocene, however, terrestrial conditions permitted
glaciation, and therefore the supposed increase in stormi-
ness caused great ice sheets.
The conditions which prevail at times of increased
storminess have been discussed in detail in Earth and
Sun. Those which apparently brought on glaciation seem
to have acted as follows : In the first place the storminess
lowered the temperature of the earth *s surface in several
ways. The most important of these was the rapid upward
convection in the centers of cyclonic storms whereby
THE GLACIAL PERIOD 118
abundant heat was carried to high levels where most of it
was radiated away into space. The marked increase in
the number of tropical cyclones which accompanies in-
creased solar activity was probably important in this
respect. Such cyclones carry vast quantities of heat and
moisture out of the tropics. The moisture, to be sure,
liberates heat upon condensing, but as condensation
occurs above the earth's surface, much of the heat
escapes into space. Another reason for low temperature
was that under the influence of the supposedly numerous
storms of Pleistocene times evaporation over the oceans
must have increased. This is largely because the velocity
of the winds is relatively great when storms are strong
and such winds are powerful agents of evaporation. But
evaporation requires heat, and hence the strong winds
lower the temperature.**
The second great condition which enabled increased
storminess to bring on glaciation was the location of the
storm tracks. Kullmer's maps, as illustrated in Fig. 2,
suggest that a great increase in solar activity, such as is
postulated in the Pleistocene, might shift the main storm
track poleward even more than it is shifted by the milder
solar changes during the twelve-year sunspot cycle. If
this is so, the main track would tend to cross North
America through the middle of Canada instead of near
the southern border. Thus there would be an increase in
precipitation in about the latitude of the Keewatin and
Labradorean centers of glaciation. From what is known
of storm tracks in Europe, the main increase in the in-
tensity of storms would probably center in Scandinavia.
Fig. 3 in Chapter V bears this out. That figure, it will be
recalled, shows what happens to precipitation when solar
»»Por fuller disciiBsion of climatic controls see 8. 8. Visher: Seventy
Laws of Climate, Annals Assoc. Am. Geographers, 1922.
^
114 CLIMATIC CHANGES
activity is increasing. A high rate of precipitation is
especially marked in the boreal storm track, that is, in
the northern United States, southern Canada, and north-
western Europe.
Another important condition in bringing on glaciation
would be the fact that when storms are numerous the
total precipitation appears to increase in spite of the
slightiy lower temperature. This is largely because of the
greater evaporation. The excessive evaporation arises
partly from the rapidity of the winds, as already stated,
and partiy from the fact that in areas where the air is
clear the sun would presumably be able to act more effec-
tively than now. It would do so because at times of abun-
dant sunspots the sun in our own day has a higher solar
constant than at times of milder activity. Our whole
hypothesis is based on the supposition that what now
happens at times of many sunspots was intensified in
glacial periods.
A fourth condition which would cause glaciation to
result from great solar activity would be the fact that
the portion of the yearly precipitation falling as snow
would increase, while the proportion of rain would dimin-
ish in the main storm track. This would arise partiy be-
cause the storms would be located farther north than
now, and partly because of the diminution in temperature
due to the increased convection. The snow in itself would
still further lower the temperature, for snow is an excel-
lent reflector of sunlight. The increased cloudiness which
would accompany the more abundant storms would also
cause an unusually great reflection of the sunlight and
still further lower the temperature. Thus at times of
many sunspots a strong tendency toward the accumula-
tion of snow would arise from the rapid convection and
consequent low temperature, from the northern location
THE GLACIAL PERIOD 116
of storms, from the increased evaporation and precipita-
tion, from the larger percentage of snowy rather than
rainy precipitation, and from the great loss of heat due
to reflection from clouds and snow.
If events at the beginning of the last glacial period
took place in accordance with the cyclonic hypothesis, as
outlined above, one of the inevitable results would be the
production of snowfields. The places where snow would
accumulate in special quantities would be central Canada,
the Labrador plateau, and Scandinavia, as well as cer-
tain mountain regions. As soon as a snowfield became
somewhat extensive, it would begin to produce striking
climatic alterations in addition to those to which it owed
its origin.* For example, within a snowfield the summers
remain relatively cold. Hence such a field is likely to be
an area of high pressure at all seasons. The fact that the
snowfield is always a place of relatively high pressure
results in outblowing surface winds except when these
are temporarily overcome by the passage of strong cy-
clonic storms. The storms, however, tend to be concen-
trated near the margins of the ice throughout the year
instead of following different paths in each of the four
seasons. This is partly because cyclonic lows always
avoid places of high pressure and are thus pushed out
of the areas where permanent snow has accumulated.
On the other hand, at times of many sunspots, as KuU-
mer has shown, the main storm track tends to be drawn
'A Many of these alterations are implied or diBCussed in the following
papers:
1. F. W. Harmer: Influence of Winds upon the Climate of the Pleisto-
cene; Qaart. Jour. QeoL Soe., Vol. 57, 1901, p. 405.
2. 0. E. P. Brooks: Meteorologicid Conditions of an Ice Sheet; Quart.
Jour. Boyal Meteorol. Soc, Vol. 40, 1914, pp. 53-70, and The Evolution of
Climate in Northwest Europe; op. cit,, Vol. 47, 1921, pp. 173-194.
3. W. H. Hobbs: The B61e of the Glacial Anticyclone in the Air Circu-
lation of the Globe; Proc. Am. Phil. Soc, Vol. 54, 1915, pp. 185-225.
4
116 CLIMATIC CHANGES
poleward, perhaps by electrical conditions. Hence when
a snowfield is present in the north, the lows, instead of
migrating mnch farther north in summer than in winter,
as they now do, would merely crowd on to the snowfield
a little farther in summer than in winter. Thus the heavy
precipitation which is usual in humid climates near the
centers of lows would take place near the advancing
margin of the snowfield and cause the field to expand
still farther southward.
The tendency toward the accumulation of snow on the
margins of the snowfields would be intensified not only
by the actual storms themselves, but by other conditions.
For example, the coldness of the snow would tend to
cause prompt condensation of the moisture brought by
the winds that blow toward the storm centers from low
latitudes. Again, in spite of the general dryness of the
air over a snowfield, the lower air contains some moisture
due to evaporation from the snow by day during the
clear sunny weather of anti-cyclones or highs. Where this
is sufficient, the cold surface of the snowfields tends to
produce a frozen fog whenever the snowfield is cooled
by radiation, as happens at night and during the passage
of highs. Such a frozen fog is an effective reflector of
solar radiation. Moreover, because ice has only half the
specific heat of water, and is much more transparent to
heat, such a ** radiation fog** composed of ice crystals is
a much less effective retainer of heat than clouds or fog
made of unfrozen water particles. Shallow fogs of this
type are described by several polar expeditions. They
clearly retard the melting of the snow and thus help the
icefield to grow.
For all these reasons, so long as storminess remained
great, the Pleistocene snowfields, according to the solar
hypothesis, must have deepened and expanded. In due
THE GLACIAL PERIOD 117
time some of the snow was converted into glacial ice.
When that occurred, the growth of the snowfield as well
as of the ice cap must have been accelerated by glacial
movement. Under snch circumstances, as the ice crowded
southward toward the source of the moisture by which it
grew, the area of high pressure produced by its low
temperature would expand. This would force the storm
track southward in spite of the contrary tendency due to
the sun. When the ice sheet had become very extensive,
the track would be crowded relatively near to the north-
em margin of the trade-wind belt. Indeed, the Pleisto-
cene ice sheets, at the time of their maximum extension,
reached almost as far south as the latitude now marking
the northern limit of the trade-wind belt in summer.
As the storm track with its frequent low pressure and the
subtropical belt with its high pressure were forced nearer
and nearer together, the barometric gradient between
the two presumably became greater, winds became
stronger, and the storms more intense.
This zonal crowding would be of special importance in
summer, at which time it would also be most pronounced.
Li the first place, the storms would be crowded far upon
the ice cap which would then be protected from the sun
by a cover of fog and cloud more fully than at any other
season. Furthermore, the close approach of the trade-
wind belt to the storm belt would result in a great in-
crease in the amount of moisture drawn from the belt of
evaporation which the trade winds dominate. In the
trade-wind belt, clear skies and liigh temperature make
evaporation especially rapid. Indeed, in spite of the vast
deserts it is probable that more than three-fourths of the
total evaporation now taking place on the earth occurs
in the belt of trades, an area which includes about one-
half of the earth's surface.
118 CLIMATIC CHANGES
The agency which could produce this increased draw-
ing northward of moisture from the trade-wind belt
would be the winds blowing into the lows. According to
the cyclonic hypothesis, many of these lows would be so
strong that they would temporarily break down the sub-
tropical belt of high pressure which now usually prevails
between the trades and the zone of westerly winds. This
belt is even now often broken by tropical cyclones. If the
storms of more northerly regions temporarily destroyed
the subtropical high-pressure belt, even though they still
remained on its northern side, they would divert part of
the trade winds. Hence the air which now is carried
obliquely equatorward by those winds would be carried
spirally northward into the cyclonic lows. Precipitation
in the storm track on the margin of the relatively cold ice
sheet would thus be much increased, for most winds from
low latitudes carry abundant moisture. Such a diversion
of moisture from low latitudes probably explains the
deficiency of precipitation along the heat equator at
times of solar activity, as shown in Fig. 3. Taken as a
whole, the summer conditions, according to the cyclonic
hypothesis, would be such that increased evaporation in
low latitudes would cooperate with increased storminess,
cloudiness, and fog in higher latitudes to preserve and
increase the accumulation of ice upon the borders of the
ice sheet The greater the storminess, the more this would
be true and the more the ice sheet would be able to hold
its own against melting in summer. Such a combination
of precipitation and of protection from the sun is espe-
cially important if an ice sheet is to grow.
The meteorologist needs no geologic evidence that the
storm track was shoved equatorward by the growth of
the ice sheet, for he observes a similar shifting whenever
a winter ^s snow cap occupies part of the normal storm
THE GLACIAL PERIOD 119
tract The geologist, however, may welcome geologic
evidence that such an extreme shift of the storm track
actually occurred during the Pleistocene. Harmer, in
1901, first pointed out the evidence which was repeated
with approval by Wright of the Ireland Geological Sur-
vey in 1914/ According to these authorities, numerous
boulders of a distinctive chalk were deposited by Pleisto-
cene icebergs along the coast of Ireland. Their distribu-
tion shows that at the time of maximum glaciation the
strong winds along the south coast of Ireland were from
the northeast while today they are from the southwest.
Such a reversal could apparently be produced only by a
southward shift of the center of the main storm track
from its present position in northern Ireland, Scotland,
and Norway to a position across northern France, central
Germany, and middle Russia. This would mean that while
now the centers of the lows conamonly move northeast-
ward a short distance north of southern Ireland, they
formerly moved eastward a short distance south of Ire-
land. It will be recalled that in the northern hemisphere
the winds spiral into a low counter-clockwise and that
they are strongest near the center. When the centers pass
not far north of a given point, the strong winds therefore
blow from the west or southwest, while when the centers
pass just south of that point, the strong winds come from
the east or northeast.
In addition to the consequences of the crowding of the
storm track toward the trade-wind belt, several other
conditions presumably operated to favor the growth of
the ice sheet. For example, the lowering of the sea level
by the removal of water to form the snowfields and
glaciers interfered with warm currents. It also increased
the rate of erosion, for it was equivalent to an uplift of
B W. B. Wright: The Quaternary lee Age, 1914, p. 100.
120 CLIMATIC CHANGES
all the land. One consequence of erosion and weathering
was presumably a diminution of the carbon dioxide in
.the atmosphere, for although the ice covered perhaps a
tenth of the lands and interfered with carbonation to that
extent, the removal of large quantities of soil by acceler-
ated erosion on the other nine-tenths perhaps more than
counterbalanced the protective effect of the ice. At the
same time, the general lowering of the temperature of the
ocean as well as the lands increased the ocean's capacity
for carbon dioxide and thus facilitated absorption. At a
temperature of 50°F. water absorbs 32 per cent more
carbon dioxide than at 68°. The high waves produced by
the severe storms must have had a similar effect on a
small scale. Thus the percentage of carbon dioxide in the
atmosphere was presumably diminished. Of less signifi-
cance than these changes in the lands and the air, but
perhaps not negligible, was the increased salinity of the
ocean which accompanied the removal of water to form
snow, and the increase of the dissolved mineral load of
the rejuvenated streams. Increased salinity slows up the
deep-sea circulation, as we shall see in a later chapter.
This increases the contrasts from zone to zone.
At times of great solar activity the agencies mentioned
above would apparently cooperate to cause an advance of
ice sheets into lower latitudes. The degree of solar activ-
ity would have much to do with the final extent of the ice
sheets. Nevertheless, certain terrestrial conditions would
tend to set limits beyond which the ice would not greatly
advance unless the storminess were extraordinarily
severe. The most obvious of these conditions is the loca-
tion of oceans and of deserts or semi-arid regions. The
southwestward advance of the European ice sheet and
the southeastward advance of the Labradorean sheet in
America were stopped by the Atlantic. The semi-aridity
THE GLACIAL PERIOD 121
of the Great Plains, produced by their position in the lee
of the Bocky Mountains, stopped the advance of the
Keewatin ice sheet toward the southwest. The advance of
the European ice sheet southeast seems to have been
stopped for similar reasons. The cessation of the advance
would be brought about in such an area not alone by the
light precipitation and abundant sunshine, but by the
dryness of the air, and also by the power of dust to ab-
sorb the sun's heat. Much dust would presumably be
drawn in from the dry regions by passing cyclonic storms
and would be scattered over the ice.
The advance of the ice is also slowed up by a rugged
topography, as among the Appalachians in northern
Pennsylvania. Such a toi)ography besides opposing a
physical obstruction to the movement of the ice provides
bare south-facing slopes which the sun warms effectively.
Such warm slopes are unfavorable to glacial advance.
The rugged topography was perhaps quite as effective as
the altitude of the Appalachians in causing the conspicu-
ous northward dent in the glacial margin in Pennsyl-
vania. Where glaciers lie in mountain valleys the advance
beyond a certain point is often interfered with by the
deployment of the ice at the mouths of gorges. Evapora-
tion and melting are more rapid where a glacier is broad
and thin than where it is narrow and thick, as in a gorge.
Again, where the topography or the location of oceans or
dry areas causes the glacial lobes to be long and narrow,
the elongation of the lobe is apparently checked in sev-
eral ways. Toward the end of the lobe, melting and
evaporation increase rapidly because the planetary
westerly winds are more likely to overcome the glacial
winds and sweep across a long, narrow lobe than across
a broad one. As they cross the lobe, they accelerate
evaporation, and probably lessen cloudiness, with a con-
122 CLIMATIC CHANGES
sequent augmentation of melting. Moreover, although
lows rarely cross a broad ice sheet, they do cross a
narrow lobe. For example, Nansen records that strong
lows occasionally cross the narrow southern part of the
Greenland ice sheet. The longer the lobe, the more likely
it is that lows will cross it, instead of following its mar-
gin. Lows which cross a lobe do not yield so much snow
to the tip as do those which follow the margin. Hence
elongation is retarded and finally stopped even without
a change in the earth 's general climate.
Because of these various reasons the advances of the
ice during the several epochs of a glacial period might
be approximately equal, even if the durations of the
periods of storminess and low temperature were differ-
ent. Indeed, they might be sub-equal, even if the periods
differed in intensity as well as length. Differences in the
periods would apparently be manifested less in the ex-
tent of the ice than in the depth of glacial erosion and in
the thickness of the terminal moraines, outwash plains,
and other glacial or glacio-fluvial formations.
Having completed the consideration of the conditions
leading to the advance of the ice, let us now consider the
condition of North America at the time of maximum
glaciation.^ Over an area of nearly four million square
miles, occupying practically all the northern half of the
continent and part of the southern half, as appears in
Fig. 6, the surface was a monotonous and almost level
plain of ice covered with snow. When viewed from a
high altitude, all parts except the margins must have
presented a uniformly white and sparkling appearance.
Along the margins, however, except to the north, the
« The description of the distribution of the iee sheet is based on T. G.
Chamberlin 's wall map of North America at the wn^imp Tn of glaciation,
1913.
124 CLIMATIC CHANGES
whiteness was irregular, for the view must have included
not only fresh snow, but moving clouds and dirty snow
or ice. Along the borders where melting was in progress
there was presumably more or less spottedness due to
morainal material or glacial debris brought to the sur-
face by ice shearage and wastage. Along the dry south-
western border it is also possible that there were numer-
ous dark spots due to dust blown onto the ice by the
wind.
The great white sheet with its ragged border was
roughly circular in form, with its center in central
Canada. Yet there were many departures from a per-
fectly circular form. Some were due to the oceans, for,
except in northern Alaska, the ice extended into the
ocean all the way from New Jersey around by the north
to Washington. On the south, topographic conditions
made the margin depart from a simple arc From New
Jersey to Ohio it swung northward. In the Mississippi
Valley it reached far south; indeed most of the broad
wedge between the Ohio and the Missouri rivers was
occupied by ice. From latitude 37° near the junction of
the Missouri and the Mississippi, however, the ice margin
extended almost due north along the Missouri to central
North Dakota. It then stretched westward to the Rockies.
Farther west lowland glaciation was abundant as far
south as western Washington. In the Bockies, the Cas-
cades, and the Sierra Nevadas glaciation was common as
far south as Colorado and southern California, respec-
tively, and snowfields were doubtless extensive enough to
make these ranges ribbons of white. Between these lofty
ranges lay a great unglaciated region, but even in the
Great Basin itself, in spite of its present aridity, certain
ranges carried glaciers, while great lakes expanded
widely.
THE GLACIAL PERIOD 126
In this vast field of snow the glacial ice slowly crept
outward, possibly at an average speed of half a foot a
day, but varying from almost nothing in winter at the
north, to several feet a day in summer at the south/ The
force which caused the movement was the presence of
the ice piled up not far from the margins. Almost cer-
tainly, however, there was no great dome from the
center in Canada outward, as some early writers as-
sumed. Such a dome would require that the ice be many
thousands of feet thick near its center. This is impos-
sible because of the fact that ice is more voluminous than
water (about 9 per cent near the freezing point). Hence
when subjected to sufficient pressure it changes to the
liquid form. As friction and internal heat tend to keep
the bottom of a glacier warm, even in cold regions, the
probabilities are that only under very special conditions
was a continental ice sheet much thicker than about 2500
feet. In Antarctica, where the temperature is much lower
than was probably attained in the United States, the ice
sheet is nearly level, several expeditions having traveled
hundreds of miles with practically no change in altitude.
In Shackleton's trip almost to the South Pole, he en-
countered a general rise of 3000 feet in 1200 miles. Moun-
tains, however, projected through the ice even near the
pole and the geologists conclude that the ice is not very
thick even at the world 's coldest point, the South Pole.
Along the margin of the ice there were two sorts of
movement, much more rapid than the slow creep of the
ice. One was produced by the outward drift of snow
carried by the outblowing dry winds and the other and
more important was due to the passage of cyclonic
storms. Along the border of the ice sheet, except at the
7 Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H. Hobbs:
Characteristics of Existing Glaciers, 1911.
126 CLIMATIC CHANGES
north, storm presumably closely followed storm. Their
movement, we judge, was relatively slow until near the
southern end of the Mississippi lobe, but when this point
was passed they moved much more rapidly, for then they
could go toward instead of away from the far northern
path which the sun prescribes when solar activity is
great. The storms brought much snow to the icefield,
perhaps sometimes in favored places as much as the hun-
dred feet a year which is recorded for some winters in the
Sierras at present. Even the unglaciated intermontane
Great Basin presumably received considerable precipi-
tation, perhaps twice as much as its present scanty
supply. The rainfall was enough to support many lakes,
one of which was ten times as large as Great Salt Lake ;
and grass was doubtless abimdant upon many slopes
which are now dry and barren. The relatively heavy
precipitation in the Great Basin was probably due pri-
marily to the increased number of storms, but may also
have been much influenced by their slow eastward move-
ment. The lows presumably moved slowly in that general
region not only because they were retarded and turned
from their normal path by the cold ice to the east, but
because during the summer the area between the Sierra
snowfields on the west and the Rocky Moimtain and Mis-
sissippi Valley snowfields on the east was relatively
warm. Hence it was normally a place of low pressure
and therefore of inblowing winds. Slow-moving lows are
much more effective than fast-moving ones in drawing
moisture northwestward from the Gulf of Mexico, for
they give the moisture more time to move spirally first
northeast, under the influence of the normal south-
westerly winds, then northwest and finally southwest as
it approaches the storm center. In the case of the present
lows, before much moisture-laden air can describe such
THE GLACIAL PERIOD 127
a circuit, first eastward and then westward, the storm
center has nearly always moved eastward across the
BocMes and even across the Great Plains. A result of this
is the regular decrease in precipitation northward, north-
westward, and westward from the Gulf of Mexico.
Along the part of the glacial margins where for more
than 3000 miles the North American ice entered the
Atlantic and the Pacific oceans, myriads of great blocks
broke off and floated away as stately icebergs, to scatter
boulders far over the ocean floor and to melt in warmer
climes. Where the margin lay upon the lands numerous
streams issued from beneath the ice, milk-white with
rock flour, and built up great outwash plains and valley
trains of gravel and sand. Here and there, just beyond
the ice, marginal lakes of strange shapes occupied valleys
which had been dammed by the advancing ice. In many
of them the water level rose until it reached some low
point in the divide and then overflowed, forming rapids
and waterfalls. Indeed, many of the waterfalls of the
eastern United States and Canada were formed in just
this way and not a few streams now occupy courses
through ridges instead of parallel to them, as in pre-
glacial times.
In the zone to the south of the continental ice sheet,
the plant and animal life of boreal, cool temperate, and
warm temperate regions commingled curiously. Heather
and Arctic willow crowded out elm and oak; musk ox,
hairy mammoth, and marmot contested with deer, chip-
munk, and skunk for a chance to live. Near the ice on
slopes exposed to the cold glacial gales, the immigrant
boreal species were dominant, but not far away in more
protected areas the species that had formerly lived there
held their own. In Europe during the last two advances
of the great ice sheet the caveman also struggled with
128 CLIMATIC CHANGES
fierce animals and a fiercer climate to maintain life in an
area whose habitabiUty had long been decreasing.
The next step in our history of gladation is to outline
the disappearance of the ice sheets. When a decrease in
solar activity produced a corresponding decrease in
storminesSy several influences presumably combined to
cause the disappearance of the ice. Most of their results
are the reverse of those which brought on glaciation. A
few special aspects, however, some of which have been
discussed in Earth and Sun, ought to be brought to mind.
A diminution in storminess lessens upward convection,
wind velocity, and evaporation, and these changes, if they
occurred, must have united to raise the temperature of
the lower air by reducing the escape of heat. Again a
decrease in the number and intensity of tropical cyclones
presumably lessened the amount of moisture carried into
mid-latitudes, and thus diminished the precipitation. The
diminution of snowfall on the ice sheets when storminess
diimnished was probably highly important. The amount
of precipitation on the sheets was presumably lessened
still further by changes in the storminess of middle
latitudes. When storminess diminishes, the lows follow a
less definite path, as Kullmer's maps show, and on the
average a more southerly path. Thus, instead of all the
lows contributing snow to the ice sheet, a large fraction
of the relatively few remaining lows would bring rain to
areas south of the ice sheet. As storminess decreased, the
trades and westerlies probably became steadier, and thus
carried to high latitudes more warm water than when
often interrupted by storms. Steadier southwesterly
winds must have produced a greater movement of atmos-
pheric as well as oceanic heat to high latitudes. The
warming due to these two causes was probably the chief
reason for the disappearance of the European ice sheet
THE GLACIAL PERIOD 129
and of those on the Pacific coast of North America. The
two greater American ice sheets, however, and the
glaciers elsewhere in the lee of high mountain ranges,
probably disappeared chiefly because of lessened pre-
cipitation. If there were no cyclonic storms to draw mois-
ture northward from the Gulf of Mexico, most of North
America east of the Eocky Mountain barrier would be
arid. Therefore a diminution of storminess would be
particularly effective in causing the disappearance of ice
sheets in these regions.
That evaporation was an especially important factor
in causing the ice from the Keewatin center to disappear,
is suggested by the relatively small amount of water-
sorted material in its drift. In South Dakota, for ex-
ample, less than 10 per cent of the drift is stratified.* On
the other hand, Salisbury estimates that perhaps a third
of the Labradorean drift in eastern Wisconsin is crudely
stratified, about half of that in New Jersey, and more
than half of the drift in western Europe.
When the sun's activity began to diminish, all these
conditions, as well as several others, would cooperate to
cause the ice sheets to disappear. Step by step with their
disappearance, the amelioration of the climate would
progress so long as the period of solar inactivity con-
tinued and storms were rare. If the inactivity continued
long enough, it would result in a fairly mild climate in
high latitudes, though so long as the continents were
emergent this mildness would not be of the extreme type.
The inauguration of another cycle of increased disturb-
ance of the Sim, with a marked increase in storminess,
would inaugurate another glacial epoch. Thus a succes-
sion of glacial and inter-glacial epochs might continue so
long as the sun was repeatedly disturbed.
8S. 8. Visher: The Geography of South Dakota; S. D. Geol. Sury., 1918.
CHAPTER VIII
SOME PROBLEMS OF GLACIAL PERIODS
HAYING outlined in general terms the ooming of
the ice sheets and their disappearance, we are
now ready to discuss certain problems of com-
pelling climatic interest. The discussion will be grouped
under five heads: (I) the localization of glaciation; (II)
the sudden coming of glaciation; (III) peculiar varia-
tions in the height of the snow line and of glaciation;
(IV) lakes and other evidences of humidity in ungla-
ciated regions during the glacial epochs ; (V ) glaciation
at sea level and in low latitudes in the Permian and
Proterozoic eras. The discussion of perhaps the most
difficult of all climatic problems of glaciation, that of the
succession of cold glacial and mild inter-glacial epochs,
has been postponed to the next to the final chapter of this
book. It cannot be properly considered until we take up
the history of solar disturbances.
I. The first problem, the localization of the ice sheets,
arises from the fact that in both the Pleistocene and the
Permian periods glaciation was remarkably limited. In
neither period were all parts of high latitudes glaciated ;
yet in both cases glaciation occurred in large regions in
lower latitudes. Many explanations of this localization
have been offered, but most are entirely inadequate. Even
hypotheses with something of proven worth, such as
those of variations in volcanic dust and in atmospheric
SOME PROBLEMS OF GLACIAL PERIODS 181
carbon dioxide, fail to account for localization. The
cyclonic form of the solar hypothesis, however, seems to
afford a satisfactory explanation.
The distribution of the ice in the last glacial period is
well known, and is shown in Fig. 6. Four-fifths of the
ice-covered area, which was eight million square miles,
more or less, was near the borders of the North Atlantic
in eastern North America and northwestern Europe.
The ice spread out from two great centers in North
America, the Labradorean east of Hudson Bay, and the
Keewatin west of the bay. There were also many glaciers
in the western mountains, especially in Canada, while
subordinate centers occurred in Newfoundland, the Adi-
Tondacks, and the White Mountains. The main ice sheet
at its maximum extension reached as far south as lati-
tude 39^ in Kansas and Kentucky, and 37^ in Illinois.
Huge boulders were transferred more than one thousand
miles from their source in Canada. The northward ex-
tension was somewhat less. Indeed, the northern margin
of the continent was apparently relatively little glaciated
and much of Alaska unglaciated. Why should northern
Kentucky be glaciated when northern Aiaska was not f
In Europe the chief center from which the continental
glacier moved was the Scandinavian highlands. It pushed
across the depression now occupied by the Baltic to
southern Russia and across the North Sea depression
to England and Belgium. The Alps formed a center of
considerable importance, and there were minor centers
in Scotland, Ireland, the Pyrenees, Apennines, Caucasus,
and Urals. In Asia numerous ranges also contained large
glaciers, but practically all the glaciation was of the
alpine type and very little of the vast northern lowland
was covered with ice.
In the southern hemisphere glaciation at low latitudes
182 CLIMATIC CHANGES
was less striking than in the northern hemisphere. Most
of the increase in the areas of ice was confined to moun-
tains which today receive heavy precipitation and still
contain small glaciers. Indeed, except for relatively slight
gladation in the Australian Alps and in Tasmania, most
of the Pleistocene glaciation in the southern hemisphere
was merely an extension of existing glaciers, such as
those of south Chile, New Zealand, and the Andes. Never-
theless, fairly extensive glaciation existed much nearer
the equator than is now the case.
In considering the localization of Pleistocene glacia-
tion, three main factors must be taken into account,
namely, temperature, topography, and precipitation. The
absence of glaciation in large parts of the Arctic regions
of North America and of Asia makes it certain that low
temperature was not the controlling factor. Aiside from
Antarctica, the coldest place in the world is northeastern
Siberia. There for seven months the average temperature
is below 0°C., while the mean for the whole year is
below — 10° C. If the temperature during a glacial period
averaged S^'G. lower than now, as is commonly supposed,
this part of Siberia would have had a temperature below
freezing for at least nine months out of the twelve even if
there were no snowfield to keep the summers cold. Yet
even under such conditions no glaciation occurred, al-
though in other places, such as parts of Canada and
northwestern Europe, intense glaciation occurred where
the mean temperature is much higher.
The topography of the lands apparently had much
more influence upon the localization of glaciation than
did temperature. Its effect, however, was always to cause
glaciation exactly where it would be expected and not in
unexpected places as actually occurred. For example, in
North America the western side of the Canadian Bockies
SOME PROBLEMS OF GLACIAL PERIODS 188
suffered intense glaciation, for there precipitation was
heavy because the westerly winds from the Pacific are
forced to give up their moisture as they rise. In the same
way the western side of the Sierra Nevadas was much
more heavily glaciated than the eastern side. In similar
fashion the windward slopes of the Alps, the Caucasus,
the Himalayas, and many other mountain ranges suf-
fered extensive glaciation. Low temperature does not
seem to have been the cause of this glaciation, for in that
case it is hard to see why both sides of the various ranges
did not show an equal percentage of increase in the size
of their icefields.
From what has been said as to temperature and topog-
raphy, it is evident that variations in precipitation have
had much more to do with glaciation than have variations
in temperature. In the Arctic lowlands and on the lee-
ward side of mountains, the slight development of glacia-
tion appears to have been due to scarcity of precipita-
tion. On the windward side of mountains, on the other
hand, a notable increase in precipitation seems to have
led to abundant glaciation. Such an increase in precipi-
tation must be dependent on increased evaporation and
this could arise either from relatively high temperature
or strong winds. Since the temperature in the glacial
period was lower than now, we seem forced to attribute
the increased precipitation to a strengthening of the
winds. If the westerly winds from the Pacific should in-
crease in strength and waft more moisture to the western
side of the Canadian Rockies,' or if similar winds in-
creased the snowfall on the upper slopes of the Alps or
the Tian-Shan Mountains, the glaciers would extend
lower than now without any change in temperature.
Although the incompetence of low temperature to cause
glaciation, and the relative unimportance of the moun-
184. CLIMATIC CHANGES
tains in northeastern Canada and northwestern Europe
throw most glacial hypotheses out of court, they are in
harmony with the cyclonic hypothesis. The answer of
that hypothesis to the problem of the localization of ice
sheets seems to be found in certain maps of storminess
and rainfall in relation to solar activity. In Fig. 2 a
marked belt of increased storminess at times of many
sunspots is seen in southern Canada. A comparison of
this with a series of maps given in Earth and Sun shows
that the stormy belt tends to migrate northward in har-
mony with an increase in the activity of the sun *s atmos-
phere. If the sun were sufficiently active the belt of
maximum storminess would apparently pass through the
Keewatin and Labradorean centers of glaciation instead
of well to the south of them, as at present. It would
presumably cross another center in Greenland, and then
would traverse the fourth of the great centers of Pleisto-
cene glaciation in Scandinavia. It would not succeed in
traversing northern Asia, however, any more than it
does now, because of the great high-pressure area which
develops there in winter. When the ice sheets expanded
from the main centers of glaciation, the belt of storms
would be pushed southward and outward. Thus it might
give rise to minor centers of glaciers such as the Patri-
cian between Hudson Bay and Lake Superior, or the
centers in Ireland, Cornwall, Wales, and the northern
Ural Mountains. As the main ice sheets advanced, how-
ever, the minor centers would be overridden and the
entire mass of ice would be merged into one vast expanse
in the Atlantic portion of each of the two continents.
In this connection it may be well to consider briefly the
most recent hypothesis as to the growth and hence the
localization of glaciation. In 1911 and more fully in 1915,
SOME PROBLEMS OF GLACIAL PERIODS 185
Hobbs,* advanced the anti-cydonic hypothesis of the
origin of ice sheets. This hypothesis has the great merit
of focasing attention upon the fact that ice sheets are
pronounced anti-cydonic regions of high pressure. This
is proved by the strong outblowing winds which pre-
vail along their margins. Such winds must, of course, be
balanced by inward-moving winds at high levels. Abun-
dant observations prove that such is the case. For
example, balloons sent up by Barkow near the margin of
the Antarctic ice sheet reveal the occurrence of inblow-
ing winds, although they rarely occur below a height of
9000 meters. The abundant data gathered by Guervain
on the coast of Greenland indicate that outblowing winds
prevail up to a height of about 4000 meters. At that
height inblowing winds commence and increase in fre-
quency until at an altitude of over 5000 meters they be-
come more common than outblowing winds. It should be
noted, however, that in both Antarctica and Greenland,
although the winds at an elevation of less than a thousand
meters generally blow outward, there are frequent and
decided departures from this rule, so that *' variable
winds'^ are quite commonly mentioned in the reports of
expeditions and balloon soimdings.
The undoubted anti-cyclonic conditions which Hobbs
thus calls to the attention of scientists seem to him to
necessitate a peculiar mechanism in order to produce
the snow which feeds the glaciers. He assumes that the
winds which blow toward the centers of the ice sheets
at high levels carry the necessary moisture by which the
glaciers grow. When the air descends in the centers of
the highs, it is supposed to be chilled on reaching the sur-
iW. H. Hobbs: Gharacteristics of EziBtmg Glaciers, 1911. The Bdle of
the Glaeial Antieyclones in the Air Circulation of the Globe; Proc. Am.
PhiL Soc., Vol. 54, 1915, pp. 185-225.
186 CLIMATIC CHANGES
face of the ice, and hence to give up its moisture in the
form of minute crystals. This conclusion is doubtful for
several reasons. In the first place, Hobbs does not seem
to appreciate the importance of the variable winds which
he quotes Arctic and Antarctic explorers as describing
quite frequently on the edges of the ice sheets. They are
one of many signs that cyclonic storms are fairly fre-
quent on the borders of the ice though not in its interior.
Thus there is a distinct and sufficient form of precipita-
tion actually at work near the margin of the ice, or
exactly where the thickness of the ice sheet would lead
us to expect.
Another consideration which throws grave doubt on
the anti-cyclonic hypothesis of ice sheets is the small
amount of moisture possible in the highs because of their
low temperature. Suppose, for the sake of argument,
that the temperature in the middle of an ice sheet aver-
ages 20°F. This is probably much higher than the actual
fact and therefore unduly favorable to the anti-cyclonic
hypothesis. Suppose also that the decrease in tempera-
ture from the earth ^s surface upward proceeds at the
rate of l^'F. for each 300 feet, which is 50 per cent less
than the actual rate for air with only a slight amount of
moisture, such as is found in cold regions. Then at a
height of 10,000 feet, where the inblowing winds begin
to be felt, the temperature would be — ^20°F. At that
temperature the air is able to hold approximately 0.166
grain of moisture per cubic foot when fully saturated.
This is an exceedingly small amount of moisture and even
if it were all precipitated could scarcely build a glacier.
However, it apparently would not be precipitated because
when such air descends in the center of the anti-cyclone
it is warmed adiabatically, that is, by compression. On
reaching the surface it would have a temperature of 20°
SOME PROBLEMS OF GLACIAL PERIODS 187
and would be able to hold 0.898 grain of water vapor per
cubic foot ; in other words, it would have a relative hu-
midity of about 18 per cent. Under no reasonable assump-
tion does the upper air at the center of an ice sheet
appear to reach the surface with a relative humidity of
more than 20 or 25 per cent. Such air cannot give up
moisture. On the contrary, it absorbs it and tends to
diminish rather than increase the thickness of the sheet
of ice and snow. But after the surplus heat gained by
descent has been lost by radiation, conduction, and
evaporation, the air may become super-saturated with
the moisture picked up while warm. Hobbs reports that
explorers in Antarctica and Greenland have frequently
observed condensation on their clothing. If such moisture
is not derived directly from the men*s own bodies, it is
apparently picked up from the ice sheet by the descending
air, and not added to the ice sheet by air from aloft.
The relation of all this to the localization of ice sheets
is this. If Hobbs' anti-cyclonic hypothesis of glacial
growth is correct, it would appear that ice sheets should
grow up where the temperature is lowest and the high-
pressure areas most persistent ; for instance, in northern
Siberia. It would also appear that so far as the topog-
raphy permitted, the ice sheets ought to move out uni-
formly in all directions ; hence the ice sheet ought to be
as prominent to the north of the Keewatin and Labra-
dorean centers as to the south, which is by no means the
case. Again, in mountainous regions, such as the glacial
areas of Alaska and Chile, the glaciation ought not to
be confined to the windward slope of the mountains so
closely as is actually the fact. In each of these cases the
glaciated region was large enough so that there was
probably a true anti-cyclonic area comparable with that
now prevailing over southern Greenland. In both places
188 CLIMATIC CHANGES
the correlation between gladation and mountain ranges
seems much too close to support the anti-cyclonic hy-
pothesis, for the inblowing winds which on that hypothe-
sis bring the moisture are shown by observation to occur
at heights far greater than that of all but the loftiest
ranges.
II. The sudden coming of glaciation is another prob-
lem which has been a stumbling-block in the way of every
glacial hypothesis. In his Climates of Geologic Times,
Schuchert states that the fossils give almost no warning
of an approaching catastrophe. If glaciation were solely
due to uplift, or other terrestrial changes aside from vul-
canism, Schuchert holds that it would have come slowly
and the stages preceding glaciation would have affected
life sufficiently to be recorded in the rocks. He considers
that the suddenness of the coming of glaciation is one
of the strongest arguments against the carbon dioxide
hypothesis of glaciation.
According to the cyclonic hypothesis, however, the
suddenness of the oncoming of glaciation is merely what
would be expected on the basis of what happens today.
Changes in the sun occur suddenly. The sunspot cycle is
only eleven or twelve years long, and even this short
period of activity is inaugurated more suddenly than it
declines. Again the climatic record derived from the
growth of trees, as given in Figs. 4 and 5, also shows that
marked changes in climate are initiated more rapidly
than they disappear. In tiiis connection, however, it must
be remembered that solar activity may arise in various
ways, as will appear more fully later. Under certain con-
ditions storminess may increase and decrease slowly.
III. The height of the snow line and of glaciation fur-
nishes another means of testing glacial hypotheses. It is
well established that in times of glaciation the snow line
SOME PROBLEMS OF GLACIAL PERIODS 189
was depressed everywhere, but least near the equator.
For example, according to Penck, permanent snow ex-
tended 4000 feet lower than now in the Alps, whereas
it stood only 1500 feet below the present level near the
equator in Venezuela. This unequal depression is not
readily accounted for by any hypothesis depending solely
upon the lowering of temperature. By the carbon dioxide
and the volcanic dust hypotheses, the temperature pre-
sumably was lowered amost equally in all latitudes, but
a little more at the equator than elsewhere. If glaciation
were due to a temporary lessening of the radiation re-
ceived from the sun, such as is demanded by the thermal
solar hypothesis, and by the longer periods of CrolPs
hypothesis, the lowering would be distinctly greatest at
the equator. Thus, according to all these hypotheses, the
snow line should have been depressed most at the equator,
instead of least.
The cyclonic hypothesis explains the lesser depression
of the snow line at the equator as due to a diminution of
precipitation. The effectiveness of precipitation in this
respect is illustrated by the present great difference in
the height of the snow line on the humid and dry sides of
mountains. On the wet eastern side of the Andes near the
equator, the snow line lies at 16,000 feet; on the dry
western side, at 18,500 feet. Again, although the humid
side of the Himalayas lies toward the south, the snow line
has a level of 15,000 feet, while farther north, on the dry
side, it is 16,700 feet.* The fact that the snow line is lower
near the margin of the Alps than toward the center
points in the same direction. The bearing of all this on
the glacial period may be judged by looking again at Fig.
3 in Chapter V. This shows that at times of sunspot
activity and hence of augmented storminess, the precipir
SB. D. Salisbary: Physiographj, 1919.
140 CLIMATIC CHANGES
tation diminishes near the heat equator, that is, where
the average temperature for the whole year is highest.
At present the great size of the northern continents and
their consequent high temperature in summer, cause the
heat equator to lie north of the **real" equator, except
where Australia draws it to the southward.* When large
parts of the northern continents were covered with ice,
however, the heat equator and the true equator were
probably much closer than now, for the continents could
not become so hot. If so, the diminution in equatorial
precipitation, which accompanies increased storminess
throughout the world as a whole, would take place more
nearly along the true equator than appears in Fig. 3.
Hence so far as precipitation alone is concerned, we
should actually expect that the snow line near the equator
would rise a little during glacial periods. Another factor,
however, must be considered. Koppen's data, it will be
remembered, show that at times of solar activity the
earth's temperature falls more at the equator than in
higher latitudes. If this effect were magnified it would
lower the snow line. The actual position of the snow line
at the equator during glacial periods thus appears to be
the combined effect of diminished precipitation, which
would raise the line, and of lower temperature, which
would bring it down.
Before leaving this subject it may be well to recall that
the relative lessening of precipitation in equatorial lati-
tudes during the glacial epochs was probably caused by
the diversion of moisture from the trade-wind belt. This
diversion was presumably due to the great number of
tropical cyclones and to the fact that the cyclonic storms
of middle latitudes also drew much moisture from the
trade-wind belt in summer when the northern position of
s Griffith Taylor: Australian Meteorology^ 1920, p. 283.
SOME PROBLEMS OF GLACIAL PERIODS 141
the sun drew that belt near the storm track which was
forced to remain south of the ice sheet. Such diversion
of moisture out of the trade-wind belt must diminish the
amount of water vapor that is carried by the trades to
equatorial regions; hence it would lessen precipitation
in the belt of so-called equatorial calms, which lies along
the heat equator rather than along the geographical
equator.
Another phase of the vertical distribution of glaciation
has been the subject of considerable discussion. In the
Alps and in many other mountains the glaciation of the
Pleistocene period appears to have had its upper limit
no higher than today. This has been variously inter-
preted. It seems, however, to be adequately explained
as due to decreased precipitation at high altitudes during
the cold periods. This is in spite of the fact that precipi-
tation in general increased with increased storminess.
The low temperature of glacial times presimiably induced
condensation at lower altitudes than now, and most of
the precipitation occurred upon the lower slopes of the
mountains, contributing to the lower glaciers, while little
of it fell upon the highest glaciers. Above a moderate
altitude in all lofty mountains the decrease in the amount
of precipitation is rapid. In most cases the decrease
begins at a height of less than 3000 feet above the base
of the main slope, provided the slope is steep. The colder
the air, the lower the altitude at which this occurs. For
example, it is much lower in winter than in summer.
Indeed, the higher altitudes in the Alps are sunny in
winter even where there are abundant clouds lower down.
IV. The presence of extensive lakes and other evidences
of a pluvial climate during glacial periods in non-glaci-
ated regions which are normally dry is another of the
facts which most glacial hypotheses fail to explain satis-
144 CLIMATIC CHANGES
the region of salt lakes in the Old World. Judging by
these maps, which illustrate what has happened since
careful meteorological records were kept, an increase in
solar activity is accompanied by increased rainfall in
large parts of what are now semi-arid and desert regions.
Such precipitation would at once cause the level of the
lakes to rise. Later, when ice sheets had developed in
Europe and America, the high-pressure areas thus caused
might force the main storm belt so far south that it would
lie over these same arid regions. The increase in tropical
hurricanes at times of abundant sunspots may also have
a bearing on the climate of regions that are now arid.
During the glacial period some of the hurricanes prob-
ably swept far over the lands. The numerous tropical
cyclones of Australia, for example, are the chief source
of precipitation for that continent." Some of the stronger
cyclones locally yield more rain in a day or two than
other sources yield in a year.
V. The occurrence of widespread glaciation near the
tropics during the Permian, as shown in Fig. 7, has given
rise to much discussion. The recent discovery of glacia-
tion in latitudes as low as 30° in the Proterozoic is corre-
spondingly significant. In all cases the occurrence of
glaciation in low and middle latitudes is probably due to
the same general causes. Doubtless the position and alti-
tude of the mountains had something to do with the
matter. Yet taken by itself this seems insufficient. Today
the loftiest range in the world, the Himalayas, is almost
unglaciated, although its southern slope may seem at first
thought to be almost ideally located in this respect. Some
parts rise over 20,000 feet and certain lower slopes re-
ceive 400 inches of rain per year. The small size of the
Himalayan glaciers in spite of these favorable conditions
10 Griffith Taylor: Australian Meteorology, 1920, p. 189.
146 CLIMATIC CHANGES
is apparently due largely to the seasonal character of tlie
monsoon winds. The strong ontblowing monsoons of
winter cause about half the year to be very dry with clear
skies and dry winds from the interior of Asia. In aU low
latitudes the sun rides high in the heavens at midday,
even in winter, and thus melts snow fairly effectively in
clear weather. This is highly unfavorable to glaciatioiu
The inblowing southern monsoons bring all their mois-
ture in midsummer at just the time when it is least effec-
tive in producing snow. Conditions similar to those now-
prevailing in the Himalayas must accompany any great
uplift of the lands which produces high mountains and
large continents in subtropical and middle latitudes.
Hence, uplift alone cannot account for extensive glacia-
tion in subtropical latitudes during the Permian and
Proterozoic.
The assumption of a great general lowering of tem-
perature is also not adequate to explain glaciation in
subtropical latitudes. In the first place this would reqtdre
a lowering of many degrees, — ^far more than in the Pleis-
tocene glacial period. The marine fossils of the Permian,
however, do not indicate any such condition. In the
second place, if the lands were widespread as they ap-
pear to have been in the Permian, a general lowering of
temperature would diminish rather than increase the
present slight efficiency of the monsoons in producing
glaciation. Monsoons depend upon the difference between
the temperatures of land and water. If the general tem-
perature were lowered, the reduction would be much less
pronounced on the oceans than on the lands, for water
tends to preserve a uniform temperature, not only be-
cause of its mobility, but because of the large amount of
heat given out when freezing takes place, or consumed in
evaporation. Hence the general lowering of temperature
SOME PROBLEMS OF GLACIAL PERIODS 147
would make the contrast between continents and oceans
less than at present in summer, for the land temperature
would be brought toward that of the ocean. This would
diminish the strength of the inblowing summer mon-
soons and thus cut off part of the supply of moisture.
Evidence thjat this actually happened in the cold four-
teenth century has already been given in Chapter VI.
On the other hand, in winter the lands would be much
colder than now and the oceans only a little colder, so
that the dry outblowing monsoons of the cold season
would increase in strength and would also last longer
than at present. In addition to all this, the mere fact of
low temperature would mean a general reduction in the
amount of water vapor in the air. Thus, from almost
every point of view a mere lowering of temperature
seems to be ruled out as a cause of Permian glaciation.
Moreover, if the Permian or Proterozoic glacial periods
were so cold that the lands above latitude 30*^ were snow-
covered most of the time, the normal surface winds in
subtropical latitudes would be largely equatorward, just
as the winter monsoons now are. Hence little or no mois-
ture would be available to feed the snowfields which give
rise to the glaciers.
It has been assumed by Marsden Manson and others
that increased general cloudiness would account for the
subtropical glaciation of the Permian and Proterozoic.
Granting for the moment that there could be universal
persistent cloudiness, this would not prevent or counter-
act the outblowing anti-cyclonic winds so characteristic
of great snowfields. Therefore, under the hypothesis of
general cloudiness there would be no supply of moisture
to cause glaciation in low latitudes. Indeed, persistent
cloudiness in all higher latitudes would apparently de-
prive the Himalayas of most of their present moisture.
148 CLIMATIC CHANGES
for the interior of Asia would not become hot in summer
and no inblowing monsoons would develop. In fact, winds
of all kinds would seemingly be scarce, for they arise
almost wholly from contrasts of temperature and hence
of atmospheric pressure. The only way to get winds and
hence precipitation would be to invoke some other agency,
such as cyclonic storms, but that would be a departure
from the supposition that glaciation arose from cloudi-
ness.
Let us now inquire how the cyclonic hypothesis
accounts for glaciation in low latitudes. We will first
consider the terrestrial conditions in the early Permian,
the last period of glaciation in such latitudes. Geologists
are almost universally agreed that the lands were excep-
tionally extensive and also high, especially in low lati-
tudes. One evidence of this is the presence of abundant
conglomerates composed of great boulders. It is also
probable that the carbon dioxide in the air during the
early Permian had been reduced to a minimum by the
extraordinary amount of coal formed during the preced-
ing period. This would tend to produce low temperature
and thus make the conditions favorable for glaciation as
soon as an accentuation of solar activity caused unusual
storminess. If the storminess became extreme when ter-
restrial conditions were thus universally favorable to
glaciation, it would presumably produce glaciation in low
latitudes. Numerous and intense tropical cyclones would
carry a vast amount of moisture out of the tropics, just
as now happens when the sun is active, but on a far
larger scale. The moisture would be precipitated on the
equatorward slopes of the subtropical mountain ranges.
At high elevations this precipitation would be in the form
of snow even in summer. Tropical cyclones, however, as
is shown in Earth and Sun, occur in the autumn and
SOME PROBLEMS OF GLACIAL PERIODS 149
winter as well as in summer. For example, in the Bay of
Bengal the number recorded in October is fifty, the
largest for any month; while in November it is thirty-
f onr, and December fourteen as compared with an aver-
age of forty-two for the months of July to September.
From January to March, when sunspot numbers aver-
aged more than forty, the number of tropical hurricanes
was 143 per cent greater than when the sunspot numbers
averaged below forty. During the months from April to
June, which also would be times of considerable snowy
precipitation, tropical hurricanes averaged 58 per cent
more numerous with sunspot niunbers above forty than
with numbers below forty, while from July to September
the difference amounted to 23 per cent. Even at this
season some snow falls on the higher slopes, while the
increased cloudiness due to numerous storms also tends
to preserve the snow. Thus a great increase in the fre-
quency of sunspots is accompanied by increased intensity
of tropical hurricanes, especially in the cooler autumn and
spring months, and results not only in a greater accumu-
lation of snow but in a decrease in the melting of the
snow because of more abundant clouds. At such times as
the Permian, the general low temperature due to rapid
convection and to the scarcity of carbon dioxide pre-
sumably joined with the extension of the lands in pro-
ducing great high-pressure areas over the lands in middle
latitudes during the winters, and thus caused the more
northern, or mid-latitude type of cyclonic storms to be
shifted to the equatorward side of the continents at that
season. This would cause an increase of precipitation in
winter as well as during the months when tropical hurri-
canes abound. Many other circumstances would cooper-
ate to produce a sunilar result. For example, the general
low temperature would cause the sea to be covered with
160 CLIMATIC CHANGES
ice in lower latitudes than now, and would help to create
high-pressure areas in middle latitudes, thus driving the
storms far south. If the sea water were fresher than now,
as it probably was to a notable extent in the Proterozoic
and perhaps to some slight extent in the Permian, the
higher freezing point would also further the extension
of the ice and help to keep the storms away from high
latitudes. If to this there is added a distribution of land
and sea such that the volume of the warm ocean currents
flowing from low to high latitudes was diminished, as
appears to have been the case, there seems to be no diffi-
culty in explaining the subtropical location of the main
glaciation in both the Permian and the Proterozoic. An
increase of storminess seems to be the key to the whole
situation.
One other possibility may be mentioned, although little
stress should be laid on it. In Earth and Sun it has been
shown that the main storm track in both the northern
and southern hemispheres is not concentric with the
geographical poles. Both tracks are roughly concentric
with the corresponding magnetic poles, a fact which-may
be important in connection with the hypothesis of an elec-
trical effect of the sun upon terrestrial storminess. The
magnetic poles are known to wander considerably. Such
wandering gives rise to variations in the direction of
the magnetic needle from year to year. In 1815 the com-
pass in England pointed 241/2° W. of N. and in 1906
17° 45' W. Such a variation seems to mean a change of
many miles in the location of the north magnetic pole.
Certain changes in the daily march of electromagnetic
phenomena over the oceans have led Bauer and his asso-
ciates to suggest that the magnetic poles may even be
subject to a slight daily movement in response to the
changes in the relative positions of the earth and sun.
SOME PROBLEMS OF GLACIAL PERIODS 161
Thus there seems to be a possibility that a pronounced
change in the location of the magnetic pole in Permian
times, for example, may have had some connection with
a shifting in the location of the belt of storms. It must be
clearly understood that there is as yet no evidence of any
such change, and the matter is introduced merely to call
attention to a possible line of investigation.
Any hypothesis of Permian and Proterozoic gladation
must explain not only the glaciation of low latitudes but
the lack of glaciation and tiie accumulation of red desert
beds in high latitudes. The facts already presented seem
to explain this. Glaciation could not occur extensively in
high latitudes partly because during most of the year the
air was too cold to hold much moisture, but still more
because the winds for the most part must have blown
outward from the cold northern areas and the cyclonic
storm belt was pushed out of high latitudes. Because of
these conditions precipitation was apparently limited to
a relatively small number of storms during the smnmer.
Hence great desert areas must have prevailed at high
latitudes. Great aridity now prevails north of the Hima-
layas and related ranges, and red beds are accumulating
in the centers of the great deserts, such as those of the
Tarim Basin and the Transcaspian. The redness is not
due to the original character of the rock, but to intense
oxidation, as appears from the fact that along the edges
of the desert and wherever occasional floods carry sedi-
ment far out into the midst of the sand, the material has
the ordinary brownish shades. As soon as one goes out
into the places where the sand has been exposed to the air
for a long time, however, it becomes pink, and then red.
Such conditions may have given rise to the high degree
of oxidation in the famous Permian red beds. If the air
of the early Permian contained an unusual percentage of
152 CLIMATIC CHANGES
oxygen because of the release of that gas by the great
plant beds which formed coal in the preceding era, as
Chamberlin has thought probable, the tendency to pro-
duce red beds would be still further increased.
It must not be supposed, however, that these condi-
tions would absolutely limit glaciation to subtropical
latitudes. The presence of early Permian glaciation in
North America at Boston and in Alaska and in the Falk-
land Islands of the South Atlantic Ocean proves that at
least locally there was sufficient moisture to form glaciers
near the coast in relatively high latitudes. The possibility
of this would depend entirely upon the form of the lands
and the consequent course of ocean currents. Even in
those high latitudes cyclonic storms would occur unless
they were kept out by conditions of pressure such as have
been described above.
The marine faunas of Permian age in high latitudes
have been interpreted as indicating mild oceanic tempera-
tures. This is a point which requires further investiga-
tion. Warm oceans during times of slight solar activity
are a necessary consequence of the cyclonic hypothesis,
as will appear later. The present cold oceans seem to be
the expectable result of the Pleistocene glaciation and of
the present relatively disturbed condition of the sun. If
a sudden disturbance threw the solar atmosphere into
violent commotion within a few thousand years during
Permian times, glaciation might occur as described above,
while the oceans were still warm. In fact their warmth
would increase evaporation while the violent cyclonic
storms and high winds would cause heavy rain and keep
the air cool by constantly raising it to high levels where
it would rapidly radiate its heat into space.
Nevertheless it is not yet possible to determine how
warm the oceans were at the actual time of the Permian
SOME PROBLEMS OF GLACIAL PERIODS 168
glaciation. Some faunas formerly reported as Permian
are now known to be considerably older. Moreover, others
of undoubted Permian age are probably not strictly con-
temporaneous with the glaciation. So far back in the
geological record it is very doubtful whether we can date
fossils within the limits of say 100,000 years. Yet a dif-
ference of 100,000 years would be more than enough to
allow the fossils to have lived either before or after the
glaciation, or in an inter-glacial epoch. One such epoch
is known to have occurred and nine others are suggested
by the inter-stratification of glacial till and marine sedi-
ments in eastern Australia. The warm currents which
would flow poleward in inter-glacial epochs must have
favored a prompt reintroduction of marine faunas driven
out during times of glaciation. Taken all and all, the
Permian glaciation seems to be accounted for by the
cyclonic hypothesis quite as well as does the Pleistocene.
In both these cases, as weU as in the various pulsations
of historic times, it seems to be necessary merely to mag-
nify what is happening today in order to reproduce the
conditions which prevailed in the past. If the conditions
which now prevail at times of sunspot minima were mag-
nified, they would give the mild conditions of inter-glacial
epochs and similar periods. If the conditions which now
prevail at times of sunspot maxima are magnified a little
they seem to produce periods of climatic stress such as
those of the fourteenth century. If they are magnified
still more the result is apparently glacial epochs like
those of the Pleistocene, and if they are magnified to a
still greater extent, the result is Permian or Proterozoic
glaciation. Other factors must indeed be favorable, for
climatic changes are highly complex and are unques-
tionably due to a combination of circumstances. The point
which is chiefly emphasized in this book is that among
154 CLIMATIC CHANGES
those several circumstances, changes in cyclonic storms
due apparently to activity of the snn's atmosphere must
always be reckoned.
CHAPTER IX
THE ORIGIN OF LOESS
ONE of the most remarkable formations associ-
ated with glacial deposits consists of vast sheets
of the fine-grained, yellowish, wind-blown ma-
terial called loess. Somewhat peculiar climatic condi-
tions evidently prevailed when it was formed. At present
similar deposits are being laid down only near the lee-
ward margin of great deserts. The famous loess deposits
of China in the lee of the Desert of Gobi are examples.
During the Pleistocene period, however, loess accumu-
lated in a broad zone along the margin of the ice sheet
at its maximum extent. In the Old World it extended
from France across Germany and through the Black
Earth region of Russia into Siberia. In the New World
a still larger area is loess-covered. In the Mississippi
Valley, tens of thousands of square miles are mantled by
a layer exceeding twenty feet in thickness and in many
places approaching a hundred feet. Neither the North
American nor the European deposits are associated with
a desert. Indeed, loess is lacking in the western and
drier parts of the great plains and is best developed in
the well-watered states of Iowa, Illinois, and Missouri.
Part of the loess overlies the non-glacial materials of the
great central plain, but the northern portions overlie the
drift deposits of the first three glaciations. A few traces
of loess are associated with the Kansan and Illinoian,
the second and third glaciations, but most of the Ameri-
I
166 CLIMATIC CHANGES
can loess appears to have been formed at approximately
the time of the lowan or fourth glaciation, while only a
little overlies the drift sheets of the Wisconsin age. The j
loess is thickest near the margin of the lowan till sheet
and thins progressively both north and south. The
thinning southward is abrupt along the stream divides,
but very gradual along the larger valleys. Indeed, loess is
abundant along the bluffs of the Mississippi, especially
the east bluff, almost to the Gulf of Mexico.^
It is now generally agreed that all typical loess is wind
blown. There is still much question, however, as to its
time of origin, and thus indirectly as to its climatic im-
plications. Several American and European students
have thought that the loess dates from inter-glacial times.
On the other hand, Penck has concluded that the loess
was formed shortly before the commencement of the
glacial epochs ; while many American geologists hold that
the loess accumulated while the ice sheets were at ap-
proximately their maximum size. W. J. McGee, Cham-
berlin and Salisbury, Keyes, and others lean toward this
view. In this chapter the hypothesis is advanced that it
was formed at the one other possible time, namely, imme-
diately following the retreat of the ice.
These four hypotheses as to the time of origin of loess
imply the following differences in its climatic relations.
If loess was formed during typical inter-glacial epochs,
or toward the close of such epochs, profound general
aridity must seemingly have prevailed in order to kill
off the vegetation and thus enable the wind to pick up
sufficient dust. If the loess was formed during times of
extreme glaciation when the glaciers were supplying
large quantities of fine material to outflowing streams,
less aridity would be required, but there must have been
1 Chamberlin and Salisbury: Geology, 1906^ Vol. Ill, pp. 405-412.
THE ORIGIN OF LOESS 167
sharp contrasts between wet seasons in summer when
the snow was melting and dry seasons in winter when
the storms were forced far south by the glacial high pres-
sare. Alternate floods and droughts would thus affect
broad areas along the streams. Hence arises the hypothe-
sis that the wind obtained the loess from the flood plains
of streams at times of maximum glaciation. If the loess
was formed during the rapid retreat of the ice, alternate
summer floods and winter droughts would still prevail,
but much material could also be obtained by the winds
not only from flood plains, but also from the deposits
exposed by the melting of the ice and not yet covered by
vegetation.
The evidence for and against the several hypotheses
may be stated briefly. In support of the hypothesis of the
inter-glacial origin of loess, Shimek and others state that
the glacial drift which lies beneath the loess commonly
gives evidence that some time elapsed between the dis-
appearance of the ice and the deposition of the loess. For
example, abundant shells of land snails in the loess are
not of the sort now found in colder regions, but resemble
those found in the drier regions. It is probable that if
they represented a glacial epoch they would be depauper-
ated by the cold as are the snails of far northern regions.
The gravel pavement discussed below seems to be strong
evidence of erosion between the retreat of the ice and
the deposition of the loess.
Turning to the second hypothesis, namely, that the
loess accumulated near the close of the inter-glacial epoch
rather than in the midst of it, we may follow Penck. The
mammalian fossils seem to him to prove that the loess
was formed while boreal animals occupied the region, for
they include remains of the hairy manmaoth, woolly rhi-
noceros, and reindeer. On the other hand, the typical
158 CLIMATIC CHANGES
inter-glacial beds not far away yield remains of species
characteristic of milder climates, such as the elephant,
the smaller rhinoceros, and the deer. In connection with
these facts it should be noted that occasional remains of
tundra vegetation and of trees are found beneath the
loess, while in the loess itself certain steppe animals,
such as the common gopher or spermaphyl, are found.
Penck interprets this as indicating a progressive desicca-
tion culminating just before the oncoming of the next ice
sheet.
The evidence advanced in favor of the hypothesis that
the loess was formed when glaciation was Sar its maxi-
mum includes the fact that if the loess does not represent
the outwash from the lowan ice, there is little else that
does, and presumably there must have been outwash.
Also the distribution of loess along the margins of
streams suggests that much of the material came from
the flood plains of overloaded streams flowing from the
melting ice.
Although there are some points in favor of the hy-
pothesis that the loess originated (1) in strictly inter-
glacial times, (2) at the end of inter-glacial epochs, and
(3) at times of full glaciation, each hypothesis is much
weakened by evidence that supports the others. The evi-
dence of boreal animals seems to disprove the hypothesis
that the loess was formed in the middle of a mild inter-
glacial epoch. On the other hand, Penck 's hypothesis as
to loess at the end of inter-glacial times fails to account
for certain characteristics of the lowest part of the loess
deposits and of the underlying topography. Instead of
normal valleys and consequent prompt drainage such as
ought to have developed before the end of a long inter-
glacial epoch, the surface on which the loess lies shows
many undrained depressions. Some of these can be seen
THE ORIGIN OF LOESS 169
in exposed banks, while many more are inferred from the
presence of shells of pond snails here and there in the
overlying loess. The pond snails presumably lived in
shallow pools occupying depressions in the uneven sur-
face left by the ice. Another reason for questioning
whether the loess was formed at the end of an inter-
glacial epoch is that this hypothesis does not provide a
reasonable origin for the material which composes the
loess. Near the Alps where the loess deposits are small
and where glaciers probably persisted in the inter-glacial
epochs and thus supplied flood plaia material in large
quantities, this does not appear important. In the broad
upper Mississippi Basin, however, and also in the Black
Earth region of Russia there seems to be no way to get
the large body of material composing the loess except by
assuming the existence of great deserts to windward.
But there seems to be little or no evidence of such deserts
where they could be effective. The mineralogical char-
acter of the loess of lowan age proves that the material
came from granitic rocks, such as formed a large part of
the drift. The nearest extensive outcrops of granite are
in the southwestern part of the United States, nearly a
thousand miles from Iowa and Illinois. But the loess is
thickest near the ice margin and thins toward the south-
west and in other directions, whereas if its source were
the southwestern desert, its maximum thickness would
probably be near the margin of the desert.
The evidence cited above seems inconsistent not only
with the hypothesis that the loess was formed at the end
of an inter-glacial epoch, but also with the idea that it
originated at times of maximum glaciation either from
river-borne sediments or from any other source. A
further and more convincing reason for this last con-
clusion is the probability and almost the certainty that
160 CLIMATIC CHANGES
when the ice advanced, its front lay close to areas where
the vegetation was not much thinner than that which
today prevails under similar climatic conditions. If the
average temperature of glacial maxima was only 6°G.
lower than that of today, the conditions just beyond the
ice front when it was in the loess region from southern
Illinois to Minnesota would have been like those now pre-
vailing in Canada from New Brunswick to Winnipeg.
The vegetation there is quite different from the grassy,
semi-arid vegetation of which evidence is found in the
loess. The roots and stalks of such grassy vegetation are
generally agreed to have helped produce the columnar
structure which enables the loess to stand with almost
vertical surfaces.
We are now ready to consider the probability that loess
accumulated mainly during the retreat of the ice. Such a
retreat exposed a zone of drift to the outflowing glacial
winds. Most glacial hypotheses, such as that of uplift,
or depleted carbon dioxide, call for a gradual retreat
of the ice scarcely faster than the vegetation could ad-
vance into the abandoned area. Under the solar-cyclonic
hypothesis, on the other hand, the climatic changes may
have been sudden and hence the retreat of the ice may
have been much more rapid than the advance of vegeta-
tion. Now wind-blown materials are derived from places
where vegetation is scanty. Scanty vegetation on good
soil, it is true, is usually due to aridity, but may also
result because the time since the soil was exposed to the
air has not been long enough for the soil to be sufficiently
weathered to support vegetation. Even when weathering
has had full opportunity, as when sand bars, mud flats,
and flood plains are exposed, vegetation takes root only
slowly. Moreover, storms and violent winds may prevent
the spread of vegetation, as is seen on sandy beaches even
THE ORIGIN OF LOESS 161
in distinctly humid regions like New Jersey and Den-
mark. Thus it appears that unless the retreat of the ice
were as slow as the advance of vegetation, a barren area
of more or less width must have bordered the retreating
ice and formed an ideal source of loess.
Several other lines of evidence seemingly support the
conclusion that the loess was formed during the retreat
of the ice. For example, Shimek, who has made almost
a lifelong study of the lowan loess, emphasizes the fact
that there is often an accumulation of stones and pebbles
at its base. This suggests that the underlying till was
eroded before the loess was deposited upon it. The first
reaction of most students is to assume that of coui'se
this was due to running water. That is possible in many
cases, but by no means in all. So widespread a sheet of
gravel could not be deposited by streams without destroy*-
ing the irregular basins and hollows of which we have
seen evidence where the loess lies on glacial deposits. On
the other hand, the wind is competent to produce a simi*-
lar gravel pavement without disturbing the old topog-
raphy. ** Desert pavements'' are a notable feature in most
deserts. On the edges of an ice sheet, as Hobbs has made
us realize, the commonest winds are outward. They often
attain a velocity of eighty miles an hour in Antarctica
and Greenland. Such winds, however, usually decline
rapidly in velocity only a few score miles from the ice.
Thus their effect would be to produce rapid erosion
of the freshly bared surface near the retreating ice.
The pebbles would be left behind as a pavement, while
sand and then loess would be deposited farther from the
ice where the winds were weaker and where vegetation
was beginning to take root. Such a decrease in wind
velocity may explain the occasional vertical gradation
from gravel through sand to coarse loess and then to
162 CLIMATIC CHANGES
normal fine loess. As the ice sheet retreated the wind in
any given place would gradually become less violent.
As the ice continued to retreat the area where loess was
deposited would follow at a distance, and thus each part
of the gravel pavement would in turn be covered with the
loess.
The hypothesis that loess is deposited while the ice is
retreating is in accord with many other lines of evidence.
For example, it accords with the boreal character of the
mammal remains as described above. Again, the advance
of vegetation into the barren zone along the front of the
ice would be delayed by the strong outblowing winds.
The common pioneer plants depend largely on the wind
for the distribution of their seeds, but the glacial winds
would carry them away from the ice rather than toward
it. The glacial winds discourage the advance of vegeta-
tion in another way, for they are drying winds, as are
almost all winds blowing from a colder to a warmer
region. The fact that remains of trees sometimes occur
at the bottom of the loess probably means that the depo-
sition of loess extended into the forests which almost
certainly persisted not far from the ice. This seems more
likely than that a period of severe aridity before the ad-
vance of the ice killed the trees and made a steppe or
desert. Penck's chief argument in favor of the formation
of loess before the advance of the ice rather than after,
is that since loess is lacking upon the youngest drift sheet
in Europe it must have been formed before rather than
after the last or Wiirm advance of the ice. This breaks
down on two counts. First, on the corresponding (Wis-
consin) drift sheet in America, loess is present, — ^in small
quantities to be sure, but unmistakably present. Second,
there is no reason to assume that conditions were identi-
cal at each advance and retreat of the ice. Indeed, the
THE ORIGIN OF LOESS 168
fact that in Europe, as in the United States, nearly all
the loess was formed at one time, and only a little is asso-
ciated with the other ice advances, points clearly against
Penck's fundamental assumption that the accumulation
of loess was due to the approach of a cold climate.
Having seen that the loess was probably formed during
the retreat of the ice, we are now ready to inquire what
conditions the cyclonic hypothesis would postulate in the
loess areas during the various stages of a glacial cycle.
Fig. 2, in Chapter IV, gives the best idea of what would
apparently happen in North America, and events in Europe
would presumably be similar. During the nine maximmn
years on which Fig. 2 is based the sunspot numbers aver-
aged seventy, while during the nine minimum years they
averaged less than five. It seems fair to suppose that the
maximum years represent the average conditions which
prevailed in the past at times when the sun was in a
median stage between the full activity which led to glacia-
tion and the mild activity of the minimum years which
appear to represent inter-glacial conditions. This would
mean that when a glacial period was approaching, but
before an ice sheet had accumulated to any great extent,
a crescent-shaped strip from Montana through Illinois to
Maine would suffer a diminution in storminess ranging
up to 60 per cent as compared with inter-glacial condi-
tions. This is in strong contrast with an increase in
storminess amounting to 75 or even 100 per cent both in
the boreal storm belt in Canada and in the subtropical
belt in the Southwest. Such a decrease in storminess in
the central United States would apparently be most
noticeable in summer, as is shown in Earth and Sun.
Hence it would have a maximum effect in producing
aridity. This would favor the formation of loess, but it is
doubtful whether the aridity would become extreme
164 CLIMATIC CHANGES
enough to explain such vast deposits as are found
throughout large parts of the Mississippi Basin. That
would demand that hundreds of thousands of square miles
should become almost absolute desert, and it is not prob-
able that any such thing occurred. Nevertheless, accord-
ing to the cyclonic hypothesis the period inmiediately
before the advent of the ice would be relatively dry in
the central United States, and to that extent favorable to
the work of the wind.
As the climatic conditions became more severe and the
ice sheet expanded, the dryness and lack of storms would
apparently diminish. The reason, as has been explained,
would be the gradual pushing of the storms southward
by the high-pressure area which would develop over the
ice sheet. Thus at the height of a glacial epoch there
would apparently be great storminess in the area where
the loess is found, especially in sunmaer. Hence the
cyclonic hypothesis does not accord with the idea of great
deposition of loess at the time of maximum glaciation.
Finally we come to the time when the ice was retreat-
ing. We have already seen that not only the river flood
plains, but also vast areas of fresh glacial deposits would
be exposed to the winds, and would remain without vege-
tation for a long time. At that very time the retreat of
the ice sheet would tend to permit the storms to follow
paths determined by the degree of solar activity, in place
of the far southerly paths to which the high atmospheric
pressure over the expanded ice sheet had previously
forced them. In other words, the conditions shown in
Fig. 2 would tend to reappear when the sun^s activity
was diminishing and the ice sheet was retreating, just as
they had appeared when the sun was becoming more
active and the ice sheet was advancing. This time, how-
ever, the semi-arid conditions arising from the scarcity
THE ORIGIN OF LOESS 165
of storms would prevail in a region of glacial deposits
and widely spreading river deposits, few or none of
which would be covered with vegetation. The conditions
would be almost ideal for eolian erosion and for the
transportation of loess by the wind to areas a little more
remote from the ice where grassy vegetation had made a
start
The cyclonic hypothesis also seems to offer a satis-
factory explanation of variations in the amount of loess
associated with the several glacial epochs. It attributes
these to differences in the rate of disappearance of the
ice, which in turn varied with the rate of decline of solar
activity and storminess. This is supposed to be the reason
why the lowan loess deposits are much more extensive
than those of the other epochs, for the lowan ice sheet
presumably accomplished part of its retreat much more
suddenly than the other ice sheets.* The more sudden the
retreat, the greater the barren area where the winds
could gather fine bits of dust. Tempoirary readvances may
also have been so distributed and of such intensity that
they frequently accentuated the condition shown in Fig.
2, thus making the central United States dry soon after
the exposure of great amounts of glacial debris. The
closeness with which the cyclonic hypothesis accords with
the facts as to the loess is one of the pleasant surprises of
the hypothesis. The first draft of Fig. 2 and the first out-
lines of the hypothesis were framed without thought of
the loess. Yet so far as can now be seen, both agree
closely with the conditions of loess formation.
sit maj have retreated soon after reaching its mazimnm. If so, the
general lack of thick terminal moraines would be explained. See page 122.
CHAPTER X
CAUSES OF MILD GEOLOGICAL CLIMATES
IN discussions of climate, as of most subjects, a
peculiar psychological phenomenon is observable.
Everyone sees the necessity of explaining conditions
different from those that now exist, but few realize that
present conditions may be abnormal, and that they need
explanation just as much as do others. Because of this
tendency glaciation has been discussed with the greatest
fullness, while there has been much neglect not only of
the periods when the climate of the earth resembled that
of the present, but also of the vastly longer periods when
it was even milder than now.
How important the periods of mild climate have been
in geological times may be judged from the relative
length of glacial compared with inter-glacial epochs, and
still more from the far greater relative length of the mild
parts of periods and eras when compared with the severe
parts. Recent estimates by R. T. Chamberlin^ indicate
that according to the consensus of opinion among geolo-
gists the average inter-glacial epoch during the Pleisto-
cene was about five times as long as the average glacial
epoch, while the whole of a given glacial epoch averaged
five times as long as the period when the ice was at a
maximum. Climatic periods far milder, longer, and more
monotonous than any inter-glacial epoch appear repeat-
1 BoUin T. Ghamberlin : Personal Communication.
CAUSES OF MILD GEOLOGICAL CLIMATES 167
edly during the course of geological history. Our task in
this chapter is to explain them.
Knowlton^ has done geology a great service by col-
lecting the evidence as to the mild type of climate which
has again and again prevailed in the past. He lays special
stress on botanical evidence since that pertains to the
variable atmosphere of the lands, and hence furnishes a
better guide than does the evidence of animals that lived
in the relatively unchanging water of the oceans. The
nature of the evidence has already been indicated in
various parts of this book. It includes palms, tree ferns,
and a host of other plants which once grew in regions
which are now much too cold to support them. With this
must be placed the abundant reef-building corals and
other warmth-loving marine creatures in latitudes now
much too cold for them. Of a piece with this are the condi-
tions of inter-glacial epochs in Europe, for example,
when elephants and hippopotamuses, as well as many
species of plants from low latitudes, were abundant.
These conditions indicate not only that the climate was
warmer than now, but that the contrast from season to
season was much less. Indeed, Ejiowlton goes so far as
to say that ** relative uniformity, mildness, and compara-
tive equability of climate, accompanied by high hmnidity,
have prevailed over the greater part of the earth, extend-
ing to, or into, polar circles, during the greater part of
geologic time — since, at least, the Middle Paleozoic. This
is the regular, the ordinary, the normal condition. * * . . .
**By many it is thought that one of the strongest argu-
ments against a gradually cooling globe and a humid,
non-zonally disposed climate in the ages before the Pleis-
tocene is the discovery of evidences of glacial action
2 F. H. Enowlton : Evolution of Geologic Glimatee ; Bull. Geol. Soe. Am.,
VoL 30, 1919, pp. 499-566.
168 CLIMATIC CHANGES
practically throughout the entire geologic column.
Hardly less than a dozen of these are now known, ranging
in age from Huronian to Eocene. It seems to be a very
general assumption by those who hold this view that
these evidences of glacial activities are to be classed as
ice ageSy largely comparable in effect and extent to the
Pleistocene refrigeration, but as a matter of fact only
three are apparently of a magnitude to warrant such
designation. These are the Huronian glaciation, that of
the * Permo-Carbonif erous, ' and that of the Pleistocene.
The others, so far as available data go, appear to be
explainable as more or less local manifestations that had
no widespread effect on, for instance, ocean tempera-
tures, distribution of life, et cetera. They might well have
been of the type of ordinary mountain glaciers, due en-
tirely to local elevation and precipitation. ' * . . . * * If the
sun had been the principal source of heat in pre-Pleisto-
cene time, terrestrial temperatures would of necessity
have been disposed in zones, whereas the whole trend of
this paper has been the presentation of proof that these
temperatures were distinctly non-zonal. Therefore it
seems to follow that the sun — at least the present small-
angle sun — could not have been the sole or even the prin-
cipal source of heat that warmed the early oceans. ' '
Kiiowlton is so strongly impressed by the widespread
fossil floras that usually occur in the middle parts of the
geological periods, that as Schuchert' puts it, he neglects
the evidence of other kinds. In the middle of the periods
and eras the expansion of the warm oceans over the con-
tinents was greatest, while the lands were small and
hence had more or less insular climates of the oceanic
type. At such times, the marine fauna agrees with the
sChas. Schuchert: Beview of Knowlton's Evolution of Geological Cli-
mates, in Am. Jour. Sci., 1921.
CAUSES OF MILD GEOLOGICAL CLIMATES 169
flora in indicating a mild climate. Large colony-forming
foraminifera, stony corals, shelled cephalopods, gastro-
pods and thick-shelled bivalves, generally the cemented
forms, were common in the Far North and even in the
Arctic. This occurred in the Silurian, Devonian, Penn-
sylvanian, and Jurassic periods, yet at other times, such
as the Cretaceous and Eocene, such forms were very
greatly reduced in variety in the northern regions or else
wholly absent. These things, as Schuchert' says, can only
mean that Ejiowlton is right when he states that '^ cli-
matic zoning such as we have had since the beginning of
the Pleistocene did not obtain in the geologic ages prior
to the Pleistocene.'^ It does not mean, however, that
there was a ** non-zonal arrangement and that the tem-
perature of the oceans was everywhere the same and
** without widespread effect on the distribution of life.*'
Students of paleontology hold that as far back as we
can go in the study of plants, there are evidences of sea-
sons and of relatively cool climates in high latitudes. The
cycads, for instance, are one of the types most often used
as evidence of a warm climate. Yet Wieland,^ who has
made a lifelong study of these plants, says that many of
them ** might well grow in temperate to cool climates.
Until far more is learned about them they should at least
be held as valueless as indices of tropic climates." The
inference is **that either they or their close relatives had
the capacity to live in every clime. There is also a sus-
picion that study of the associated ferns may compel re-
vision of the long-accepted view of the universality of
tropic climates throughout the Mesozoic." Nathorst is
quoted by Wieland as saying, * * I think . . . that during
the time when the Gingkophytes and Cycadophytes domi-
^G. B. Wieland: Distribution and Belationships of the Cyeadeoids; Am.
Jour. Bot, Vol. 7, 1920, pp. 125-145.
170 CLIMATIC CHANGES
natedy many of them must have adapted themselves for
living in cold climates also. Of this I have not the least
doubt/'
Another important line of evidence which Knowlton
and others have cited as a proof of the non-zonal arrange-
ment of climate in the past, is the vast red beds which are
found in the Proterozoic, late Silurian, Devonian, Per-
mian, and Triassic, and in some Tertiary formations.
These are believed to resemble laterite, a red and highly
oxidized soil which is found in great abundance in equa-
torial regions. Knowlton does not atteitipt to show that
the red beds present equatorial characteristics in other
respects, but bases his conclusion on the statement that
'^red beds are not being formed at the present time in
any desert region. '^ This is certainly an error. As has
already been said, in both the Transcaspian and TaMa
Makan deserts, the color of the sand regularly changes
from brown on the borders to pale red far out in the
desert. Kuzzil Kum, or Bed Sand, is the native name.
The sands in the center of the desert apparently were
originally washed down from the same mountains as
those on the borders, and time has turned them red.
Since the same condition is reported from the Arabian
Desert, it seems that redness is characteristic of some of
the world's greatest deserts. Moreover, beds of salt and
gypsum are regularly found in red beds, and they can
scarcely originate except in deserts, or in shallow ahnost
landlocked bays on the coasts of deserts, as appears to
have happened in the Silurian where marine fossils are
found interbedded with gypsum.
Again, Ejiowlton says that red beds cannot indicate
deserts because the plants found in them are not
'* pinched or depauperate, nor do they indicate xero-
phytic adaptations. Moreover, very considerable deposits
CAUSES OF MILD GEOLOGICAL CLIMATES 171
of coal are found in red beds in many parts of the world,
which implies the presence of swamps but little above
sea-level.*^
Students of desert botany are likely to doubt the force
of these considerations. As MacDougaP has shown, the
variety of plants in deserts is greater than in moist
regions. Not only do xerophytic desert species prevail,
but halophytes are present in the salty areas, and hygro-
phytes in the wet swampy areas, while ordinary meso-
phytes prevail along the water courses and are washed
down from the mountains. The ordinary plants, not the
xerophytes, are the ones that are chiefly preserved since
they occur in most abundance near streams where deposi-
tion is taking place. So far as swamps are concerned, few
are of larger size than those of Seistan in Persia, Lop
Nor in Chinese Turkestan, and certain others in the midst
of the Asiatic deserts. Streams flowing from the moun-
tains into deserts are almost sure to form large swamps,
such as those along the Tarim Biver in central Asia.
Lake Chad in Africa is another example. In it, too, reeds
are very numerous.
Putting together the evidence on both sides in this dis-
puted question, it appears that throughout most of geo-
logical time there is some evidence of a zonal arrange-
ment of climate. The evidence takes the form of traces of
cool climates, of seasons, and of deserts. Nevertheless,
there is also strong evidence that these conditions were
in general less intense than at present and that times of
relatively warm, moist climate without great seasonal
extremes have prevailed very widely during periods
much longer than those when a zonal arrangement as
8D. T. MacDongal: Botanical Features of North American Deserts;
Carnegie Instit. of Wash., No. 99, 1908.
172 CLIMATIC CHANGES
marked as that of today prevailed. As Schuchert* puts it :
* * Today the variation on land between the tropics and the
poles is roughly between 110** and — 60°F., in the oceans
between 85^ and Sl^'F. In the geologic past the tempera-
ture of the oceans for the greater parts of the periods
probably was most often between 85° and 55°F,, while on
land it may have varied between 90'' and 0°F. At rare
intervals the extremes were undoubtedly as great as they
are today. The conclusion is therefore that at all times
the earth had temperature zones^ varying between the
present-day intensity and times which were almost with-
out such belts, and at these latter times the greater part
of the earth had an almost uniformly mild climate, with-
out winters. ' '
It is these mild climates which we must now attempt
to explain. This leads us to inquire what would happen to
the climate of the earth as a whole if the conditions which
now prevail at times of few sunspots were to become
intensified. That they could become greatly intensified
seems highly probable, for there is good reason to think
that aside from the sunspot cycle the sun's atmosphere
is in a disturbed condition. The prominences which
sometimes shoot out hundreds of thousands of miles
seem to be good evidence of this. Suppose that the sun's
atmosphere should become very quiet. This would appar-
ently mean that cyclonic storms would be much less
numerous and less severe than during the present times
of sunspot minima. The storms would also apparently
follow paths in middle latitudes somewhat as they do
now when sunspots are fewest. The first effect of such a
condition, if we can judge from what happens at present,
would be a rise in the general temperature of the earth,
because less heat would be carried aloft by storms.
• Loe, cit.
CAUSES OF MILD GEOLOGICAL CLIMATES 178
Today, as is shown in Earth and Sun, a difference of
perhaps 10 per cent in the average storminess during
periods of sunspot maxima and minima is correlated with
a difference of S'^C. in the temperature at the earth's
surface. This includes not only an actual lowering of
0.6'' C. at times of sunspot maxima, but the overcoming
of the effect of increased insolation at such times, an
effect which Abbot calculates as about 2.5° C. If the
storminess were to be reduced to one-half or one-quarter
its present amount at sunspot minima, not only would the
loss of heat by upward convection in storms be dimin-
ished, but the area covered by clouds would diminish so
that the sun would have more chance to warm the lower
air. Hence the average rise of temperature might amount
to as much at 5° or 10° C.
Another effect of the decrease in storminess would be
to make the so-called westerly winds, which are chiefly
southwesterly in the northern hemisphere and north-
westerly in tiie southern hemisphere, more strong and
steady than at present. They would not continually suffer
interruption by cyclonic winds from other directions, as
is now the case, and would have a regularity like that
of the trades. This conclusion is strongly reenforced in
a paper by Clayton^ which came to hand after this chap-
ter had been completed. From his studies of the solar
constant and the temperature of the earth which are
described in Earth and Sun, he reaches the following
conclusion: **The results of these researches have led
me to believe : 1. That if there were no variation in solar
radiation the atmospherio motions would establish a
stable system with exchanges of air between equator and
pole and between ocean and land, in which the only varia-
7 H. H. Clayton : Variation in Solar Badiation and the Weather ; Smiths.
Miec ColL, Vol. 71, No. 3, Washington, 1920.
174 CLIMATIC CHANGES
tions would be daily and annual changes set in operation
by the relative motions of the earth and sun. 2. The exist-
ing abnormal changes, which we call weather, have their
origins chiefly, if not entirely, in the variations of solar
radiation. * *
If cyclonic storms and ** weather^' were largely elimi-
nated and if the planetary system of winds with its
steady trades and southwesterlies became everywhere
dominant, the regularity and volume of the poleward-
flowing currents, such as the Gulf Stream and the
Atlantic Drift in one ocean, and the Japanese Current in
another, would be greatly increased. How important this
is may be judged from the work of Helland-Hansen and
Nansen.* These authors find that with the passage of each
cyclonic storm there is a change in the temperature of
the surface water of the Atlantic Ocean. Winds at right
angles to the course of the Drift drive the water first in
one direction and then in the other but do not advance it
in its course. Winds with an easterly component, on the
other hand, not only check the Drift but reverse it, driv-
ing the warm water back toward the southwest and
allowing cold water to well up in its stead. The driving
force in the Atlantic Drift is merely the excess of the
winds with a westerly component over those with an
easterly component.
Suppose that the numbers in Fig. 8 represent the
strength of the winds in a certain part of the North
Atlantic or North Pacific, that is, the total number of
miles moved by the air per year. In quadrant A of the
left-hand part all the winds move from a more or less
southwesterly direction and produce a total movement
SB. Helland-Hansen and F. Nansen: Temperature Variations in tlie
North Atlantic Ocean and in the Atmosphere; Misc. Ck>Il., Smiths. Inst., Vol.
70, No. 4, Washington, 1920.
CAUSES OF MILD GEOLOGICAL CLIMATES 176
B
\7
A
A
D
B'
1 c
"\:
y\
yT 60
Fig. 6. Effect of diminution of storms on
movement of water.
of the air amounting to thirty units per year. Those
coming from points between north and west move twenty-
five units ; those between north and east, twenty units ;
and those between east and south, twenty-five units.
Since the movement of the winds in quadrants B and
D is the same, these winds have no effect in producing
currents. They merely move the water back and forth,
and thus give it time to lose whatever heat it has brought
from more southerly latitudes. On the other hand, since
the easterly winds in quadrant C do not wholly check the
currents caused by the westerly winds of quadrant A,
the effective force of the westerly winds amounts to ten,
or the difference between a force of thirty in quadrant A
and of twenty in quadrant C. Hence the water is moved
forward toward the northeast, as shown by the thick
part of arrow A.
Now suppose that cyclonic storms should be greatly
reduced in number so that in the zone of prevailing
westerlies they were scarcely more numerous than tropi-
176 CLIMATIC CHANGES
cal hurricanes now are in the trade-wind belt. Then the
more or less southwesterly winds in quadrant A' in the
right-hand part of Fig. 8 would not only become more
frequent but would be stronger than at present. The
total movement from that quarter might rise to sixty
units, as indicated in the figure. In quadrants B' and D^
the movement would fall to fifteen and in quadrant C to
ten. B' and D' would balance one another as before. The
movement in A', however, would exceed that in C by fifty
instead of ten. In other words, the current-making force
would become five times as great as now. The actual
effect would be increased still more, for the winds from
the southwest would be stronger as well as steadier if
there were no storms. A strong wind which causes white-
caps has much more power to drive the water forward
than a weaker wind which does not cause whitecaps. In a
wave without a whitecap the water returns to practically
the original point after completing a circle beneath the
surface. In a wave with a whitecap, however, the cap
moves forward. Any increase in velocity beyond the rate
at which whitecaps are formed has a great influence upon
the amount of water which is blown forward. Several
times as much water is drifted forward by a persistent
wind of twenty miles an hour as by a ten-mile wind.*
In this connection a suggestion which is elaborated in
Chapter XIII may be mentioned. At present the salinity
of the oceans checks the general deep-sea circulation and
thereby increases the contrasts from zone to zone. In the
past, however, the ocean must have been fresher than
now. Hence the circulation was presumably less impeded,
and the transfer of heat from low latitudes to high was
facilitated.
• The dimatie significance of ocean currents is well discussed in OroU'e
Climate and Time^ 1875, and his Climate and Cosmogony, 1889.
CAUSES OF MILD GEOLOGICAL CLIMATES 177
Consider now the magnitude of the probable efEect of
a diminution in storms. Today off the coast of Norway
in latitude 65^N. and longitude 10°E., the mean tempera-
ture in January is 2°C. and in July 12°C. This represents
a plus anomaly of about 22° in January and 2° in July;
that isy the Norwegian coast is warmer than the normal
for its latitude by these amounts. Suppose that in some
past time the present distribution of lands and seas pre-
vailed, but Norway was a lowland where extensive de-
posits could accumulate in great flood plains. Suppose,
also, that the sun's atmosphere was so inactive that few
cyclonic storms occurred, steady winds from the west-
southwest prevailed, and strong, uninterrupted ocean
currents brought from the Caribbean Sea and Gulf of
Mexico much greater supplies of warm water than at
present. The Norwegian winters would then be warmer
than now not only because of the general increase in tem-
perature which the earth regularly experiences at sun-
spot minima, but because the currents would accentuate
this condition. Li summer similar conditions would pre-
vail except that the warming effect of the winds and
currents would presumably be less than in winter, but
this might be more than balanced by the increased heat
of the 8^ during the long smmner days, for storms and
clouds would be rare.
If such conditions raised the winter temperature only
8®C. and the summer temperature 4°C., the climate would
be as warm as that of the northern island of New Zealand
(latitude 35M3°S.). The flora of that part of New Zea-
land is subtropical and includes not only pines and
beeches, but palms and tree ferns. A climate scarcely
warmer than that of New Zealand would foster a flora
like that which existed in far northern latitudes during
some of the milder geological periods. If, however, the
178 CLIMATIC CHANGES
general temperature of the earth's surface were raised
5** because of the scarcity of storms, if the currents were
strong enough so that they increased the present anomaly
by 50 per cent, and if more persistent sunshine in summer
raised the temperature at that season about 4°0., the
January temperature would be 18°C. and the July tem-
perature 22 ""C. These figures perhaps make summer and
winter more nearly alike than was ever really the case in
such latitudes. Nevertheless, they show that a diminution
of storms and a consequent strengthening and steadying
of the southwesterlies might easily raise the temperature
of the Norwegian coast so high that corals could flourish
within the Arctic Circle.
Another factor would cooperate in producing mild
temperatures in high latitudes during the winter, namely,
the fogs which would presumably accumulate. It is well
known that when saturated air from a warm ocean is
blown over the lands in winter, as happens so often in the
British Islands and around the North Sea, fog is formed.
The effect of such a fog is indeed to shut out the sun's
radiation, but in high latitudes during the winter when
the sun is low, this is of little importance. Another effect
is to retain the heat of the earth itself. When a constant
supply of warm water is being brought from low lati-
tudes this blanketing of the heat by the fog becomes of
great importance. In the past, whenever cyclonic storms
were weak and westerly winds were correspondingly
strong, winter fogs in high latitudes must have been much
more widespread and persistent than now.
The bearing of fogs on vegetation is another interest-
ing point. If a region in high latitudes is constantly pro-
tected by fog in winter, it can support types of vegetation
characteristic of fairly low latitudes, for plants are
oftener killed by dry cold than by moist cold. Indeed,
CAUSES OF MILD GEOLOGICAL CLIMATES 179
excessive evaporation from the plant induced by dry
cold when the evaporated water cannot be rapidly re-
placed by the movement of sap is a chief reason why
large plants are winterkilled. The growing of trans-
planted palms on the coast of sonthwestem Ireland, in
spite of its location in latitude SO'^N., is possible only be-
cause of the great fogginess in winter due to the marine
climate. The fogs prevent the escape of heat and ward off
killing frosts. The tree ferns in latitude 46° S. in New
Zealand, already referred to, are often similarly pro-
tected in winter. Therefore, the relative frequency of fogs
in high latitudes when storms were at a minimum would
apparently tend not merely to produce mild winters but
to promote tropical vegetation.
The strong steady trades and southwesterlies which
would prevail at times of slight solar activity, according
to our hypothesis, would have a pronounced effect on the
water of the deep seas as well as upon that of the surface.
In the first place, the deep-sea circulation would be has-
tened. For convenience let us speak of the northern hemi-
sphere. In the past, whenever the southwesterly winds
were steadier than now, as was probably the case when
cyclonic storms were relatively rare, more surface water
than at present was presumably driven from low latitudes
and carried to high latitudes. This, of course, means that
a greater volume of water had to flow back toward the
equator in the lower parts of the ocean, or else as a cool
surface current. The steady southwesterly winds, how-
ever, would interfere with south-flowing surface currents,
thus compelling the polar waters to find their way
equatorward beneath the surface. In low latitudes the
polar waters would rise and their tendency would be to
lower the temperature. Hence steadier westerlies would
make for lessened latitudinal contrasts in climate not
180 CLIMATIC CHANGES
only by driving more warm water poleward but by caus-
ing more polar water to reach low latitudes.
At this point a second important consideration must be
faced. Not only would the deep-sea circulation be has-
tened, but the ocean depths might be warmed. The deep
parts of the ocean are today cold because they receive
their water from high latitudes where it sinks because of
low temperature. Suppose, however, that a diminution in
storminess combined with other conditions should permit
corals to grow in latitude 70°N. The ocean temperature
would then have to average scarcely lower than 20**C.
and even in the coldest month the water could scarcely
fall below about 15^0. Under such conditions, if the polar
ocean were freely connected with the rest of the oceans,
no part of it would probably have a temperature much
below 10° C, for there would be no such thing as ice caps
and snowfields to reflect the scanty sunlight and radiate
into space what little heat there was. On the contrary,
during the winter an almost constant state of dense f og-
giness would prevail. So great would be the blanketing
effect of this that a minimum monthly temperature of
10° C. for the coldest part of the ocean may perhaps be
too low for a time when corals thrived in latitude 70°.
The temperature of the ocean depths cannot perma-
nently remain lower than that of the coldest parts of the
surface. Temporarily this might indeed happen when a
solar change first reduced the storminess and strength-
ened the westerlies and the surface currents. Gradually,
however, the persistent deep-sea circulation would bring
up the colder water in low latitudes and carry downward
the water of medium temperature at the coldest part of
the surface. Thus in time the whole body of the ocean
would become warm. The heat which at present is carried
away from the earth's surface in storms would slowly
CAUSES OF MELD GEOLOGICAL CLIMATES 181
accumulate in the oceans. As the process went on, all
parts of the ocean's surface would become warmer, for
equatorial latitudes would be less aad less cooled by cold
water from below, while the water blown from low lati-
tudes to high would be correspondingly warmer. The
warming of the ocean would come to an end only with
the attainment of a state of equilibrium in which the loss
of heat by radiation and evaporation from the ocean's
surface equaled the loss which xmder other circumstances
would arise from the rise of warm air in cyclonic storms.
When once the oceans were warmed, they would form an
extremely strong conservative force tending to preserve
an equable climate in all latitudes and at all seasons.
According to the solar cyclonic hypothesis such condi-
tions ought to have prevailed throughout most of geo-
logical time. Only after a strong and prolonged solar
disturbance with its consequent storminess would condi-
tions like those of today be expected.
In this connection another possibility may be men-
tioned. It is commonly assumed that the earth's axis is
held steadily in one direction by the fact that the rotating
earth is a great gyroscope. Having been tilted to a cer-
tain position, perhaps by some extraneous force, the axis
is supposed to maintain that position until some other
force intervenes. Cordeiro," however, maintains that this
is true only of an absolutely rigid gyroscope. He believes
that it is mathematically demonstrable that if an elastic
gyroscope be gradually tilted by some extraneous force,
and if that force then ceases to act, the gyroscope as a
whole will oscillate back and forth. The earth appears to
be slightly elastic. Cordeiro therefore applies his for-
mulae to it, on the following assumptions: (1) That the
original position of the axis was nearly vertical to the
10 p. J. B. Cordeiro: The Gyroscope, 1913.
182 CLIMATIC CHANGES
plane of the ecliptic in which the earth revolves around
the sun ; (2) that at certain times the inclination has been
even greater than now; and (3) that the position of the
axis with reference to the earth has not changed to any
great extent, that is, the earth *s poles have remained
essentially stationary with reference to the earth, al-
though the whole earth has been gyroscopically tilted
back and forth repeatedly.
With a vertical axis tiie daylight and darkness in all
parts of the earth would be of equal duration, being
always twelve hours. There would be no seasons, and the
climate would approach the average condition now ex-
perienced at the two equinoxes. On the whole the climate
of high latitudes would give the impression of being
milder than now, for there would be less opportimity for
the accumulation of snow and ice with their strong cool-
ing effect On the other hand, if the axis were tilted more
than now, the winter nights would be longer and the
winters more severe than at present, and there would be
a tendency toward glaciation. Thus Oordeiro accounts
for alternating mild and glacial epochs. The entire swing
from the vertical position to the maximum indination
and back to the vertical may last millions of years de-
pending on the earth's degree of elasticity. The swing
beyond the vertical position in the other direction would
be equally prolonged. Since the axis is now supposed to
be much nearer its maximum than its minimum degree of
tilting, the duration of epochs having a climate more
severe than that of the present would be relatively short,
while the mild epochs would be long.
Cordeiro's hypothesis has been almost completely
ignored. One reason is that his treatment of geological
facts, and especially his method of riding rough-shod
over widely accepted conclusions, has not commended his
CAUSES OF MILD GEOLOGICAL CLIMATES 188
work to geologists. Therefore they have not deemed it
worth while to urge mathematicians to test the assump-
tions and methods by which he reached his results. It is
perhaps mifair to test Cordeiro by geology, for he lays
no claim to being a geologist In mathematics he labors
under the disadvantage of having worked outside the
usual professional channels, so that his work does not
seem to have been subjected to sufficiently critical
analysis.
Without expressing any opinion as to the value of
Cordeiro *s results we feel that the subject of the earth *s
gyroscopic motion and of a possible secular change in
the direction of the axis deserves investigation for two
chief reasons. In the first place, evidences of seasonal
changes and of seasonal uniformity seem to occur more
or less alternately in the geological record. Second, the
remarkable discoveries of Gamer and Allard^^ show that
the duration of daylight has a pronounced effect upon
the reproduction of plants. We have referred repeatedly
to the tree ferns, corals, and other forms of life which
now live in relatively low latitudes and which cannot
endure strong seasonal contrasts, but which once lived
far to the north. On the other hand, Sayles," for example,
finds that microscopical examination of the banding of
ancient shales and slates indicates distinct seasonal band-
ing like that of recent Pleistocene clays or of the Squan-
tum slate formed during or near the Permian glacial
period. Such seasonal banding is found in rocks of vari-
ous ages: (a) Huronian, in cobalt shales previously
reported by Coleman ; (b) late Proterozoic or early Cam-
XI W. W. Qarner and H. A. Allard: Flowering and Fmition of Plants
as Ck>ntrolled by Length of Day; Yearbook Dept. Agri., 1920, pp. 377-400.
i2Beport of Oonunittee on Sedimentation, National Research Council,
AprU, 1922.
184 CLIMATIC CHANGES
brian, in Hiwassee slate ; (c) lower Cambrian^ in Geor-
gian slates of Vermont; (d) lower Ordovician, in Geoxgia
(Bockmart slate), Tennessee (Athens shale), Vermont
(slates), and Quebec (Beekmantown formation) ; and (e)
Permian in Massachusetts (Squantum slate). How far
the periods during which such evidence of seasons was
recorded really alternated with mild periods, when tropi-
cal species lived in high latitudes and the contrast of
seasons was almost or wholly lacking, we have as yet no
means of knowing. If periods characterized by marked
seasonal changes should be found to have alternated with
those when the seasons were of little importance, the fact
would be of great geological significance.
The discoveries of Gamer and Allard as to the effect
of light on reproduction began with a peculiar tobacco
plant which appeared in some experiments at Washing-
ton. The plant grew to unusual size, and seemed to
promise a valuable new variety. It formed no seeds, how-
ever, before the approach of cold weather. It was there-
fore removed to a greenhouse where it flowered and
produced seed. In succeeding years the flowering was
likewise delayed till early winter, but finally it was dis-
covered that if small plants were started in the green-
house in the early fall they flowered at the same time as
the large ones. Experiments soon demonstrated that the
time of flowering depends largely upon the length of the
daily period when the plants are exposed to light. The
same is true of many other plants, and there is great
variety in the conditions which lead to flowering. Some
plants, such as witch hazel, appear to be stimulated to
bloom by very short days, while others, such as evening
primrose, appear to require relatively long days. So
sensitive are plants in this respect that Gamer and
Allard, by changing the length of the period of light, have
CAUSES OF MILD GEOLOGICAL CLIMATES 186
caused a flowerbud in its early stages not only to stop
developing but to return once more to a vegetative shoot.
Common iris, which flowers in May and June, will not blossom
under ordinary conditions when grown m the greenhouse in
winter, even under the same temperature conditions that prevail
in early summer. Again, one variety of soy beans will regularly
begin to flower in June of each year, a second variety in July,
and a third in August, when all are planted on the same date.
There are no temperature differences during the summer months
which could explain these differences in time of flowering; and,
since ''internal causes" alone cannot be accepted as furnishing
a satisfactory explanation, some external factor other than tem-
perature must be responsible.
The ordinary varieties of cosmos regularly flower in the fall
in northern latitudes if they are planted in the spring or summer.
If grown in a warm greenhouse during the winter months the
plants also flower readily, so that the cooler weather of fall is
not a necessary condition. If successive plantings of cosmos are
made in the greenhouse during the late winter and early spring
months, maintaining a uniform temperature throughout, the
plantings made after a certain date will fail to blossom promptly,
but, on the contrary, will continue to grow till the following fall,
thus flowering at the usual season for this species. This curious
reversal of behavior with advance of the season cannot be attrib-
uted to change in temperature. Some other factor is responsible
for the failure of cosmos to blossom during the summer months.
In this respect the behavior of cosmos is just the opposite of that
observed in iris.
Certain varieties of soy beans change their behavior in a
peculiar manner with advance of the summer season. The variety
known as Biloxi, for example, when planted early in the spring
in the latitude of Washington, D. C, continues to grow through-
out the summer, flowering in September. The plants maintain
growth without flowering for fifteen to eighteen weeks, attaining
a height of five feet or more. As the dates of successive plantings
are moved forward through the months of June and July, how-
186 CLIMATIC CHANGES
ever, there is a marked tendency for the plants to cut short the
period of growth which precedes flowering. This means^ of course,
that there is a tendency to flower at approximately the same time
of year regardless of the date of planting. As a necessary con-
sequence, the size of the plants at the time of flowering is reduced
in proportion to the delay in planting.
The bearing of this on geological problems lies in a
query which it raises as to the ability of a genus or family
of plants to adapt itself to days of very different length
from those to which it is wonted. Could tree ferns, gink-
gos, cycads, and other plants whose usual range of loca-
tion never subjects them to daylight for more than
perhaps fourteen hours or less than ten, thrive and re-
produce themselves if subjected to periods of daylight
ranging all the way from nothing up to about twenty-
four hours! No answer to this is yet possible, but the
question raises most interesting opportunities of in-
vestigation. If Cordeiro is right as to the earth's elastic
gyroscopic motion, there may have been certain periods
when a vertical or almost vertical axis permitted the
days to be of almost equal length at all seasons in all
latitudes. If such an absence of seasons occurred when
the lands were low, when the oceans were extensive and
widely open toward the poles, and when storms were
relatively inactive, the result might be great mildness of
climate such as appears sometimes to have prevailed in
the middle of geological eras. Suppose on the other hand
that the axis should be tilted more than now, and that
the lands should be widely emergent and the storm belt
highly active in low latitudes, perhaps because of the
activity of the sun. The conditions might be favorable for
glaciation at latitudes as low as those where the Permo-
Carboniferous ice sheets appear to have centered. The
possibilities thus suggested by Cordeiro 's hypothesis are
CAUSES OF MILD GEOLOGICAL CLIMATES 187
80 interesting that the gyroscopic motion of the earth
onght to be investigated more thoroughly. Even if no
such gyroscopic motion takes place, however, the other
causes of mild climate discussed in this chapter may
be enough to explain all the observed phenomena.
Many important biological consequences might be
drawn from this study of mild geological climates, but
this book is not the place for them. In the first chapter
we saw that one of the most remarkable features of the
climate of the earth is its wonderful uniformity through
hundreds of millions of years. As we come down through
the vista of years the mild geological periods appear to
represent a return as nearly as possible to this standard
condition of mdformity. Certakt changes of the earth
itself, as we shall see in the next chapter, may in the long
run tend slightly to change the exact conditions of this
climatic standard, as we might perhaps call it. Yet they
act so slowly that their effect during hundreds of millions
of years is still open to question. At most they seem
merely to have produced a slight increase in diversity
from season to season and from zone to zone. The normal
climate appears still to be of a milder type than that
which happens to prevail at present. Some solar condi-
tion, whose possible nature will be discussed later, seems
even now to cause the number of cyclonic storms to be
greater than normal. Hence the earth's climate still
shows something of the great diversity of seasons and
of zones which is so marked a characteristic of glacial
epochs.
CHAPTER XI
TERRESTRIAL CAUSES OF CLIMATIC CHANGES
•
THE major portion of this book has been concerned
with the explanation of the more abmpt and ex-
treme changes of climate. This chapter and the
next consider two other sorts of climatic changes, the
slight secular progression during the hundreds of mil-
lions of years of recorded earth history, and especially
the long slow geologic oscillations of millions or tens of
millions of years. It is generally agreed among geologists
that the progressive change has tended toward greater
extremes of climate ; that is, greater seasonal contrasts,
and greater contrasts from place to place and from zone
to zone.^ The slow cyclic changes have been those that
favored widespread glaciation at one extreme near the
ends of geologic periods and eras, and mild temperatures
even in subpolar regions at the other extreme during the
medial portions of the periods.
As has been pointed out in an earlier chapter, it has
often been assumed that all climatic changes are due to
terrestrial causes. We have seen, however, that there is
strong evidence that solar variations play a large part in
modifying the earth 's climate. We have also seen that no
known terrestrial agency appears to be able to produce
the abrupt changes noted in recent years, the longer
iChas. Schuehert: The Earth's Changing Surface and Climate daring
Geologic Time; in Lull: The Evolution of the Earth and Its Inhabitants,
1918, p. 55.
TERRESTRIAL CAUSES OF CHANGES 189
cycles of historical times, or geological changes of the
shorter type, such as glaciation. Nevertheless, terrestrial
changes doubtless have assisted in producing both the
progressive change and the slow cyclic changes recorded
in the rocks, and it is the purpose of this chapter and the
two that follow to consider what terrestrial changes have
taken place and the probable effect of such changes.
The terrestrial changes that have a climatic signifi-
cance are numerous. Some, such as variations in the
amount of volcanic dust in the higher air, have been con-
sidered in an earlier chapter. Others are too imperfectly
known to warrant discussion, and in addition there are
presumably others which are entirely unknown. Doubt-
less some of these little known or unknown changes have
been of importance in modifying climate. For example,
the climatic influence of vegetation, animals, and man
may be appreciable. Here, however, we shall confine our-
selves to purely physical causes, which will be treated in
the following order : First, those concerned with the solid
parts of the earth, namely: (I) amount of land; (II) dis-
tribution of land; (in) height of land; (IV) lava flows;
and (V) internal heat. Second, those which arise from
the salinity of oceans, and third, those depending on the
composition and amount of atmosphere.
The terrestrial change which appears indirectly to
have caused the greatest change in climate is the con-
traction of the earth. The problem of contraction is
highly complex and is as yet only imperfectly understood.
Since only its results and not its processes influence cli-
mate, the following section as far as page 196 is not
necessary to the general reader. It is inserted in order to
explain why we assume that there have been oscillations
between certain types of distribution of the lands.
The extent of the earth ^s contraction may be judged
190 CLIMATIC CHANGES
from the shrinkage indicated by the shortening of the
rock formations in folded mountains such as the Alps,
JuraSy Appalachians, and Caucasus. Gteologists are con-
tinually discovering new evidence of thrust faults of
great magnitude where masses of rock are thrust bodily
over other rocks, sometimes for many miles. Therefore,
the estimates of the amount of shrinkage based on the
measurements of folds and faults need constant revision
upward. Nevertheless, they have already reached a con-
siderable figure. For example, in 1919, Professor ,A. Heim
estimated the shortening of the meridian passing through
the modem Alps and the ancient Hercynian and Cale-
donian mountains as fully a thousand miles in Europe,
and over five hundred miles for the rest of this meridian.*
This is a radial shortening of about 250 miles. Possibly
the shrinkage has been even greater than this. Chamber-
lin' has compared the density of the earth, moon, Mars,
and Venus with one another, and found it probable that
the radial shrinkage of the earth may be as much as
570 miles. This result is not so different from Heim's as
appears at first sight, for Heim made no allowance for
unrecognized thrust faults and for the contraction inci-
dent to metamorphism. Moreover, Heim did not include
shrinkage during the first half of geological time before
the above-mentioned mountain systems were upheaved.
According to a well-established law of physics, con-
traction of a rotating body results in more rapid rotation
and greater centrifugal force. These conditions must in-
crease the earth's equatorial bulge and thereby cause
changes in the distribution of land and water. Opposed
to the rearrangement of the land due to increased rota-
i Quoted by J. Cornet : Cours de G^ologie, 1920, p. 330.
ST. C. Ohamberlin: The Order of Magnitude of the Shrinkage of the
Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.
TERRESTRIAL CAUSES OF CHANGES 191
tion caused by contraction, there has presumably been
another rearrangement due to tidal retardation of the
earth's rotation and a consequent lessening of the equa-
torial bulge. G. H. Darwin long ago deduced a relatively
large retardation due to lunar tides. A few years ago
W. D. MacMillaUy on other assumptions, deduced only a
negligible retardation. Still more recently Taylor* has
studied the tides of the Irish Sea, and his work has led
Jeffreys" and Brown* to conclude that there has been con-
siderable retardation, perhaps enough, according to
Brown, to equal the acceleration due to the earth's con-
traction. From a prolonged and exhaustive study of the
motions of the moon Brown concludes that tidal friction
or some other cause is now lengthening the day at the rate
of one second per thousand years^ or an hour in almost
four million years if the present rate continues. He makes
it dear that the retardation due to tides would not corre-
spond in point of time with the acceleration due to con-
traction. The retardation would occur slowly, and would
take place chiefly during the long quiet periods of geo-
logic history, whUe the acceleration would occur rapidly
at times of diastrophic deformation. As a consequence,
the equatorial bulge would alternately be reduced at a
slow rate, and then somewhat suddenly augmented.
The less rigid any part of the earth is, the more quickly
it responds to the forces which lead to bulging or which
tend to lessen the bulge. Since water is more fluid than
land, the contraction of the earth and the tidal retarda-
tion presumably tend alternately to increase and decrease
the amount of water near the equator more than the
«G. I. Taylor: PhiloBophical Transactions, A. 220, 1919, pp. 1-33;
Monthlj Notiees Bojal Astron. Soc, Jan., 1920, Vol. 80, p. 308.
fi J. Jeffreys : Monthlj Notices. Boyal Astron. Soc, Jan., 1920, Vol. 80,
p. 309.
9 £. W. Brown : personal communication.
n
192 CLIMATIC CHANGES
amount of land. Thus, throughout geological history we
should look for cyclic changes in the relative area of the
lands within the tropics and similar changes of opposite
phase in higher latitudes. The extent of the change would
depend upon (a) the amount of alteration in the speed
of rotation, and (b) the extent of low land in low lati-
tudes and of shallow sea in high latitudes. According to
Slichter's tables, if the earth should rotate in twenty-
three hours instead of twenty-four, the great Amazon
lowland would be submerged by the inflow of oceanic
water, while wide areas in Hudson Bay, the North Sea,
and other northern regions, would become land because
the ocean water would flow away from them.'
Following the prompt equatorward movement of water
which would occur as the speed of rotation increased,
there must also be a gradual movement or creepage of the
solid rocks toward the equator, that is, a bulging of the
ocean floor and of the lands in low latitudes, with a con-
sequent emergence of the lands there and a relative
rise of sea level in higher latitudes. Tidal retardation
would have a similar effect. Suess" has described wide-
spread elevated strand lines in the tropics which he in-
terprets as indicating a relatively sudden change in sea
level, though he does not suggest a cause of the change.
However, in speaking of recent geological times, Suess
reports that a movement more recent than the old
strands ^^was an accumulation of water toward the
equator, a diminution toward the poles, and (it appears)
as though this last movement were only one of the many
oscillations which succeed each other with the same tend-
ency, i.e., with a positive excess at the equator, a nega-
• 7 G. S. Blichter: The Rotational Period of a Heterogeneous Spheroid; in
Contributions to the Fundamental Problems of Geology, by T. C. Gfaimt-
berlin, et al,, Carnegie Inst, of Wash., No. 107, 1909.
8 E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.
TERRESTRIAL CAUSES OF CHANGES 198
tive excess at the poles/' (Vol. II, p. 551.) This creepage
of the rocks equatorward seemingly might favor the
growth of mountains in tropical and subtropical regions,
because it is highly improbable that the increase in the
bulge would go on in all longitudes with perfect uni-
formity. Where it went on most rapidly mountains would
arise. That such irregularity of movement has actually
occurred is suggested not only by the fact that many
Cenozoic and older mountain ranges extend east and
west, but by the further fact that these include some of
our greatest ranges, many of which are in fairly low lati-
tudes. The Himalayas, the Javanese ranges, and the half-
submerged Caribbean chains are examples. Such moun-
tains suggest a thrust in a north and south direction
which is just what would happen if the solid mass of the
earth were creeping j&rst equatorward and then poleward.
A fact which is in accord with the idea of a periodic
increase in the oceans in low latitudes because of renewed
bulging at the equator is the exposure in moderately
high latitudes of the greatest extent of ancient rocks.
This seems to mean that in low latitudes the frequent
deepening of the oceans has caused the old rocks to be
largely covered by sediments, while the old lands in
higher latitudes have been left more fully exposed to
erosion.
Another suggestion of such periodic equatorward move-
ments of the ocean water is found in the reported contrast
between the relative stability with which the northern part
of North America has remained slightly above sea level
except at times of widespread submergence, while the
southern parts have suffered repeated submergence al-
ternating with great emergence.* Furthermore, although
• Chas. Sehnehert: The Earth's Changing Surface and Climate; in Lull:
The Evolution of the Earth and Its Inhabitants, 1918, p. 78.
194 CLIMATIC CHANGES
the northern part of North America has been generally
exposed to erosion since the Proterozoic, it has supplied
much less sediment than have the more southern land
areas." This apparently means that much of Canada has
stood relatively low, while repeated and profound uplift
alternating with depression has occurred in subtropical
latitudes, apparently in adjustment to changes in the
earth's speed of rotation. The uplifts generally followed
the times of submergence due to equatorward movement
of the water, though the buckling of the crust which ac-
companies shrinkage doubtless caused some of the sub-
mergence. The evidence that northern North America
stood relatively low throughout much of geological time
depends not only on the fact that little sediment came to
the south from the north, but also on the fact that at
times of especially widespread epicontinental seas, the
submergence was initiated at the north." This is espe-
cially true for Ordovician, Silurian, Devonian, and Juras-
sic times in North America. General submergence of this
kind is supposed to be due chiefly to the overflowing of
the ocean when its level is slowly raised by the deposition
of sediment derived from the erosion of what once were
continental highlands but later are peneplains. The fact
that such submergence began in high latitudes, however,
seems to need a further explanation. The bulging of the
rock sphere at the equator and the consequent displace-
ment of some of the water in low latitudes would furnish
such an explanation, as would also a decrease in the speed
of rotation induced by tidal retardation, if that retarda-
tion were great enough and rapid, enough to be geologi-
cally effective.
10 J. Barrell: Bhythms and the Measurement of Geologic Time; BnlL
Geol. Soc. Am., Vol. 28, 1917, p. 838.
11 Chas. Schuchert : loc, cit., p. 78.
TERRESTRIAL CAUSES OF CHANGES 196
The climatic effects of the earth's contraction, which
we shall shortly discuss, are greatly complicated by the
fact that contraction has taken place irregularly. Such
irregularity has occurred in spite of the fact that the
processes which cause contraction have probably gone
on quite steadily throughout geological history. These
processes include the chemical reorganization of the min-
erals of the crust, a process which is illustrated by the
metamorphism of sedimentary rocks into crystalline
forms. The escape of gases through volcanic action or
otherwise has been another important process.
Although the processes which cause contraction prob-
ably go on steadily, their effect, as Chamberlin" and
others have pointed out, is probably delayed by inertia.
Thus the settling of the crust or its movement on a large
scale is delayed. Perhaps the delay continues until the
stresses become so great that of themselves they over-
come the inertia, or possibly some outside agency, whose
nature we shall consider later, reenf orces the stresses and
gives the slight impulse which is enough to release them
and allow the earth's crust to settle into a new state of
equilibrium. When contraction proceeds actively, the
ocean segments, being largest and heaviest^ are likely to
settle most, resulting in a deepening of the oceans and an
emergence of the lands. Following each considerable con-
traction there would be an increase in the speed of rota-
tion. The repeated contractions with consequent growth
of the equatorial bulge would alternate with long quiet
periods during which tidal retardation would again de-
crease the speed of rotation and hence lessen the bulge.
The result would be repeated changes of distribution of
land and water, with consequent changes in climate.
1ST. 0. Ghamberlin: DiastTophism, the Ultimate Basis of Correlation;
Jour. Geol., Vol. 16, 1909; Chas. Schnehert: loo. cit.
196 CLIMATIC CHANGES
L We shall now consider the climatic e£fect of the
repeated changes in the relative amounts of land and
water which appear to have resulted from the earth's
contraction and from changes in its speed of rotation.
During many geologic epochs a larger portion of the
earth was covered with water than at present For ex-
ample, during at least twelve out of about twenty epochs.
North America has suffered extensive inundations^^' and
in general the extensive submergence of Europe, the
other area well known geologically, has coincided with
that of North America. At other times, the ocean has
been less extensive than now, as for example during the
recent glacial period, and probably during several of
the glacial periods of earlier date. Each of the numerous
changes in the relative extent of the lands must have
resulted in a modification of climate.^^ This modification
would occur chiefly because water becomes warm far
more slowly than land, and cools off far more slowly.
An increase in the lands would cause changes in several
climatic conditions, (a) The range of temperature be-
tween day and night and between summer and winter
would increase, for lands become warmer by day and in
summer than do oceans, and cooler at night and in
winter. The higher summer temperature when the lands
are widespread is due chiefly to the fact that the land, if
not snow-covered, absorbs more of the sun's radiant
energy than does the ocean, for its reflecting power is
low. The lower winter temperature when lands are wide-
spread occurs not only because they cool off rapidly but
"Pirsson-Schuchert: Textbook of Geology, 1915, Vol. n, p. 982; Chaa.
Schuchert: Pideogeography of North America; BulL Geol. Soc. Anu, VoL
20, pp. 427-606; reference on p. 499.
14 The general subject of the climatic significance of continentality is
discussed by G. E. P. Brooks: Gontinentality and Temperature; Quarts
Jour. Bojal MeteoroL Soc, April, 1917, and Get, 1918.
TERRESTRIAL CAUSES OF CHANGES 197
because the reduced oceans cannot give them so much
heat. Moreover, the larger the land, the more generally
do the winds blow outward from it in winter and thus
prevent the ocean heat from being carried inland. So
long as the ocean is not frozen in high latitudes, it is
generally the chief source of heat in winter, for the nights
are several months long near the poles, and even when
the sun does shine its angle is so low that reflection from
the snow is very great. Furthermore, although on the
average there is more reflection from water than from
land, the opposite is true in high latitudes in winter
when the land is snow-covered while the ocean is rela-
tively dark and is roughened by the waves. Another
factor in causing large lands to have extremely low tem-
perature in winter is the fact that in proportion to their
size they are less protected by fog and cloud than are
smaller areas. The belt of cloud and fog which is usually
formed when the wind blows from the ocean to the rela-
tively cold land is restricted to the coastal zone. Thus the
larger the land, the smaller the fraction in which loss of
heat by radiation is reduced by clouds and fogs. Hence
an increase in the land area is accompanied by an in-
crease in the contrasts in temperature between land and
water.
(b) The contrasts in temperature thus produced must
cause similar contrasts in atmospheric pressure, and
hence stronger barometric gradients, (c) The strong
gradients would mean strong winds, flowing from land
to sea or from sea to land, (d) Local convection would
also be strengthened in harmony with the expansion of
the lands, for the more rapid heating of land than of
water favors active convection.
(e) As the extent of the ocean diminished, there would
normally be a decrease in the amount of water vapor for
198 CLIMATIC CHANGES
three reasons: (1) Evaporation from the ocean is the
great source of water vapor. Other conditions being
equaly the smaller the ocean becomes, the less the evapo-
ration. (2) The amount of water vapor in the air dimin-
ishes as convection increases, since upward convection
is a chief method by which condensation and precipita-
tion are produced, and water vapor removed from the
atmosphere. (3) Nocturnal cooling sufficient to produce
dew and frost is very much more common upon land than
upon the ocean. The formation of dew and frost dimin-
ishes the amount of water vapor at least temporarily,
(f) Any diminution in water vapor produced in these
ways, or otherwise, is significant because water vapor is
the most essential part of the atmosphere so far as regu-
lation of temperature is concerned. It tends to keep the
days from becoming hot or the nights cold. Therefore
any decrease in water vapor would increase the diurnal
and seasonal range of temperature, making the climate
more extreme and severe. Thus a periodic increase in the
area of the continents would clearly make for periodic
increased climatic contrasts, with great extremes, a type
of climatic change which has recurred again and again.
Indeed, each great glaciation accompanied or followed
extensive emergence of the lands."
Whether or not there has been a progressive increase
from era to era in the area of the lands is uncertain.
Good authorities disagree widely. There is no doubt,
however, that at present the lands are more extensive
than at most times in the past, though smaller, perhaps,
than at certain periods. The wide expanse of lands helps
explain the prominence of seasons at present as com-
pared with the past.
iBChas. Schuchert: Climates of Geologic Time; in The Climatic Factor;
Carnegie Institution, 1914, p. 286.
TERRESTRIAL CAUSES OF CHANGES 199
II. The contraction of the earth, as we have seen, has
produced great changes in the distribution as well as in
the extent of land and water. Large parts of the present
continents have been covered repeatedly by the sea, and
extensive areas now covered with water have been land.
In recent geological times, that is, during the Pliocene and
Pleistocene, much of the present continental shelf, the
zone less than 600 feet below sea level, was land. If the
whole shelf had been exposed, the lands would have been
greater than at present by an area larger than North
Ajnerica. When the lands were most elevated, or a little
earlier. North America was probably connected with
Asia and almost with Europe. Asia in turn was appar-
ently connected with the larger East Indian islands. In
much earlier times land occupied regions where now the
ocean is fairly deep. Groups of islands, such as the East
Indies and Malaysia and perhaps the West Indies, were
united into widespreading land masses. Figs. 7 and 9,
illustrating the paleography of the Permian and the
Cretaceous periods, respectively, indicate a land distri-
bution radically different from that of today.
So far as appears from the scattered facts of geologi-
cal history, the changes in the distribution of land seem
to have been marked by the following characteristics : (1)
Accompanying the differentiation of continental and
oceanic segments of the earth's crust, the oceans have
become somewhat deeper, and their basins perhaps
larger, while the continents, on the average, have been
more elevated and less subject to submergence. Hence
there have been less radical departures from the present
distribution during the relatively recent Cenozoic era
than in the ancient Paleozoic because the submergence of
continental areas has become less general and less fre-
quent. For example, the last extensive epeiric or interior
200 CLIMATIC CHANGES
sea in North America was in the Cretaceous, at least ten
million years ago, and according to Barrell perhaps fifty
million, while in Europe, according to de Lapparent," a
smaller share of the present continent has been sub-
merged since the Cretaceous than before. Indeed, as in
North America, the submergence has decreased on the
average since the Paleozoic era. (2) The changes in dis-
tribution of land which have taken place during earth
history have been cyclic. Bepeatedly, at the close of each
of the score or so of geologic periods, the continents
emerged more or less, while at the close of the groups of
periods known as eras, the lands were especially large
and emergent. After each emergence, a gradual encroach-
ment of the sea took place, and toward the close of sev-
eral of the earlier periods, the sea appears to have
covered a large fraction of the present land areas. (3) On
the whole, the amount of land in the middle and high lati-
tudes of the northern hemisphere appears to have in-
creased during geologic time. Such an increase does not
require a growth of the continents, however, in the
broader sense of the term, but merely that a smaller
fraction of the continent and its shelf should be sub-
merged. (4) In tropical latitudes, on the other hand, the
extent of the lands seems to have decreased, apparently
by the growth of the ocean basins. South America and
Africa are thought by many students to have been con-
nected, and Africa was united with India via Mada-
gascar, as is suggested in Fig. 9. The most radical cyclic
as well as the most radical progressive changes in land
distribution also seem to have taken place in tropical
regions."
Although there is much evidence of periodic increase
10 A. de Lapparent : Traits de GMog^e, 1906.
17 Chas. Schuchert : Historical Qeology, 1915, p. 464.
e
202 CLIMATIC CHANGES
of the sea in equatorial latitudes and of land in high lati-
tudes, it has remained for the zoologist Metcalf to pre-
sent a very pretty bit of evidence that at certain times
submergence along the equator coincided with emergence
in high latitudes, and vice versa. Certain fresh water
frogs which carry the same internal parasite are confined
to two widely separated areas in tropical and south tem-
perate America and in Australia. The extreme improba-
bility that both the frogs and the parasites could have
originated independently in two unconnected areas and
could have developed by convergent evolution so that
they are almost identical in the two continents makes it
almost certain that there must have been a land con-
nection between South America and Australia, presum-
ably by way of Antarctica. The facts as to the parasites
seem also to prove that while the land connection existed
there was a sea across South America in equatorial lati-
tudes. The parasite infests not only the frogs but the
American toads known as Bufo. Now Bufo originated
north of the equator in America and differs from the
frogs which originated in southern South America in
not being found in Australia. This raises the question of
how the frogs could go to Australia via Antarctica carry-
ing the parasite with them, while the toads could not go.
Metcalf 's answer is that the toads were cut off from the
southern part of South America by an equatorial sea
until after the Antarctic connection between the Old
World and the New was severed.
As Patagonia let go of Antarctica by subsidence of the inter-
vening land area, there was a probable concomitant rise of land
through what is now middle South America and the northern
and southern portions of this continent came together.^*
18 M. M. Metcalf: Upon an important method of studying problems of
relationship and of geographical distribution; Proceedings National Acad-
emy of Sciences, Vol. 6, July, 1920, pp. 432-433.
TERRESTRIAL CAUSES OF CHANGES 208
These various changes in the earth's crust have given
rise to certain specific types of distribution of the lands,
which will now be considered. We shall inquire what cli-
matic conditions would arise from changes in (a) the
continuity of the lands from north to south, (b) the
amount of land in tropical latitudes, and (c) the amount
of land in middle and high latitudes.
(a) At present the westward drift of warm waters, set
in motion by the trade winds, is interrupted by land
masses and turned poleward, producing the important
Gulf Stream Drift and Japan Current in the northern
hemisphere, and corresponding, though less important,
currents in the southern hemisphere. During the past,
quite different sets of ocean currents doubtless have
existed in response to a different distribution of land.
Repeatedly, in the mid-Cretaceous (Fig. 9) and several
other periods, the present American barrier to the west-
ward-moving tropical current was broken in Central
America. Even if the supposed continent of ' * Gondwana
Land*' extended from Africa to South America in equa-
torial latitudes, strong currents must still have flowed
westward along its northern shore under the impulse of
the peculiarly strong trade winds which the equatorial
land would create. Nevertheless at such times relatively
little warm tropical water presumably entered the North
Atlantic, for it escaped into the Pacific. At several other
times, such as the late Ordovician and mid-Devonian,
when the isthmian barrier existed, it probably turned an
important current northward into what is now the Mis-
sissippi Basin instead of into the Atlantic. There it
traversed an epeiric, or mid-continental sea open to both
north and south. Hence its effectiveness in warming
Arctic regions must have been quite different from that
of the present Gulf Stream.
204 CLIMATIC CHANGES
(b) We will next consider the influences of changes in
the amount of equatorial and tropical land. As such lands
are much hotter than the corresponding seas, the inten-
sity and width of the equatorial belt of low pressure must
be great when they are extensive. Hence the trade winds
must have been stronger than now whenever tropical
lands were more extensive than at present. This is be-
cause the trades are produced by the convection due to
excessive heat along the heat equator. There the air
expands upward and flows poleward at high altitudes.
The trade wind consists of air moving toward the heat
equator to take the place of the air which there rises.
When the lands in low latitudes were wide the trade
winds must also have dominated a wide belt. The greater
width of the trade-wind belt today over Africa than over
the Atlantic illustrates the matter. The belt must have
been still wider when GK)ndwana Land was large, as it is
believed to have been during the Paleozoic era and the
early Mesozoic.
An increase in the width of the equatorial belt of
low pressure under the influence of broad tropical lands
would be accompanied not only by stronger and more
widespread trade winds, but by a corresponding strength-
ening of the subtropical belts of high pressure. The chief
reason would be the greater expansion of the air in the
equatorial low pressure belt and the consequent more
abundant outflow of air at high altitudes in the form of
anti-trades or winds returning poleward above the trades.
Such winds would pile up the air in the region of the high-
pressure belt. Moreover, since the meridians converge as
one proceeds away from the equator, the air of the pole-
ward-moving anti-trades tends to be crowded as it
reaches higher latitudes, thus increasing the pressure.
Unless there were a corresponding increase in tropical
TERRESTRIAL CAUSES OF CHANGES 206
cyclones, one of the most prominent results of the
strengthened trades and the intensified subtropical high-
pressure belt at times of broad lands in low latitudes
would be great deserts. It will be recalled that the trade-
wind lowlands and the extra-tropical belt of highs are the
great desert belts at present. The trade-wind lowlands
are desert because air moving into warmer latitudes
takes up water except where it is cooled by rising on
mountain-sides. The belt of highs is arid because there,
too, air is being warmed, but in this case by descending
from aloft.
Again, if the atmospheric pressure in the subtropical
belt should be intensified, the winds flowing poleward
from this belt would necessarily become stronger. These
would begin as southwesterlies in the northern hemi-
sphere and northwesterlies in the southern. In the pre-
ceding chapter we have seen that such winds, especially
when cyclonic storms are few and mild, are a powerful
agent in transferring subtropical heat poleward. If the
strength of the westerlies were increased because of
broad lands in low latitudes, their efficacy in transferring
heat would be correspondingly augmented. It is thus
evident that any change in the extent of tropical lands
during the geologic past must have had important cli-
matic consequences in changing the velocity of the
atmospheric circulation and in altering the transfer of
heat from low latitudes to high. When the equatorial and
tropical lands were broad the winds and currents must
have been strong, much heat must have been carried
away from low latitudes, and the contrast between low
and high latitudes must have been relatively slight. As
we have already remarked, leading paleogeographers
believe that changes in the extent of the lands have been
especially marked in low latitudes, and that on the aver-
206 CLIMATIC CHANGES
age there has been a decrease in the extent of land within
the tropics. Gk>ndwana Land is the greatest illnstration
of this. In the same way, on the nnmerons paleogeo-
graphic maps of North America, most paleogeographers
have shown fairly extensive lands sonth of the latitude of
the United States during most of the geologic epochs.^*
(c) There is evidence that during geologic history the
area of the lands in middle and high latitudes, as well as
in low latitudes, has changed radically. An increase in
such lands would cause the winters to grow colder. This
would be partly because of the loss of heat by radiation
into the cold dry air over the continents in winter, and
partly because of increased reflection from snow and
frost, which gather much more widely upon the land than
upon the ocean. Furthermore, in winter when the conti-
nents are relatively cold, there is a strong tendency for
winds to blow out from the continent toward the ocean.
The larger the land the stronger this tendency. In Asia
it gives rise to strong winter monsoons. The effect of
such winds is illustrated by the way in which the wester-
lies prevent the Gulf Stream from warming the eastern
United States in winter. The Gulf Stream warms north-
western Europe much more than the United States be-
cause, in Europe, the prevaUing winds are onshore.
Another effect of an increase in the area of the lands in
middle and high latitudes would be to interpose bar-
riers to oceanic circulation and thus lower the tempera-
ture of polar regions. This would not mean glaciation in
high latitudes, however, even when the lands were wide-
spread as in the Mesozoic and early Tertiary. Students
of glaciology are more and more thoroughly convinced
i^Chas. Schuchert: Paleogeography of North America; Bull. GeoL Soc.
Am., Vol. 20, 1910; and Willis, Salisburj, and others: Outlines of Geologic
History, 1910.
TERRESTRIAL CAUSES OF CHANGES 207
that glaciation depends on the availability of moisture
even more than upon low temperature.
In conclusion it may be noted that each of the several
climatic influences of increased land area in the high
latitudes would tend to increase the contrasts between
land and sea, between winter and summer, and between
low latitudes and high. In other words, so far as the
effect upon high latitudes themselves is concerned, an
expansion of the lands there would tend in the same
direction as a diminution in low latitudes. In so far as
the general trend of geological evolution has been toward
more land in high latitudes and less in low, it would help
to produce a progressive increase in climatic diversity
such as is faintly indicated in the rock strata. On the
other hand, the oscillations in the distribution of the
lands, of which geology affords so much evidence, must
certainly have played an important part in producing the
periodic changes of climate which the earth has under-
gone.
in. Throughout geological history there is abundant
evidence that the process of contraction has led to
marked differences not only in the distribution and area
of the lands, but in their height. On the whole the lands
have presumably increased in height since the Protero-
zoic, somewhat in proportion to the increased differentia-
tion of continents and oceans.^^ If there has been such an
increase, the contrast between the climate of ocean and
land must have been accentuated, for highlands have a
greater diurnal and seasonal range of temperature than
do lowlands. The ocean has very little range of either
sort. The large range at high altitudes is due chiefly to
the small quantity of water vapor, for this declines
so Chas. Schnchert : The Earth 's Changing Surface and Climate ; in Lull :
The Evolution of the Earth and Its Inhabitants, 1918, p. 50.
208 CLIMATIC CHANGES
steadily with increased altitude. A diminution in the
density of the other constituents of the air also decreases
the blanketing effect of the atmosphere. In conformity
with the great seasonal range in temperature at times
when the lands stand high, the direction of the wind
would be altered. When the lands are notably warmer
than the oceans, the winds commonly flow from land to
sea, and when the continents are much colder than the
oceans, the direction is reversed. The monsoons of Asia
are examples. Strong seasonal winds disturb the normal
planetary circulation of the trade winds in low latitudes
and of the westerlies in middle latitudes. They also inter-
fere with the ocean currents set in motion by the planet-
ary winds. The net result is to hinder the transfer of
heat from low latitudes to high, and thus to increase the
contrasts between the zones. Local as well as zonal con-
trasts are also intensified. The higher the land, the
greater, relatively speaking, are the cloudiness and pre-
cipitation on seaward slopes, and the drier the interior*
Indeed, most highlands are arid. Henry's*^ recent study
of the vertical distribution of rainfall on mountain-sides
indicates that a decrease sets in at about 3500 feet in the
tropics and only a little higher in mid-latitudes.
In addition to the main effects upon atmospheric cir-
culation and precipitation, each of the many upheavals
of the lands must have been accompanied by many minor
conditions which tended toward diversity. For example,
the streams were rejuvenated, and instead of meandering
perhaps over vast flood plains they intrenched their
channels and in many cases dug deep gorges. The water
table was lowered, soil was removed from considerable
areas, the bare rock was exposed, and the type of domi-
21 A. J. Henry: The Deerease of Precipitation with Altitude; Monthlj
Weather Beview, Vol. 47, 1919, pp. 33-41.
TERRESTRIAL CAUSES OF CHANGES 209
nant vegetation altered in many places. An almost barren
ridge may represent all that remains of what was once a
vast forested flood plain. Thus, increased elevation of the
land produces contrasted conditions of slope, vegetation,
availability of ground water, exposure to wind and so
forth, and these unite in diversifying climate. Where
mountains are formed, strong contrasts are sure to
occur. The windward slopes may be very rainy, while
neighboring leeward slopes are parched by a dry foehn
wind. At the same time the tops may be snow-covered.
Increased local contrasts in climatic conditions are
known to influence the intensity of cyclonic storms," and
these affect the climatic conditions of all middle and high
latitudes, if not of the entire earth. The paths followed
by cyclonic storms are also altered by increased contrast
between land and water. When the continents are notably
colder than the neighboring oceans, high atmospheric
pressure develops on the lands and interferes with the
passage of lows, which are therefore either deflected
around the continent or forced to move slowly.
The distribution of lofty mountains has an even more
striking climatic effect than the general uplift of a region.
In Proterozoic times there was a great range in the Lake
Superior region ; in the late Devonian the Acadian moun-
tains of New England and the Maritime Provinces of
Canada possibly attained a height equal to the present
Eockies. Subsequently, in the late Paleozoic a significant
range stood where the Ouachitas now are. Accompanying
the uplift of each of these ranges, and all others, the
climate of the surrounding area, especially to leeward,
must have been altered greatly. Many extensive salt de-
22Cha8. F. Brooks: Monthly Weather Eeview, Vol. 46, 1918, p. 511; and
also A. J. Henry and others: Weather Forecasting in the United States,
1913.
210 CLIMATIC CHANGES
posits found now in fairly humid regions, for example,
the Pennsylvanian and Permian deposits of Kansas and
Oklahoma, were probably laid down in times of local
aridity due to the cutting off of moisture-bearing winds
by the mountains of Llanoria in Louisiana and Texas.
Hence such deposits do not necessarily indicate periods
of widespread and profound aridity.
When the causes of ancient glaciation were first con-
sidered by geologists, about the middle of the nineteenth
century, it was usually assumed that the glaciated areas
had been elevated to great heights, and thus rendered
cold enough to permit the accumulation of glaciers. The
many glaciers occurring in the Alps of central Europe
where gladology arose doubtless suggested this explana-
tion. However, it is now known that most of the ancient
glaciation was not of the alpine type, and there is ade-
quate proof that the glacial periods cannot be explained
as due directly and solely to uplift. Nevertheless, up-
heavals of the lands are among the most important fac-
tors in controlling climate, and variations in the height
of the lands have doubtless assisted in producing climate
oscillations, especially those of long duration. Moreover,
the progressive increase in the height of the lands has
presumably played a part in fostering local and zonal
diversity in contrast with the relative uniformity of
earlier geological times.
IV. The contraction of the earth has been accompanied
by volcanic activity as well as by changes in the extent,
distribution, and altitude of the lands. The probable part
played by volcanic dust as a contributory factor in pro-
ducing short sudden climatic variations has already been
discussed. There is, however, another though probably
less important respect in which volcanic activity may
have had at least a slight climatic significance. The oldest
TERRESTRIAL CAUSES OF CHANGES 211
known rocks, those of the Archean era, contain so much
igneous matter that many students have assumed that
they show that the entire earth was once liquid. It is now
considered that they merely indicate igneous activity of
great magnitude. In the later part of Proterozoic time,
during the second quarter of the earth *s history accord-
ing to Schuchert's estimate, there were again vast out-
flowings of lava. In the Lake Superior district, for ex-
ample, a thickness of more than a mile accumulated over
a large area, and lavas are common in many areas where
rocks of this age are known. The next quarter of the
earth's history elapsed without any correspondingly
great outflows so far as is known, though several lesser
ones occurred. Toward the end of the last quarter, and
hence quite recently from the geological standpoint,
another period of outflows, perhaps as noteworthy as
that of the Proterozoic, occurred in the Cretaceous and
Tertiary.
The climatic effects of such extensive lava flows would
be essentially as follows : In the first place so long as the
lavas were hot they would set up a local system of con-
vection with inflowing winds. This would interfere at
least a little with the general winds of the area. Again,
where the lava flowed out into water, or where rain fell
upon hot lava, there would be rapid evaporation which
would increase the rainfall. Then after the lava had
cooled, it would still influence climate a trifle in so far as
its color was notably darker or lighter than that of the
average surface. Dark surfaces absorb solar heat and
become relatively warm when the sun shines upon them.
Dark objects likewise radiate heat more rapidly than
light-colored objects. Hence they cool more rapidly at
night, and in the winter. As most lavas are relatively
dark they increase the average diurnal range of tempera-
212 CLIMATIC CHANGES
tore. Hence even after they are cool they increase the
climatic diversity of the land.
The amount of heat given to the atmosphere by an
extensive lava flow, though large according to human
standards, is small compared with the amount received
from the sun by a like area, except during the first few
weeks or months before the lava has formed a thick
crust. Furthermore, probably only a small fraction of
any large series of flows occurred in a given century or
millennium. Moreover, even the largest lava flows covered
an area of only a few hundredths of one per cent of the
earth's surface. Nevertheless, the conditions which mod-
ify climate are so complicated that it would be rash to
state that this amount of additional heat has been of
no climatic significance. like the proverbial ** straw that
broke the camel's back," the changes it would surely
produce in local convection, atmospheric pressure, and
the direction of the wind may have helped to shift the
paths of storms and to produce other complications whidi
were of appreciable climatic significance.
V. The last point which we shall consider in connection
with the effect of the earth's interior upon climate is
internal heat The heat given off by lavas is merely a
small part of that which is emitted by the earth as a
whole. In the earliest part of geological history enough
heat may have escaped from the interior of the earth to
exert a profound influence on the climate. Ejiowlton,**
as we have seen, has recently built up an elaborate theory
on this assumption. At present, however, accurate meas-
urements show that the escape of heat is so slight that
it has no appreciable influence except in a few volcanic
28 F. H. Knowlton: Evolution of Geologic Climates; BulL Oeol. See. Am.^
Vol. 30, Dec, 1919, pp. 499-566.
TERRESTRIAL CAUSES OF CHANGES 218
areas. It is estimated to raise the average temperature
of the earth's surface less than O.l'^C."
In order to contribute enough heat to raise the surface
temperature 1°0., the temperature gradient from the
interior of the earth to the surface would need to be ten
times as great as now, for the rate of conduction varies
directly with the gradient. If the gradient were ten times
as great as now, the rocks at a depth of two and one-half
miles would be so hot as to be almost liquid according to
Barrell's" estimates. The thick strata of unmetamor-
phosed Paleozoic rocks indicate that such high tempera-
tures have not prevailed at such slight depths since the
Proterozoic. Furthermore, the fact that the climate was
cold enough to permit gladation early in the Proterozoic
era and at from one to three other times before the open-
ing of the Paleozoic suggests that the rate of escape of
heat was not rapid even in the first half of the earth's
recorded history. Yet even if the general escape of heat
has never been large since the beginning of the better-
known part of geological history, it was prestmiably
greater in early times than at present.
If there actually has been an appreciable decrease in
the amount of heat given out by the earth's interior, its
effects would agree with the observed conditions of the
geological record. It would help to explain the relative
mildness of zonal, seasonal, and local contrasts of climate
in early geological times, but it would not help to explain
the long oscillations from era to era which appear to have
been of much greater importance. Those oscillations, so
far as we can yet judge, may have been due in part to
solar changes, but in large measure they seem to be
24 iTalbert, quoted by I. Bowman: Forest Physiography, 1911, p. 63.
3s J. Barrell: Rhytiims and the Measurement of Geologic Time; BulL
Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.
214 CLBIATIC CHANGES
explained by variations in the extent, distribution, and
altitude of the lands. Such variations appear to be the
inevitable result of the earth 's contraction.
CHAPTER XII
POST-GLACIAL CRUSTAL MOVEMENTS AND
CLIMATIC CHANGES
A N interesting practical application of some of the
/% preceding generalizations is found in an attempt
1 \ by 0. E. P. Brooks^ to interpret post-glacial
climatic changes almost entirely in terms of crustal move-
ment. We believe that he carries the matter much too far,
but his discussion is worthy of rather full recapitulation,
not only for its theoretical value but because it gives a
good summary of post-glacial changes. His climatic table
for northwest Europe as reprinted from the annual re-
port of the Smithsonian Institution for 1917, p. 366, is
as follows :
Phase
Climate
Date
1.
The Last Great Glaeia-
tion.
Aretic climate.
30,000-18,000 B. C.
2.
The Betreat of the
Severe continental
Glaeien.
climate.
18,000-6000 B. C.
3.
The Ga&tinental Phase.
Continental climate.
6000-4000 B. C.
4.
The Maritime Phase.
Warm and moist.
4000-3000 B. C.
5.
The Later Forest Phase.
Waim and dry.
3000-1800 B. C.
6.
The Peat-Bog Phase.
Cooler and moister.
1800 B. C.-300 A. D.
7.
The Beeent Phase.
Becoming drier.
300 A. D.-
Brooks bases his chronology largely on De (Jeer^s
measurements of the annual layers of clay in lake
1 C. E. P. Brooks : The Evolution of Climate in Northwest Europe. Quart.
Jour. Boyal Meteorol. Soc, Vol, 47, 1921, pp. 173-194.
216 CLIMATIC CHANGES
bottoms but makes mnch use of other evidence. Accord-
ing to Brooks the last glacial epoch lasted roughly from
30,000 to 18,000 B. C, but this includes a slight ameliora-
tion of climate followed by a readvance of the ice, known
as the Buhl stage. During the time of maximum glacia-
tion the British Isles stood twenty or thirty feet higher
than now and Scandinavia was ** considerably'' more
elevated. The author believes that this caused a fall of
VG. in the temperature of the British Isles and of 2^C.
in Scandinavia. By an ingenious though not wholly con-
vincing method of calculation he concludes that this
lowering of temperature, aided by an increase in the area
of the lands, sufficed to start an ice sheet in Scandinavia.
The relatively small area of ice cooled the air and gave
rise to an area of high barometric pressure. This in turn
is supposed to have caused further expansion of the ice
and to have led to full-fledged glaciation.
About 18,000 B. C. the retreat of the ice began in good
earnest. Even though no evidence has yet been found,
Brooks believes there must have been a change in the dis-
tribution of land and sea to account for the diminution of
the ice. The ensuing millenniums formed the Magdale-
nian period in human history, the last stage of the Paleo-
lithic, when man lived in caves and reindeer were abun-
dant in central Europe.* At first the ice retreated very
slowly and there were periods when for scores of years
the ice edge remained stationary or even readvanced.
About 10,000 B. C. the edge of the ice lay along the
southern coast of Sweden. During the next 2000 years it
withdrew more rapidly to about 59**N* Then came the
Fennoscandian pause, or Gschnit^ stage, when for about
2 H. F. Osbom: Men of the Old Stone Age, N. T., 1915; J. M. Tjrler:
The New Stone Age in Northwestern Europe, N. T., 1920.
POST-GLACIAL CRUSTAL MOVEMENTS 217
200 years the ice edge remained in one position, forming
a great moraine. Brooks suggests that this pause about
8000 B. C. was due to the closing of the connection be-
tween the Atlantic Ocean and the Baltic Sea and the
synchronous opening of a connection between the Baltic
and the White Seas, whereby cold Arctic waters replaced
the warmer Atlantic waters. He notes, however, that
about 7500 B. 0. the obliquity of the ecliptic was probably
nearly 1** greater than at present. This he calculates to
have caused the climate of Germany and Sweden to be
VF. colder than at present in winter and 1°F. warmer in
summer.
The next climatic stage was marked by a rise of tem-
perature till about 6000 B. C. During this period the ice
at first retreated, presumably because the climate was
ameliorating, although no cause of such amelioration is
assigned. At length the ice lay far enough north to allow
a connection between the Baltic and the Atlantic by way
of Lakes Wener and Wetter in southern Sweden. This is
supposed to have warmed the Baltic Sea and to have
caused the climate to become distinctly milder. Next the
land rose once more so that the Baltic was separated
from the Atlantic and was converted into the Ancylus
lake of fresh water. The southwest Baltic region then
stood 400 feet higher than now. The result was the Daun
stage, about 5000 B. C, when the ice halted or perhaps
readvanced a little, its front being then near Bagunda
in about latitude 63°. Why such an elevation did not
cause renewed glaciation instead of merely the slight
Daun pause. Brooks does not explain, although his calcu-
lations as to the effect of a slight elevation of the land
during the main period of glaciation from 30,000 to
18,000 B. C. would seem to demand a marked readvance.
218 CLIMATIC CHANGES
After 5000 B. C. there ensued a period when the cli-
mate, although still distinctly continental, was relatively
mild. The winters, to be sure, were stiU cold but the
summers were increasingly warm. In Sweden, for ex-
ample, the types of vegetation indicate that the smnmer
temperature was 7°F. higher than now. Storms, Brooks
assumes, were comparatively rare except on the outer
fringe of Great Britain. There they were sufficiently
abundant so that in the Northwest they gave rise to the
first Peat-Bog period, during which swamps replaced
forests of birch and pine. Southern and eastern England,
however, probably had a dry continental climate. Even
in northwest Norway storms were rare as is indicated by
remains of forests on islands now barren because of the
strong winds and fierce storms. Farther east most parts
of central and northern Europe were relatively dry. This
was the early Neolithic period when man advanced from
the use of unpolished to polished stone implements.
Not far from 4000 B. C. the period of continental cK-
mate was replaced by a comparatively moist maritime
climate. Brooks believes that this was because sub-
mergence opened the mouth of the Baltic and caused the
fresh Ancylus lake to give place to the so-called Litorina
sea. The temperature in Sweden averaged about 3°F.
higher than at present and in southwestern Norway 2**.
More important than this was the small annual range of
temperature due to the fact that the summers were cool
while the winters were mild. Because of the presence of
a large expanse of water in the Baltic region, storms, as
our author states, then crossed Great Britain and fol-
lowed the Baltic depression, carrying the moisture far
inland. In spite of the additional moisture thus available
the snow line in southern Norway was higher than now.
At this point Brooks turns to other parts of the world.
POST-GLACIAL CRUSTAL MOVEMENTS 219
He states that not far from 4000 B. C, a submergence
of the lands, rarely amounting to more than twenty-five
feet, took place not only in the Baltic region but in Ire-
land, Iceland, Spitzbergen, and other parts of the Arctic
Ocean, as well as in the White Sea, Greenland, and the
eastern part of North America. Evidences of a mild cli-
mate are found in all those places. Similar evidence of a
mild warm climate is found in East Africa, East Aus-
tralia, Tierra del Fuego, and Antarctica. The dates are
not established with certainty but* they at least fall in the
period immediately preceding the present epoch. In ex-
planation of these conditions Brooks assumes a universal
change of sea level. He suggests with some hesitation
that this may have been due to one of Pettersson's
periods of maximum * * tide-generating force. ' ^ According
to Pettersson the varying positions of the moon, earth,
and sun cause the tides to vary in cycles of about 9, 90,
and 1800 years, though the length of the periods is not
constant. When tides are high there is great movement
of oc^an waters and hence a great mixture of the water
at different latitudes. This is supposed to cause an
amelioration of climate. The periods of maximum and
minimum tide-generating force are as follows :
Maxima 3600 B. C. 2100 B. C. 350 B. C. A. D. 1434
Minima 2800 B. C. 1200 B. C. A. D. 630
Brooks thinks that the big trees in California and the
Norse sagas and Germanic myths indicate a rough agree-
ment of climatic phenomena with Pettersson 's last three
dates, while the mild climate of 4000 B. C. may really
belong to 3500 B. C. He gives no evidence confirming
Pettersson 's view at the other three dates.
To return to Brooks ' sketch of the relation of climatic
pulsations to the altitude of the lands, by 3000 B. C, that
220 CLIMATIC CHANGES
is, toward the close of the Neolithic period, further eleva-
tion is supposed to have taken place over the central
latitudes of western Europe. Southern Britain, which had
remained constantly above its present level ever since
30,000 B. C, was perhaps ninety feet higher than now.
Ireland was somewhat enlarged by elevation, the Straits
of Dover were almost closed, and parts of the present
North Sea were land. To these conditions Brooks ascribes
the prevalence of a dry continental climate. The storms
shifted northward once more, the winds were mild, as
seems to be proved by remains of trees in exposed places ;
and forests replaced fields of peat and heath in Britain
and Germany. The summers were perhaps warmer than
now but the winters were severe. The relatively dry cli-
mate prevailed as far west as Ireland. For example, in
Drumkelin Bog in Donegal County a corded oak road and
a two-story log cabin appear to belong to this time. Four-
teen feet of bog lie below the floor and twenty-six above.
This period, perhaps 3000-2000 B. C, was the legendary
heroic age of Ireland when **the vigour of the Irish
reached a level not since attained." This, as Brooks
points out, may have been a result of the relatively dry
climate, for today the extreme moisture of Ireland seems
to be a distinct handicap. In Scandinavia, civilization, or
at least the stage of relative progress, was also high at
this time.
By 1600 B. C. the land had assumed nearly its pres-
ent level in the British Isles and the southern Baltic
region, while northern Scandinavia still stood lower than
now. The climate of Britain and Germany was so humid
that there was an extensive formation of peat even on
high ground not before covered. This moist stage seems
to have lasted almost to the time of Christ, and may have
been the reason why the Romans described Britain as
POST-GLACIAL CRUSTAL MOVEMENTS 221
peculiarly wet and damp. At this point Brooks again de-
parts from northwest Europe to a wider field :
It is possible that we have to attribute this damp period in
Northwest Europe to some more general cause, for Ellsworth
Huntington's curves of tree-growth in California and climate
in Western Asia both show moister conditions from about
1000 B. C. to A. D. 200, and the same author believes that the
Mediterranean lands had a heavier rainfall about 500 B. G. to
A. D. 200. It seems that the phase was marked by a general in-
crease of the storminess of the temperate regions of the northern
hemisphere at least, with a maximum between Ireland and North
Germany, indicating probably that the Baltic again became the
favourite track of depressions from the Atlantic.
Brooks ends his paper with a brief resume of glacial
changes in North Ainerica, but as the means of dating
events are unreliable the degree of synchronism with
Europe is not clear. He sums up his conclusions as
follows :
On the whole it appears that though there is a general simi-
larity in the climatic history of the two sides of the North
Atlantic, the changes are not really contemporaneous, and such
relationship as appears is due mainly to the natural similarity
in the geographical history of two regions both recovering from
an Ice Age, and only very partially to world-wide pulsations
of climate. Additional evidence on this head will be available
when Baron de Geer publishes the results of his recent investiga-
tions of the seasonal glacial clays of North America, especially
if, as he hopes, he is able to correlate the banding of these clays
with the growth-rings of the big trees.
When we turn to the northwest of North America, this is
brought out very markedly. For in Yukon and Alaska the Ice
Age was a very mild affair compared with its severity in eastern
America and Scandinavia. As the land had not a heavy ice-load
to recover from, there were no complicated geographical
222 CLIMATIC CHANGES
changes. Also, there were no fluctuations of climate, but simply
a gradual passage to present conditions. The latter circumstance
especially seems to show that the emphasis laid on geographical
rather than astronomical factors of great climatic changes is
not misplaced.
Brooks ' painstaking discussion of post-glacial climatic
changes is of great value because of the large body of
material which he has so carefully wrought together. His
strong belief in the importance of changes in the level of
the lands deserves serious consideration. It is difficult,
however, to accept his final conclusion that such changes
are the main factors in recent climatic changes. It is al-
most impossible, for example, to believe that movements
of the land could produce almost the same series of
climatic changes in Europe, Central Asia, the western
and eastern parts of North America, and the southern
hemisphere. Yet such changes appear to have occurred
during and since the glacial period. Again there is no
evidence whatever that movements of the land have any-
thing to do with the historic cycles of climate or with the
cycles of weather in our own day, which seem to be the
same as glacial cycles on a small scale. Also, as Dr.
Simpson points out in discussing Brooks' paper, there
appears ^^no solution along these lines of the problem
connected with rich vegetation in both polar circles and
the ice-age which produced the ice-sheet at sea-level in
Northern India. ' ' Nevertheless, we may well believe that
Brooks is right in holding that changes in the relative
level and relative area of land and sea have had im-
portant local effects. While they are only one of the
factors involved in climatic changes, they are certainly
one that must constantly be kept in mind.
CHAPTER XIII
THE CHANGING COMPOSITION OF OCEANS AND
ATMOSPHERE
HAVING discussed the climatic effect of move-
ments of the earth's crust during the course of
geological time, we are now ready to consider
the corresponding effects due to changes in the movable
envelopes — ^the oceans and the atmosphere. Variations in
the composition of sea water and of air, and in the
amount of air must ahnost certainly have occurred, and
must have produced at least slight climatic consequences.
It should be pointed out at once that such variations
appear to be far less important climatically than do
movements of the earth 's crust and changes in the activ-
ity of the sun. Moreover, in most cases, they are not
reversible as are the crustal and solar phenomena. Hence,
while most of them appear to have been unimportant so
far as climatic oscillations and fluctuations are concerned,
they seemingly have aided in producing the slight secular
progression to which we have so often referred.
There is general agreement among geologists that the
ocean has become increasingly saline throughout the
ages. Indeed, calculations of the rate of accumulation of
salt have been a favorite method of arriving at estimates
of the age of the ocean, and hence of the earliest marine
sediments. So far as known, however, no geologist or
climatologist has discussed the probable climatic effects
224 CLIMATIC CHANGES
of increased salinity. Yet it seems clear that an increase
in salinity must have a slight effect npon climate.
Salinity affects climate in fonr ways: (1) It appre-
ciably influences the rate of evaporation; (2) it alters
the freezing point; (3) it produces certain indirect
effects through changes in the absorption of carbon
dioxide; and (4) it has an effect on oceanic circulation.
(1) According to the experiments of Mazelle and
Okada, as reported by Klriimmel,^ evaporation from ordi-
nary sea water is from 9 to 30 per cent less rapid than
from fresh water under similar conditions. The varia-
tion from 9 to 30 per cent found in the experiments de-
pends, perhaps, upon the wind velocity. When salt water
is stagnant, rapid evaporation tends to result in the
development of a film of salt on the top of the water,
especially where it is sheltered from the wind. Such a film
necessarily reduces evaporation. Hence the relatively
low salinity of the oceans in the past probably had a
tendency to increase the amount of water vapor in the
air. Even a little water vapor augments slightly the
blanketing effect of the air and to that extent diminishes
the diurnal and seasonal range of temperature and the
contrast from zone to zone.
(2) Increased salinity means a lower freezing tem-
perature of the oceans and hence would have an effect
during cold periods such as the present and the Pleisto-
cene ice age. It would not, however, be of importance
during the long warm periods which form most of
geologic time. A salinity of about 3.5 per cent at present
lowers the freezing point of the ocean roughly 2°C. below
that of fresh water. If the ocean were fresh and our
winters as cold as now, all the harbors of New England
and the Middle Atlantic States would be icebound. The
1 Encyclopaedia Britannica, 11th edition: article "Ocean."
OCEANS AND ATMOSPHERE 225
Baltic Sea would also be frozen each winter, and even
the eastern harbors of the British Isles would be fre-
quently locked in ice. At high latitudes the area of per-
manently frozen oceans would be much enlarged. The
effect of such a condition upon marine life in high lati-
tudes would be like that of a change to a warmer climate.
It would protect the life on the continental shelf from the
severe battering of winter storms. It would also lessen
the severity of the winter temperature in the water for
when water freezes it gives up much latent heat, — eighty
calories per cubic centimeter. Part of this raises the
temperature of the underlying water.
The expansion of the ice near northern shores would
influence the life of the lands quite differently from that
of the oceans, It would act like an addition of land to the
continents and would, therefore, increase the atmospheric
contrasts from zone to zone and from continental interior
to ocean. In summer the ice upon the sea would tend to
keep the coastal lands cool, very much as happens now
near the Arctic Ocean, where the ice floes have a great
effect through their reflection of light and their absorp-
tion of heat in melting. In winter the virtual enlargement
of the continents by the addition of an ice fringe would
decrease the snowfall upon the lands. Still more im-
portant would be the effect in intensifying the anti-
cyclonic conditions which normally prevail in winter not
only over continents but over ice-covered oceans. Hence
the outblowing cold winds would be strengthened.^ The
net effect of all these conditions would apparently be a
diminution of snowfall in high latitudes upon the lands
even though the summer snowfall upon the ocean and the
so. E. P. Brooks: The Meteorological Conditions of an Ice Sheet and
Their Bearing on the Desiccation of the Globe; Quart. Jour. Bojal Meteorol.
Soc., Vol. 40, 1914, pp. 53-70.
226 CLIMATIC CHANGES
coasts may have increased. This condition may have been
one reason why widespread glaciation does not appear to
have prevailed in high latitudes during the Proterozoic
and Permian glaciations, even though it occurred farther
south. If the ocean during those early glacial epochs
were ice-covered down to middle latitudes, a lack of ex-
tensive glaciation in high latitudes would be no more
surprising than is the lack of Pleistocene glaciation in
the northern parts of Alaska and Asia. Great ice sheets
are impossible without a large supply of moisture.
(3) Among the indirect effects of salinity one of the
chief appears to be that the low salinity of the water in
the past and the greater ease with which it froze presum-
ably allowed the temperature of the entire ocean to be
slightly higher than now. This is because ice serves as a
blanket and hinders the radiation of heat from the under-
lying water. The temperature of the ocean has a climatic
sig3iificance not only directly, but indirectly through its
influence on the amount of carbon dioxide held by the
oceans. A change of even 1°C. from the present mean
temperature of 2°C. would alter the ability of the entire
ocean to absorb carbon dioxide by about 4 per cent. This,
according to F. W. Clarke,' is because the oceans contain
from eighteen to twenty-seven times as much carbon
dioxide as the air when only the free carbon dioxide is
considered, and about seventy times as much according
to Johnson and Williamson* when the partially combined
carbon dioxide is also considered. Moreover, the capacity
of water for carbon dioxide varies sharply with the tem-
perature.* Hence a rise in temperature of only 1®C.
would theoretically cause the oceans to give up from 30
• Data of GeochemiBtry, Fourth Ed., 1920; BulL No. 695, U. 8. OeoL
Survey.
* Quoted by Schuchert in The Evolution of the Earth.
s Smithsonian Physical Tables, Sixth Bevision, 1914, p. 142.
OCEANS AND ATMOSPHERE 227
to 280 times as much carbon dioxide as the air now holds.
This, however, is on the unfounded assumption that the
oceans are completely saturated. The important point is
merely that a slight change in ocean temperature would
cause a disproportionately large change in the amount
of carbon dioxide in the air with all that this impUes in
respect to blanketing the earth, and thus altering tem-
perature.
(4) Another and perhaps the most important effect of
salinity upon climate depends upon the rapidity of the
deep-sea circulation. The circulation is induced by differ-
ences of temperature, but its speed is affected at least
slightly by salinity. The vertical circulation is now domi-
nated by cold water from subpolar latitudes. Except in
closed seas like the Mediterranean the lower portions
of the ocean are near the freezing point. This is because
cold water sinks in high latitudes by reason of its su-
perior density, and then ** creeps'' to low latitudes. There
it finally rises and replaces either the water driven pole-
ward by the winds, or that which has evaporated from
the surface.*
During past ages, when the sea water was less salty,
the circulation was presumably more rapid than now.
This was because, in tropical regions, the rise of cold
<) Chamberlin, in a verj Buggestive article "On a possible reversal of
oceanic circulation" (Jour, of G^L, Vol. 14, pp. 363-373, 1906), discusses
the probable climatic consequences of a reversal in the direction of deep-
flea circulation. It is not whoUj bejond the bounds of possibility that
in the course of ages the increasing drainage of salt from the lands not
onlj hy nature but by man's activities in agriculture and drainage, may
ultimately cause such a reversal by increasing the ocean's salinity until the
more saline tropical portion is heavier than the cooler but fresher subpolar
waters. If that should happen, Greenland, Antarctica, and the northern
shores of America and Asia would be warmed by the tropical heat which
had been transferred poleward beneath the surface of the ocean, without
lofls en route. Subpolar regions, under such a condition of reversed deep-sea
eireulation, might have a mild climate. Indeed, they might be among the
world's most favorable regions climatically.
228 CLIMATIC CHANGES
water is hindered by the sinMng of warm surface water
which is relatively dense because evaporation has re-
moved part of the water and caused an accumulation of
salt. According to Kriimmel and Mill/ the surface salin-
ity of the subtropical belt of the North Atlantic commonly
exceeds 3.7 per cent and sometimes reaches 3.77 per cent,
whereas the underlying waters have a salinity of less
than 3.5 per cent and locally as little as 3.44 per cent
The other oceans are slightly less saline than the North
Atlantic at all depths^ but the vertical salinity gradients
along the tropics are similar. According to the Smith-
sonian Physical Tables, the difference in salinity between
the surface water and that lying below is equivalent to
a difference of .003 in density, where the density of fresh
water is taken as 1.000. Since the decrease in density pro-
duced by warming water from the temperature of its
greatest density (4°C.) to the highest temperatures
which ever prevail in the ocean (30°C. or SG^'F.) is only
.004, the more saline surface waters of the dry tropics
are at most times almost as dense as the less saline but
colder waters beneath the surface, which have come from
higher latitudes. During days of especially great evapo-
ration, however, the most saline portions of the surface
waters in the dry tropics are denser than the underlying
waters and therefore sink, and produce a temporary local
stagnation in the general circulation. Such a sinking of
the warm surface waters is reported by Kriimmel, who
detected it by means of the rise in temperature which it
produces at considerable depths. If such a hindrance to
the circulation did not exist, the velocity of the deep-sea
movements would be greater.
If in earlier times a more rapid circulation occurred,
low latitudes must have been cooled more than now by
7 Encydopcedia Britaxmica : article ' ' Ocean. ' '
OCEANS AND ATMOSPHERE 229
the rise of cold waters. At the same time higher latitudes
were presmnably warmed by a greater flow of warm
water from tropical re^ons because less of the surface
heat sank in low latitudes. Such conditions would tend to
lessen the climatic contrast between the different lati-
tudes. Hence, in so far as the rate of deep-sea circulation
depends upon salinity, the slowly increasing amount of
salt in the oceans must have tended to increase the con-
trasts between low and high latitudes. Thus for several
reasons, the increase of saUnity during geologic history
seems to deserve a place among the minor agencies which
help to explain the apparent tendency toward a secular
progression of climate in the direction of greater con-
trasts between tropical and subpolar latitudes.
Changes in the composition and amount of the atmos-
phere have presumably had a climatic importance greater
than that of changes in the salinity of the oceans. The
atmospheric changes may have been either progressive
or cyclic, or both. In early times, according to the nebular
hypothesis, the atmosphere was much more dense than
now and contained a larger percentage of certain con-
stituents, notably carbon dioxide and water. The plane-
tesimal hypothesis, on the other hand, postulates an in-
crease in the density of the atmosphere, for according to
this hypothesis the density of the atmosphere depends
upon the power of the earth to hold gases, and this power
increases as the earth grows bigger with the infall of
material from without.*
Whichever hypothesis may be correct, it seems prob-
able that when life first appeared on the land the at-
mosphere resembled that of today in certain fundamental
respects. It contained the elements essential to life, and
8 Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132, 1906; and T. C.
Chamberlin: The Origin of the Earth, 1916.
280 CLIMATIC CHANGES
its blanketing effect was such as to maintain tempera-
tures not greatly different from those of the present. The
evidence of this depends largely upon the narrow limits
of temperature within which the activities of modem
life are possible, and upon the cumulative evidence that
ancient life was essentiaUy similar to the types now
living. The resemblance between some of the oldest
forms and those of today is striking. For example,
according to Professor Schuchert:* **Many of the living
genera of forest trees had their origin in the Cretaceous,
and the giant sequoias of California go back to the Trias-
sic, while Ginkgo is known in the Permian. Some of the
fresh-water molluscs certainly were living in the early
periods of the Mesozoic, and the lung-fish of today
(Ceratodus) is known as far back as the Triassic and is
not very unlike other lung-fishes of the Devonian. The
higher vertebrates and insects, on the other hand, are
very sensitive to their environment, and therefore do not
extend back generically beyond the Cenozoic, and only in
a few instances even as far as the Oligocene. Of marine
invertebrates the story is very different, for it is well
known that the horseshoe crab (Limulus) lived in the
Upper Jurassic, and Nautilus in the Triassic, with forms
in the Devonian not far removed from this genus. Still
longer-ranging genera occur among the brachiopods, for
living Lingula and Crania have specific representatives
as far back as the early Ordovician. Among living f ora-
minifers, Lagena, Globigerina, and Nodosaria are known
in the later Cambrian or early Ordovician. In the Middle
Cambrian near Field, British Columbia, Walcott has
found a most varied array of invertebrates among which
are crustaceans not far removed from living forms.
Zoologists who see these wonderful fossils are at once
> Personal communication.
OCEANS AND ATMOSPHERE 281
struck with their modernity and the little change that has
taken place in certain stocks since that far remote time.
Back of the Paleozoic, little can be said of life from the
generic standpoint, since so few fossils have been re-
covered, but what is at hand suggests that the marine
environment was similar to that of today. ' '
At present, as we have repeatedly seen, little growth
takes place either among animals or plants at tempera-
tures below 0°C. or above 40"* C, and for most species
the limiting temperatures are about 10° and 30°. The
maintenance of so narrow a scale of temperature is a
function of the atmosphere, as well as of the sun. Without
an atmosphere, the temperature by day would mount
fatally wherever the sun rides high in the sky. By night
it would fall everywhere to a temperature approaching
absolute zero, that is — 273 °C. Some such tempera-
ture prevails a few miles above the earth's surface,
beyond the effective atmosphere. Indeed, even if the
atmosphere were almost as it is now, but only lacked one
of the minor constituents, a constituent which is often
actually ignored in statements of the composition of the
air, life would be impossible. Tyndall concludes that if
water vapor were entirely removed from the atmosphere
for a single day and night, all life — except that which is
dormant 1 the form of seeds, eggs, or spores-would be
exterminated. Part would be killed by the high tempera-
ture developed by day when the sun was high, and part,
by the cold night.
The testimony of ancient glaciation as to the slight
difference in the climate and therefore in the atmos-
phere of early and late geological times is almost as clear
as that of life. Just as life proves that the earth can never
have been extremely cold during hundreds of millions of
years, so glaciation in moderately low latitudes near
282 CLIMATIC CHANGES
the dawn of earth history and at several later times,
proves that the earth was not particularly hot even in
those early days. The gentle progressive change of climate
which is recorded in the rocks appears to have been only
in slight measure a change in the mean temperature of
the earth as a whole, and almost entirely a change in the
distribution of temperature from place to place and
season to season. Hence it seems probable that neither
the earth's own emission of heat, nor the supply of solar
heat, nor the power of the atmosphere to retain heat can
have been much greater a few hundred million years ago
than now. It is indeed possible that these three factors
may have varied in such a way that any variation in one
has been offset by variations of the others in the opposite
direction. This, however, is so highly improbable that it
seems advisable to assume that all three have remained
relatively constant. This conclusion together with a
realization of the climatic significance of carbon dioxide
has forced most of the adherents of the nebular hypothe-
sis to abandon their assumption that carbon dioxide, the
heaviest gas in the air, was very abundant until taken
out by coal-forming plants or combined with the calcium
oxide of igneous rocks to form the limestone secreted
by animals. In the same way the presence of sun cracks
in sedimentary rocks of all ages suggests that the air
cannot have contained vast quantities of water vapor
such as have been assumed by Knowlton and others in
order to account for the former lack of sharp climatic
contrast between the zones. Such a large amount of water
vapor would ahnost certainly be accompanied by well-
nigh universal and continual cloudiness so that there
would be little chance for the pools on the earth 's water-
soaked surface to dry up. Furthermore, there is only one
way in which such cloudiness could be maintained and
OCEANS AND ATMOSPHERE 288
that is by keeping the air at an ahnost constant tempera-
ture night and day. This would require that the chief
source of warmth be the interior of the earth, a condition
which the Proterozoic, Permian, and other widespread
glaciations seem to disprove.
Thus there appears to be strong evidence against the
radical changes in the atmosphere which are sometimes
postulated. Yet some changes must have taken place, and
even minor changes would be accompanied by some sort
of climatic effect. The changes would take the form of
either an increase or a decrease in the atmosphere as a
whole, or in its constituent elements. The chief means by
which the atmosphere has increased appear to be as
follows: (a) By contributions from the interior of the
earth via volcanoes and springs and by the weathering of
igneous rocks with the consequent release of their en-
closed gases ;^® (b) by the escape of some of the abundant
gases which the ocean holds in solution ; (c) by the arrival
on the earth of gases from space, either enclosed in
meteors or as free-flying molecules; (d) by the release of
gases from organic compounds by oxidation, or by ex-
halation from animals and plants. On the other hand, one
or another of the constituents of the atmosphere has pre-
sumably decreased (a) by being locked up in newly
formed rocks or organic compounds; (b) by being dis-
solved in the ocean; (c) by the escape of molecules into
space; and (d) by the condensation of water vapor.
The combined effect of the various means of increase
and decrease depends partly on the amount of each con-
stituent received from the earth's interior or from space,
and partly on the fact that the agencies which tend to
deplete the atmosphere are highly selective in their
10 B. T. Chamberlin : Oases in Bocks, Carnegie Inst, of Wash., No. 106,
1908.
284 CLIMATIC CHANGES
action. Our knowledge of how large a quantity of new
gases the air has received is very scanty, but judging by
present conditions the general tendency is toward a slow
increase chiefly because of meteorites, volcanic action,
and the work of deep-seated springs. As to decrease, the
case is clearer. This is because the chemically active
gases, oxygen, CO2, and water vapor, tend to be locked
up in the rocks, while the chemically inert gases, nitrogen
and argon, show almost no such tendency. Though oxy-
gen is by far the most abimdant element in the earth's
crust, making up more than 50 per cent of the total, it
forms only about one-fifth of the air. Nitrogen, on the
other hand, is very rare in the rocks, but makes up nearly
four-fifths of the air. It would, therefore, seem probable
that throughout the earth's history, there has been a
progressive increase in the amount of atmospheric nitro-
gen, and presumably a somewhat corresponding increase
in the mass of the air. On the other hand, it is not clear
what changes have occurred in the amount of atmos-
pheric oxygen. It may have increased somewhat or
perhaps even notably. Nevertheless, because of the
greater increase in nitrogen, it may form no greater per-
centage of the air now than in the distant past.
As to the absolute amounts of oxygen, Barrell"
thought that atmospheric oxygen began to be present
only after plants had appeared. It will be recalled that
plants absorb carbon dioxide and separate the carbon
from the oxygen, using the carbon in their tissues and
setting free the oxygen. As evidence of a paucity of
oxygen in the air in early Proterozoic times, Barrell
cites the fact that the sedimentary rocks of that remote
11 J. Barrell : The Origin of the Earth, in Evolution of the Earth and
Its Inhabitants, 1918, p. 44, and more fully in an unpublished manuaeript.
OCEANS AND ATMOSPHERE 286
time commonly are somewhat greyish or greenish-grey
wackesy or other types, indicating incomplete oxidation.
He admits, however, that the stupendous thicknesses of
red sandstones, quartzite, and hematitic iron ores of the
later Proterozoic prove that by that date there was an
abundance of atmospheric oxygen. If so, the change from
paucity to abundance must have occurred before fossils
were numerous enough to give much clue to climate.
However, Barrell's evidence as to a former paucity of
atmospheric oxygen is not altogether convincing. In the
first place, it does not seem justifiable to assume that
there could be no oxygen until plants appeared to break
down the carbon dioiide, for some oxygen is contributed
by volcanoes,** and lightning decomposes water into its
elements. Part of the hydrogen thus set free escapes into
space, for the earth's gravitative force does not appear
great enough to hold this lightest of gases, but the oxy-
gen remains. Thus electrolysis of water results in the
accumulation of oxygen. In the second place, there is no
proof that the ancient greywackes are not deoxidized
sediments. Light colored rock formations do not neces-
sarily indicate a paucity of atmospheric oxygen, for such
rocks are abundant even in recent times. For example,
the Tertiary formations are characteristically light
colored, a result, however, of deoxidation. Finally, the
fact that sedimentary rocks, irrespective of their age,
contain an average of about 1.5 per cent more oxygen
than do igneous rocks," suggests that oxygen was pres-
ent in the air in quantity even when the earliest shales
and sandstones were formed, for atmospheric oxygen
seems to be the probable source of the extra oxygen they
12 p. W. Clarke: Data of Geochemistry, Fourth Ed., 1920, Bull. No. 695,
U. S. GeoL Survey, p. 256.
"P. W. Clarke: loc. cit,, pp. 27-34 et al.
286 CLIMATIC CHANGES
contain. The formation of these particular sedimentary
rocks by weathering of igneous rocks involves only a
little carbon dioxide and water. Although it seems prob-
able that oxygen was present in the atmosphere even at
the beginning of the geological record, it may have been
far less abundant then than now. It may have been re-
moved from the atmosphere by animals or by the oxida-
tion of the rocks almost as rapidly as it was added by
volcanoes, plants, and other agencies.
After this chapter was in typBy St. John*" announced
his interesting discovery that oxygen is apparently lack-
ing in the atmosphere of Venus. He considers that this
proves that Venus has no life. Furthermore he concludes
that so active an element as oxygen cannot be abundant
in the atmosphere of a planet unless plants continually
supply large quantities by breaking down carbon dioxide.
But even if the earth has experienced a notable in-
crease in atmospheric oxygen since the appearance of
life, this does not necessarily involve important dimatic
changes except those due to increased atmospheric den-
sity. This is because oxygen has very little effect upon
the passage of light or heat, being transparent to all but
a few wave lengths. Those absorbed are chiefly in the
ultra violet.
The distinct possibility that oxygen has increased in
amount, makes it the more likely that there has been an
increase in the total atmosphere, for the oxygen would
supplement the increase in the relatively inert nitrogen
and argon, which has presumably taken place. The cli-
matic effects of an increase in the atmosphere include, in
the first place, an increased scattering of light as it
approaches the earth. Nitrogen, argon, and oxygen all
i<* Chas. E. St. John : Science Service Press Reports from the Mt. Wilson
Observatory, May, 1922,
OCEANS AND ATMOSPHERE 287
scatter the short waves of light and thus interfere with
their reaching the earth. Abbot and Fowle," who have
carefully studied the matter, believe that at present the
scattering is quantitatively important in lessening insola-
tion. Hence our supposed general increase in the volume
of the air during part of geological times would tend to
reduce the amount of solar energy reaching the earth's
surface. On the other hand, nitrogen and argon do not
appear to absorb the long wave lengths known as heat,
and oxygen absorbs so little as to be almost a non-
absorber. Therefore the reduced penetration of the air
by solar radiation due to the scattering of light would
apparently not be neutralized by any direct increase in
the blanketing effect of the atmosphere, and the tempera-
ture near the earth's surface would be slightly lowered
by a thicker atmosphere. This would diminish the amount
of water vapor which would be held in the air, and
thereby lower the temperature a trifle more.
In the second place, the higher atmospheric pressure
which would result from the addition of gases to the
air would cause a lessening of the rate of evaporation,
for that rate declines as pressure increases. Decreased
evaporation would presumably still further diminish the
vapor content of the atmosphere. This would mean a
greater daily and seasonal range of temperature, as is
very obvious when we compare clear weather with cloudy.
Cloudy nights are relatively warm while clear nights are
cool, because water vapor is an almost perfect absorber
of radiant heat, and there is enough of it in the air on
moist nights to interfere greatly with the escape of the
heat accumulated during the day. Therefore, if atmos-
1^ Abbot and Fowle: Annals Astrophyaical Observatoiy; Smiths. Inst.,
Vol. II, 1908, p. 163.
F. £. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst.,
Vol. 69, 1918.
288 CLIMATIC CHANGES
pheric moisture were formerly much more abundant
than noWy the temperature must have been much more
uniform. The tendency toward climatic severity as time
went on would be still further increased by the cooling
which would result from the increased wind velocity dis-
cussed below; for cooling by convection increases with
the velocity of the wind, as does cooling by conduction.
Any persistent lowering of the general temperature of
the air would affect not only its ability to hold water
vapor, but would produce a lessening in the amount of
atmospheric carbon dioxide, for the colder the ocean
becomes the more carbon dioxide it can hold in solution.
When the oceanic temperature falls, part of the atmos-
pheric carbon dioxide is dissolved in the ocean. This
minor constituent of the air is important because
although it forms only 0.003 per cent of the earth's at-
mosphere, Abbot and Fowle *s" calculations indicate that
it absorbs over 10 per cent of the heat radiated outward
from the earth. Hence variations in the amount of carbon
dioxide may have caused an appreciable variation in
temperature and thus in other climatic conditions.
Humphreys, as we have seen, has calculated that a
doubling of the carbon dioxide in the air would directly
raise the earth's temperature to the extent of 1.3*^0., and
a halving would lower it a like amount. The indirect
results of such an increase or decrease might be greater
than the direct results, for the change in temperature
due to variations in carbon dioxide would alter the
capacity of the air to hold moisture.
Two conditions would especially help in this respect;
first, changes in nocturnal cooling, and second, changes
in local convection. The presence of carbon dioxide dimin-
ishes nocturnal cooling because it absorbs the heat radi-
iB Abbot and Fowle: loc. dt., p. 172.
OCEANS AND ATMOSPHERE 289
ated by the earth, and re-radiates part of it back again.
Hence with increased carbon dioxide and with the
consequent wanner nights there would be less nocturnal
condensation of water vapor to form dew and frost.
Local convection is influenced by carbon dioxide because
this gas lessens the temperature gradient. In general, the
less the gradient, that is, the less the contrast between
the temperature at the surface and higher up, the less
convection takes place. This is illustrated by the seasonal
variation in convection. In summer, when the gradient is
steepest, convection reaches its maximum. It will be re-
called that when air rises it is cooled by expansion, and
if it ascends far the moisture is soon condensed and
precipitated. Indeed, local convection is considered by
C. P. Day to be the chief agency which keeps the lower
air from being continually saturated with moisture. The
presence of carbon dioxide lessens convection because it
increases the absorption of heat in the zone above the
level in which water vapor is abundant, thus warming
these higher layers. The lower air may not be warmed
correspondingly by an increase in carbon dioxide if
Abbot and Fowle are right in stating that near the earth's
surface there is enough water vapor to absorb practicaUy
all the wave lengths which carbon dioxide is capable of
absorbing. Hence carbon dioxide is chiefly effective at
heights to which the low temperature prevents water
vapor from ascending. Carbon dioxide is also effective
in cold winters and in high latitudes when even the lower
air is too cold to contain much water vapor. Moreover,
carbon dioxide, by altering the amount of atmospheric
water vapor, exerts an indirect as well a& a direct effect
upon temperature.
Other effects of the increase in air pressure which we
are here asstmiing during at least the early part of geo-
240 CLIMATIC CHANGES
logical times are corresponding changes in barometric
contrasts, in the strength of winds, and in the mass of air
carried by the winds along the earth's surface. The in-
crease in the mass of the air would reenf orce the greater
velocity of the winds in their action as eroding and trans-
porting agencies. Because of the greater weight of the
air, the winds would be capable of picking up more dust
and of carrying it farther and higher; while the increased
atmospheric friction would keep it aloft a longer time.
The significance of dust at high levels and its relation to
solar radiation have already been discussed in connection
with volcanoes. It will be recalled that on the average it
lowers the surface temperature. At lower levels, since
dust absorbs heat quickly and gives it out quickly, its
presence raises the temperature of the air by day and
lowers it by night. Hence an increase in dustiness tends
toward greater extremes.
-From all these considerations it appears that if the
atmosphere has actually evolved according to the suppo-
sition which is here tentatively entertained, the general
tendency of the resultant climatic changes must have
been partly toward long geological oscillations and partly
toward a general though very slight increase in climatic
severity and in the contrasts between the zones. This
seems to agree with the geological record, although the
fact that we are living in an age of relative climatic
severity may lead us astray.
The significant fact about the whole matter is that the
three great types of terrestrial agencies, namely, those
of the earth's interior, those of the oceans, and those of
the air, all seem to have suffered changes which lead to
slow variations of climate. Many reversals have doubt-
less taken place, and the geologic oscillations thus in-
duced are presumably of much greater importance than
OCEANS AND ATMOSPHERE 241
the progressive diangey yet so far as we can tell the
purely terrestrial changes throughout the hundreds of
millions of years of geological time have tended toward
complexity and toward increased contrasts from conti-
nent to ocean, from latitude to latitude, from season to
season, and from day to night.
Throughout geological history the slow and almost
imperceptible differentiation of the earth's surface has
been one of the most noteworthy of all changes. It has
been opposed by the extraordinary conservatism of the
universe which causes the average temperature today to
be so like that of hundreds of millions of years ago that
many types of life are almost identical. Nevertheless, the
differentiation has gone on. Often, to be sure, it has pre-
sumably been completely masked by the disturbances of
the solar atmosphere which appear to have been the
cause of the sharper, shorter climatic pulsations. But
regardless of cosmic conservatism and of solar impulses
toward change, the slow differentiation of the earth's
surface has apparently given to the world of today much
of the geographical complexity which is so stimulating
a factor in organic evolution. Such complexity — such
diversity from place to place — appears to be largely
accounted for by purely terrestrial causes. It may be
regarded as the great terrestrial contribution to the
climatic environment which guides the development of
Ufe.
CHAPTER XIV
THE EFFECT OF OTHER BODIES ON THE SUN
IF solar activity is really an important factor in
causing climatic changes, it behooves us to subject
the sun to the same kind of inquiry to which we
have subjected the earth. We have inquired into the na-
ture of the changes through which the earth *s crust, the
oceans, and the atmosphere have influenced the climate
of geological times. It has not been necessary, however,
to study the origin of the earth, nor to trace its earlier
stages. Our study of the geological record begins only
when the earth had attained practically its present mass,
essentially its present shape, and a climate so similar to
that of today that life as we know it was possible. In
other words, the earth had passed the stages of infancy,
childhood, youth, and early maturity, and had reached
full maturity. As it still seems to be indefinitely far from
old age, we infer that during geological times its relative
changes have been no greater than those which a man
experiences between the ages of perhaps twenty-five and
forty.
Similar reasoning applies with equal or greater force
to the sun. Because of its vast size it presumably passes
through its stages of development much more slowly than
the earth. In the first chapter of this book we saw that
the earth 's relative uniformity of climate for hundreds of
millions of years seems to imply a similar uniformity in
solar activity. This accords with a recent tendency among
EFFECT OF OTHER BODIES ON THE SUN 248
astronomers who are more and more recognizing that the
stars and the solar system possess an extraordinary de-
gree of conservatism. Changes that once were supposed
to take place in thousands of years are now thought to
have required millions. Hence in this chapter we shall
assume that throughout geological times the condition
of the sun has been almost as at present. It may have
been somewhat larger, or different in other ways, but it
was essentially a hot, gaseous body such as we see today
and it gave out essentially the same amount of energy.
This assumption will affect the general validity of what
follows only if it departs widely from the truth. With this
assumption, then, let us inquire into the degree to which
the sun's atmosphere has probably been disturbed
throughout geological times.
In Earth and Sun, as already explained, a detailed
study has led to the conclusion that cyclonic storms are
influenced by the electrical action of the sun. Such ac-
tion appears to be most intense in sunspots, but appar-
ently pertains also to other disturbed areas in the sun 's
atmosphere. A study of sunspots suggests that their
true periodicity is almost if not exactly identical with
that of the orbital revolution of Jupiter, 11.8 years. Other
investigations show numerous remarkable coincidences
between sunspots and the orbital revolution of the other
planets, including especially Saturn and Mercury. This
seems to indicate that there is some truth in the hypothe-
sis that sunspots and other related disturbances of the
solar atmosphere owe their periodicity to the varying
effects of the planets as they approach and recede from
the sun in their eccentric orbits and as they combine or
oppose their effects according to their relative positions.
This does not mean that the energy of the solar disturb-
ances is supposed to come from the planets, but merely
244 CLIMATIC CHANGES
that their variations act Uke the turning of a switch to
determine when and how violently the internal forces of
the sun shall throw the solar atmosphere into commotion.
This hypothesis is by no means new, for in one form or
another it has been advocated by Wolfer, Birkeland,
E. W. Brown, Schuster, Arctowski, and others.
The agency through which the planets influence the
solar atmosphere is not yet clear. The suggested agencies
are the direct pull of gravitation, the tidal effect of the
planets, and an electro-magnetic effect. In Earth and
Sun the conclusion is reached that the first two are out
of the question, a conclusion in which E. W. Brown
acquiesces. Unless some unknown cause is appealed to,
this leaves an electro-magnetic hypothesis as the only one
which has a reasonable foundation. Schuster inclines to
this view. The conclusions set forth in Earth and Sun as
to the electrical nature of the sun^s influence on the earth
point somewhat in the same direction. Hence in this
chapter we shall inquire what would happen to the sun,
and hence to the earth, on their journey through space,
if the solar atmosphere is actually subject to disturbance
by the electrical or other effects of other heavenly bodies.
It need hardly be pointed out that we are here venturing
into highly speculative ground, and that the verity or
falsity of the conclusions reached in this chapter has
nothing to do with the validity of the reasoning in pre-
vious chapters. Those chapters are based on the assump-
tion that terrestrial causes of climatic changes are sup-
plemented by solar disturbances which produce their
effect partly through variations in temperature but also
through variations in the intensity and paths of cyclonic
storms. The present chapter seeks to shed some light on
the possible causes and sequence of solar disturbances.
Let us begin by scanning the available evidence as to
EFFECT OF OTHER BODIES ON THE SUN 245
solar disturbances previous to the time when accurate
sunspot records are available. Two rather slender bits of
evidence point to cycles of solar activity lasting hundreds
of years. One of these has already been discussed in
Chapter VI, where the climatic stress of the fourteenth
century was described. At that time sunspots are known
to have been imusually numerous, and there were great
climatic extremes. Lakes overflowed in Central Asia;
storms, droughts, floods, and cold winters were unusually
severe in Europe; the Caspian Sea rose with great
rapidity; the trees of California grew with a vigor un-
known for centuries ; the most terrible of recorded fam-
ines occurred in England and India; the Eskimos were
probably driven south by increasing snowiness in Green-
land; and the Mayas of Yucatan appear to have made
their last weak attempt at a revival of civilization under
the stimulus of greater storminess and less constant
rainfall.
The second bit of evidence is foun4 in recent ex-
haustive studies of periodicities by Turner^ and other
astronomers. They have sought every possible natural
occurrence for which a numerical record is available for
a long period. The most valuable records appear to be
those of tree growth, Nile floods, Chinese earthquakes,
and sunspots. Turner reaches the conclusion that all four
types of phenomena show the same periodicity, namely,
cycles with an average length of about 260 to 280 years.
He suggests that if this is true, the cycles in tree growth
and in floods, both of which are climatic, are probably
due to a non-terrestrial cause. The fact that the sunspots
1 H. H. Turner: On a Long Period in Chinese Earthquake Beeords; Mon.
Not. Boyal Astron. Soc., Vol. 79, 1919, pp. 531-539; VoL 80, 1920, pp. 617-
619; Long Period Terms in the Growth of Trees; idem, pp. 793-808.
246 CLIMATIC CHANGES
show similar cycles suggests that the sun^s variations
are the cause.
These two bits of evidence are far too slight to form
the foundation of any theory as to changes in solar
activity in the geological past. Nevertheless it may be
helpful to set forth certain possibilities as a stimulus to
further research. For example, it has been suggested that
meteoric bodies may have fallen into the sun and caused
it suddenly to flare up, as it were. This is not impossible,
although it does not appear to have taken place since
men became advanced enough to make careful observa-
tions. Moreover, the meteorites which now fall on the
earth are extremely small, the average size being com-
puted as no larger than a grain of wheat. The largest
ever found on the earth *s surface, at Bacubirito in
Mexico, weighs only about fifty tons, while within the
rocks the evidences of meteorites are extremely scanty
and insignificant. If meteorites had fallen into the sun
often enough and of sufficient size to cause glacial fluctua-
tions and historic pulsations of climate, it seems highly
probable that the earth would show much more evidence
of having been similarly disturbed. And even if the sun
should be bombarded by large meteors the result would
probably not be sudden cold periods, which are the most
notable phenomena of the earth's climatic history, but
sudden warm periods followed by slow cooling. Neverthe-
less, the disturbance of the sun by collision with meteoric
matter can by no means be excluded as a possible cause
of climatic variations.
Allied to the preceding hypothesis is Shapley's* nebu-
lar hypothesis. At frequent intervals, averaging about
2 Harlow Shapley: Note on a Posedble Factor in Geologic Climatee;
Jonr. Geol., Vol. 29, No. 4, May, 1921; Notsb and Variable Stare, Pub.
Astron. Soc. Pac, No. 194, Aug., 1921.
EFFECT OF OTHER BODIES ON THE SUN 247
once a year during the last thirty years, astronomers have
discovered what are known as novae. These are stars
which were previously faint or even invisible, bnt which
flash suddenly into brilliancy. Often their light-giving
power rises seven or eight magnitudes— a thousand-fold.
In addition to the spectacular novae there are numerous
irregular variables whose briUiancy changes in every
ratio from a few per cent up to several magnitudes. Most
of them are located in the vicinity of nebulae, as is also
the case with novae. This, as well as other facts, makes it
probable that all these stars are * * friction variables, ' ' as
Shapley calls them. Apparently as they pass through the
nebulae they come in contact with its highly diffuse
matter and thereby become bright much as the earth
would become bright if its atmosphere were filled with
millions of almost infinitesimally small meteorites. A star
may also lose brilliancy if nebulous matter intervenes
between it and the observer. If our sun has been sub-
jected to any of these changes some sort of climatic effect
must have been produced.
In a personal communication Shapley amplifies the
nebular climatic hypothesis as follows :
Within 700 light years of the sun in many directions (Taurus,
Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebu-
losity, some bright, most of them dark. The probability that stars
moving in the general region of such clouds will encounter this
material is very high, for the clouds fill enormous volumes of
space, — e.g., probably more than a hundred thousand cubic light
years in the Orion region, and are presumably composed of rare-
fied gases or of dust particles. Probably throughout all our
part of space such nebulosity exists (it is all around us, we are
sure), but only in certain regions is it dense enough to affect
conspicuously the stars involved in it. If a star moving at high
velocity should collide with a dense part of such a nebulous
248 CLIMATIC CHANGES
cloud, we should probably have a typical nova. If the relative
velocity of nebulous material and star were low or moderate, or
if the material were rare, we should not expect a conspicuous
effect on the star's light.
In the nebulous region of Orion, which is probably of un-
usually high density, there are about 100 known stars, varying
between 20% and 80% of their total light — aU of them irregu-
larly — some slowly, some suddenly. Apparently they are
''friction variables." Some of the variables suddenly lose 40%
of their light as if blanketed by nebulous matter. In the Trifid
Nebula there are variables like those of Orion, in Messier 8 also,
and probably many of the 100 or so around the Bho Ophiuchi
region belong to this kind.
I believe that our sun could not have been a typical nova, at
least not since the Archeozoic, that is for perhaps a billion years.
I believe we have in geological climates final proof of this, be-
cause an increase in the amount of solar radiation by 1000 times
as in the typical nova, would certainly punctuate emphatically
the life cycle on the earth, even if the cause of the nova would
not at the same time eliminate the smaller planets. But the sun
may have been one of these miniature novae or friction vari-
ables; and I believe it very probable that its wanderings through
this part of space could not long leave its mean temperature
unaffected to the amount of a few per cent.
One reason we have not had this proposal insisted upon
before is that the data back of it are mostly new — ^the Orion
variables have been only recently discovered and studied, the
distribution and content of the dark nebulae are hardly as yet
generally known.
This interesting hypothesis cannot be hastily dis-
missed. If the sun should pass through a nebula it seems
inevitable that there would be at least slight climatic
effects and perhaps catastrophic effects through the
action of the gaseous matter not only on the sun but on
the earth's own atmosphere. As an explanation of the
EFFECT OF OTHER BODIES ON THE SUN 249
general climatic conditions of the past, however, Shapley
points out that the hypothesis has the objection of being
vagae, and that nebulosity should not be regarded as
more than **a possible factor/' One of the chief difficul-
ties seems to be the enormously wide distribution of as
yet undiscovered nebulous matter which must be assumed
if any large share of the earth's repeated climatic
changes is to be ascribed to such matter. If such matter
is actually abimdant in space, it is hard to see how any
but the nearest stars would be visible. Another objection
is that there is no known nebulosity near at hand with
which to connect the climatic vicissitudes of the last
glacial period. Moreover, the known nebulae are so much
less numerous than stars that the chances that the sun
will encounter one of them are extremely slight. This,
however, is not an objection, for Shapley points out that
during geological times the sun can never have varied
as much as do the novae, or even as most of the friction
variables. Thus the hypothesis stands as one that is worth
investigating, but that cannot be finally rejected or ac-
cepted until it is made more definite and until more in-
formation is available.
Another suggested cause of solar variations is the rela-
tively sudden contraction of the sun such as that which
sometimes occurs on the earth when continents are up-
lifted and mountains upheaved. It seems improbable that
this could have occurred in a gaseous body like the sun.
Lacking, as it does, any solid crust which resists a change
of form, the sun probably shrinks steadily. Hence any
climatic effects thus produced must be extremely gradual
and must tend steadily in one direction for millions of
years.
Still another suggestion is that the tidal action of the
stars and other bodies which may chance to approach
\
\
I
I
260 CLIMATIC CHANGES
the sun's path may cause disturbances of the solar
atmosphere. The vast kaleidoscope of space is never
quiet. The sun, the stars, and all the other heavenly-
bodies are moving, often with enormous speed. Hence the
effect of gravitation upon the sun must vary constantly
and irregularly, as befits the geological requirements. In
the case of the planets, however, the tidal effect does not
seem competent to produce the movements of the solar
atmosphere which appear to be concerned in the incep-
tion of sunspots. Moreover, there is only the most remote
probability that a star and the sun will approach near
enough to one another to produce a pronounced gravita-
tional disturbance in the solar atmosphere. For instance,
if it be assumed that changes in Jupiter's tidal effect on
the sun are the main factor in regulating the present dif-
ference between sunspot maxima and sunspot minima,
the chances that a star or some non-luminous body of
similar mass will approach near enough to stimulate
solar activity and thereby bring on glaciation are only
one in twelve billion years, as will be explained below.
This seems to make a gravitational hypothesis im-
possible.
Another possible cause of solar disturbances is that
the stars in their flight through space may exert an
electrical influence which upsets the equilibrium of the
solar atmosphere. At first thought this seems even more
impossible than a gravitational effect. Electrostatic
effects, however, differ greatly from those of tides. They
vary as the diameter of a body instead of as its mass;
their differentials also vary inversely as the square of
the distance instead of as the cube. Electrostatic effects
also increase as the fourth power of the temperature or
at least would do so if they followed the law of black
bodies ; they are stimulated by the approach of one body
EFFECT OF OTHER BODIES ON THE SUN 251
to another; and they are cumulative, for if ions arrive
from space they must accumulate until the body to which
they have come begins to discharge them. Hence, on the
basis of assumptions such as those used in the preceding
paragraph, the chances of an electrical disturbance of
the solar atmosphere sufficient to cause glaciation on the
earth may be as high as one in twenty or thirty million
years. This seems to put an electrical hypothesis within
the bounds of possibility. Further than that we cannot
now go. There may be other hypotheses which fit the facts
much better, but none seems yet to have been suggested.
In the rest of this chapter the tidal and electrical
hypotheses of stellar action on the sun will be taken up
in detail. The tidal hypothesis is considered because in
discussions of the effect of the planets it has hitherto
held almost the entire field. The electrical hypothesis will
be considered because it appears to be the best yet sug-
gested, although it still seems doubtful whether electrical
effects can be of appreciable importance over such vast
distances as are inevitably involved. The discussion of
both hypotheses will necessarily be somewhat technical,
and will appeal to the astronomer more than to the lay-
man. It does not form a necessary part of this book, for
it has no bearing on our main thesis of the effect of the
sun on the earth. It is given here because ultimately the
question of changes in solar activity during geological
times must be faced.
In the astronomical portion of the following discus-
sion we shall follow Jeans' in his admirable attempt at a
mathematical analysis of the motions of the universe.
Jeans divides the heavenly bodies into five main types:
(1) Spiral nebulae, which are thought by some astrono-
sj. H. Jeans: Problems of Cosmogony and Stellar Dynamics, 0am-
bridge, 1919.
262 CLIMATIC CHANGES
mers to be systems like our own in the making, and by
others to be independent universes lying at vast distances
beyond the limits of our Galactic universe, as it is called
from the Galaxy or Milky Way. (2) Nebulae of a smaller
type, called planetary. These lie within the Galactic por-
tion of the universe and seem to be early stages of what
may some day be stars or solar systems. (3) Binary or
multiple stars, which are extraordinarily numerous. In
some parts of the heavens they form 50 or even 60 per
cent of the stars and in the galaxy as a whole they seem to
form ** fully one third.'* (4) Star clusters. These consist
of about a hundred groups of stars in each of which the
stars move together in the same direction with approxi-
mately the same velocity. These, like the spiral nebulae,
are thought by some astronomers to lie outside the limits
of the galaxy, but this is far from certain. (5) The solar
system. According to Jeans this seems to be unique. It
does not fit into the general mathematical theory by
which he explains spiral nebulae, planetary nebulae, binary
stars, and star clusters. It seems to demand a special
explanation, such as is furnished by tidal disruption due
to the passage of the sun close to another star.
The part of Jeans ' work which specially concerns us is
his study of the probability that some other star will
approach the sun closely enough to have an appreciable
gravitative or electrical effect, and thus cause disturb-
ances in the solar atmosphere. Of course both the star
and the sun are moving, but to avoid circumlocution we
shall speak of such mutual approaches simply as ap-
proaches of the sun. For our present purpose the most
fundamental fact may be summed up in a quotation from
Jeans in which he says that most stars ^^show evidence
of having experienced considerable disturbance by other
systems ; there is no reason why our solar system should
J
EFFECT OF OTHER BODIES ON THE SUN 268
be expected to have escaped the common fate." Jeans
gives a careful calculation from which it is possible to
derive some idea of the probability of any given degree of
approach of the sun and some other star. Of course all
such calculations must be based on certain assumptions.
The assumptions made by Jeans are such as to make the
probability of close approaches as great as possible. For
example, he allows only 560 million years for the entire
evolution of the sun, whereas some astronomers and
geologists would put the figure ten or more times as
high. Nevertheless, Jeans* assumptions at least show
the order of magnitude which we may expect on the basis
of reasonable astronomical conclusions.
According to the planetary hypothesis of sunspots, the
difference in the effect of Jupiter when it is nearest and
farthest from the sun is the main factor in starting the
sunspot cycle and hence the corresponding terrestrial
cycle. The climatic difference between sunspot maxima \
and minima, as measured by temperature, apparently ; ,.
amounts to at least a twentieth and perhaps a tenth of / / *'
the difference between the climate of the last glacial /
epoch and the present. We may suppose, then, that a body ^ ^
which introduced a gravitative or electrical factor twenty '
times as great as the difference in Jupiter's effect at its
maximum and minimum distances from the sun would ,'
cause a glacial epoch if the effect lasted long enough. Of \
course the other planets combine their effects with that /
of Jupiter, but for the sake of simplicity we will leave
the others out of account. The difference between Jupi-
ter's maximum and minimum tidal effect on the sun
amounts to 29 per cent of the planet's average effect.
The corresponding difference, according to the electrical
hypothesis, is about 19 per cent, for electrostatic action
varies as the square of the distance instead of as the cube.
264 CLIMATIC CHANGES
Let us assume that a body exerting four times Jupiter's
present tidal effect and placed at the average distance of
Jupiter from the sun would disturb the sun^s atmosphere
twenty times as much as the present difference between
sunspot maxima and minima, and thus, perhaps, cause a
glacial period on the earth.
On the basis of this assumption our first problem is to
estimate the frequency with which a star, visible or
dark, is likely to approach near enough to the sun to
produce a tidaX effect four times that of Jupiter. The
number of visible stars is known or at least well esti-
mated. As to dark stars, which have grown cool, Arrhe-
nius believed that they are a hundred times as numerous
as bright stars; few astronomers believe that there are
less than three or four times as many. Dr. Shapley of
the Harvard Observatory states that a new investigation
of the matter suggests that eight or ten is probably a
maximum figure. Let us assume that nine is correct.
The average visible star, so far as measured, has a mass
about twice that of the sun, or about 2100 times that of
Jupiter. The distances of the stars have been measured
in hundreds of cases and thus we can estimate how many
stars, both visible and invisible, are on an average con-
tained in a given volume of space. On this basis Jeans
estimates that there is only one chance in thirty billion
years that a visible star will approach within 2.8 times
the distance of Neptune from the sun, that is, within about
eight billion miles. If we include the invisible stars the
chances become one in three billion years. In order to
produce four times the tidal effect of Jupiter, however,
the average star would have to approach within about
four billion miles of the sun, and the chances of that
are only one in twelve billion years. The disturbing star
EFFECT OF OTHER BODIES ON THE SUN 265
would be only 40 per cent farther from the sun than
Neptune, and would almost pass within the solar system. ,
Even though Jeans holds that the frequency of the ?
mutual approach of the sun and a star was probably ( i\
much greater in the distant past than at present, the
figures just given lend little support to the tidal hypothe-
sis. In fact, they apparently throw it out of court. It will
be remembered that Jeans has made assumptions which
give as high a frequency of stellar encounters as is con-
sistent with the astronomical facts. We have assumed
nine dark stars for every bright one, which may be a
liberal estimate. Also, although we have assumed that a
disturbance of the sun's atmosphere sufficient to cause
a glacial period would arise from a tidal effect only \
twenty times as great as the difference in Jupiter's effect
when nearest the sun and farthest away, in our computa-
tions this has actually been reduced to thirteen. With aU
these favorable assumptions the chances of a stellar ap-
proach of the sort here described are now only one in
twelve billion years. Yet within a hundred million years, (
according to many estimates of geological time, and \
almost certainly within a billion, there have been at least
half a dozen glaciations.
Our use of Jeans' data interposes another and equally
insuperable difficulty to any tidal hypothesis. Four bil-
lion miles is a very short distance in the eyes of an
astronomer. At that distance a star twice the size of the
sun would attract the outer planets more strongly than
the sun itself, and might capture them. If a star should
come within four billion miles of the sun, its effect in
distorting the orbits of all the planets would be great.
If this had happened often enough to cause all the gla-
ciations known to geologists, the planetary orbits would
be strongly elliptical instead of almost circular. The con-
i
256 CLIMATIC CHANGES
siderations here advanced militate so strongly against
the tidal hypothesis of solar disturbances that it seems
scarcely worth while to consider it further.
Let us turn now to the electrical hypothesis. Here the
conditions are fundamentally different from those of the
tidal hypothesis. In the first place the electrostatic effect
of a body has nothing to do with its mass, but depends on
the area of its surface ; that is, it varies as the square of
the radius. Second, the emission of electrons varies ex-
ponentially. If hot glowing stars follow the same law as
black bodies at lower temperatures, the emission of
electrons, like the emission of other kinds of energy,
varies as the fourth power of the absolute temperature.
In other words, suppose there are two black bodies, other-
wise alike, but one with a temperature of 2V C. or 300°
on the absolute scale, and the other with 600° on the
absolute scale. The temperature of one is twice as high
as that of the other, but the electrostatic effect will be
sixteen times as great.* Third, the number of electrons
^This fact is so important and at the same time so surprising to the
lajman, that a quotation from The Electron Theory of Matter by O. W.
Bichardson, 1914, pp. 326 and 334 is here added.
''It is a very familiar fact that when material bodies are heated they
emit electromagnetic radiations, in the form of thermal, luminous, and
actinic rays, in appreciable quantities. Such an effect is a natural consequence
of the electron and kinetic theories of matter. On the kinetic theory, tem-
perature is a measure of the yiolence of the motion of the ultimate par-
tides; and we have seen that on the electron theory, electromagnetic
radiation is a consequence of their acceleration. The calculation of tills
emission from the standpoint of the electron theory alone is a very complex
problem which takes us deeply into the structure of matter and which has
probably not yet been satisfactorily resolved. Fortunately, we can find out
a great deal about these phenomena by the application of general prin-
ciples like the conservation of energy and the second law of thermo-
dynamics without considering special assumptions about the ultimate con-
stitution of matter. It is to be borne in mind that the emission under
consideration occurs at all temperatures although it is more marked the
higher the temperature. . . . The energy per unit volume, in vacuo, of the
radiation in equilibrium in an enclosure at the absolute temperature, T, is
equal to a universal constant. A, multiplied by the fourth power of the
EFFECT OF OTHER B0DU5S ON THE SUN 267
that reach a given body varies inversely as the square of
the distance, instead of as the cube which is the case
with tide-making forces.
In order to use these three principles in calculating
the effect of the stars we must know the diameters, dis-
tances, temperature, and number of the stars. The dis-
tances and number may safely be taken as given by Jeans
in the calculations already cited. As to the diameters, the
measurements of the stars thus far made indicate that
the average mass is about twice that of the sun. The
average density, as deduced by Shapley* from the move-
ments of double stars, is about one-eighth the solar
density. This would give an average diameter about two
and a half times that of the sun. For the dark stars, we
shall assume for convenience that they are ten times as
numerous as the bright ones. We shall also assume that
their diameter is half that of the sun, for being cool they
must be relatively dense, and that their temperature is
the same as that which we shall assume for Jupiter.
As to Jupiter we shall continue our former assumption
that a body with four times the effectiveness of that
planet, which here means with twice as great a radius,
would disturb the sun enough to cause glaciation. It
would produce about twenty times the electrostatic effect
absolute temperature. Since the intensitj of the radiation is equal to the
energy per unit yolume multiplied bj the velocitj of light, it follows that
the former must also be proportionid to the fourth power of the absolute
temperature. Moreover, if E is the total emission from unit area of a
perfectly black body, we see from p. 330 that E=A'T«, where A' is a new
universal constant. This result is usually known as Stefan's Law. It was
suggested by Stefan in the inaccurate form that the total radiant energy
of emission from bodies varies as the fourth power of the absolute tempera-
ture, as a generalization from the results of experiments. The credit for
showing that it is a consequence of the existence of radiation pressure
oombined with the principles of thermodynamics is due to BartoU and
Boltzmann. ' '
8 Quoted by Moulton in his Introduction to Astronomy.
258 CLIMATIC CHANGES
which now appears to be associated with the difference in
Jupiter's effect at maximum and minimum. The tempera-
ture of Jupiter must also be taken into account. The
planet is supposed to be hot because its density is low,
being only about 1.25 that of water. Nevertheless, it is
probably not luminous, for as Moulton* puts it, shadows
upon it are black and its moons show no sign of illumina-
tion except from the sun. Hence a temperature of about
600° C, or approximately 900** on the absolute scale,
seems to be tiie highest that can reasonably be assigned
to the cold outer layer whence electrons are emitted. As
to the temperature of the sun, we shall adopt the conmion
estimate of about GSOO^'C. on the absolute scale. The
other stars will be taken as averaging the same, although
of course they vary greatly.
When Jeans' method of calculating the probability of
a mutual approach of the sun and a star is applied to the
assumptions given above, the results are as shown in
Table 5. On that basis the dark stars seem to be of
negligible importance so far as the electrical hypothesis
is concerned. Even though they may be ten times as
numerous as the bright ones there appears to be only
one chance in 130 billion years that one of them will ap-
proach the sun closely enough to cause the assumed dis-
turbance of the solar atmosphere. On the other hand, if
all the visible stars were the size of the sun, and as hot
as that body, their electrical effect would be fourfold
that of our assumed dark star because of their size, and
2401 times as great because of their temperature, or ap-
proximately 10,000 times as great. Under such conditions
the theoretical chance of an approach that would cause
glaciation is one in 130 million years. If the average
visible star is somewhat cooler than the sun and has a
8 Introduction to Astronomy.
EFFECT OF OTHER BODIES ON THE SUN 259
radius about two and one-half times as great, as appears
to be the fact, the chances rise to one in thirty-eight mil-
lion years. A slight and wholly reasonable change in our
assumptions would reduce this last figure to only five or
ten million. For instance, the earth's mean temperature
during the glacial period has been assumed as lO^'C.
lower than now, but the difference may have been only 6"^.
Again, the temperature of the outer atmosphere of Jupi-
ter where the electrons are shot out may be only 500° or
700° absolute, instead of 900°. Or the diameter of the
average star may be five or ten times that of the sun,
instead of only two and one-half times as great. All this,
however, may for the present be disregarded. The essen-
tial point is that even when the assumptions err on the
side of conservatism, the results are of an order of magni-
tude which puts the electrical hypothesis within the
bounds of possibility, whereas similar assumptions put
the tidal hypothesis, with its single approach in twelve
billion years, far beyond those limits.
The figures for Betelgeuse in Table 5 are interesting.
At a meeting of the American Association for the Ad-
vancement of Science in December, 1920, Michelson
reported that by measurements of the interference of
light coming from the two sides of that bright star in
Orion, the observers at Mount Wilson had confirmed the
recent estimates of three other authorities that the star 's
diameter is about 218 million miles, or 250 times that of
the Sim. If other stars so much surpass the estimates of
only a decade or two ago, the average diameter of all the
visible stars must be many times that of the sun. The low
figure for Betelgeuse in section D of the table means that
if all the stars were as large as Betelgeuse, several might
often be near enough to cause profound disturbances of
the solar atmosphere. Nevertheless, because of the low
260
CLIMATIC CHANGES
TABLE 5
THEORETICAL PROBABILITY OF STELLAR |
APPROACHES 1
1
Dark stars
Sun
Average
SUir
4
BetOgeuee
A. Approximate
radius in miles
430,000
860,000
2,150,000
218,000,000
B. Assumed tem-
perature above
absolute zero..
900° C.
6300° C.
5400° C.
3150° C.
0. Approximate
theoretical dis-
tance at which
star would
cause solar dis-
turbance great
enough to cause
glaciation (bil-
lions^ of miles).
1.2
120
220
3200
D. Average in-
terval between
approaches
close enough to
cause glacia-
tion if all stars
were of given
type. Years..
130,000,000,0008
130,000,000
38,000,000
700,000
temperature of the giant red stars of the Betelgeuse type,
the distance at which one of them would produce a given
electrical effect is only about five times the distance at
which our assumed average star would produce the same
effect. This, to be sure, is on the assumption that the
7 The term billions, here and elsewhere, is used in the American senaey lO^.
> The assumed number of stars here is ten times as great as in the other
parts of this line.
EFFECT OF OTHER BODIES ON THE SUN 261
radiation of energy from incandescent bodies varies
according to temperature in the same ratio as the radia-
tion from black bodies. Even if this assumption departs
somewhat from the truth, it still seems almost certain
that the lower temperature of the red compared with the
high temperature of the wMte stars must to a consider-
able degree reduce the difference in electrical effect which
would otherwise arise from their size.
Thus far in our attempt to estimate the distance at
which a star might disturb the sun enough to cause gla-
ciation on the earth, we have considered only the star's
size and temperature. No account has been taken of the
degree to which its atmosphere is disturbed. Yet in the
case of the sun this seems to be one of the most important
factors. The magnetic field of sunspots is sometimes 50
or 100 times as strong as that of the sun in general. The
strength of the magnetic field appears to depend on the
strength of the electrical currents in the solar atmos-
phere. But the intensity of the sunspots and, by inference,
of the electrical currents, may depend on the electrical
action of Jupiter and the other planets. If we apply a
similar line of reasoning to the stars, we are at once led
to question whether the electrical activity of double stars
may not be enormously greater than that of isolated
stars like the sun.
If this line of reasoning is correct, the atmosphere of
every double star must be in a state of commotion vastly
greater than that of the sun's atmosphere even when it
is most disturbed. For example, suppose the sun were
accompanied by a companion of equal size at a distance
of one million miles, which would make it much like many
known double stars. Suppose also that in accordance with
the general laws of physics the electrical effect of the
two suns upon one another is proportional to the fourth
262 CLIMATIC CHANGES
power of the temperature, the square of the radius, and
the inverse square of the distance. Then the effect of each
sun upon the other would be sixty billion (6 x 10^^) times
as great as the present electrical effect of Jupiter upon
the sun. Just what this would mean as to the net effect
of a pair of such suns upon the electrical potential of
other bodies at a distance we can only conjecture. The
outstanding fact is that the electrical conditions of a
double star must be radically different and vastly more
intense than those of a single star like the sun.
This conclusion carries weighty consequences. At pres-
ent twenty or more stars are known to be located within
about 100 trillion miles of the sun (five par sees, as the
astronomers say), or 16.5 light years. According to the
assumptions employed in Table 5 an average single star
would influence the sun enough to cause glaciation if it
came within approximately 200 billion miles. If the star
were double, however, it might have an electrical capacity
enormously greater than that of the sun. Then it would
be able to cause glaciation at a correspondingly great
distance. Today Alpha Centauri, the nearest known star,
is about twenty-five trillion miles, or 4.3 light years from
the sun, and Sirius, the brightest star in the heavens, is
about fifty trillion miles away, or 8.5 light years. If these
stars were single and had a diameter three times that of
the sun, and if they were of the same temperature as has
been assumed for Betelgeuse, which is about fifty times as
far away as Alpha Centauri, the relative effects of the
three stars upon the sun would be, approximately, Betel-
geuse 700, Alpha Centauri 250, Sirius 1. But Alpha Cen-
tauri is triple and Sirius double, and both are much hotter
than Betelgeuse. Hence Alpha Centauri and even Sirius
may be far more effective than Betelgeuse.
The two main components of Alpha Centauri are sepa-
EFFECT OF OTHER BODIES ON THE SUN 268
rated by an average distance of about 2,200,000,000 miles,
or somewhat less than that of Neptmie from the sim. A
third and far fainter star, one of the faintest yet meas-
ured, revolves around them at a great distance. In mass
and brightness the two main components are about like
the sun, and we will assume that the same is true of their
radius. Then, according to the assumptions made above,
their effect in disturbing one another electrically would
be about 10,000 times the total effect of Jupiter upon the
sun, or 2500 times the effect that we have assumed to be
necessary to produce a glacial period. We have already
seen in Table 5 that, according to our assumptions, a
single star like the sim would have to approach within
120 billion miles of the solar system, or within 2 per cent
of a light year, in order to cause glaciation. By a similar
process of reasoning it appears that if the mutual elec-
trical excitation of the two main parts of Alpha Centauri,
regardless of the third part, is proportional to the ap-
parent excitation of the sun by Jupiter, Alpha Centauri
would be 5000 times as effective as the sun. In other
words, if it came within 8,500,000,000,000 miles of the sun,
or 1.4 light years, it would so change the electrical condi-
tions as to produce a glacial epoch. In that case Alpha
Centauri is now so near that it introduces a disturbing
effect equal to about one-sixth of the effect needed to
cause glaciation on the earth. Sirius and perhaps others
of the nearer and brighter or larger stars may also create
appreciable disturbances in the electrical condition of the
sun's atmosphere, and may have done so to a much
greater degree in the past, or be destined to do so in the
future. Thus an electrical hypothesis of solar disturb-
ances scenes to indicate that the position of the sun in
respect to other stars may be a factor of great impor-
tance in determining the earth's climate.
/
CHAPTER XV
THE SUN'S JOURNEY THROUGH SPACE
HAVING gained some idea of the nature of the
electrical hypothesis of solar disturbances and
of the possible effect of other bodies upon the
sun's atmosphere, let us now compare the astronomical
data with those of geology. Let us take up five chief
points for which the geologist demands an explanation^
and which any hypothesis must meet if it is to be per-
manently accepted. These are (1) the irregular intervals
at which glacial periods occur; (2) the division of glacial
periods into epochs separated sometimes by hundreds
of thousands of years; (3) the length of glacial periods
and epochs; (4) the occurrence of glacial stages and his-
toric pulsations in the form of small climatic waves
superposed upon the larger waves of glacial epochs ; (5)
the occurrence of climatic conditions much milder than
those of today, not only in the middle portion of the great
geological eras, but even in some of the recent inter-
glacial epochs.
1. The irregular duration of the interval from one
glacial epoch to another corresponds with the irregular
distribution of the stars. If glaciation is indirectly due
to stellar influences, the epochs might fall close together,
or might be far apart. If the average interval were ten
million years, one interval might be thirty million or
more and the next only one or two hundred thousand.
THE SUN'S JOURNEY THROUGH SPACE 266
According to Schuchert, the known periods of glacial or
semi-glacial climate have been approximately as follows :
LIST OF GLACIAL PERIODS
1. Archeozoic.
(^ of geological time or perhaps much more)
No known glacial periods.
2. Proterosoic.
(% of geological time)
a. Oldest known glacial period near base of Proterozoic in
Canada. Evidence widely distributed.
b. Indian glacial period; time unknown.
c. African glacial period; time unknown.
d. Glaciation near end of Proterozoic in Australia, Norway,
and China.
3. Paleozoic.
(% of geological time)
a. Late OrdoTician(f). Local in Arctic Norway.
b. Silurian. Local in Alaska.
c. Early Devonian. Local in South Africa.
d. Early Permian. World-wide and very severe.
4. Mesozoic and Cenozoic.
(% of geological time)
a-b. None definitely determined during Mesozoic, although
there appears to have been periods of cooling (a) in the
late Triassic, and (b) in the late Oretacic, with at least
local glaciation in early Eocene.
c. Severe glacial period during Pleistocene.
This table suggests an interesting inquiry. During the
last few decades there has been great interest in ancient
glaciation and geologists have carefully examined rocks
of all ages for signs of glacial deposits. In spite of the
large parts of the earth which are covered with deposits
belonging to the Mesozoic and Cenozoic, which form the
266 CLIMATIC CHANGES
I last quarter of geological time, the only signs of actual
glaciation are those of the great Pleistocene period and a
few local occurrences at the end of the Mesozoic or be-
I ginning of the Cenozoic. Late in the Triassic and early
in the Jurassic, the dinaate appears to have been rigor-
ous, although no tillites have been found to demonstrate
glaciation. In the preceding quarter, that is, the Paleo-
zoic, the Permian glaciation was more severe than that
of the Pleistocene, and the Devonian than that of the
Eocene, while the Ordovician evidences of low tempera-
ture are stronger than those at the end of the Triassic.
In view of the fact that rocks of Paleozoic age cover
much smaller areas than do those of later age, the three
Paleozoic glaciations seem to indicate a relative fre-
quency of glaciation. Going back to the Proterozoic, it
is astonishing to find that evidence of two highly de-
veloped glacial periods, and possibly four, has been dis-
covered. Since the Indian and the African glaciations of
Proterozoic times are as yet undated, we cannot be sure
that they are not of the same date as the others. Never-
theless, even two is a surprising number, for not only
are most Proterozoic rocks so metamorphosed that pos-
sible evidences of glacial origin are destroyed, but rocks
of that age occupy far smaller areas than either those
of Paleozoic or, still more, Mesozoic and Cenozoic age.
Thus the record of the last three-quarters of geological
time suggests that if rocks of all ages were as abundant
and as easily studied as those of the later periods, the
frequency of glacial periods would be found to increase
as one goes backward toward the beginnings of the
earth's history. This is interesting, for Jeans holds that
the chances that the stars would approach one another
were probably greater in the past than at present. This
conclusion is based on the assumption that our universe
THE SUN'S JOURNEY THROUGH SPACE 267
is like the spiral nebulae in which the orbits of the varions
members are nearly circular during the younger stages.
Jeans considers it certain that in such cases the orbits
will gradually become larger and more elliptical because
of the attraction of one body for another. Thus as time
goes on the stars will be more widely distributed and
the chances of approach will diminish. K this is correct,
the agreement between astronomical theory and geologi-
cal conclusions suggests that the two are at least not in
opposition.
The first quarter of geological time as well as the last
three must be considered in this connection. During the
Archeozoic, no evidence of glaciation has yet been dis-
covered. This suggests that the geological facts disprove
the astronomical theory. But our Imowledge of early
geological times is extremely limited, so limited that
lack of evidence of glaciation in the Archeozoic may have
no significance. Archeozoic rocks have been studied
minutely over a very small percentage of the earth's land
surface. Moreover, they are highly metamorphosed so
that, even if glacial tills existed, it would be hard to
recognize them. Third, according to both the nebular and
the planetesimal hypotheses, it seems possible that
during the earliest stages of geological history the
earth's interior was somewhat warmer than now, and the
surface may have been warmed more than at present by
conduction, by lava flows, and by the fall of meteorites.
If the earth during the Archeozoic period emitted enough
heat to raise its surface temperature a few degrees, the
heat would not prevent the development of low forms of
life but might effectively prevent all glaciation. This
does not mean that it would prevent changes of climate,
but merely changes so extreme that their record would
be preserved by means of ice. It will be most interesting
268 CLIMATIC CHANGES
to see whether future investigations in geology and
astronomy indicate either a semi-uniform distribution of
glacial periods throughout the past, or a more or less
regular decrease in frequency from early times down to
the present
2. The Pleistocene glacial period was divided into at
least four epochs, while in the Permian at least one
inter-gladal epoch seems certain, and in some places the
alternation between glacial and non-glacial beds suggests
no less than nine. In the other glaciations the evidence is
not yet clear. The question of periodicity is so important
that it overthrows most glacial hypotheses. Indeed, had
their authors known the facts as established in recent
years, most of the hypotheses would never have been
advanced. The carbon dioxide hypothesis is the only one
which was framed with geologically rapid climatic alter-
nations in mind. It certainly explains the facts of perio-
dicity better than does any of its predecessors, but even
so it does not account for the intimate way in which
variations of aU degrees from those of the weather up to
glacial epochs seem to grade into one another.
According to our stellar hypothesis, occasional groups
of glacial epochs would be expected to occur dose to-
gether and to form long glacial periods. This is because
many of the stars belong to groups or clusters in which
the stars move in parallel paths. A good example is the
cluster in the Hyades, where Boss has studied thirty-nine
stars with special care.^ The stars are grouped about a
center about 130 light years from the sun. The stars
themselves are scattered over an area about thirty
light years in diameter. They average about the same
distance apart as do those near the sun, but toward the
1 Lewis Bobs: Convergent of a Moving Cluster in Taurus; Astronom.
Jour., Vol. 26, No. 4, 1908, pp. 31-36.
THE SUN'S JOURNEY THROUGH SPACE 269
center of the group they are somewhat closer together.
The whole thirty-nine sweep forward in essentially
parallel paths. Boss estimates that 800^000 years ago
the cluster was only half as far from the sun as at pres-
enty but probably that was as near as it has been during
recent geological times. All of the thirty-nine stars of this
duster, as Moulton^ puts it, ^^are much greater in light-
giving power than the sun. The luminosities of even the
five smallest are from five to ten times that of the sun^
while the largest are one hundred times greater in light-
giving power than our own luminary. Their masses are
probably much greater than that of the sun. * ' If the sun
were to pass through such a duster, first one star and
then another might come so near as to cause a profound
disturbance in the sun's atmosphere.
3. Another important point upon which a glacial hy-
pothesis may come to grief is the length of the periods
or rather of the epochs which compose the periods.
During the last or Pleistocene gladal period the evidence
in America and Europe indicates that the inter-glacial
epochs varied in length and that the later ones were
shorter than the earlier. Chamberlin and Salisbury, from
a comparison of various authorities, estimate that the
intervals from one glacial epoch to another form a de-
clining series, which may be roughly expressed as fol- //
lows: 16-8-4-2-1, where unity is the interval from the
dimaz of the late Wisconsin, or last glacial epoch, to the
present. Most authorities estimate the culmination of the
late Wisconsin glaciation as twenty or thirty thousand (
years ago. Penck estimates the length of the last inter- ^
glacial period as 60,000 years and the preceding one as /
240,000.' B. T. Chamberlin, as already stated, finds that
sF. B. Moulton: in Introduction to Astronomy, 1016.
B A. Penek: Die Alpen im Eiszeitalter, Leipzig, 1909.
270 CLIMATIC CHANGES
the consensus of opinion is that inter-glacial epochs have
averaged five times as long as glacial epochs. The actual
duration of the various gladations probably did not vary
in so great a ratio as did the intervals from one glada-
tion to another. The main point, however, is the irregu-
larity of the various periods.
The relation of the stellar electrical hypothesis to the
length of glacial epochs may be estimated from column
C, in Table 5. There we see that the distances at which
a star might possibly disturb the sun enough to cause
glaciation range all the way from 120 billion miles in
the case of a small star like the sun, to 3200 billion in
the case of Betelgeuse, while for double stars the figure
may rise a hundred times higher. From this we can cal-
culate how long it would take a star to pass from a point
where its influence would first amount to a quarter of the
assumed maximum to a similar point on the other side of
the sun. In making these calcidations we will assume that
the relative rate at which the star and the sun approach
each other is about twenty-two miles per second, or 700
million miles per year, which is the average rate of
motion of all the known stars. According to the distances
in Table 5 this gives a range from about 500 years up to
about 10,000, which might rise to a million in the case of
double stars. Of course the time might be relatively short
if the sun and a rapidly moving star were approaching
one another almost directly, or extremely long if the sun
and the star were moving in almost the same direction
and at somewhat similar rates, — a condition more
common than the other. Here, as in so many other cases,
the essential point is that the figures which we thus ob-
tain seem to be of the right order of magnitude.
4. Post-glacial climatic stages are so well known that
in Europe they have definite names. Their sequence has
THE SUN'S JOURNEY THROUGH SPACE 271
already been discussed in Chapter XIL Fossils found in
the peat bogs of Denmark and Scandinavia, for example,
prove that since the final disappearance of the conti-
nental ice cap at the close of the Wisconsin there has
been at least one period when the climate of Europe was
distinctly milder than now. Directly overlying the sheets
of glacial drift laid down by the ice there is a flora corre-
sponding to that of the present tundras. Next come re-
mains of a forest vegetation dominated by birches and
poplars, showing that the climate was growing a little
warmer. Third, there follow evidences of a still more
favorable climate in the form of a forest dominated by
pines ; fourth, one where oak predominates ; and fifth, a
flora similar to that of the Black Forest of Germany,
indicating that in Scandinavia the temperature was then
decidedly higher than today. This fifth flora has retreated
southward once more, having been driven back to its
present latitude by a slight recurrence of a cool stormy
dimate.^ In central Asia evidence of post-glacial stages
is f oimd not only in five distinct moraines but in a corre-
sponding series of elevated strands surrounding salt
lakes and of river terraces in non-glaciated arid regions.'
In historic as well as prehistoric times, as we have
already seen, there have been climatic fluctuations. For
instance, the twelfth or thirteenth century B. C. appears
to have been almost as mild as now, as does the seventh
century B. C. On the other hand about 1000 B. C, at the
time of Christ, and in the fourteenth century there were
times of relative severity. Thus it appears that both on
« B. D. Salisbuiy: Physical (Geography of the Pleistocene, in Outlines of
Geologic History, by Willis and Salisbury, 1910, pp. 273-274.
B Davis, Pumpelly, and Huntington : Explorations in Turkestan, Carnegie
Inst, of Wash., No. 26, 1905.
In North America the stages have been the subject of intensive studies
on the part of Taylor, Leverett, Goldthwait, and many others.
272 CLIMATIC CHANGES
a large and on a small scale pulsations of climate are the
rule. Any hypothesis of climatic changes must satisfy
the periods of these pulsations. These conditions furnish
a problem which makes difficulty for almost all hypothe-
ses of climatic change. According to the present hypothe-
siSy earth movements such as are discussed in Chapter
XII may cooperate with two astronomical factors. One is
the constant change in the positions of the stars, a change
which we have already called kaleidoscopic, and the other
is the fact that a large proportion of the stars are double
or multiple. When one star in a group approaches the
sun closely enough to cause a great solar disturbance,
numerous others may approach or recede and have a
minor effect. Thus, whenever the sun is near groups of
stars we should expect that the earth would show many
minor climatic pulsations and stages which might or
might not be connected with glaciation. The historic
pulsations shown in the curve of tree growth in CaU-
fomia, Fig. 4, are the sort of changes that would be
expected if movements of the stars have an effect on the
solar atmosphere.
Not only are fully a third of all the visible stars double,
as we have already seen, but at least a tenth of these are
known to be triple or multiple. In many of the double
stars the two bodies are close together and revolve so
rapidly that whatever periodicity they might create in
the sun *s atmosphere would be very short In the triplets,
however, the third star is ordinarily at least ten times
as far from the other two as they are from each other,
and its period of rotation sometimes runs into hundreds
or thousands of years. An actual multiple star in the
constellation Polaris will serve as an example. The main
star is believed by Jeans to consist of two parts which
are almost in contact and whirl around each other with
THE SUN'S JOURNEY THROUGH SPACE 278
extraordinary speed in four days. If this is true they
must keep each other's atmospheres in a state of intense
commotion. Much farther away a third star revolves
around this pair in twelve years. At a much greater dis-
tance a fourth star revolves around the common center
of gravity of itself and the other three in a period which
may be 20,000 years. Still more complicated cases prob-
ably exist. Suppose such a system were to traverse a
path where it would exert a perceptible influence on the
sun for thirty or forty thousand years. The varying
movements of its members would produce an intricate
series of cycles which might show all sorts of major and
minor variations in length and intensity. Thus the varied
and irregular stages of glaciation and the pulsations of
historic times might be accoimted for on the hypothesis
of the proximity of the sun to a multiple star, as well as
on that of the less pronounced approach and recession
of a number of stars. In addition to all this, an almost
infinitely complex series of climatic changes of long and
short duration might arise if the sun passed through a
nebula.
5. We have seen in Chapter VIII that the contrast
between the somewhat severe climate of the present and
the generally mild climate of the past is one of the great
geological problems. The glacial period is not a thing
of the distant past. (Geologists generally recognize that
it is still with us. Greenland and Antarctica are both
shrouded in ice sheets in latitudes where fossil floras
prove ^that at other periods the climate was as mild as
in England or even New Zealand. The present glaciated
regions, be it noted, are on the polar borders of the
world's two most stormy oceanic areas, just where ice
would be exi)ected to last longest according to the solar
cyclonic hypothesis. In contrast with the semi-glacial
I
/
>■-.
274 CLIMATIC CHANGES
conditions of the present^ the last inter-glacial epoch was
so mild that not only men bnt elephants and hippopota-
muses flourished in central Europe, while at earlier times
in the middle of long eras, such as the Paleozoic and
MesozoiCy corals, cycads, and tree ferns flourished within
the Arctic circle.
If the electro-stellar hypothesis of solar disturbances
proves well founded, it may explain these peculiarities.
Periods of mild climate would represent a return of the
sun and the earth to their normal conditions of quiet. At
such times the atmosphere of the sun is assimied to be
little disturbed by sunspots, faculse, prominences, and
other allied evidences of movements ; and the rice-g^rain
structure is perhaps the most prominent of the solar
markings. The earth at such times is supposed to be
correspondingly free from cyclonic storms. Its winds are
then largely of the purely planetary type, such as trade
winds and westerlies. Its rainfaU also is largely planet-
ary rather than cyclonic. It falls in places such as the
heat equator where the air rises under the influence of
heat, or on the windward slopes of mountains, or in re-
gions where warm winds blow from the ocean over cold
lands.
According to the electro-stellar hypothesis, the condi-
tions which prevailed during hundreds of millions of
years of mild climate mean merely that the solar system
was then in parts of the heavens where stars — especially
double stars — ^were rare or small, and electrical disturb-
ances correspondingly weak. Today, on the other hand,
the sun is fairly near a number of stars, many of which
are large doubles. Hence it is supposed to be disturbed,
although not so much as at the height of the last glacial
epoch.
After the preceding parts of this book had been
THE SUN'S JOURNEY THROUGH SPACE 276
written, the assistance of Dr. Schlesinger made it pos-
sible to test the electro-stellar hypothesis by comparing
actual astronomical dates with the dates of climatic or
solar phenomena. In order to make this possible. Dr.
Schlesinger and his assistants have prepared Table 6,
giving the position, magnitude, and motions of the thirty-
eight nearest stars, and especially the date at which each
was nearest the sun. In column 10 where the dates are
given, a minus sign indicates the past and a plus sign the
future. Dr. Shapley has kindly added column 12, giving
the absolute magnitudes of the stars, that of the sun
being 4.8, and column 13, showing their luminosity or
absolute radiation, that of the sun being unity. Finally,
column 14 shows the eflfective radiation received by the
sun from each star when the star is at a minimum dis-
tance. Unity in this case is the effect of a star like the
sun at a distance of one light year.
It is well known that radiation of all kinds, including
light, heat, and electrical emissions, varies in direct pro-
portion to the exposed surface, that is, as the square
of the radius of a sphere, and inversely as the square of
the distance. From black bodies, as we have seen, the
total radiation varies as the fourth power of the abso-
lute temperature. It is not certain that either light or
electrical emissions from incandescent bodies vary in
quite this same proportion, nor is it yet certain whether
luminous and electrical emissions vary exactly together.
Nevertheless they are closely related. Since the light
coming from each star is accurately measured, while no
information is available as to electrical emissions, we
have followed Dr. Shapley *s suggestion and used the
luminosity of the stars as the best available measure of
total radiation. This is presumably an approximate
measure of electrical activity, provided some allowance
TABLE
THIRTY-EIGHT STARS HAVING
a)
Groombr. 34 0»»12».7
*iy Cassiop 43 .0
43 .9
*jrTucanffi 1 12 .4
T Ceti 39 .4
dsEridani 3 15 .9
•eEridani 28 .2
•40(0)« Eridani 4 10 .7
Cordoba Z. 243 5 7.7
Wei88e592 26 .4
•a Can. Maj. (Sinus). . . 6 40 .7
•aCamMin. (Procyon). 7 34 .1
•Fedorenko 1457-8 9 7 .6
Groombr. 1618 10 5.3
Wei88e234 14 .2
Lalande 21185 57 .9
Lalande 21258 11 .5
12 .0
Lalande 25372 13 40 .7
*a Centauri 14 32 .8
•^Bootes 14 46 .8
♦Lalande 27173 51 .6
Wei88el259 16 41 .4
Lacaille 7194 17 11 .5
*/3 416 12 .1
Argel -0.17415-6 .... 37 .0
Barnard's star ..:.... 52 .9
•70pOphiuehi 18 .4
*S2398 41 .7
<r Draconis 19 32 .5
*a AquilaB (Altair) .... 45 .9
*61Cygni 21 2 .4
Lacaille 8760 11 .4
€lndi 55 .7
*Kruger 60 22 24 .4
Lacaille 9352 59 .4
Lalande 46650 23 44 .0
C. G. A. 32416 59 .5
* Double star.
(3) (4) (6) {6)
+43*'27'
+57 17
+ 4 55
-69 24
-16 28
Is
8.1
. 3.6
12.3
5.0
3.6
Ma
F8
PO
P8
KO
2". 89
1 .24
3 .01
.39
1 .92
+ 3
+ 10
+"12
- 16
-43 27
- 9 48
- 7 49
-44 59
- 3 42
4.3
3.8
4.5
9.2
8.8
G5
KO
G5
E2
E2
3 .16
.97
4 .08
8 .75
2 .22
+.87
+ 16
- 42
+242
-16 35 -1.6 AO 1 .32 — 8
+ 5 29 0.5 P5 1 .24 - 4
+53 7 7.9 Ma 1 .68 + 10
+49 58 6.8 K5p 1 .45 — 30
+20 22 9.0 ... .49
+36 38 7.6 Mb 4 .78 - 87
+44 2 8.5 E5 4 .52 + 65
-57 2 12.0 ... 2 .69
+15 26 8.5 K5 2 .30
-60 25 0.2 G 3 .68 + 22
JL7 + 4
1 .96 -i- 20
.37
.97
1 .19 — 4
+68 26 90 K 1 .33
+ 4 25 9.7 Mb 10 .30 — 80
+ 2 31 4.3 K 1 .13
+59 29 8.8 K 2 .31
+69 29 4.8 G5 1 .84 +28
+19 31
-20 58
+33 41
-46 32
-34 53
4.6 K5p
5.8 Ep
8.4 ...
5.7 K
5.9 E5
+ 8 36
+38 15
-39 15
-57 12
+57 12
1.2
5.6
6.6
4.8
9.2
A5
K5
G
£5
.66
5 .20
3 .53
4 .70
.87
— 33
— 64
+ 13
— 39
-36 26
+ 1 52
-37 51
7.1
8.7
8.2
K
Ma
G
6 .90
1 .39
6 .05
+ 12
+ 26
6
LARGEST KNOWN PARALLAXES
(7) («)
(9)
(10)
(m
(Ii)
(XS)
a^)
Ir 1
I
igtx
Il5
sis
« 3
^
4*
d
^ -S -i S «
lllll
".28 " .
.28
11.6
- 4000
8.1
10.3
0.0063
0.000051
.18
.19
17.1
- 47000
3.5
4.9
0.91
0.003110
.24
• •
. . . •
• • • •
14.2
0.00017
.16
.23
14.2
-264000
4.2
6.0
0.33
0.001610
.32
.37
8.8
+ 46000
3.3
6.1
0.30
0.003840
.16
.22
14.8
- 33000
3.6
5.3
0.63
0.002960
.31
.46
7.1
-106000
3.0
6.3
0.25
0.004970
.21
.23
14.2
+ 19000
4.3
6.1
0.30
0.001470
.32
.68
4.8
- 10000
7.6
11.7
0.0017
0.000074
.17
• •
. . • •
....
9.9
0.009
.37
.41
8.0
+ 65000
-1.8
1.2
27.50
0.429000
.31
32
10.2
+ 34000
0.5
3.0
5.25
0.051300
.16
,16
20.4
- 24000
7.9
8.9
0.023
0.000055
.18
23
14.2
+ 69000
6.3
8.1
0.048
0.000238
.19
• .
• • • •
• • • •
10.4
0.0057
.41
76
4.3
+ 20000
6.2
10.7
0.0044
0.000238
.19
.22
14.8
- 20000
8.2
9.9
0.009
0.000041
.34
. .
• ■ • •
• • • •
14.7
0.00011
.19
• .
• • • •
• • • •
9.9
0.009
.76 1.
.03
3.2
- 28000
-0.5
4.6
1.20
0.117500
.17
22
14.8
-598000
4.0
5.8
0.40
0.001815
.18
,19
17.1
- 36000
5.6
7.1
0.12
0.000412
.18
• •
....
• • • •
9.7
0.011
.19
• •
....
• • • m
7.1
0.12
.17
.17
19.2
+ 21000
5.7
7.1
. 0.12
0.000329
.22
• •
....
• • • •
10.8
0.004
.53
.70
4.7
+ 10000
9.1
13.3
0.0025
0.000114
.19
• •
• • • •
....
5.7
0.44
.29
.• •
• • • •
....
11.1
0.0030
.20
.23
14.2
- 49000
4.5
6.3
0.25
0.001238
.21
.51
6.4
+117000
-0.7
2.8
6.30
0.153600
.30
.38
8.6
+ 19000
5.1
8.0
0.053
0.000715
.25
.26
12.6
- 11000
6.6
8.6
0.030
0.000189
.28
.31
10.5
+ 17000
4.6
7.0
0.13
0.001230
.26
• •
....
• • • •
11.3
0.0025
.29
.29
11.2
- 3000
7.1
9.4
0.014
0.000111
.17
» • •
....
• • • •
9.9
0.009
.22
.22
14.8
- 7000
8.2
9.9
0.009
0.000041
278 CLIMATIC CHANGES
be made for disturbances by outside bodies such as com-
panion stars. Hence the inclusion of column 14.
On the basis of column 14 and of the movements and
distances of the stars as given in the other columns Fig.
10 has been prepared. This gives an estimate of the
approximate electrical energy received by the sun from
the nearest stars for 70,000 years before and after the
present. It is based on the twenty-six stars for which
complete data are available in Table 6. The inclusion of
the other twelve would not alter the form of the curve,
for even the largest of them would not change any part
by more than about half of 1 per cent, if as much.
Nor would the curve be visibly altered by the omission
of all except four of the twenty-six stars actually used.
The four that are important, and their relative lumi-
nosity when nearest the sun, are Sirius 429,000, Altair
153,000, Alpha Centauri 117,500, and Procyon 51,300.
The figure for the next star is only 4970, while for this
star combined with the other twenty-one that are imim-
portant it is only 24,850.
Figure 10 is not carried more than 70,000 years into
the past or into the future because the stars near the
sun at more remote times are not included among the
thirty-eight having the largest known parallaxes. That
is, they have either moved away or are not yet near
enough to be included. Indeed, as Dr. Schlesinger
strongly emphasizes, there may be swiftly moving, bright
or gigantic stars which are now quite far away, but whose
inclusion would alter Fig. 10 even within the limits of
the 140,000 years there shown. It is almost certain, how-
ever, that the most that these would do would be to raise,
but not obliterate, the minima on either side of the main
maximum.
In preparing Fig. 10 it has been necessary to make
IS
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08
2
O
in
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8
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280 CLIMATIC CHANGES
allowance for double stars. Passing by the twenty-two
unimportant stars, it appears that the companion of
Sinus is eight or ten magnitudes smaller than that star,
while the companions of Procyon and Altair are five or
more magnitudes smaller than their bright comrades.
This means that the luminosity of the faint components
is at most only 1 per cent of that of their bright com-
panions and in the case of Sirius not a hundredth of 1
per cent. Hence their inclusion would have no visible
effect on Fig. 10. In Alpha Centauri, on the other hand^
the two components are of almost the same magnitude.
For this reason the effective radiation of that star as
given in column 14 is doubled in Fig. 10, while for
another reason it is raised still more. The other reason
is that if our inferences as to the electrical effect of the
sun on the earth and of the planets on the sun are cor-
rect, double stars, as we have seen, must be much more
effective electrically than single stars. By the same
reasoning two bright stars close together must excite
one another much more than a bright star and a very
faint one, even if the distances in both cases are the same.
So, too, other things being equal, a triple star must be
more excited electrically than a double star. Hence in
preparing Fig. 10 aU double stars receive double weight
and each part of Alpha Centauri receives an additional
50 per cent because both parts are bright and because
they have a third companion to help in exciting them.
According to the electro-stellar hypothesis. Alpha Cen-
tauri is more important climatically than any other star
in the heavens not only because it is triple and bright, but
because it is the nearest of all stars, and moves fairly
rapidly. Sirius and Procyon move slowly in respect to
the sun, only about eleven and eight kilometers per
second respectively, and their distances at minimum are
THE SUN'S JOURNEY THROUGH SPACE 281
fairly large, that is, 8 and 10.2 light years. Hence their
effect on the snn changes slowly. Altair moves faster,
about twenty-six kilometers per second, and its minimum
distance is 6.4 light years, so that its effect changes fairly
rapidly. Alpha Centauri moves about twenty-four kilo-
meters per second, and its minimum distance is only 3.2
light years. Hence its effect changes very rapidly, the
change in its apparent Imninosity as seen from the sun
amounting at maximum to about 30 per cent in 10,000
years against 14 per cent for Altair, 4 for Sirius, and 2
for Procyon. The vast majority of the stars change so
much more slowly than even Procyon that their effect is
almost uniform. All the stars at a distance of more than
perhaps twenty or thirty light years may be regarded as
sending to the sun a practically unchanging amount of
radiation. It is the bright stars within this limit which
are important, and their importance increases with their
proximity, their speed of motion, and the brightness and
number of their companions. Hence Alpha Centauri
causes the main maximum in Fig. 10, while Sirius, Altair,
and Procyon combine to cause a general rise of the curve
from the past to the future.
Let us now interpret Fig. 10 geologically. The low posi-
tion of the curve fifty to seventy thousand years ago
suggests a mild inter-glacial climate distinctly less severe
than that of the present. Geologists say that such was the
case. The curve suggests a glacial epoch culminating
about 28,000 years ago. The best authorities put the di-
max of the last glacial epoch between twenty-five and
thirty thousand years ago. The curve shows an ameliora-
tion of climate since that time, although it suggests that
there is still considerable severity. The retreat of the ice
from North America and Europe, and its persistence in
Greenland and Antarctica agree with this. And the curve
282 CLIMATIC CHANGES ^ ^
indicates that the change of climate is still persisting, a
conclusion in harmony with the evidence as to historic
changes.
If Alpha Centauri is really so important, the effect of
its variations, provided it has any, ought perhaps to be
evident in the sun. The activity of the star 's atmosphere
presumably varies, for the orbits of the two components
have an eccentricity of 0.51. Hence during their period
' of revolution, 81.2 years, the distance between them
; ranges from 1,100,000,000 to 3,300,000,000 miles. They
were at a minimum distance in 1388, 1459, 1550, 1631,
1713, 1794, 1875, and wiU be again in 1956. In Fig.
11, showing sunspot variations, it is noticeable that the
years 1794 and 1875 come just at the ends of periods of
unusual solar activity, as indicated by the heavy hori-
zontal line. A^similar. periods of ,ig:eat,activity^seeDas to
:, ^ »,. I ' n^ have begun about 1914. If its duration eguals the ^verage
. .;\of its fwA prftf^pAQggArfl^ if TiHii ftTld^ahPPi^iPJQ Back in
the fourteenth century a period of excessive solar ac-
tivity, which has already been described, culminated from
' 1370 to 1385, or just before the two parts of Alpha Cen-
tauri were at a minimum distance. Thus in three and
perhaps four cases the sun has been unusually active
during a time when the two parts of the star were most
rapidly approaching each other and when their atmos-
pheres were presumably most disturbed and their elec-
trical emanations strongest.
The fact that Alpha Centauri, the star which would be
expected most strongly to influence the sun, and hence
the earth, was nearest the sun at the climax of the last
glacial epoch, and that today the solar atmosphere is
most active when the star i3 presumably most disturbed
may be of no significance. It is given for what it is worth.
Its importance lies not in the fact that it proves any-
*
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284 CLIMATIC CHANGES
thing, but that no contradiction is found when we test
the electro-stellar hypothesis by facts which were not
thought of when the hypothesis was framed. A vast
amount of astronomical work is still needed before the
matter can be brought to any definite conclusion. In case
the hypothesis stands firm, it may be possible to use the
star,^ a hdp in determinig th/ex«* chronology of the
later part of geological times. If the hypothesis is dis-
proved, it will merely leave the question of solar varia-
tions where it is today. It will not influence the main
conclusions of this book as to the causes and nature of
climatic changes. Its value lies in the fact that it calls
attention to new lines of research.
CHAPTER XVI
THE EARTH'S CRUST AND THE SUN
A LTHOUGH the problems of this book may lead far
/% afield, they ultimately bring us back to the earth
1 \ and to the present. Several times in the preceding
pages there has been mention of the fact that periods of
extreme climatic fluctuations are closely associated with
great movements of the earth 's crust whereby mountains
are uplifted and continents upheaved. In attempting to
explain this association the general tendency has been
to look largely at the past instead of the present. Hence
it has been almost impossible to choose among three
possibilities^ all beset with diflSculties. First, the move-
ments of the crust may have caused the climatic fluctua-
tions ; second, climatic changes may cause crustal move-
ments ; and third, variations in solar activity or in some
other outside agency may give rise to both types of terres-
trial phenomena.
The idea that movements of the earth's crust are the
main cause of geological changes of climate is becoming
increasingly untenable as the complexity and Rapidity of
climatic changes become more clear, especially during
post-glacial times. It implies that the earth's surface
moves up and down with a speed and facility which
appear to be out of the question. If volcanic activity be
invoked the problem becomes no clearer. Even if volcanic
dust should fill the air frequently and completely, neither
its presence nor absence would produce such peculiar f ea-
286 CLIMATIC CHANGES
tures as the localization of glaciers^ the distribution of
loesSy and the mild climate of most parts of geological
time. Nevertheless, because of the great difficulties pre-
sented by the other two possibilities many geologists
still hold that directly or indirectly the greater climatic
changes have been mainly due to movements of the
earth 's crust and to the reaction of the crustal movements
on the atmosphere.
The possibility that climatic changes are in themselves
a cause of movements of the earth's crust seems so im-
probable that no one appears to have investigated it with
any seriousness. Nevertheless, it is worth while to raise
the question whether climatic extremes may cooperate
with other agencies in setting the time when the earth's
crust shall be deformed.
As to the third possibility, it is perfectly logical to
ascribe both climatic changes and crustal deformation to
some outside agency, solar or otherwise, but hitherto
there has been so little evidence on this point that such
an ascription has merely begged the question. If heavenly
bodies should approach the earth closely enough so that
their gravitational stresses caused crustal deformation,
all life would presumably be destroyed. As to the sun,
there has hitherto been no conclusive evidence that it is
related to crustal movements, although various writers
have made suggestions along this line. In this chapter
we shall carry these suggestions further and shall see
that they are at least worthy of study.
As a preliminary to this study it may be well to note
that the coincidence between movements of the earth's
crust and climatic changes is not so absolute as is some-
times supposed. For example, the profound crustal
changes at the end of the Mesozoic were not accompanied
by widespread glaciation so far as is yet known, although
THE EARTH'S CRUST AND THE SUN 287
the temperature appears to have been lowered. Nor was
the violent volcanic and diastrophic activity in the Mio-
cene associated with extreme climates. Indeed, there
appears to have been little contrast from zone to zone,
for figs, bread fruit trees, tree ferns, and other plants of
low latitudes grew in Greenland. Nevertheless, both at
the end of the Mesozoic and in the Miocene the climate
may possibly have been severe for a time, although the
record is lost. On the other hand, Kirk's recent discovery
of glacial till in Alaska between beds carrying an un-
doubted Middle Silurian fauna indicates glaciation at a
time when there was Uttle movement of the crust so far
as yet appears.^ Thus we conclude that while climatic
changes and crustal movements usually occur together,
they may occur separately.
According to the solar-cyclonic hypothesis such a con-
dition is to be expected. If the sun were especially active
when the terrestrial conditions prohibited glaciation,
changes of climate would still occur, but they would be
milder than under other circumstances, and would leave
little record in the rocks. Or there might be glaciation in
high latitudes, such as that of southern Alaska in the
Middle Silurian, and none elsewhere. On the other hand,
when the sun was so inactive that no great storminess
occurred, the upheaval of continents and the building of
moxmtains might go on without the formation of ice
sheets, as apparently happened at the end of the Meso-
zoic. The lack of absolute coincidence between glaciation
and periods of widespread emergence of the lands is
evident even today, for there is no reason to suppose
that the lands are notably lower or less extensive now
than they were during the Pleistocene glaciation. In
fact, there is much evidence that many areas have risen
lE. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci., 1918, p. 511.
288 CLIMATIC CHANGES
since that time. Yet glaciation is now far less extensive
than in the Pleistocene. Any attempt to explain this dif-
ference on the basis of terrestrial changes is extremely
difficult, for the shape and altitude of continents and
mountains have not changed much in twenty or thirty
thousand years. Yet the present moderately mild epochs
like the puzzUng inter-glacial epochs of earlier times, is
easily explicable on the assumption that the sun's atmos-
phere mky sometimes vary in harmony witii cmstal
activity, but does not necessarily do so at all times.
Turning now to the main problem of how climatic
changes may be connected with movements of the earth's
crust, let us follow our usual method and examine what
is happening today. Let us first inquire whether earth-
quakes, which are one of the chief evidences that crustal
movements are actually taking place in our own times,
show any connection with sunspots. In order to test this,
we have compared MUne^s Catalogue of Destructive
Earthquakes from 1800 to 1899, with Wolf's stmspot
numbers for the same period month by month. The earth-
quake catalogue, as its compiler describes it, ^4s an
attempt to give a list of earthquakes which have an-
nounced changes of geological importance in the earth's
crust; movements which have probably resulted in the
creation or the extension of a line of fault, the vibrations
accompanying which could, with proper instruments,
have been recorded over a continent or the whole surface
of our world. Small earthquakes have been excluded,
while the number of large earthquakes both for ancient
and modem times has been extended. As an illustration
of exclusion, I may mention that between 1800 and 1808,
which are years taken at random, I find in Mallet 's cata-
logue 407 entries. Only thirty-seven of these, which were
accompanied by structural damage, have been retained.
THE EARTH'S CRUST AND THE SUN
289
Other catalogaes such as those of Perry and Fuchs have
been treated similarly.'"
If the earthquakes in such a caref iilly selected list bear
a distinct relation to sunspots, it is at least possible and
perhaps probable that a similar relation may exist be-
tween solar activity and geological changes in the earth 's
crust The result of the comparison of earthquakes and
sxmspots is shown in Table 7. The first column gives the
sunspot numbers ; the second, the number of months that
had the respective spot numbers during the century from
1800 to 1899. Column C shows the total number of earth-
quakes during the months having any particular degree
of spottedness ; while D, which is the significant column,
gives the average number of destructive earthquakes per
month under each of the six conditions of solar spotted-
TABLE 7
DESTRUCTIVE EARTHQUAKES FROM 1800 TO
1899 COMPARED WITH SUNSPOTS
A
B
C
2> E
Average Number
F
Average
number
Number
number of earth-
of earth-
of months
Number
of earth' quakes in
quakes in
Sunspot
per Wolf's
of earth-
quakes per succeeding
succeeding
nwnhers
Table
quakes
month month
month
0- 15
344
522
1.52 512
1.49
15- 30
194
306
1.58 310
1.60
30- 50
237
433
1.83 439
1.85
50- 70
195
402
2.06 390
2.00
70-100
135
286
2.12 310
2.30
over 100
95
218
2.30 175
1.84
s J. Milne: Catalogue of DestnictiTe Earthquakes; Bep. Brit. Asso. Adv.
8ci., 1911.
290 CLIMATIC CHANGES
ness. The regularity of coltunn D is so great as to make
it almost certain that we are here dealing with a real
relationship. Column F, which shows the average number
of earthquakes in the month succeeding any given condi-
tion of the sun^ is stiU more regular except for the last
entry.
The chance that six numbers taken at random will
arrange themselves in any given order is one in 720. In
other words, there is one chance in 720 that the regularity
of column D is accidental. But column F is as regular as
column D except for the last entry. If columns D and E
were independent there would be one chance in about
500,000 that the six numbers in both columns would
fall in the same order, and one chance in 14,400 that
five numbers in each would fall in the same order.
But the two columns are somewhat related, for although
the after-shocks of a great earthquake are never included
in Milne's table, a world-shaking earthquake in one
region during a given month probably creates conditions
that favor similar earthquakes elsewhere during the next
month. Hence the probability that we are dealing with a
purely accidental arrangement in Table 7 is less than one
in 14,400 and greater than one in 500,000. It may be one
in 20,000 or 100,000. In any event it is so slight that there
is high probability that directly or indirectly sunspots
and earthquakes are somehow connected.
In ascertaining the relation between sunspots and
earthquakes it would be well if we could employ the strict
method of correlation coefficients. This, however, is im-
possible for the entire century, for the record is by no
means homogeneous. The earlier decades are represented
by only about one-fourth as many earthquakes as the
later ones, a condition which is presumably due to lack of
information. This makes no difference with the method
THE EARTH'S CRUST AND THE SUN 291
employed in Table 7, since years with many and few sun-
spots are distributed almost equally throughout the
entire nineteenth century, but it renders the method of
correlation coefficients inapplicable. During the period
from 1850 onward the record is much more nearly homo-
geneous, though not completely so. Even in these later
decades, however, allowance must be made for the fact
that there are more earthquakes in winter than in
summer, the average number per month for the fifty
years being as follows :
Jan. 2.8 May 2.4 Sept. 2.5
Feb. 2.4 June 2.3 Oct. 2.6
Mar. 2.5 July 2.4 Nov. 2.7
Apr. 2.4 Aug. 2.4 Dec. 2.8
The correlation coefficient between the departures from
these monthly averages and the corresponding depar-
tures from the monthly averages of the sunspots for the
same period, 1850-1899, are as follows :
Sunspots and earthquakes of same month: -|-0.042, or 1.5
times the probable error.
Sunspots of a given month and earthquakes of that month
and the next : -{-0.084, or 3.1 times the probable error.
Sunspots of three consecutive months and earthquakes of
three consecutive months allowing a lag of one month, i.e., son-
spots of January, February, and March compared with earth-
quakes of February, March, and April; sunspots of February,
March, and April with earthquakes of March, April, and May,
etc. ; +0.112, or 4.1 times the probable error.
These coefficients are all small, but the number of in-
dividual cases, 600 months, is so large that the probable
error is greatly reduced, being only ±0.027 or ±0.028.
Moreover, the nature of our data is such that even if
292 CLBiATIC CHANGES
there is a strong connection between solar changes and
earth movements^ we should not expect a large correla-
tion coefficient. In the first place, as already mentioned,
the earthquake data are not strictly homogeneous.
Second, an average of about two and one-half strong
earthquakes per month is at best only a most imperfect
indication of the actual movement of the earth's crust.
Third, the sunspots are only a partial and imperfect
measure of the activity of the sun's atmosphere. Fourth,
the relation between solar activity and earthquakes is
almost certainly indirect. In view of all these conditions,
the regularity of Table 7 and the fact that the most im-
portant correlation coefficient rises to more than four
times the probable error makes it almost certain that the
solar and terrestrial phenomena are really connected.
We are now confronted by the perplexing question of
how this connection can take place. Thus far only three
possibilities present themselves, and each is open to
objections. The chief agencies concerned in these three
possibilities are heat, electricity, and atmospheric pres-
sure. Heat may be dismissed very briefly. We have seen
that the earth's surface becomes relatively cool when
the sun is active. Theoretically even the slightest change
in the temperature of the earth's surface must influence
the thermal gradient far into the interior and hence cause
a change of volume which might cause movements of the
crust. Practically the heat of the surface ceases to be of
appreciable importance at a depth of perhaps twenty
feet, and even at that depth it does not act quickly enough
to cause the relatively prompt response which seems to be
characteristic of earthquakes in respect to the sun.
The second possibility is based on the relationship
between solar and terrestrial electricity. When the sun
is active the earth's atmospheric electrical potential is
THE EARTH'S CRUST AND THE SUN 298
subject to slight variations. It is well known that when
two opposing points of an ionized solution are oppositely
charged electrically^ a current passes through the liquid
and sets up electrolysis whereby there is a segregation
of materials, and a consequent change in the volume of
the parts near the respective electrical poles. The same
process takes place, although less freely, in a hot mass
such as forms the interior of the earth. The question
arises whether internal electrical currents may not pass
between the two oppositely charged poles of the earth,
or even between the great continental masses and the
regions of heavier rock which underlie the oceans. Could
this lead to electrolysis, hence to differentiation in vol-
ume, and thus to movements of the earth's crust? Could
the results vary in harmony with the sunt Bowie* has
shown that numerous measurements of the strength and
direction of the earth's gravitative pull are explicable
only on the assumption that the upheaval of a continent
or a mountain range is due in part not merely to pres-
sure, or even to flowage of the rocks beneath the crust,
but also to an actual change in volume whereby the rocks
beneath the continent attain relatively great volume and
those under the oceans a small volume in proportion to
their weight. The query arises whether this change of
volume may be related to electrical currents at some
depth below the earth's surface.
The objections to this hypothesis are numerous. First,
there is little evidence of electrolytic differentiation in
the rocks. Second, the outer part of the earth's crust is a
very poor conductor so that it is doubtful whether even
a high degree of electrification of the surface would have
much effect on the interior. Third, electrolysis due to any
s Wm. Bowie: Lecture before the Geological Club of Yale University.
See Am. Jour. Sci., 1921.
294 CLIMATIC CHANGES
such mild causes as we have here postulated must be an
extremely slow process, too slow, presumably, to have
any appreciable result within a month or two. Other
objections join with these three in making it seem im-
probable that the sun^s electrical activity has any direct
effect upon movements of the earth's crust.
The third, or meteorological hypothesis, which makes
barometric pressure the main intermediary between solar
activity and earthquakes, seems at first sight almost as
improbable as the thermal and electrical hypotheses.
Nevertheless, it has a certain degree of observational
support of a kind which is wholly lacking in the other two
cases. Among the extensive writings on the periodicity of
earthquakes one main fact stands out with great dis-
tinctness : earthquakes vary in number according to the
season. This fact has already been shown incidentally in
the table of earthquake frequency by months. If allow-
ance is made for the fact that February is a short month,
there is a regular decrease in the frequency of severe
earthquakes from December and January to June. Since
most of Milne's earthquakes occurred in the northern
hemisphere, this means that severe earthquakes occur in
winter about 20 per cent of tener than in summer.
The most thorough investigation of this subject seems
to have been that of Davisson.^ His results have been
worked over and amplified by Knott,* who has tested
them by Schuster's exact mathematical methods. His re-
sults are given in Table 8.' Here the northern hemisphere
^Chas. Davisson: On the Annual and Semi-annual Seismie Perioda;
B07. Soc. of London, Philosophical Transactional Vol. 184, 1893, 1107 fP.
6G. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.
« In Table 8 the first column indicates the region ; the second, the dates ;
and the third, the number of shocks. The fourth column gives the month in
which the annual maximum occurs when the crude figures are smoothed by
the use of overlapping six-monthly means. In other words, the average for
each successive six months has been placed in the middle of the period.
THE EARTH'S CRUST AND THE SUN
295
TABLE 8
SEASONAL MARCH OF EARTHQUAKES
AFTER DAVISSON
AND KNOT!'
A
B
C
D
E
F
G
i
1
i
^
^
%*
5f
^5sl
Region
Limiti
Date*
1^
Maxim
Month
•••
1
Exptci
Amplii
^ •*{ *» s
tf U H^ S
Northern Hefmisphere
223-1850
5879
Dec.
0.110
0.023
4.8
Northern Hemisphere
1865-1884
8133
Dec.
0.290
0.020
14.5
Europe
1865-1884
5499
Dec.
0.350
0.024
14.6
Europe
306-1843
1961
Dec.
0.220
0.040
5.5
Southeast Europe
1859-1887
3470
Dec.
0.210
0.030
7.0
Vesuvius District
1865-1883
513
Dec.
0.250
0.078
3.2
Italy:
Old Tromometre
1872-1887
61732
Dec.
0.490
0.007
70.0
Old Tromometre
1876-1887
38546
Dec.
0.460
0.009
49.5
Normal Tromometre
1876-1887
38546
Dec.
0.490
0.009
52.8
Balkan, etc.
1865-1884
624
Dec.
0.270
0.071
3.8
Hungazy, etc.
1865-1884
384
Dec.
0.310
0.090
3.4
Italy
1865-1883
2350
Dec. (Sept.)
0.140
0.037
3.8
Grecian Archip.
1859-1881
3578
Dec-Jan.
0.164
0.030
5.5
Austria
1865-1884
461
Jan.
0.370
0.083
4.4
Switzerland, etc.
1865-1883
524
Jan.
0.560
0.077
7.3
Asia
1865-1884
458
Feb.
0.330
0.083
4.0
North America
1865-1884
552
Nov.
0.350
0.075
4.7
California
1850-1886
949
Oct.
0.300
0.058
5.2
Japan
1878-1881
246
Dec.
0.460
0.113
4.1
Japan
1872-1880
367
Dec.-Jan.
0.256
0.093
2.8
Japan
1876-1891
1104
Feb.
0.190
0.053
3.6
Japan
1885-1889
2997
Oct.
0.080
0.032
2.5
Zante
1825-1863
1326
Aug.
0.100
0.049
2.0
Italy, North of Naples
1 1865-1883
1513
Sept. (Nov.)
0.210
0.046
4.6
East Indies
1873-1881
515
Aug., Oct.,
or Dec.?
0.071!
0.078
0.9
Malay Archip.
1865-1884
598
May
0.190
0.072
2.6
New Zealand
1869-1879
585
Aug.-Sept.
0.203
0.073
2.8
Chile
1873-1881
212
July
0.480
0.122
3.9
Southern Hemisphere
1865-1884
751
July
0.370
0.065
5.7
New Zealand
1868-1890
641
March, May
0.050
0.070
0.7
Chile
1865-1883!
316
July, Dec
0.270
0.100
2.7
Peru, Bolivia
1865-1884
350
July
0.480
0.095
5.1
296 CLIMATIC CHANGES
is placed first ; then come the East Indies and the Malay
Archipelago lying close to the equator; and finally the
southern hemisphere. In the northern hemisphere prac-
tically all the maxima come in the winter, for the month
of December appears in fifteen cases out of the twenty-
five in column D, while January, February, or November
appears in six others. It is also noticeable that in sixteen
cases out of twenty-five the ratio of the actual to the ex-
pected amplitude in column G is four or more, so that a
real relationship is indicated, while the ratio falls below
three only in Japan and Zante. The equatorial data,
unlike those of the northern hemisphere, are indefinite,
for in the East Indies no month shows a marked maxi-
mum and the expected amplitude exceeds the actual am-
plitude. Even in the Malay Archipelago, which shows a
maximum in May, the ratio of actual to expected ampli-
tude is only 2.6. Turning to the southern hemisphere, the
winter months of that hemisphere are as strongly marked
by a maximum as are the winter months of the northern
Thus the average of January to Jnne^ iDelaaiye, ib placed between March
and April, that for February to July between April and May, and bo on.
Thia method eliminatee the minor fluctuations and also all periodicitiea
having a duration of less than a year. If there were no annual periodicity
the smoothing would result in practically the same figure for each month.
The column marked ''Amplitude" gives the range from the highest month
to the lowest divided by the number of earthquakes and then corrected
according to Schuster's method which is well known to mathematicians,
but which is so confusing to the layman that it will not be described. Next,
in the column marked "Expected Amplitude/' we have the amplitude that
would be expected if a series of numbers corresponding to the earthquake
numbers and having a similar range were arranged in accidental order
throughout the year. This also is calculated by Schuster's method in which
the expected amplitude is equal to the square root of "pi" divided by the
number of shocks. When the actual amplitude is four or more times the
expected amplitude, the probability that there is a real periodicity in the
observed phenomena becomes so great that we may regard it as practically
certain. If there is no periodicity the two are equal. The last column gives
the number of times by which the actual exceeds the expected ampUtude,
and thus is a measure of the probability that earthquakes vary system-
atically in a period of a year.
THE EARTH'S CRUST AND THE SUN 297
hemisphere. July or August appears in five out of six
cases. Here the ratio between the actual and expected
ampUtudes is not so great as in the northern hemisphere.
Nevertheless, it is practically four in Chile, and exceeds
five in Peru and Bolivia, and in the data for the entire
southern hemisphere.
The whole relationship between earthquakes and the
seasons in the northern and southern hemispheres is
summed up in Fig. 12 taken from Knott The northern
hemisphere shows a regular diminution in earthquake
frequency from December until June, and an increase
the rest of the year. In the southern hemisphere the
course of events is the same so far as summer and winter
are concerned, for August with its maximum comes in
winter, while February with its minimum comes in
summer. In the southern hemisphere the winter month
of greatest seismic activity has over 100 per cent more
earthquakes than the summer month of least activity. In
the northern hemisphere this difference is about 80 per
cent, but this smaller figure occurs partly because the
northern data include certain interesting and signifi-
cant regions like Japan and China where the usual condi-
tions are reversed.^ If equatorial regions were included
in Fig. 12, they would give an almost straight line.
The connection between earthquakes and the seasons is
so strong that almost no students of seismology question
it, although they do not agree as to its cause. A meteoro-
logical hypothesis seems to be the only logical explana-
tion.* Wherever sufficient data are available, earthquakes
7 N. F. Drake: Destructive Earthquakes in China; Bull. Seism. Soc Am.,
VoL 2, 1912, pp. 40-91, 124-133.
s The only other explanation that seems to have any standing is the
peyehologieal hypothesis of Montessns de Ballore as given in Les Tremble-
ments de Terre. He attributes the apparent seasonal variation in earth-
quakes to the fact that in winter people are within doors, and hence notice
298 CLIMATIC CHANGES
appear to be most numerous when climatic conditions
cause the earth 's surface to be most heavily loaded or to
change its load most rapidly. The main factor in the
loading is apparently atmospheric pressure. This acts in
two ways. First, when the continents become cold in
winter the pressure increases. On an average the air
at sea level presses upon the earth's surface at the rate
of 14.7 pounds per square inch, or over a ton per square
foot, and only a little short of thirty million tons per
square mile. An average difference of one inch between
the atmospheric pressure of summer and winter over ten
million square miles of the continent of Asia, for ex-
ample, means that the continent's load in winter is about
ten million million tons heavier than in summer. Second,
the changes in atmospheric pressure due to the passage
of storms are relatively sharp and sudden. Hence they
are probably more effective than the variations in the
load from season to season. This is suggested by the
rapidity with which the terrestrial response seems to
follow the supposed solar cause of earthquakes. It is also
suggested by the fact that violent storms are frequently
followed by violent earthquakes. * ' Earthquake weather, * '
as Dr. Schlesinger suggests, is a common phrase in the
typhoon region of Japan, China, and the East Indies.
During tropical hurricanes a change of pressure amount-
ing to half an inch in two hours is common. On Septem-
movements of the earth much more than in summer when thej are out of
doors. There is a similar difference between people's habits in high lati-
tudes and low. Undoubtedly this does have a marked effect upon the degree
to which minor earthquake shocks are noticed. Nevertheless, de Ballore's
contention, as well as any other psychological explanation, is completely
upset by two facts: First, instrumental records show the same seasonal dis-
tribution as do records based on direct observation, and instruments cer-
tainly are not influenced by the seasons. Second, in some places, notably
China, as ]>rake has shown, the summer rather than the winter is very
decidedly the time when earthquakes are most frequent
THE EARTH'S CRUST AND THE SUN
299
ber 22, 1885, at False Point lighthotise on the Bay of
Bengal, the barometer fell about an inch in six hours,
then nearly an inch and a half in not much over two
hours, and finally rose fully two inches inside of two
hours. A drop of two inches in barometric pressure
means that a load of about two million tons is removed
S
.A
£
a
<
J °
V
a
9
•= s a
i < »
>
o
Z
i I
Fig. 12. Seasonal distribution of earthquakes.
{After Da/vi8son and Knott,)
Northern Hemisphere. Southern Hemisphere.
800 CLIMATIC CHANGES
from each square mile of land ; the corresponding rise of
pressure means the addition of a similar load. Such a
storm, and to a less degree every other storm, strikes a
blow upon the earth's surface, first by removing millions
of tons of pressure and then by putting them on again.*
Such storms, as we have seen, are much more frequent
and severe when sunspots are numerous than at other
times. Moreover, as Veeder*** long ago showed, one of the
most noteworthy evidences of a connection between sun-
spots and the weather is a sudden increase of pressure in
certain widely separated high pressure areas. In most
parts of the world winter is not only the season of
highest pressure and of most frequent changes of
Veeder's type, but also of severest storms. Hence a
meteorological hypothesis would lead to the expectation
that earthquakes would occur more frequentiy in winter
than in summer. On the Chinese coast, however, and also
on the oceanic side of Japan, as well as in some more
tropical regions, the chief storms come in summer in the
form of typhoons. These are the places where earth-
quakes also are most abundant in summer. Thus, wher-
ever we turn, storms and the related barometria changes
seem to be most frequent and severe at the very times
when earthquakes are also most frequent.
Other meteorological factors, such as rain, snow,
winds, and currents, probably have some effect on earth-
• A eomparison of tropical hurricaneB with earthquakea is interesting.
Taking all the hurricanee recorded in Augost, September, and October, from
1880 to 1899, and the corresponding earthquakes in Milne's catalogue, the
correlation coefficient between hurricanes and earthquakes is -|-0,236, with a
probable error of ±0.082, the month being used as the unit. This is not a
large correlation, yet when it is remembered that the hurricanes represent
only a small part of the atmospheric disturbances in any given month, it
suggests that with fuller data the correlation might be large.
10 Ellsworth Huntington: The (Geographic Work of Br. M. A. Veeder;
Geog. Bev., Vol. 3, March and April, 1917, Nos. 3 and 4.
THE EARTH'S CRUST AND THE SUN 801
quakes through their ability to load the earth's crust.
The coming of vegetation may also help. These agencies,
however, appear to be of small importance compared
with the storms. In high latitudes and in regions of
abundant storminess most of these factors generally
combine with barometric pressure to produce frequent
changes in the load of the earth's crust, especially in
winter. In low latitudes, on the other hand, there are few
severe storms, and relatively little contrast in pressure
and vegetation from season to season ; there is no snow ;
and the amount of ground water changes little. With this
goes the twofold fact that there is no marked seasonal
distribution of earthquakes, and that except in certain
local volcanic areas, earthquakes appear to be rare. In
proportion to the areas concerned, for example, there is
little evidence of earthquakes in equatorial Africa and
South America.
The question of the reality of the connection between
meteorological conditions and crustal movements is so
important that every possible test should be applied. At
the suggestion of Professor Schlesinger we have looked
up a very ingenious line of inquiry. During the last
decades of the nineteenth century, a long series of ex-
tremely accurate observations of latitude disclosed a fact
which had previously been suspected but not demon-
strated, namely, that the earth wabbles a little about its
axis. The axis itself always points in the same direction,
and since the earth slides irregularly around it the lati-
tude of all parts of the earth keeps changing. Chandler
has shown that the wabbling thus induced consists of
two parts. The first is a movement in a circle with a
radius of about fifteen feet which is described in approxi-
mately 430 days. This so-called Eulerian movement is a
normal gyroscopic motion like the slow gyration of a
802 CLIMATIC CHANGES
spinning top. This depends on purely astronomical
causeSy and no terrestrial cause can stop it or eliminate
it The period appears to be constant, but there are cer-
tain puzzling irregularities. The usual amplitude of this
movement, as Schlesinger" puts it, **is about 0".27, but
twice in recent years it has jumped to (^^40. Such a
change could be accounted for by supposing that the
earth had received a severe blow or a series of milder
blows tending in the same direction. ' ' These blows, which
were originally suggested by Helmert are most interest-
ing in view of our suggestion as to the blows struck by
storms.
The second movement of the pole has a period of a
year, and is roughly an ellipse whose longest radius is
fourteen feet and the shortest, four feet; or, to put it
technically, there is an annual term with a maximum
amplitude of about 0".20. This, however, varies irregu-
larly. The result is that the pole seems to wander over
the earth's surface in the spiral fashion illustrated in
Fig. 13. It was early suggested that this peculiar wan-
dering of the pole in an annual period must be due to
meteorological causes. Jeffreys" has investigated the
matter exhaustively. He assumes certain reasonable
values for the weight of air added or subtracted from
different parts of the earth's surface according to the
seasons. He also considers the effect of precipitation,
vegetation, and polar ice, and of variations of tempera-
ture and atmospheric pressure in their relation to move-
ments of the ocean. Then he proceeds to compare all
11 Frank Schlesinfi^er : Variations of Latitude; Their Bearing upon Our
Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc., Vol. 54,
1915, pp. 351-358. Also Smithsonian Beport for 1916, pp. 248-254.
13 Harold Jeif reys : Causes Contributory to the Annual Variations of
Latitude; Monthly Notices, Boyal Astronomical Soe., Vol. 76, 1916, pp.
499-525.
THE EARTH'S CRUST AND THE SUN
808
these with the actual wandering of the pole from 1907
to 1913. While it is as yet too early to say that any
special movement of the pole was due to the specific
meteorological conditions of any particular year,
Jeffreys ' work makes it clear that meteorological causes,
especially atmospheric pressure, are sufficient to cause
the observed irregular wanderings. Slight wanderings
may arise from various other sources such as movements
of the rocks when geological faults occur or the rush of
a great wave due to a submarine earthquake. So far as
Uff»
Fig. 13. Wandering of the
pole from 1890 to 1898.
{After Moulton.)
known, however, all these other agencies cause insignifi-
cant displacements compared with those arising from
movements of the air. This fact coupled with the mathe-
matical certainty that meteorological phenomena must
produce some wandering of the pole, has caused most
astronomers to accept Jeffreys ' conclusion. If we follow
their example we are led to conclude that changes in
atmospheric pressure and in the other meteorological
conditions strike blows which sometimes shift the earth
804 CLIMATIC CHANGES
several feet from its normal position in respect to the
axis.
If the foregoing reasoning is correct, the great and
especially the sadden departures from the smooth
gyroscopic circle described by the pole in the Eulerian
motion would be expected to occur at about the same time
as unusual earthquake activity. This brings us to an
interesting inquiry carried out by Milne" and amplified
by Knott.^* Taking Albrecht's representation of the
irregular spiral-like motion of the pole, as given in Fig.
13, they show that there is a preponderance of severe
earthquakes at times when the direction of motion of the
earth in reference to its axis departs from the smooth
Eulerian curve. A summary of their results is given in
Table 9. The table indicates that during the period from
1892 to 1905 there were nine different times when the
curve of Fig. 13 changed its direction or was deflected by
less than 10° during a tenth of a year. In other words,
during those periods it did not curve as much as it ought
according to the Eulerian movement. At such times there
were 179 world-shaking earthquakes, or an average of
about 19.9 per tenth of a year. According to the other
lines of Table 9, in thirty-two cases the deflection during
a tenth of a year was between 10° and 25°, while in fifty-
six cases it was from 25° to 40°. During these periods
the curve remained close to the Eulerian path and the
world-shaking earthquakes averaged only 8.2 and 12.9.
Then, when the deflection was high, that is, when meteoro-
logical conditions threw the earth far out of its Eule-
rian course, the earthquakes were again numerous, the
number rising to 23.4 when the deflection amounted to
more than 55°.
1* John Milne : British Association Beports for 1903 and 1906.
14 G. G. Knott: The Physics of Earthquake Phenomena, Oxford, 1908.
THE EARTH'S CRUST AND THE SUN
805
TABLE 9
■
DEFLECTION OF PATH
OF POLE
COMPARED
WITH EARTHQUAKES
No. of
No. of
Average No.
Deflection Deflections
EarthquaJces
of Earthquakes
O-IO' 9
179
19.9
10-25* 32
263
8.2
25-40» 56
722
12.9
40-65 • 19
366
19.3
over 55* 7
164
23.4
In order to test this conclusion in another way we have
followed a suggestion of Professor Schlesinger. Under
his advice the Eulerian motion has been eliminated and
a new series of earthquake records has been compared
with the remaining motions of the poles which presum-
ably arise largely from meteorological causes. For this
purpose use has been made of the very full records of
earthquakes published under the auspices of the Liter-
national Seismological Commission for the years 1903
to 1908, the only years for which they are available.
These include every known shock of every description
which was either recorded by seismographs or by direct
observation in any part of the world. Each shock is given
the same weight, no matter what its violence or how
closely it follows another. The angle of deflection has
been measured as Milne measured it, but since the Eule-
rian motion is eliminated, our zero is approximately
the normal condition which would prevail if there were
no meteorological complications. Dividing the deflections
into six equal groups according to the size of the angle,
we get the result shown in Table 10.
806
CLIMATIC CHANGES
TABLE 10
EARTHQUAKES IN 1908-1908 COMPARED
WITH DEPARTURES OF THE PRO-
JECTED CURVE OF THE EARTH'S
AXIS FROM THE EULE-
RIAN POSITION
AvercLge angle of deflection
{10 periods of \{q year each)
— ICS**
11.5*
26.8 •
40.2 •
54.7'
90.3'
Average daily nwnher
of earthquakes
8.31
8.35
8.23
8.14
8.86
11.81
\
Here where some twenty thousand earthquakes are
employed the result agrees closely with that of Milne for
a di£Ferent series of years and for a much smaller number
of earthquakes. So long as the path of the pole departs
less than about 45^ from the smooth gyroscopic Eulerian
path, the number of earthquakes is almost constant^ about
eight and a quarter per day. When the angle becomes
large, however, the number increases by nearly 50 per
cent Thus the work of Milne, Knott, and Jeffreys is con-
firmed by a new investigation. Apparently earthquakes
and crustal movements are somehow related to sudden
changes in the load imposed on the earth's crust by
meteorological conditions.
This conclusion is quite as surprising to the authors
as to the reader — ^perhaps more so. At the beginning of
this investigation we had no faith whatever in any im-
THE EARTH'S CRUST AND THE SUN 807
portant relation between dimate and earthquakes. At its ^
end we are inclined to believe that the relation is close
and important.
It must not be supposed, however, that meteorological
conditions are the cause of earthquakes and of move-
ments of the earth 's crust. Even though the load that the
climatic agencies can impose upon the earth 's crust runs
into millions of tons per square mile, it is a trifle com-
pared with what the crust is able to support. There is,
however, a great difference between the cause and the
occasion of a phenomenon. Suppose that a thick sheet of
glass is placed under an increasing strain. If the strain
is applied slowly enough, even so rigid a material as glass
will ultimately bend rather than break. But suppose that
while the tension is high the glass is tapped. A gentle
tap may be followed by a tiny crack. A series of little
taps may be the signal for small cracks to spread in
every direction. A few slightly harder taps may cause
the whole sheet to break suddenly into many pieces. Yet
even the hardest tap may be the merest trifle compared
with the strong force which is keeping the glass in a state
of strain and which would ultimately bend it if given
time.
The earth as a whole appears to stand between steel
and glass in rigidity. It is a matter of common observa-
tion that rocks stand high in this respect and in the
consequent difficulty with which they can be bent without '
breaking. Because of the earth's contraction the crust
endures a constant strain, which must gradually become
enormous. This strain is increased by the fact that sedi-
ment is transferred from the lands to the borders of the
sea and there forms areas of thick accumulation. From
this has arisen the doctrine of isostasy, or of the equali-
zation of crustal pressure. An important illustration of
808 CLBIATIC CHANGES
this is the oceanward and equatorial creep which has
been described in Chapter XL There we saw that when
the lands have once been raised to high levels or when a
shortening of the earth's axis by contraction has in-
creased the oceanic bulge at the equator, or when the
reverse has happened because of tidal retardation, the
outer part of the earth appears to creep slowly back
toward a position of perfect isostatic adjustment. If the
sun had no influence upon the earth, either direct or
indirect, isostasy and other terrestrial processes might
flex the earth's crust so gradually that changes in the
form and height of the lands would always take place
slowly, even from the geological point of view. Thus
erosion would usually be able to remove the rocks as
rapidly as they were domed above the general level. If
this happened, mountains would be rare or unknown, and
hence climatic contrasts would be far less marked than is
actually the case on our earth where crustal movements
have repeatedly been rapid enough to produce mountains.
Nature's methods rarely allow so gradual an adjust-
ment to the forces of isostasy. While the crust is under a
1 strain, not only because of contraction, but because of
changes in its load through the transference of sediments
and the slow increase or decrease in the bulge at the
equator, the atmosphere more or less persistently carries
on the tapping process. The violence of that process
varies greatly, and the variations depend largely on the
severity of the climatic contrasts. If the main outlines of
the cyclonic hypothesis are reUable, one of the first effects
of a disturbance of the sim's atmosphere is increased
storminess upon the earth. This is accompanied by in-
creased intensity in almost every meteorological process.
The most important effect, however, so far as the earth's
crust is concerned would apparently be the rapid and
THE EARTH'S CRUST AND THE SUN 809
intense changes of atmospheric pressure which would
arise from the swift passage of one severe storm after
another. Each storm would be a little tap on the tensely
strained crust. Any single tap might be of little conse-
quence, even though it involved a change of a billion
tons in the pressure on an area no larger than the state
of Rhode Island. Yet a rapid and irregular succession of
such taps might possibly cause the crust to crack, and
finally to collapse in response to stresses arising from
the shrinkage of the earth.
Another and perhaps more important effect of varia-
tions in storminess and especially in the location of the
stormy areas would be an acceleration of erosion in some
places and a retardation elsewhere. A great increase in
rainfall may almost denude the slopes of soil, while a
diminution to the point where much of the vegetation
dies off has a similar effect. If such changes should take
place rapidly, great thicknesses of sediment might be
concentrated in certain areas in a short time, thus dis-
turbing the isostatic adjustment of the earth ^s crust. This
might set up a state of strain which would ultimately
have to be relieved, thus perhaps initiating profound
crustal movements. Changes in the load of the earth *s
crust due to erosion and the deposition of sediment, no
matter how rapid they may be from the geological stand-
point, are slow compared with those due to changes in
barometric pressure. A drop of an inch in barometric
pressure is equivalent to the removal of about five inches
of solid rock. Even under the most favorable circum-
stances, the removal of an average depth of five inches
of rock or its equivalent in soil over millions of square
miles would probably take several hundred years, while
the removal of a similar load of air might occur in half
a day or even a few hours. Thus the erosion and depo-
810 CLIMATIC CHANGES
sition due to climatic variations presumably play tiieir
part in crustal deformation chiefly by producing crustal
stresses, while the storms, as it were, strike sharp, sudden
blows.
Suppose now that a prolonged period of world-wide
mild climate, such as is described in Chapter X, should
permit an enormous accumulation of stresses due to con-
traction and tidal retardation. Suppose that then a
sudden change of climate should produce a rapid shifting
of the deep soil that had accumulated on the lands, with a
corresponding localization and increase in strains. Sup-
pose also that frequent and severe storms play their part,
whether great or small, by producing an intensive tapping
of the crust. In such a case the ultimate collapse would
be correspondingly great, as would be evident in the
succeeding geological epoch. The sea floor might sink
lower, the continents might be elevated, and mountain
ranges might be shoved up along lines of special weak-
ness. This is the story of the geological period as known
to historical geology. The force that causes such move-
ments would be the pull of gravity upon the crust sur-
rounding the earth's shrinking interior. Nevertheless
climatic changes might occasionally set the date when the
gravitative puU would finaUy overcome inertia, and thus
usher in the crustal movements that close old geologic
periods and inaugurate new ones. This, however, could
occur only if the crust were under sufficient strain. As
Lawson^' says in his discussion of the '' elastic rebound
theory," the sudden shifts of the crust which seem to be
the underlying cause of earthquakes ''can occur only
after the accumulation of strain to a limit and . . . this
accumulation involves a slow creep of the region affected.
IB A. 0. LawBon: The Mobility of the Coast Ranges of California; Uniy.
of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.
f
I
/
I
I
THE EARTH'S CRUST AND THE SUN 811
In the long periods between great earthquakes the energy
necessary for such shocks is being stored up in the rocks
as elastic compression."
If a period of intense storminess should occur when
the earth as a whole was in such a state of strain, the
sudden release of the strains might lead to terrestrial
changes which would alter the climate still further, mak-
ing it more extreme, and perhaps penmtting the stormi- i
ness due to the solar disturbances to bring about gla-
ciation. At the same time if volcanic activity should , , ^
increase it would add its quota to the tendency toward J f/ ^
glaciation. Nevertheless, it might easily happen that a
very considerable amount of crustal movement would
take place without causing a continental ice sheet or even
a marked alpine ice sheet. Or again, if the strains in the
earth ^s crust had already been largely released through
other agencies before the stormy period began, the cli-
mate might become severe enough to cause glaciation
in high latitudes without leading to any very marked
movements of the earth's crust, as apparently happened
in the Mid-Silurian period.
CONCLUSION
Here we must bring this study of the earth 's evolution
to a close. Its fundamental principle has been that the
present, if rightly understood, affords a full key to the
past. With this as a guide we have touched on many
hypotheses, some essential and some unessential to the
general line of thought. The first main hypothesis is that
the earth's present climatic variations are correlated
with changes in the solar atmosphere. This is the key-
note of the whole book. It is so well established, however,
812 CLIMATIC CHANGES
that it ranks as a theory rather than as an hypothesis.
Next comes the hypothesis that variations in the solar
atmosphere influence the earth 's climate chiefly by caus-
ing variations not only in temperature but also in
atmospheric pressure and thus in storminess, wind, and
. rainfall. This, too, is one of the essential foundations on
which the rest of the book is built, but though this
cyclonic hypothesis is still a matter of discussion, it
seems to be based on strong evidence. These two hypothe-
ses might lead us astray were they not balanced by
another. This other is that many climatic conditions are
due to purely terrestrial causes, such as the form and
altitude of the lands, the degree to which the continents
are united, the movement of ocean currents, the activity
of volcanoes, and the composition of the atmosphere and
the ocean. Only by combining the solar and the terrestrial
can the truth be perceived. Finally, the last main hypothe-
sis of this book holds that if the climatic conditions which
now prevail at times of solar activity were magnified
• sufficiently and if they occurred in conjunction with cer-
tain important terrestrial conditions of which there is
/ good evidence, they would produce most of the notable
\ phenomena of glacial periods. For example, they would
/' explain such puzzling conditions as the localization and
periodicity of glaciation, the formation of loess, and the
\ occurrence of glaciation in low latitudes during Permian
' and Proterozoic times. The converse of this is that if
the conditions which now prevail at times when the sun
is relatively inactive should be intensified, that is, if
the sun's atmosphere should become calmer than now,
and if the proper terrestrial conditions of topographic
form and atmospheric composition should prevail, there
would arise the mild climatic conditions which appear
to have prevailed during the greater part of geological
/
I
THE EARTH'S CRUST AND THE SUN 818
time. In short, there seems thus far to be no phase of
the climate of the past which is not in harmony with an
hypothesis which combines into a single unit the three
main hypotheses of this book, solar, cyclonic, and terres-
trial.
Outside the main line of thought lie several other
hypotheses. Several of these, as well as some of the main
hypotheses, are discussed chiefly in Earth and Sun, but
as they are given a practical application in this book
they deserve a place in this final sunmaary. Each of these
secondary hypotheses is in its way important. Yet any or
all may prove untrue without altering our main conclu-
sions. This point cannot be too strongly emphasized, for
there is always danger that differences of opinion as to
minor hypotheses and even as to details may divert at-
tention from the main point. Among the non-essential
hypotheses is the idea that the sun's atmosphere influ-
ences that of the earth electrically as well as thermally.
This idea is still so new that it has only just entered the
stage of active discussion, and naturally the weight of
opinion is against it. Although not necessary to the main
purpose of this book, it plays a minor role in the chapter
dealing with the relation of the sun to other astronomical
bodies. It also has a vital bearing on the further advance
of the science of meteorology and the art of weather
forecasting. Another secondary hypothesis holds that
sunspots.are set in motion by the planets. Whether the
effect is gravitational or more probably electrical, or
perhaps of some other sort, does not concern us at pres-
ent, although the weight of evidence seems to point
toward electronic emissions. This question, like that of
the relative parts played by heat and electricity in terres-
trial climatic changes, can be set aside for the moment.
What does concern us is a third hypothesis, namely, that
814 CLIMATIC CHANGES
if the planets really determine the periodicity of sun-
spotSy even though not supplying the energy, the sun in
its flight through space must have been repeatedly and
more strongly influenced in the same way by many other
heavenly bodies. In that case, climatic changes like those
of the present, but sometimes greatly magnified, have pre-
sumably arisen because of the constantly changing posi-
tion of the solar system in respect to other parts of the
universe. Finally, the fourth of our secondary hypotheses
postulates that at present the date of movements of the
earth ^s crust is often determined by the fact that storms
and other meteorological conditions keep changing the
load upon first one part of the earth's surface and then
upon another. Thus stresses that have accumulated in the
earth's isostatic shell during the preceding months are
released. In somewhat the same way epochs of extreme
storminess and rapid erosion in the past may possibly
have set the date for great movements of the earth's
crust. This hypothesis, like the other three in our secon-
dary or non-essential group, is still so new that only the
first steps have been taken in testing it. Yet it seems to
deserve careful study.
In testing all the hypotheses here discussed, primary
and secondary alike, the first necessity is a far greater
amount of quantitative work. In this book there has been
a constant attempt to subject every hypothesis to the test
of statistical facts of observation. Nevertheless, we have
been breaking so much new ground that in many cases
exact facts are not yet available, while in others they
can be properly investigated only by specialists in
physics, astronomy, or mathematics. In most cases the
next great step is to ascertain whether the forces here
called upon are actually great enough to produce the
observed results. Even though they act only as a means
THE EARTH'S CRUST AND THE SUN 816
of releasing the far greater forces due to the contraction
of the earth and the sun, they need to be rigidly tested
as to their ability to play even this minor role. Still
another line of study that cries aloud for research is a
fuller comparison between earthquakes on the one hand
and meteorological conditions and the wandering of the
poles on the other. Finally, an extremely interesting and
hopeful quest is the determination of the positions and
movements of additional stars and other celestial bodies,
the faint and invisible as well as the bright, in order to
ascertain the probable magnitude of their influence upon
the sun and thus upon the earth at various times in the
past and in the future. Perhaps we are even now ap-
proaching some star that will some day give rise to a
period of climatic stress like that of the fourteenth cen-
tury, or possibly to a glacial epoch. Or perhaps the varia-
tions in others of the nearer stars as well as Alpha
Oentauri may show a close relation to changes in the sim.
Throughout this volume we have endeavored to dis-
cover new truth concerning the physical environment
that has molded the evolution of all life. We have seen
how delicate is the balance among the forces of nature,
even though they be of the most stupendous magnitude.
We have seen that a disturbance of this balance in one
of the heavenly bodies may lead to profound changes in
another far away. Yet during the billion years, more or
less, of which we have knowledge, there appears never
to have been a complete cataclysm involving the destruc-
tion of all life. One star after another, if our hypothesis
is correct, has approached the solar system closely
enough to set the atmosphere of the sun in such commo-
tion that great changes of climate have occurred upon
the earth. Yet never has the solar system passed so dose
to any other body or changed in any other way suffi-
\
816 CLIMATIC CHANGES
ciently to blot out all living things. The effect of climatic
changes has always been to alter the environment and
therefore to destroy part of the life of a given time, but
with this there has invariably gone a stimulus to other
organic types. New adaptations have occurred, new lines
of evolutionary progress have been initiated, and the net
result has been greater organic diversity and richness.
Temporarily a great change of climate may seem to
retard evolution, but only for a moment as the geologist
counts time. Then it becomes evident that the march of
progress has actually been more rapid than usual. Thus
the main periods of climatic stress are the most conspicu-
ous milestones upon the upward path toward more varied
adaptation. The end of each such period of stress has
found the life of the world nearer to the high mentality
which reaches out to the utmost limits of space, of time,
and of thought in the search for some explanation of the
meaning of the universe. Each approach of the sun to
other bodies, if such be the cause of the major climatic
changes, has brought the organic world one step nearer
to the solution of the greatest of all problems, — ^the prob-
lem of whether there is a psychic goal beyond the mental
goal toward which we are moving with ever accelerating
speed. Throughout the vast eons of geological time the
adjustment of force to force, of one body of matter to
another, and of the physical environment to the organic
response has been so delicate, and has tended so steadily
toward the one main line of mental progress that there
seems to be a purpose in it all. If the cosmic uniformity
of climate continues to prevail and if the uniformity is
varied by changes as stimulating as those of the past, the
imagination can scarcely picture the wonders of the
future. In the course of millions or even billions of years
the development of mind, and perhaps of soul, many excel
. THE EARTH'S CRUST AND THE SUN 817
that of today as far as the highest known type of men-
tality excels the primitive plasma from which all life
appears to have arisen.
INDEX
* Indicates illustratioiiB.
Abbot, G. G., cited, 45, 52, 237, 238,
239.
Abosknn, 104.
Africa, earthquakes, 301; East, see
East Africa; lakes, 143; North,
see North Africa.
African glaciation, 266.
Air, see Atmosphere.
Alaska, glacial till in, 287; Ice Age
in, 221.
Albrecht, cited, 304.
Alexander, march of, 88 f .
Allard, H. A., cited, 183, 184.
Alpha Centauri, companion of, 280;
distance from sun, 262; lumi-
nosity, 278; speed of, 281; varia-
tions, 282.
Alps, loess in, 159; precipitation in,
141; snow level in, 139.
Altair, companion of, 280; lumi-
nosity, 278; speed of, 281.
Amazon forest, temperature, 17.
Ancylus lake, 217.
Andes, snow line, 139.
Animals, climate and, 1.
Antarctica, mild climate, 219; thick-
ness of ice in, 125; winds, 135,
161.
Anti-cyclonic hypothesis, 135 ff.
Appalachians, effect on ice sheet,
121.
Arabia, civilization in, 67.
Aral, Sea of, 108.
Arehean rocks, 211.
Archeozoic, 3 f.; climate of, 267.
Arctic Ocean, submergence, 219.
Arctowski, H., cited, 29, 46, 244.
Argon, increase of, 236.
Arizona, rainfall, 89, 108; trees
measured in, 73.
Arrhenius, 8., cited, 36, 254.
Arsis, of pulsation, 24.
Asbjom Selsbane, corn of, 101.
Asia, atmospheric pressure, 298 ; cen-
tral, changes of climate, * 75 ; cen-
tral, post-glacial climate, 271;
climate, 66; glaciation in, 131;
storminess in, 60; western, cli-
mate in, 84 f.
Atlantic Ocean, storminess, 57.
Atmosphere, changes, 19 f., 229;
composition of, 223-241; effect
on temperature, 231.
Atmospheric circulation, glaciation
and, 42.
Atmospheric electricity, solar rela-
tions of, 56.
Atmospheric pressure, earthquakes
and, 298; evaporation and, 237;
increase in, 239; redistribution of,
49; variation, 53.
Australia, East, mild climate, 219;
precipitation, 144.
Axis, earth's, 48; wabbling of, 301.
Bacon, Sir Francis, cited, 27.
Bacubirito, meteor at, 246.
Baltic Sea, as lake, 217; freezing
of, 100; ice, 26; storm-floods, 99;
submergence, 219.
Bardsson, Ivar, 106.
Barkow, cited, 135.
Barometric pressure, solar relations
of, 56.
BarreU, J., cited, 3, 200, 213, 234.
BartoU, A. G., cited, 257.
820
INDEX
Bauer, L. A., cited, 150.
Beaches, nnder water, 97.
BeadneU, H. J. L., cited, 143.
Belachistan, rainfall, 89.
Bengal, Bay of, cyclones in, 149.
Bengal, famine in, 104 f .
Berlin, rainfall and temperature, 93.
Betelgeuse, 259 f.; distance from
sun, 262.
Bible, climatic evidence in, 91 f . ;
palms in, 92.
Binary stars, 252.
Birkeland, K., cited, 244.
Black Earth region, loess in, 159.
Boca, Cal., correlation coefficients,
83, 85.
Boltzmann, L., cited, 257.
Bonneville, Lake, 142, 143.
Borkum, storm-flood in, 99.
Boss, L. cited, 268, 269.
Botanical evidence of mild clinuites,
167 ff.
Boulders, on Irish coast, 119.
Bowie, W., cited, 293.
Bowman, I., cited, 213.
Britain, forests, 220; level of land,
220.
British Isles, height of land, 111;
temperature, 216.
Brooks, C. E. P., cited, 115, 143, 196,
215, 225.
Brooks, C. F., cited, 209.
Brown, E. W., cited, 191, 244.
Bruckner, E., cited, 27.
Bruckner periods, 27 f .
Bufo, habiUt of, 202.
Buhl stage, 216.
BuU, Dr., cited, 100, 101.
Butler, H. C, cited, 66, 67 ff., 70,
76.
California, changes of climate, * 75 ;
correlations of rainfall, 86; meas-
urements of sequoias in, 73, 74 ff.;
rainfall, 108.
Cambrian period, 4f.
Canada, storminees, 53 f., 57; storm
tracks in, 113.
Cape Farewell, shore ice at, 105.
Carbon dioxide, erosion and, 119 f.;
from volcanoes, 23; hypothesis,
139; importance of, 9, 11 f.; in
Permian, 148; in atmosphere, 20,
96, 238; in ocean, 226; nebular
hypothesis and, 232; theory of
glaciation, 36 ff.
Caribbean mountains, origin of, 193.
Carnegie Institution of Washington,
74.
Caspian Sea, climatic stress, 104;
rainfall, 107 f.; rise and fall, 27;
ruins in, 71.
Cenozoic, 'climate, 266; fossils, 21.
Central America, Maya ruins, 95.
Chad, Lake, swamps of, 171.
Chamberlin^ B. T., cited, 166, 233,
269.
Chamberlin, T. C, cited, 19, 36, 38,
39, 42 f., 48, 122, 125, 152, 156,
190, 195, 227, 269.
Chandler, 8. C, cited, 301.
Chinese earthquakes, periodicity of,
245.
Chinese, sunspot observations, 108 f.
Chinese Turkestan, desiccation in,
66.
Chronology, glacial, 215.
aarke, F. W., cited, 226, 235.
aayton, H. H., cited, 173 f .
Climate, effect of contraction,
189 ff.; effect of salinity, 224; in
history, 64-97; uniformity, 1-15;
variability, 16-32.
Climates, mild, causes of, 166-187;
mild, periods of, 274.
Climatic changes, and crustal move-
ments, 285 ff . ; hypotheses of, 33-
50 ; mountain-building and, * 25 ;
post-glacial crustal movements
and, 215-222; terrestrial causes
of, 188-214.
Climatic sequence, 16 f.
Climatic stages, post-glacial, 270.
Climatic stress, in fourteenth cen-
tury, 98-109.
Climatic uniformity, hypothesis of,
65, 71 f .
Climatic zoning, 169.
^
I
INDEX
821
i
Gloudinefls, glaeiation and, 114, 147.
Gloadfl, as protection, 197.
Colfax, CaL, eorrelation coefficients,
83.
Cologne, flood at, 99.
Compass, variations, 150.
Continental climate, yariations, 103.
Continents, effect on climate, lllf.
Contraction, effect on climate, 189 ff.,
199, 207; effect on landjB, 207;
heat of sun and, 13 f.; irregular,
195; of the earth, 18; of the sun,
249; stresses caused by, 310.
Convection, carbon dioxide, and, 239.
Corals, in high latitudes, 21, 39, 167,
178.
Cordeiro, F. J. B., cited, 181, 183,
186.
Correlation coefficients, earthquakes
and snnspots, 291 ; Jerusalem rain-
fall and sequoia growth, 83 ff.;
rainfall and tree growth, 79 ff.
Cosmos, effect of light, 185.
Cressey, G. B., cited, 80.
Cretaceous, lava, 211; mountain
ranges, 44; paleogeography, * 201;
submergence of North America,
200.
CroU, J., cited, 34 ff., 176.
CroU's hypothesis, snow line, 139.
Crust, climate and movements of,
63, 287, 310; movements of, 43;
strains in, 22.
Currents and planetary winds, 174.
Qycads, 169.
Cyclonic hypothesis, 97; loess and,
163; Permian glaeiation and, 148;
snow line, 139.
Cyclonic storms, in glacial epochs,
140 f.; solar electricity and, 243
(see Storms, Storminess).
Cyclonic vacillations, 30 f.; nature
of, 57 ff. .
Daily vibrations, 28 f.
Danube, frozen, 98.
Darwin, G. H., cited, 191.
Daun stage, 217.
Davis, W. M., cited, 271.
Davisson, C, cited, 294, 295, 299.
Day, C. P., cited, 239.
Day, length of, 18, 191.
Dead Sea, palms near, 92.
Death Valley, 142.
De Ballore, M., cited, 297, 298.
Deep-sea circulation, rapidity, 227;
salinity and, 176; solar activity
and, 179.
De Geer, S., cited, 215, 221.
De Lapparent, A., cited, 200.
Denmark, fossils, 271.
"Desert pavements," 161.
Deserts, abundant flora of, 171; and
pulsations theory, 88 ff . ; red beds
of, 170.
Devonian, climate, 266; mountains,
209.
Dog, climate and, 1.
Donegal County, Ireland, 220.
Double stars, 272, 280; electrical
effect of, 261.
Douglass, A. E., cited, 28, 73, 74 f .,
84, 85, 107.
Dragon Town, destruction of, 104,
108.
Drake, N. F., cited, 297, 298.
Droughts, and pulsations theory,
87 f.; in England, 102; in India,
104 f.
Drumkelin Bog, Ireland, log cabin
in, 220.
Dust, at high levels, 240.
Earth, crust of and the sun, 285-317;
internal heat, 212; nature of mild
climate, 274 ; position of axis, 181 ;
rigidity of, 307; temperature
gradient, 213; temperature of sur-
face, 8.
Earthquakes, and seasons, 294, 297;
and Bunspots, 288 f.; and tropical
hurricanes, 300; and wandering
of pole, 304 f.; cause of, 307;
compared with departures from
Eulerian position, 306; seasonal
distribution of, 299; seasonal
march, 295.
<' Earthquake weather," 298.
822
INDEX
East Africa^ mild climate, 219.
East Indies, earthquakes of, 296.
Eberswalde, tree growth at, 102 f .
Ecliptic, obliquity of, 217.
Electrical currents, in solar atmos-
phere, 261.
Electrical emissions, variation of,
275.
Electrical hypothesis, 150, 250 f.,
256 ff.
Electrical phenomena, storminess
and, 56.
Electricity, and earthquakes, 292;
solar, 243.
Electro-magnetic hypothesis, 244.
Electrons, solar, 56; variation of,
256.
Electro-stellar hypothesis, 274.
Elevation, climatic changes and, 39.
Engedi, palms in, 92.
England, climatic stress, 101 f.;
storminess and rainfall, 107.
Eocene, climate, 266.
Equinoxes, precession of, 96.
Erosion, storminess and, 309.
Eskimo, in Greenland, 106.
Eulerian movement, 301, 304.
Euphrates, 67.
Europe, climatic stress, 98 if., 102 f . ;
climatic table, 215; glaci&tion in,
131; ice sheet, 121; inundations
of rivers, 99; post-glacial climate,
271; rainfall, 107; submergence,
196, 200.
Evaporation, and glaciation, 112,
114; atmospheric pressure and,
237; from plants, 179; impor-
tance, 129; in trade- wind belt,
117; rapidity of, 224.
Evening primrose, effect of light,
184.
Evolution, climate and, 20; geo-
graphical complexity and, 241;
glaciation and, 33; of the earth,
311.
Faculee, cause of, 61.
False Point Lighthouse, barometric
pressure at, 299.
Famine, cause of, 103; in England,
101 f . ; in India, 104 f . ; pulsations
theory and, 87 f .
Faunas, and mild climates, 168 f.;
in Permian, 152 f .
Fennoscandian pause, 216.
Flowering, light and, 184.
Fog, and glaciation, 116; as pro-
tection, 197; temperature and,
178.
Forests, climate and, 66.
Form of the land, 43 ff .
Fossil floras, and mild climates, 168 ;
in Antarctica, 273; in Greenland,
273.
Fossils, 169, 230; and loess, 158;
Archeozoic, 3f.; Cenosoic, 21;
dating of, 153; glaeiation and,
138; in peat bogs, 271; mild cli-
mate, 167 ; Proterozoic, 4, 6 f .
Fourteenth century, climatic stress
in, 98-109.
Fowls, F. £., cited, 45, 237, 238,
239.
Freeh, F., cited, 36.
Free, E. E., cited, 142.
Freezing, salinity and, 224.
Fresno, rainfaU record, 82.
''Friction variables," 247.
Frisian Islands, storm-flood, 99.
Fritz, H., cited, 109.
Frogs, distribution of, 202.
Fuchs, cited, 289.
Galaxy, 252.
Galveston, Tex., rainfall and tem-
perature, 94.
Gamer, W. W., cited, 1S3, 184.
Gases, in air, 233.
Geographers, and climatic changes,
65 ff.
Geological time table, * 5.
Geologic oscillations, 18 f., 21 ff.,
188, 240.
Geologists, changes in ideas of, 64 f •
Germanic myths, 219.
Germany, forests, 220; growth of
trees in, 102; storms in, 102.
Gilbert, G. K., cited, 143.
INDEX
828
Glacial epochs, causes of, 268; dates
of, 216; intervals between, 264 f.;
length of, 166 f.
Glacial fluctuations, 24 if.; nature
of, 57 ff.
Glacial period, at present, 272; ice
in, 57 f.; length of, 269; list, 265;
temperature, 38.
Glaciation, and loess, 155 f.; and
movement of crust^ 287; condi-
tions favorable for. 111; extent
of, 124; hypotheses of, 33 ff.; in
southern Canada, 18; localization
of, 130 ff. ; Permian, * 145 ; solar-
' cyclonic hypothesis of, 110-129;
suddenness of, 138; upper limit
of, 141.
Goldthwait, J. W., cited, 271.
Gondwana land, 21, 204.
Gravitation, effect on sun, 250; pull
of, 244.
Great Basin, in glacial period, 126;
salt lakes in, 142.
Great Ice Age, see. Pleistocene.
Great Plains, effect on ice sheet, 120.
Greenland, climatic stress, 105 ff.;
ice, 26; rainfall, 108; storminess,
57; submergence, 219; vegetation,
21, 37, 287; winds, 135, 161.
Gregory, J. W., cited, 90 ff ., 97.
Gschnitz stage, 216.
Guatemala, ruins in, 95.
Guervain, cited, 135.
Gyroscope, earth as, 181.
Hale, G. E., cited, 56, 62.
Hamdulla, ci^^ 104.
Hann, J., cited, 66.
Hansa Union, operations of, 100.
Harmer, P. W., cited, 115, 119.
Heat, and earthquakes, 292; earth's
internal, 18.
Hedin, 8., cited, 88.
Heim, A., cited, 190.
Heligoland, flood in, 99.
Helland-Hansen, B., cited, 174.
Helmert^ P. B., cited, 302.
Henderson, L. J., cited, 9, 10, 11,
12.
Henry, A. J., cited, 94, 208.
Hercynian Mountains, 45.
High pressure and glaciation, 115,
135.
Himalayas, glaciation, 144; origin
of, 193; snow line, 139.
Himley, cited, 104.
Historic pulsations, 24 f.; nature of,
57 ff.
History, climate of, 64-97; climatic
pulsations and, 26.
Hobbs, W. H., cited, 115, 125, 135,
161.
Hot springs, temperature of, 6.
Humphreys, W. J., cited, 2, 37 f.,
45, 46, 50, 56, 238.
Hurricanes, in arid regions, 144;
sunspots and, 53.
Hyades, cluster in, 268.
Ice, accumulations, 57 f.; advances
of, 122; distribution of, 131;
drift, 105.
Ice sheets, disappearance, 128;
limits, 120; localization, 130 ff.;
rate of retreat, 165; thickness,
125.
Iceland, submergence, 219.
lowan ice sheet, rapid retreat, 165.
lowan loess, 158.
India, drought, 104 f.; famine,
104 f.; rainfaU, 108.
Indian glaciation, 266.
Inter-glacial epoch, Permian, 153.
Internal heat of earth, 212.
Ireland, Drumkelin Bog, 220; in
glacial period, 119; level of land,
220; storminess and rainfall,
107; submergence, 219.
Irish Sea, tides, 191.
Irrigation ditches, abandoned, 97.
Isostasy, 307 ff.
Italy, southern, climate of, 86 f .
Japan, earthquakes of, 296.
Javanese mountains, origin of, 193.
Jazartes, 108.
Jeans, J. H., cited, 251, 252, 253,
266, 272.
824
INDEX
Jeffreys, H., eited, 302, 303, 306.
JeffreTB, J., cited, 191.
Jericho, palms in, 92.
Jerusalem, rainfall, 86; rainfall and
temperature, 94; rainfall in, and
sequoia growth, 83 ff.
Johnson, cited, 226.
Judea, palms in, 92.
Jupiter, and sunspots, 243; effect of,
253; periodicity of, 61 f.; tem-
perature of, 258; tidal effect of,
250.
Jurassic, climate, 266; mountain
ranges, 44.
Kansas, variations of seasons, 103.
Kara Koshun marsh. Lop Nor, 104.
Keewatin center, 113; evaporation
in, 129.
Keewatin ice sheet, 121.
Kelvin, Lord, cited, 13 f.
Kejes, G. B., cited, 156.
Kirk, E., cited, 287.
Knott, C. G., cited, 294, 295, 297,
299. 304, 306.
Knowlton, F. H., cited, 167, 169,
170, 212, 232.
KSppen, W., 47, 52, 140.
Krakatoa, glaciation and, 48; vol-
canic hypothesis and, 45.
Krummel, O., cited, 224, 228.
Kullmer, G. J., cited, 113, 115, 128;
map of storminess, * 54.
Kungaspegel, sea routes described,
106.
Labor, price in England, 102.
Labradorean center of glaciation,
113.
Lahontan, Lake, 142.
Lake strands, see Strands.
Lake Superior, lava, 211.
Lakes, during glacial periods,
141 f. ; in semi-arid regions, 60 ;
of Great Basin, 126; ruins in, 97.
Land, and water, climatic effect of,
196 ff.; distribution of, 200;
form of, 43 ff . ; range of tem-
perature and, 196.
Lavas, climatic effect of, 211.
Lawson, A. G., cited, 310.
Lebanon, cedars of, 83.
Leiter, H., cited, 71.
Leverett, F., cited, 271.
Life, atmosphere and, 229 f . ; chemi-
cal characteristic of, 12; effect of
salinity, 225; of glacial period,
127; persistence of forms, 230.
Light, effect of atmosphere on, 236;
effect on plants, 184 ff.; ultra-
violet, storminess and, 56; varia-
tion of, 275.
Litorina sea, 218.
Loess, date of, 156 ff.; origin of,
155, 165.
Lop Nor, rise of, 104; swamps, 171.
Lows, and glacial lobes, 122; move-
ments of, 126 if eee Storms and
Gyclones.
Lulan, 104.
Lull, B. S., cited, 5, 188.
MacDougal, D. T., cited, 171.
McGee, W. J., cited, 156.
Macmillan, W. D., cited, 191.
Magdalenian period, 216.
Magnetic fields of sunspots, 56.
Magnetic poles, relation to storm
tracks, 150.
Makran, climate, 89; rainfall, 89.
Malay Archipelago, earthquakes of,
296.
Mallet, B., cited, 288.
Malta, rainfall, 86.
Manson, M., cited, 147.
Mayas, civilization, 26; ruins, 95.
Mayence, flood at, 99.
Mazelle, E., cited, 224.
Mediterranean, climate of, 72; rain-
fall records, 86; storminess in, 60.
Mercury, and sunspots, 243.
Mesozoic, climate, 266; crustal
changes, 286; emergence of lands,
287.
Messier, 8; variables, 248.
Metcalf , M. M., cited, 202.
Meteorological factors and earth-
quakes, 300 f.
INDEX
826
Meteorological hypothesis of cmstal
movements, 294.
Meteors, and sun's heat, 13, 246.
Michelson, A. A., cited, 259.
Middle Silurian, fauna in Alaska,
287.
Mild climates, see Climates, mild.
Milky Way, 252.
Mill, H. B., cited, 228.
Milne, J., cited, 288, 290, 294, 304,
306.
Miocene, crustal changes, 287.
Mississippi Basin, loess in, 159.
Mogul emperor, and famine, 104.
Monsoons, character of, 146; direc-
tion of, 208; Indian famines and,
105.
Moulton, F. B., cited, 13, 258, 269.
Mountain building, climatic changes
and, * 25.
Mountains, folding of, 190; rainfall,
on, 208.
Multiple stars, 252.
Nansen, F., cited, 122, 174.
Naplee, rainfall, 86.
Nathorst, cited, 169.
NebulsB, 247.
Nebular hypothesis, 232, 267.
Neolithic period, 218.
Nevada, correlations of rainfall, 86.
New England, height of land. 111.
New Mexico, rainfall, 89.
New Orleans, La., rainfall and tem-
perature, 94.
New Zealand, climate, 177; tree
ferns, 179.
Newcomb, S., cited, 52.
Nile floods,' periodicity in, 245.
Nitrogen, in atmosphere, 19.
Niya, Chinese Turkestan, desiccation
at, 66.
Nocturnal cooling, changes in, 238 f .
Norlind, A., cited, 100.
Norsemen, route to Greenland, 26.
Norse sagas, 219.
North Africa, climate of, 71; Bo-
man aqueducts in, 71.
North America, at maximum glacia-
tion, 122 ff. ; emergence of lands,
193 ; glaciation in, 131 ; height of
land. 111; interior sea in, 200;
inundations, 196; loess in, 155;
submergence of lands, 19, 21.
North Atlantic Ocean, salinity, 228.
North Sea, climatic stress, 98 ff.;
floods around, 26, 99; rainfall,
107; storminess, 57.
Northern hemisphere, earthquakes
of, 294.
Norway, decay, 100; temperature,
177.
Nov®, 247.
Oceanic circulation, carbon dioxide
and, 39 ff.
Oceanic climate, characteristics, 103.
Oceanic currents, diversion, 44; in-
fluence of land distribution, 203.
Oceans, age of, 223; composition of,
223-241; deepening of, 199; sa-
linity, 19, 223; temperature, 6, 152,
180, 226.
Okada, T., cited, 224.
Old Testament, temperature, 92.
Orbital precessions, 27.
Ordovician, climate, 266.
Organic evolution, glacial fluctua-
tions and, 26.
Orion, nebulosity near, 247; stars
near, 248.
Orontes, 67.
Osbom, H. F., cited, 216.
Owens-Searles, lakes, 142.
Oxus, 108.
Oxygen, in atmosphere, 20, 234; in
Permian, 152.
Ozone, cause of, 56.
Paleolithic, 216.
Paleozoic, climate, 266; mountains
in, 209.
Palestine, change of climate, 91 f .
Palms, climatic change and, 91 f.;
in Ireland, 179.
Palmyra, ruins of, 66.
Parallaxes of stars, 276 f.
Patrician center, 134.
826
INDEX
Peat-bog period, first, 218.
Penek, A., cited, 139, 156, 157, 158,
269.
Pennsjlyanian, life of, 26.
Periodicities, 245 f .
Periodicity, of climatic phenomena,
60 f.; of glaciation, 268; of snn-
spots, 243.
Permian, climate, 266; distribution
of glaciation, 152; glaciation, 60,
144, *145, 226; glaeiation and
mountains, 45; life of, 26; red
beds, 151; temperature, 146 f.
Peny, cited, 289.
Persia, lakes, 143; rainfaU, 89.
Pettersson, O., cited, 98 ff., 100 f.,
103, 106, 219.
Pirsson, L. V., cited, 3, 196.
Planetary hypothesis, 253, 267.
Planetary nebulae, 252.
Planets, and sunspots, 243; effect
of star on, 255; sunspot cycle
and, 62 ; temperatures, 8 f •
Plants, climate and, If.; effect of
light, 184 ff.
Pleion, defined, 29.
Pleionian migrations, 29 f.
Pleistocene, climate, 266; duration
of, 48; glaciation, 110 ff.; ice
sheets, *^123.
Pluvial climate, causes of, 143;
during glacial periods, 141.
Po, frozen, 98.
Polaris, 272.
Polar wandering, hypothesis of, 48 f .
Pole and earthquakes, 305.
Post-glacial crustal movements and
climatic changes, 215-222.
Poynting, J. H., cited, 8.
Processional hypothesis, 34 f.
Precipitation, and glaciation, 114,
133; during glacial period, 118;
snow line and, 139; temperature
and, 94.
Procyon, companion of, 280; lumi-
nosity, 278; speed of, 281.
Progressive change, 241.
Progressive desiccation, hypothesis
of, 65 ff.
Proterozoic, 4f.; fossils, 6f.; f^-
ciation, 18, 144, 226, 266; lava,
211; mountains in, 209; oceanic
salinity, 42 f.; oxygen in air, 234;
red beds, 151 ; temperature, 146 f .
Pulsations, hypothesis of, 65, 72 ff.
Pulsatory climatic changes, 72 ff.
Pulsatory hypothesis, 272.
Pumpelly, B., cited, 271.
Radiation, variation of, 275.
Radioactivity, heat of sun and, 14 f .
Rainfall, changes in, 93 f.; glacia-
tion and, 50; sunspots and, 53,
* 58, 59; tree growth and, 79.
Red beds, 151, 170.
Rhine, flood, 99; frozen, 98.
Rho Ophiuchi, variables, 248.
<<Rice grains," 61.
Richardson, O. W., cited, 256.
Rigidity, of earth, 307.
Roads, climate and, 66.
Rogers, Thorwald, cited, 101. .
Romans, aqueduct of, 71.
Rome, history of, 87.
Rotation, of earth, 18 f .
Ruden, storm-flood, 99.
Rugen, storm-flood, 99.
Ruins, as climatic evidence, 66 ; rain-
fall and, 60.
Sacramento, correlation coefibients,
82 f., 85; rainfaU, 86; rainfaU
record, 79.
Sagas, cited, 105 f.
St John, G. E., cited, 236.
Salinity, deeprsea circulation and,
176; effect on climate, 224; in
North Atlantic, 228; ocean tem-
perature and, 226; of ocean, 19,
120.
Salisbury, R. D., cited, 111, 125, 129,
139, 156, 206, 269, 271.
Salt, in ocean, 223.
San Bernardino, correlation of rain-
faU, 85.
Saturn, and sunspots, 243; sunspot
cycle and, 62.
INDEX
827
Sajles, B. W., eited, 183.
Sffandinavia, climatic stresB, 100 f.;
fomilB, 271; post-glacial climate,
271; rainfally 107; storminesSy 57,
107; temperature, 216.
Scandinavian center of glaciation,
113.
ScUesinger, F., cited, 275, 278, 298,
301, 305.
Schuchert, 0., cited, 3, 5, 23, * 25,
•123, 138, '145, 168, 169, 172,
188, 193, 196, 198, 200, * 201, 206,
211, 230, 265.
Schuster, A., cited, 61, 244, 294, 296.
Sculpture, Maya, 96.
Sea level and glaeiation, 119.
Seasonal alternations, 28 f .
Seasonal banding, 183 f .
Seasonal changes, geological, 183.
Seasons, and earthquakes, 294, 295,
297, 299; evidences of, 169.
Secular progression, 17 ff., 188.
Seistan, swamps, 171.
Sequoias, measurements of, 74 ff.;
rainfall record, 79.
SetcheD, W. A., cited, 1.
Shackleton, E., cited, 125.
Shaplej, H., cited, 246, 247, 254,
256, 275.
Shimek, E., cited, 157, 161.
Shreveport, La., rainfall and tem-
perature, 93 f.
Shrinkage of the earth, 190.
Siberia, and glaeiation, 132.
Sierras, rainfall records, 82.
Simpson, G. 0., cited, 222.
Sirius, companion of, 280; distance
from sun, 262; luminositj, 278;
speed of, 281.
Slichter, C. S., cited, 192.
Smith, J. W., cited, 73.
Snowfall, glaeiation and, 50, 114.
Snowileld, climatic effects of, 115.
Snow line, hei^t of, 138; in Andes,
139; in Himalajras, 139.
Solar activity, cycles of, 245; deep-
sea circulation and, 179; ice and,
134.
Solar constant, 114.
Solar-cyclonic hypothesis, 51-63,
287; glaeiation and, 110-129.
Solar prominences, cause of, 61.
Solar system, 252; conservation of,
243; proximity to stars, 63.
Solar variations, storms and, 31.
South America, earthquakes, 301.
South Pole, thickness of ice at, 125.
Southern hemisphere, earthquakes,
296; glaeiation in, 131 f.
Southern Pacific railroad, rainfall
records along, 82.
Soy beans, effect of light, 185 f .
Space, sun's journey through, 264-
284.
Spiral nebuls, 251 f . ; universe of,
267.
Spitzbergen, submergence, 219.
Springs, climate and, 66.
Stars, approach to sun, 253; binary,
252; clusters, 252, 268; effect on
solar atmosphere, 63; dark,- 254;
parallaxes of, 276 f.; tidal action
of, 249.
Stefan's Law, 257.
Stein, M. A., cited, 78.
Stellar approaches, probability of,
260.
Storm belt in arid regions, 144.
Storm-floods, in fourteenth century,
99.
Storminess, and erosion, 309; and
ice, 134; effect on glaeiation, 112;
sunspots and, 163; temperature
and, 94, 173.
Storms, blows of, 300, 302; inerease^
60; movement of, 125 f.; move-
ment of water and, * 175; origin
of, 30 f.; sunspots and, 28, 53;
see Cyclones and Lows.
Storm tracks, during glacial period,
117; location, 113; relation to
magnetic poles, 150; shifting of,
119.
StrandiB, climate and, 66; in semi-
arid regions, 60; of salt lakes, 142.
Suess, E., cited, 192.
Sun, and the earth's crust, 285-317;
approach to star, 253; atmosphere
828
INDEX
of, 61, 274; atmosphere of, and
weather, 52; cooling of, 49; con-
traetion of, 249; disturbances of,
172; effect of other bodies on, 242-
263; heat, 13; journey through
space, 264-284; Knowlton's hy-
pothesis of, 168.
Suncracks, 232.
Sunspot cycles, 27 f.
Sunspots, and earthquakes, 289;
causes of, 61; magnetic field of,
261; maximum of, 109; mild cli-
mates and, 172; number, 108 f.;
periodicity, 243; planetary hy-
pothesis of, 253; records, 245;
storminess and, 163; storms and,
300; temperature of earth and,
52, 173.
Bunspot variations, 282.
Swamps, as desert phenomena, 171.
8ylt, storm-flood, 99.
Syria, ciyilization in, 67; inscrip-
tions in, 76; Boman aqueducts in,
71.
Syrian Desert, ruins in, 66.
Talbert, cited, 213.
Tarim Basin, red beds, 151.
Tarim Desert, desiccation, 66.
Tarim Biver, swamps, 171.
Taylor, G., cited, 140, 144, 191, 271.
Temperature, change of in Atlantic,
174; changes in, 93; climatic
change and, 49; critical, 9; geo-
logical time and, 3 ; glacial period,
38; glaciation and, 42, 132, 139;
gradient of earth, 213; of ocean,
180; in Norway, 177; in Permian,
146 f.; in Proterozoic, 146 f.;
limits, 6 ff . ; precipitation and, 94 ;
range of, 3, 8; solar activity and,
140; storminess and, 94, 112, 173;
sunspots and, 28, 173; volcanic
eruptions and, 46; zones, 172.
Terrestial causes of climatic changes,
188-214.
Tertiary, lava, 211.
Thames, frozen, 98.
Thermal solar hypothesis, 49 f., 97.
Thermo-pleion, movements of, 30.
Thesis, of pulsations, 24.
Thiryu, storm-flood, 99.
Tian-Shan Mountains, irrigation in,
71.
Tidal action of stars, 249.
Tidal effect, of Jupiter, 253; of
planets, 244.
Tidal hypothesis, 251.
Tidal retardation, effect on land and
sea, 191; rotation of earth and,
18 f.; stress caused by, 310.
Tides, cycles of, 219.
Time, geological, see Geological
time.
Toads, distribution of, 202.
Tobacco plant, effect of light, 184.
Topography, and glaciation, 132.
TranscsBpian Basin, red beds, 151.
Tree ferns, in New Zealand, 179.
Tree growth, periodicity in, 245;
rainfall and, 79.
Trees, in Calif omia, 219; measure-
ment of, 73 ff.
Triassic, climate, 266.
Trifld Nebula, variables, 248.
Trondheim, wheat in, 101.
Trondhenas, com in, 101.
Tropical cyclones, in glacial epochs,
140f.; occurrence, 148; solar ac-
tivity and, 113.
Tropical hurricanes, earthquakes
and, 300; sunspots and, 149.
Turf an, temperature, 17.
Turner, H. H., cited, 245.
Tyler, J. M., cited, 216.
Tyndall, J., cited, 36, 37.
Typhoon region, < ' earthquake
weather," 298.
Typhoons, occurrence, 300.
United States, rainf lUl and tempera-
ture in Gulf region, 93 f . ; salt
lakes in, 142; southwestern, cli-
mate, 66 ; storminess, 53 f ., 60.
Variables, 247.
Veeder, M. A., cited, 300.
Vegetation, theory of pulsations and,
90.
INDEX
829
VennB, atmosphere of, 236.
Vefiterbjgdy inYasion of, 106.
Vicksburg, Miss., rainfall and tem-
perature, 93 f .
Volcanic activitj, climate and, 210;
movement of the earth's crust
and, 285; times of uplifting lands
and, 23.
Volcanic dust, climatic changes and,
97.
Volcanic hypothesis, climatic change
and, 45 ff.; snow Une, 139.
Volcanoes, activity of, 96.
Volga, 108.
Walcott, C. D., cited, 4, 230.
Wandering of the pole, 302.
Water, importance, 9.
Water vapor, condensation of, 56;
effect on life, 231; in atmos-
phere, 19.
Wave, effect on movement of water,
176.
Weather, changes of, 31 f. ; origin
of, 174; variations, 52.
Wells, H. G., cited, 35.
Wendingstadt, storm-flood, 99.
Westerlies, 21 f .
Wheat, price in England, 102.
White Sea, submergence, 219.
Whitney, J. D., cited, 142.
Wieland, G. R., cited, 169.
Williamson, £. D., cited, 226.
Willis, B., cited, 206.
Winds, at ice front, 162; effect on
currents, 174; glaciation and, 133;
in Antarctica, 161; in glacial
period, 119; in Greenland, 161;
planetary system of, 174; velocity,
240.
Witch hazel, effect of light, 184.
Wolf, J. B., cited, 61, 109, 288.
Wolfer, cited, 244.
Wright, W. B., cited, 35, 111, 119.
Writing, among Mayas, 96.
Yucatan, Maya civilization, 26, 107;
rainfall, 108; ruins, 95.
Yukon, Ice Age in, 221.
Zante, earthquakes of, 296.
Zonal crowding, 117.
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