Climatic Changes: Their Nature and Causes
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Title: Climatic Changes: Their Nature and Causes
Author: Ellsworth Huntington
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CLIMATIC CHANGES
THEIR NATURE AND CAUSES
PUBLISHED ON THE FOUNDATION
ESTABLISHED IN MEMORY OF
THEODORE L. GLASGOW
OTHER BOOKS BY THE SAME AUTHORS
ELLSWORTH HUNTINGTON
A. _Four books showing the development of knowledge as to Historical
Pulsations of Climate._
The Pulse of Asia. Boston, 1907. Explorations in Turkestan.
Expedition of 1903. Washington, 1905.
Palestine and Its Transformation. Boston, 1911.
The Climatic Factor, as Illustrated in Arid America. Washington,
1914.
B. _Two books illustrating the effect of climate on man._
Civilization and Climate. New Haven, 1915.
World Power and Evolution. New Haven, 1919.
C. _Four books illustrating the general principles of Geography._
Asia: A Geography Reader. Chicago, 1912.
The Red Man's Continent. New Haven, 1919.
Principles of Human Geography (with S. W. Cushing). New York,
1920.
Business Geography (with F. 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 SARGENT VISHER
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
THEIR NATURE AND CAUSES
BY
ELLSWORTH HUNTINGTON
Research Associate in Geography in Yale University
AND
STEPHEN SARGENT VISHER
Associate Professor of Geology in Indiana University
[Illustration]
NEW HAVEN
YALE UNIVERSITY PRESS
LONDON: HUMPHREY MILFORD: OXFORD UNIVERSITY PRESS
MDCCCCXXII
COPYRIGHT 1922 BY
YALE UNIVERSITY PRESS
Published 1922.
THE THEODORE L. GLASGOW MEMORIAL
PUBLICATION FUND
The present volume is the fifth work published by the Yale
University Press on the Theodore L. Glasgow Memorial Publication
Fund. This foundation was established September 17, 1918, by an
anonymous gift to Yale University in memory of Flight Sub-Lieutenant
Theodore L. Glasgow, R.N. He was born 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 CHROWDER CHAMBERLIN
OF THE UNIVERSITY OF CHICAGO
WHOSE CLEAR AND MASTERLY DISCUSSION OF THE GREAT PROBLEMS
OF TERRESTRIAL EVOLUTION HAS BEEN ONE OF THE MOST INSPIRING
FACTORS IN THE WRITING OF THIS BOOK
_There is a toy, which I have heard, and I would not have it given
over, but waited upon a little. They say it is observed 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 with little heat, and the like, and they call it the prime;
it is a thing I do the rather mention, because, computing backwards,
I have found some concurrence._
FRANCIS BACON
PREFACE
Unity is perhaps the keynote of modern science. This means unity 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 modern science means also a
growing tendency toward cooeperation, so that by working together
scientists discover 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, including man.
In the third place, this volume illustrates the unity of modern science
because it is preeminently a cooeperative product. Not only have the two
authors shared in its production, but several of the Yale Faculty have
also cooeperated. 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,
suggestions, and facts, but by paragraphs ready for the printer. In the
same way in the domain of physics, Professor Leigh Page has repeatedly
taken time to assist, and either in writing or by word of mouth has
contributed many pages. In astronomy, the same cordial cooeperation 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 suggestions 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 cooeperation. If the
main hypothesis of this book proves sound, it is largely because it has
been built up in constant consultation with men who look at the problem
from different points of vision. Our appreciation of their generous and
unstinted cooeperation 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 University of Chicago, to whom, with
his permission, we take great pleasure in dedicating this volume, has
read the 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 Harvard
University has contributed materially to the chapter 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 University of
California while he was teaching at Yale 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 XIII was written by the junior author;
his contributions are also especially numerous in Chapters X and XV; the
rest of the book was written originally by the senior author.
CONTENTS
PAGE
I. The Uniformity of Climate 1
II. The Variability of Climate 16
III. Hypotheses of Climatic Change 33
IV. The Solar Cyclonic Hypothesis 51
V. The Climate of History 64
VI. The Climatic Stress of the Fourteenth Century 98
VII. Glaciation According to the Solar Cyclonic Hypothesis 110
VIII. Some Problems of Glacial Periods 130
IX. The Origin of Loess 155
X. Causes of Mild Geological Climates 166
XI. Terrestrial Causes of Climatic Changes 188
XII. Post-Glacial Crustal Movements and Climatic Changes 215
XIII. 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 Sequoia trees 77
Fig. 6. Distribution of Pleistocene ice sheets 123
Fig. 7. Permian geography and glaciation 145
Fig. 8. Effect of diminution of storms on movement of water 175
Fig. 9. Cretaceous Paleogeography 201
Fig. 10. Climatic changes of 140,000 years as inferred 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 Rainfall and Growth of
Sequoias in California 80
4. Correlation Coefficients between Rainfall Records in
California and Jerusalem 84
5. Theoretical Probability of Stellar Approaches 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 human being can escape from the fact that his
food, clothing, shelter, recreation, occupation, health, and energy are
all profoundly influenced by his climatic surroundings. 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 protecting 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 deg.C. in average yearly temperature, and for most the
limit of successful growth is 10 deg..[1] So far as we yet know every living
species of plant and animal, including man, thrives best under definite
and limited conditions of temperature, humidity, and sunshine, and of
the composition 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 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 important 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 contradictory. All, however, have a similar effect in providing
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 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 proportions, 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 Unicellular
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 properties of radium
many of the best students do not hesitate to say a billion or a billion
and a half.[2] 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, carbonaceous shales, graphites, and iron ores.[3] Although
the Archeozoic has yielded no generally admitted fossils, yet what seem
to be massive algae and sponges have been found in Canada. On the other
hand, abundant life is believed to have been present in the oceans, for
by no other known means would it be possible to take from the air the
vast quantities of carbon that now form carbonaceous shales and
graphite.
In the next geologic era, the Proterozoic, the researches of Walcott
have shown that besides the marine algae there must have been many other
kinds of life. The Proterozoic fossils thus far discovered include not
only microscopic radiolarians such as still form the red ooze of the
deepest ocean floors, but the much more significant tubes of annelids or
worms. The presence of the annelids, which are relatively high in the
scale of organization, is generally taken to mean that more lowly forms
of animals such as coelenterates and probably even the mollusca and
primitive arthropods must already have been evolved. That there were
many kinds of marine invertebrates living in the later Proterozoic is
indicated by the highly varied life and more especially the trilobites
found in the oldest Cambrian strata of the next succeeding period. In
fact the Cambrian has sponges, primitive corals, a great variety of
brachiopods, the beginnings of gastropods, a wonderful array of
trilobites, and other lowly forms of arthropods. Since, under the
postulate of evolution, the life of that time forms an unbroken sequence
with that of the present, and since many of the early forms differ only
in minor details from those of today, we infer that the climate then was
not very different from that of today. The same line of reasoning leads
to the conclusion that even in the middle of the Proterozoic, when
multicellular marine animals must already have been common, the climate
of the earth had already for an enormous period been such that all the
lower types of oceanic invertebrates had already evolved.
TABLE 1
THE GEOLOGICAL TIME TABLE[4]
COSMIC TIME
FORMATIVE ERA. Birth and growth of the earth. Beginnings of the
atmosphere, hydrosphere, continental platforms, oceanic basins,
and possibly of life. No known geological record.
GEOLOGIC TIME
ARCHEOZOIC ERA. Origin of simplest life.
PROTEROZOIC 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 dominance
of trilobites.
_Ordovician Period._ First known fresh-water fishes.
_Silurian Period._ First known land plants.
_Devonian Period._ First known amphibians. "Table Mountain" ice
age.
_Mississippian Period._ Rise of marine fishes (sharks).
_Pennsylvanian Period._ Rise of insects and first period of marked
coal accumulation.
_Permian Period._ Rise of reptiles. Another great ice age.
MESOZOIC ERA. Age of reptile dominance.
_Triassic Period._ Rise of dinosaurs. The period closes with a
cool climate.
_Jurassic Period._ Rise of birds and flying reptiles.
_Comanchean Period._ Rise of flowering plants and higher insects.
_Cretaceous Period._ Rise of archaic or primitive mammalia.
CENOZOIC ERA. Age of mammal dominance.
_Early Cenozoic or Eocene and Oligocene time._ Rise of higher
mammals. Glaciers in early Eocene of the Laramide Mountains.
_Late Cenozoic or Miocene and Pliocene time._ Transformation of
ape like animals into man.
_Glacial or Pleistocene time._ Last great ice age.
PRESENT TIME
PSYCHOZOIC ERA. Age of man or age of reason. Includes the present or
"Recent time," estimated to be probably less than 30,000 years.
Moreover, they could live in most latitudes, for the indirect evidences
of life in the Archeozoic and Proterozoic rocks are widely distributed.
Thus it appears that at an almost incredibly early period, perhaps many
hundred million years ago, the earth's climate differed only a little
from that of the present.
The extreme limits of temperature beyond which the climate of geological
times cannot have departed can be approximately determined. Today the
warmest parts of the ocean have an average temperature of about 30 deg.C. on
the surface. Only a few forms of life live where the average temperature
is much higher than this. In deserts, to be sure, some highly organized
plants and animals can for a short time endure a temperature as high as
75 deg.C. (167 deg.F.). In certain hot springs, some of the lowest unicellular
plant forms exist in water which is only a little below the boiling
point. More complex forms, however, such as sponges, worms, and all the
higher plants and animals, seem to be unable to live either in water or
air where the temperature averages above 45 deg.C. (113 deg.F.) for any great
length of time and it is doubtful whether they can thrive permanently
even at that temperature. The obvious unity of life for hundreds of
millions of years and its presence at all times in middle latitudes so
far as we can tell seem to indicate that since the beginning of marine
life the temperature of the oceans cannot have averaged much above 50 deg.C.
even in the warmest portions. This is putting the limit too high rather
than too low, but even so the warmest parts of the earth can scarcely
have averaged much more than 20 deg. warmer than at present.
Turning to the other extreme, we may inquire how much colder than now
the earth's surface may have been since life first appeared. Proterozoic
fossils have been found in places where the present average temperature
approaches 0 deg.C. If those places should be colder than now by 30 deg.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
temperature 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 temperature of the earth's surface has
never varied more than perhaps 30 deg.C. above or below the present level.
Even this range of 60 deg.C. (108 deg.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 extremes 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 disappearance 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 deg.C. or even of 20 deg. in the average temperature of the
earth's surface may seem large when viewed from the limited standpoint
of terrestrial experience. Viewed, however, from the standpoint of
cosmic evolution, or even of the solar system, it seems a mere trifle.
Consider the possibilities. The temperature of empty space is the
absolute zero, or -273 deg.C. To this temperature all matter must fall,
provided it exists long enough and is not appreciably heated by
collisions 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 deg.C., while
thousands of miles in the interior it may rise to 20,000 deg. or 100,000 deg. 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 deg.C., or
287 deg. above the absolute zero. Toward the interior, the temperature in
mines and deep wells rises about 1 deg.C. for every 100 meters. At this rate
it would be over 500 deg.C. at a depth of ten miles, and over 5000 deg. 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[5]
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 deg.C.; at the distance of Venus, 85 deg.; the earth 27 deg.; Mars
-30 deg.; Neptune -219 deg.. 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 anywhere from near -273 deg.C. up to perhaps
5000 deg.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 deg., the earth claims only 60 deg. at
most, or less than 1 per cent. This may be remarkable, but what is far
more remarkable is that the earth's range of 60 deg. includes what seem to
be the two most critical of all possible temperatures, namely, the
freezing point of water, 0 deg.C., and the temperature 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 deg. to 40 deg.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. Henderson, 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 common
liquids approach it in the amount of additional heat required for
conversion into vapor after the temperature of vaporization has been
reached. Again, water substance, as the physicists call all forms of
H_{2}O, 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 compare with water. As
Henderson[6] 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 presence can be
detected in sea water. According to Henderson they include hydrogen,
oxygen, nitrogen, carbon, chlorine, sodium, magnesium, sulphur,
phosphorus, which are easily detected; and also arsenic, caesium, gold,
lithium, rubidium, barium, lead, boron, fluorine, iron, iodine, bromine,
potassium, cobalt, copper, manganese, nickel, silver, silicon, zinc,
aluminium, calcium, and strontium. Yet in spite of its marvelous power
of solution, water is chemically rather inert and relatively stable. It
dissolves all these elements and thousands of their compounds, but still
remains water and can easily be separated and purified. Another unique
property of water is its power of ionizing dissolved substances, a
property which makes it possible to produce electric currents in
batteries. This leads to an almost infinite array of electro-chemical
reactions which play an almost dominant role in the processes of life.
Finally, no common liquid except mercury equals water in its power 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 substances," says Henderson, "are the
common source of every one of the complicated substances which are
produced 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 deg.C. or 59 deg.F., which is close to the ideal temperature 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 almost
exactly equal to the amount in the adjacent air. Unlike oxygen,
hydrogen, and nitrogen, 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 water. In earth, air, fire, and water alike these two
substances are always associated.
"Accordingly, if water be the first primary constituent 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; together they make up the real environment and
they never part company."[7]
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
chemical formulae. No other known compounds can give off 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 without destruction is the fundamental chemical characteristic 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 deg.C. and the boiling point, where all water finally turns
into vapor. In the whole range of temperature, so far as we know, there
is no other interval where any such complex 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 consider 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 collision with another body, were
long ago abandoned. The impact of a constant supply of meteors affords
an almost equally unsatisfactory explanation. Moulton[8] 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 calculated that twenty million years ago it was
ineffective and ten million years hence it will be equally so. Moreover,
if this is the source of heat, the amount of radiation 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 tenuous
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 surface the
theoretical temperature would have been 200 deg. 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.[9]
Altogether the problem of how the sun can have remained 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 radioactivity 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 from the fact that its surface was cool enough so
that great ice sheets could accumulate on low lands within 40 deg. 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
stupendous fact that in those remote times the sun must have been
essentially the same as now, or else that some utterly unknown factor is
at work.
FOOTNOTES:
[Footnote 1: W. A. Setchell: The Temperature Interval in the
Geographical Distribution of Marine Algae; Science, Vol. 52, 1920,
p. 187.]
[Footnote 2: J. Barrell: Rhythms and the Measurements of Geologic Time;
Bull. Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.]
[Footnote 3: Pirsson and Schuchert: Textbook of Geology, 1915, pp.
538-550.]
[Footnote 4: From Charles Schuchert in The Evolution of the Earth and
Its Inhabitants: Edited by R. S. Lull, New Haven, 1918, but with
revisions by Professor Schuchert.]
[Footnote 5: J. H. Poynting: Radiation in the Solar System; Phil. Trans.
A, 1903, 202, p. 525.]
[Footnote 6: L. J. Henderson: The Fitness of the Environment, 1913.]
[Footnote 7: Henderson: _loc. cit._, p. 138.]
[Footnote 8: F. R. Moulton: Introduction to Astronomy, 1916.]
[Footnote 9: Moulton: _loc. cit._]
CHAPTER II
THE VARIABILITY OF CLIMATE
The variability of the earth's climate is almost as extraordinary as its
uniformity. This variability is made up partly of a long, slow tendency
in one direction and partly of innumerable cycles of every conceivable
duration from days, or even hours, up to millions of years. Perhaps the
easiest way to grasp the full complexity of the matter is to put the
chief types of climatic sequence in the form of a table.
+-----------------------------------------------------------+
| |
| TABLE 2 |
| |
| TYPES OF CLIMATIC SEQUENCE |
| |
| 1. Cosmic uniformity. 7. Brueckner periods. |
| 2. Secular progression. 8. Sunspot cycles. |
| 3. Geologic oscillations. 9. Seasonal alternations. |
| 4. Glacial fluctuations. 10. Pleionian migrations. |
| 5. Orbital precessions. 11. Cyclonic vacillations. |
| 6. Historical pulsations. 12. Daily vibrations. |
| |
+-----------------------------------------------------------+
In assigning names to the various types an attempt has been made to
indicate something of the nature of the sequence so far as duration,
periodicity, and general tendencies are concerned. Not even the rich
English language of the twentieth century, however, furnishes words with
enough shades of meaning to express all that 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 millions 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 becoming 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 thermometer sometimes ranges from
0 deg.F. in the morning to 60 deg. in the shade at noon. On a cloudy day in the
Amazon forest close to the seashore, on the contrary, the temperature
for months may rise to 85 deg. by day and sink no lower than 75 deg. at night.
The reasons for the secular progression of the earth's 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 terrestrial factors: first, the
condition of the earth's interior, 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 appears to be an almost negligible factor in
explaining either secular progression or geologic oscillation. According
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. The emission of internal heat, however, had probably
ceased to be of much climatic significance near the beginning of the
geological record, for in southern Canada glaciation occurred very early
in the Proterozoic era. On the other hand, the contraction of the earth
has produced remarkable effects throughout the whole of geological time.
It has lessened the earth's circumference by a thousand miles or more,
as appears from the way in which the rocks have been folded and thrust
bodily over one another. According to the laws of dynamics this must
have increased the speed of the earth's rotation, thus shortening the
day, and also having the more important effect of increasing the bulge
at the equator. On the other hand, recent investigations indicate that
tidal retardation has probably diminished the earth's rate of rotation
more than seemed probable a few years ago, thus lengthening the day and
diminishing the bulge at the equator. Thus two opposing forces have been
at work, one causing acceleration and one retardation. Their combined
effect may have been a factor in causing secular progression of climate.
It almost certainly was of much importance in causing pronounced
oscillations first one way and then the other. This matter, together
with most of those touched in these first chapters, will be expanded in
later parts of the book. On the whole the tendency appears to have been
to create climatic diversity in place of uniformity.
The increasing salinity of the oceans may have been another factor in
producing secular progression, although of slight importance in respect
to oscillations. While the oceans were still growing in volume, it is
generally assumed that they must have been almost fresh for a vast
period, although Chamberlin thinks that the change in salinity has been
much less than is usually supposed. So far as the early oceans were
fresher than those of today, their deep-sea circulation must have been
less hampered than now by the heavy saline water which is produced by
evaporation in warm regions. Although this saline water is warm, its
weight causes it to descend, instead of moving poleward in a surface
current; this descent slows up the rise of the cold water which has
moved along in the depths of the ocean from high latitudes, and thus
checks the general oceanic circulation. If the ancient oceans were
fresher and hence had a freer circulation than now, a more rapid
interchange of polar and equatorial water presumably tended to equalize
the climate of all latitudes.
Again, although the earth's atmosphere has probably changed far less
during geological times than was formerly supposed, its composition has
doubtless varied. The total volume of nitrogen has probably increased,
for that gas is so inert that when it once becomes a part of the air it
is almost sure to stay there. On the other hand, the proportions of
oxygen, carbon dioxide, and water vapor must have fluctuated. Oxygen is
taken out constantly by animals and by all the processes of rock
weathering, but on the other hand the supply is increased when plants
break up new carbon dioxide derived from volcanoes. As for the carbon
dioxide, it appears probable that in spite of the increased supply
furnished by volcanoes the great amounts of carbon which have gradually
been locked up in coal and limestone have appreciably depleted the
atmosphere. Water vapor also may be less abundant now than in the past,
for the presence of carbon dioxide raises the temperature a little and
thereby enables the air to hold more moisture. When the area of the
oceans has diminished, and this has recurred very often, this likewise
would tend to reduce the water vapor. Moreover, even a very slight
diminution in the amount of heat given off by the earth, or a decrease
in evaporation because of higher salinity in the oceans, would tend in
the same direction. Now carbon dioxide and water vapor both have a
strong blanketing effect whereby heat is prevented from leaving the
earth. Therefore, the probable reduction in the carbon dioxide and water
vapor of the earth's atmosphere has apparently tended to reduce the
climatic monotony and create diversity and complexity. Hence, in spite
of many reversals, the general tendency of changes, not only in the
earth's interior and in the oceans, but also in the atmosphere, appears
to be a secular progression from a relatively monotonous climate in
which the evolution of higher organic forms would scarcely be rapid to
an extremely diverse and complex climate highly favorable to progressive
evolution. The importance of these purely terrestrial agencies must not
be lost sight of when we come to discuss other agencies outside the
earth.
In Table 2 the next type of climatic sequence is geologic oscillation.
This means slow swings that last millions of years. At one extreme of
such an oscillation the climate all over the world is relatively
monotonous; it returns, as it were, toward the primeval conditions at
the beginning of the secular progression. At such times magnolias,
sequoias, figs, tree ferns, and many other types of subtropical plants
grew far north in places like Greenland, as is well known from their
fossil remains of middle Cenozoic time, for example. At these same
times, and also at many others before such high types of plants had
evolved, reef-making corals throve in great abundance in seas which
covered what is now Wisconsin, Michigan, Ontario, and other equally cool
regions. Today these regions have an average temperature of only about
70 deg.F. in the warmest month, and average well below freezing in winter.
No reef-making corals can now live where the temperature averages below
68 deg.F. The resemblance of the ancient corals to those of today makes it
highly probable that they were equally sensitive to low temperature.
Thus, in the mild portions of a geologic oscillation the climate seems
to have been so equable and uniform that many plants and animals could
live 1500 and at other times even 4000 miles farther from the equator
than now.
At such times the lands in middle and high latitudes were low and small,
and the oceans extended widely over the continental platforms. Thus
unhampered ocean currents had an opportunity to carry the heat of low
latitudes far toward the poles. Under such conditions, especially if the
conception of the great subequatorial continent of Gondwana land is
correct, the trade winds and the westerlies must have been stronger and
steadier than now. This would not only enable the westerlies, which are
really southwesterlies, to carry more heat than now to high latitudes,
but would still further strengthen the ocean currents. At the same time,
the air presumably contained an abundance of water vapor derived from
the broad oceans, and an abundance of atmospheric carbon dioxide
inherited from a preceding time when volcanoes contributed much carbon
dioxide to the air. These two constituents of the atmosphere may have
exercised a pronounced blanketing effect whereby the heat of the earth
with its long wave lengths was kept in, although the energy of the sun
with its shorter wave lengths was not markedly kept out. Thus everything
may have combined to produce mild conditions in high latitudes, and to
diminish the contrast between equator and pole, and between summer and
winter.
Such conditions perhaps carry in themselves the seeds of decay. At any
rate while the lands lie quiet during a period of mild climate great
strains must accumulate in the crust because of the earth's contraction
and tidal retardation. At the same time the great abundance of plants
upon the lowlying plains with their mild climates, and the marine
creatures upon the broad continental platforms, deplete the atmospheric
carbon dioxide. Part of this is locked up as coal and part as limestone
derived from marine plants as well as animals. Then something happens so
that the strains and stresses of the crust are released. The sea floors
sink; the continents become relatively high and large; mountain ranges
are formed; and the former plains and emergent portions of the
continental platforms are eroded into hills and valleys. The large size
of the continents tends to create deserts and other types of climatic
diversity; the presence of mountain ranges checks the free flow of winds
and also creates diversity; the ocean currents are likewise 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
depletion 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
climate 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-hand 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 mountains 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 pressure. 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 carbon
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 follow "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, they 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 temperature is lower than usual, snow
accumulates in high 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 the Baltic Sea from Germany to Sweden on the ice, and
the ice of Greenland advanced so much that shore ice caused the Norsemen
to change their sailing route between Iceland and the Norse colonies in
southern Greenland. At the same time in low latitudes and in parts of
the continental interior there is a tendency toward diminished rainfall
and even toward aridity and the formation of deserts. In Yucatan, for
example, a diminution in tropical rainfall in the fourteenth century
seems to have given the Mayas a last opportunity for a revival of their
decaying civilization.
[Illustration: _Fig. 1. Climatic changes and mountain building._
(_After Schuchert, in The Evolution of the Earth and Its Inhabitants,
edited by R. S. Lull._) Diagram showing the times and probable extent of
the more or less marked climate changes in the geologic history of North
America, and of its elevation into chains of mountains.]
Among the climatic sequences, glacial fluctuations are perhaps of the
most vital import from the standpoint of organic evolution; from the
standpoint of human history the same is true of climatic pulsations.
Glacial epochs have repeatedly wiped out thousands upon thousands of
species and played a part in the origin of entirely new types of plants
and animals. This is best seen when the life of the Pennsylvanian is
contrasted with that of the Permian. An historic pulsation may wipe out
an entire civilization and permit a new one to grow up with a radically
different character. Hence it is not strange that the causes of such
climatic phenomena have been discussed with extraordinary vigor. In few
realms of science has there been a more imposing or more interesting
array of theories. In this book we shall consider the more important of
these theories. A new solar or cyclonic hypothesis and the hypothesis of
changes in the form and altitude of the land will receive the most
attention, but the other 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 themselves 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 distinguished
from those of other climatic sequences.
We come now to Brueckner periods and sunspot cycles. The Brueckner 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_, Brueckner has collected an uncommonly interesting assortment of
facts as to the climate of Europe for more than two centuries. More
recently, by a study of the rate of 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 discussed 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 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
designate 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 eliminated, it appears that the course of
these various elements 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,
corresponding 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 simultaneously 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
outside 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 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
temperature 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. Probably 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 represent the
combined effect of conditions in the sun and upon the earth itself.
The last of the climatic sequences which require explanation 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
supposed that cyclonic storms were purely terrestrial in origin. Without
any adequate investigation it was assumed 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 in certain
limited belts, but not in most parts of the world. Today this view is
being rapidly modified. Undoubtedly, 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 part.
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 sequences, it is evident
that they may be divided into four great 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, because they are fairly well understood, they are deferred
until after the third group of sequences has been discussed. This group
includes glacial fluctuations, historic pulsations, Brueckner periods,
sunspot cycles, pleionian migrations, and cyclonic vacillations. The
outstanding fact in regard to all of these is that while they are
greatly 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, seasonal alternations, and daily variations. These may be
regarded 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 III
HYPOTHESES OF CLIMATIC CHANGE
The next step in our study of climate is to review the main hypotheses
as to the causes of glaciation. 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
understanding of them does much to explain the others. In reviewing the
various theories we shall 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 hypothesis 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 primarily to a diminution in the earth's
supply of heat or to a _redistribution_ of the heat through changes in
atmospheric and oceanic circulation. At present the great 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 frequent synchronism
between two great types of phenomena; 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 satisfies the somewhat
exacting requirements of the geological 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 widespread
uplifting of continents. At least during the later glacial periods the
hypothesis must explain numerous climatic epochs and stages superposed
upon a single general period of continental upheaval. Moreover, although
historical geology demands cycles of varied duration and magnitude, it
does not furnish evidence of any rigid 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 considered is open to question, although the relative
importance of existing agencies may have varied.
I. _Croll's Eccentricity Theory._ One of the most ingenious and most
carefully elaborated scientific hypotheses is Croll's[10] precessional
hypothesis as to the effect of the earth's own motions. So well was this
worked 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 hypothesis 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
spinning 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 perihelion, 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.
Croll argued that these astronomical changes must alter the earth's
climate, especially by their effect on winds and ocean currents. His
elaborate argument contains a vast amount of valuable material. Later
investigation, 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, Croll's hypothesis
demands that glaciation in the northern and southern hemisphere take
place alternately. A constantly growing collection of facts, however,
indicates that glaciation does not occur in the two hemispheres
alternately, but at the same time. Third, the hypothesis calls for the
constant and frequent repetition of glaciation at absolutely regular
intervals. The geological record shows no such regularity, for sometimes
several glacial epochs follow in relatively close succession at
irregular intervals of perhaps fifty to two hundred 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
pulsations, and the other smaller climatic variations which are
superposed upon glacial epochs and upon one another in bewildering
confusion. In spite of these objections, there can be little question
that the eccentricity of the earth's orbit and the precession of the
equinoxes with the resulting change in the season of perihelion must
have some climatic effect. Hence Croll's theory deserves a permanent
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,[11] adopting an idea suggested by Tyndall, fired
the imagination of geologists by his skillful 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 atmosphere 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 variations 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 appreciable effect on
sunlight coming toward the earth, would filter out and thus retain in
the atmosphere all the outgoing 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. According to Humphreys' calculations, a
doubling of the carbon dioxide in the air would in itself raise the
average temperature about 1.3 deg.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 appear 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 CO_{2} 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 absorbed 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
CO_{2}, that is, the outermost portions in the upper atmosphere, must
apparently 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 CO_{2}, would have an effect of more than
1.3 deg.C. A change of even 2 deg.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 deg. to 8 deg.C. lower than now.
Nevertheless, variations in atmospheric carbon dioxide do not
necessarily seem competent to produce the relatively rapid climatic
fluctuations of glacial epochs and historic pulsations as distinguished
from the longer swings of glacial periods and geological eras. In
Chamberlin'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
variations 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 descended to the ocean depths in low latitudes,
kept its heat while creeping slowly poleward, and rose in high latitudes
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 account 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 a relatively concentrated brine. Such
a brine is heavy and tends to sink, thereby setting up an oceanic
circulation the reverse of that which now prevails. At present the polar
waters sink because they are cold and hence 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 warmer and
lighter water from low latitudes which moves poleward 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
concentrated 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 prevailed, the
atmospheric CO_{2} should be depleted, and the air cooled so much that
the concentration of the equatorial 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 surface of the ocean would become cool.
The cool surface-water would absorb carbon dioxide faster than the
previous warm water had done, for heat drives off gases from water. This
would hasten the cooling of the atmosphere 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 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 deglaciation, or toward an
inter-glacial condition would arise. At such times the oceanic
circulation is not supposed 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 produced 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 established, 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 atmospheric carbon dioxide would be
depleted; the air would grow cooler; and the cycle of glaciation would
be repeated. Each cycle would be shorter than the last, for not only
would the swings diminish like those of a pendulum, but the agencies
that were causing the main depletion of the atmospheric carbon dioxide
would diminish in intensity. 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 reversed again. When the warm saline waters
of low latitudes 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 believe that it is of the utmost value as a step toward
the 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 discussed 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
possibility 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 observed 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 atmospheric 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. Chamberlin 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 such material on the sea floor
seem to indicate that the early 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
decreasing 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 producing the longer oscillations of climate from one
geological 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 phenomena dependent upon movements of
the earth's crust. 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 continents, 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 climate can scarcely be
questioned. Such changes may be introduced 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 explain glacial
fluctuations, historic pulsations, and especially 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 between
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 on glacial climates. Moreover, the marked
Permian ice age follows long after the birth of the Hercynian Mountains
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 efficient and yet able to vary rapidly and
independently, Abbot, Fowle, Humphreys, and others,[12] have concluded
that volcanic eruptions are the missing agency. In _Physics of the Air_,
Humphreys gives a careful study of the effect of volcanic dust upon
terrestrial temperature. He begins with a mathematical investigation of
the size of dust particles, and their quantity after certain eruptions.
He demonstrates that the power of such particles to deflect 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 deg.C. in the
earth's surface temperature. As in every such complicated 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
volcanic explosions were frequent enough and violent enough, the
temperature of the earth's surface would be considerably lowered.
Actual observation supports this theoretical conclusion. Humphreys
gathers together and amplifies all that he and Abbot and Fowle have
previously said as to observations of the sun's thermal radiation by
means of the pyrheliometer. This summing up 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 "phenomena 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
considerable 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,[13] for example,
has prepared numerous 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 temperatures 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."
Koeppen,[14] in his comprehensive study of temperature for a hundred
years, also presents a strong argument against the idea that volcanic
eruptions have an important 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 Koeppen, 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 case.
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 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 Krakatoan
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 Krakatoan 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 of any such size. Even though the same volcano suffered
repeated explosions, 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-glacial climate.
V. _The Hypothesis of Polar Wandering._ Another hypothesis, 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. Astrophysicists,
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
Permian 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 historic 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 due to
variations in the heat emitted by the sun 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 permanently 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 glaciation 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 heating, such as there is no evidence of in geological
history. In fact, there is far more evidence of sudden cooling 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 climate 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 thermal form of solar hypothesis is
stated by Humphreys as follows: "A change of the solar constant
obviously alters all surface temperatures by a roughly constant
percentage. Hence a decrease of the heat from the sun would in general
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
precipitation, 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 complications,
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 probably been exaggerated. Finally, although minor changes in the
amount of heat given out by the sun occur constantly 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.
FOOTNOTES:
[Footnote 10: James Croll: Climate and Time, 1876.]
[Footnote 11: T. C. Chamberlin: An attempt to frame a working hypothesis
of the cause of glacial periods on an atmospheric basis; Jour. Geol.,
Vol. VII, 1899, pp. 545-584, 667-685, 757-787.
T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp.
93-106, 655-677, and Vol. III, pp. 432-446.
S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some
investigations on carbon dioxide which have had a pronounced effect on
later conclusions.
F. Frech adopted Arrhenius' idea and developed it in a paper entitled
Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte
Rendu, Tenth (Mexico) Congr. 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 suggestion, of Tyndall that glaciation might be due to depletion of
atmospheric carbon dioxide, Chamberlin worked up the essentials of his
early views before he saw any publication from Arrhenius, to whom the
idea has often been attributed. In 1895 or earlier Chamberlin began to
give the carbon dioxide hypothesis to his students and to discuss it
before local scientific 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 competency 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 temperature 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.
Chamberlin's latest views are more like his earlier and more
conservative statement.]
[Footnote 12: C. G. Abbot and F. E. Fowle: Volcanoes and Climate;
Smiths. Misc. Coll., Vol. 60, 1913, 24 pp.
W. J. Humphreys: Volcanic dust and other factors in the production of
climatic and their possible relation to ice ages; Bull. Mount Weather
Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air,
1920.]
[Footnote 13: H. Arctowski: The Pleonian Cycle of Climatic Fluctuations;
Am. Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New
York Academy of Sciences, Vol. 24, 1914.]
[Footnote 14: W. Koeppen: Ueber mehrjaehrige Perioden der Witterung
ins besondere uezer die II-jaehrige Periode der Temperatur. Also,
Lufttemperaturen Sonnenflecke und Vulcanausbrueche; Meteorologische
Zeitschrift, Vol. 7, 1914, pp. 305-328.]
CHAPTER IV
THE SOLAR CYCLONIC HYPOTHESIS
The progress of science is made up of a vast succession 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. In the previous chapter 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. In this chapter we shall outline
still another hypothesis, the relation of which to present climatic
conditions has been fully developed in _Earth and Sun_; while its
relation to the past will be explained 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 explain 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, 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 accepted ideas, and which therefore must arouse serious
question in the minds of thoughtful readers. That, however, 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, Newcomb, and
that of the great climatologist, Koeppen, have shown beyond question that
the temperature of the earth's surface varies in harmony with variations
in the number and area of sunspots.[15] 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 certainly
owes its warmth to the sun. Yet when the sun emits the most energy, that
is, when sunspots are most numerous, 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 that 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 bring, there is a tendency toward frequent tropical
hurricanes 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 opposite way. Thus when
the sun's atmosphere is particularly 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 Atlantic coast. In a crescent-shaped central area, however,
extending from Wyoming through Missouri to Nova Scotia, the number of
storms and the amount of rainfall decrease.
[Illustration: _Fig. 2. Storminess at sunspot maxima vs. minima._
(_After Kullmer._)
Based on nine years' nearest sunspot minima and nine years' nearest
sunspot maxima in the three sunspot cycles from 1888 to 1918. Heavy
shading indicates excess of storminess when sunspots are numerous.
Figures indicate average yearly number of storms by which years of
maximum sunspots exceed those of minimum sunspots.]
The two controlling factors of any climate are the temperature and the
atmospheric pressure, for they determine 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 during times of many sunspots. The earth's surface is heated
by the rays of the sun, but 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 surface 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 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 difficult questions
which cannot yet be answered. On the whole the weight of evidence
suggests that electrical phenomena of some kind are involved, although
variations 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 concentration 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 intensifying storms. On the other hand the peculiar way in which
activity upon the margins of the sun appears to be correlated 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
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 temperature of the
upper air would influence the lower air through changes in barometric
pressure. These considerations 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 electrical hypothesis will be employed. For the
main portions 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 Scandinavia.
(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 Scandinavia, 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.
[Illustration: _Fig. 3A. Relative rainfall at times of increasing and
decreasing sunspots._
Heavy shading, more rain with increasing spots. Light shading, more rain
with decreasing spots. No data for unshaded areas.
Figures indicate percentages of the average rainfall by which the
rainfall during periods of increasing spots exceeds or falls short of
rainfall during periods of decreasing spots. The excess or deficiency is
stated in percentages of the average. Rainfall data from Walker:
Sunspots and Rainfall.]
[Illustration: _Fig. 3B. Relative rainfall at times of increasing and
decreasing sunspots._
Heavy shading, more rain with increasing spots. Light shading, more rain
with decreasing spots. No data for unshaded areas. Figures indicate
percentages of the average rainfall by which the rainfall during periods
of increasing spots exceeds or falls short of rainfall during periods of
decreasing spots. The excess or deficiency is stated in percentages of
the average. Rainfall data from Walker: Sunspots and Rainfall.]
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 temperature 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 fundamental 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 rest of 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 periodicity 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 disturbances in the sun's atmosphere. It is generally
assumed that sunspots, solar prominences, the bright clouds known as
faculae, and other phenomena denoting a perturbed state of the solar
atmosphere, are due to some cause within the sun. Yet the limitation of
these phenomena, 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 circulation, 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 described in _Earth and Sun_ support this
conclusion. The chief difficulty in accepting it hitherto has been that
although Jupiter, because of its size, would be 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 concentrated 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 momentum
from the sun's own energy. Since the planetary 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 presumably well up from the lower parts of the solar atmosphere
and thereby cause the sun 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 inaugurated.
If the preceding reasoning is correct, any disturbance of the solar
atmosphere must have an effect upon the 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
hypotheses 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 mountain 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 movement 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.
FOOTNOTES:
[Footnote 15: 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 by A. Wolfer. The number and size of the spots are both taken
into account. The numbers from 1749 to 1900 may be found in the Monthly
Weather Review for April, 1902, and from 1901 to 1918 in the same
journal for 1920.]
CHAPTER V
THE CLIMATE OF HISTORY[16]
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
numerous other publications.[17] 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 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 hypothesis of
progressive cooling and drying, an hypothesis which had much to do with
the development of the nebular hypothesis, and which has in turn been
greatly strengthened by that hypothesis. The discovery of evidence of
widespread continental glaciation, however, necessitated a modification
of this view, and succeeding years have brought to light a constantly
increasing number of glacial, or at least cool, periods distributed
throughout almost the whole of geological time. Moreover, 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 progressive desiccation has been widely
advocated. In many of the drier portions of the world, especially
between 30 deg. and 40 deg. 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 detours 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 Palmyra, in the
Syrian Desert, show that it must once have been a city like modern
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 similar 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,[18] pronounces it "ueberzeugend."
A single quotation from scores that might be used will illustrate the
conclusions of some of the most careful archaeologists.[19]
Among the regions which were once populous and highly civilized,
but 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 fully recognized and that attempts have been
made to reduce the extent of the unexplored 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 practically 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 modern 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. Presently he comes upon
a small deserted and partly ruined town composed of buildings large
and small constructed of beautifully 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
believe 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 green
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 landscape; but
the careful explorer will find on closer examination 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 considerable 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 convenience 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 practically 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.[20] 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 interesting 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 was 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.
Although the arguments in favor of a change of climate 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 Syria and North Africa during the early centuries of the
Christian era the Romans 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
convincing as is the evidence of moister conditions in the past. It is
admirably set forth, for example, in the comprehensive and ably written
monograph of Leiter on the climate of North Africa.[21] The evidence
cited there and elsewhere has led many authors strongly to advocate the
hypothesis of climatic uniformity. They have done exactly 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 progressive 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 Romans was to cause them to
construct numerous engineering 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 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
apparently the driest time during the historic period--distinctly drier
than the present--but the thirteenth 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 fortunately able to apply this test by
means of trees. The growth of vegetation depends on many factors--soil,
exposure, 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.[22] The work of Douglass[23] 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 and end of a
tree's life was the same, but it is easily possible 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 summarized 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
California. 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. Accordingly, 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 results 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 solid line. The high parts
of the line indicate rainy conditions, the low parts, dry. An
examination 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 details
more clearly, the more reliable part of the California curve, from 100
B. C. to the present time, has been reproduced in Fig. 5. This is
identical with the corresponding part of Fig. 4, except that the
vertical scale is three times as great.
[Illustration: _Fig. 4. Changes of climate in California (solid line)
and in western and central Asia (dotted line)._
Note. The curves of Figs. 4 and 5 are reproduced as published in _The
Solar Hypothesis_ in 1914. Later work, however, has indicated that in
the Asiatic curve the dash lines, which were tentatively inserted in
1914, are probably more nearly correct than the dotted lines. Still
further evidence indicates that the Asiatic curve is nearly like that of
California in its main features.]
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.[24] The dotted line is here reproduced
for its historical significance as a stage in the study of climatic
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 Stein[25] in central Asia
makes it clear that the contrast between the water supply about 200 B.C.
and in the preceding and following centuries was greater than was
supposed on the basis of the scanty evidence available when the dotted
line of Fig. 4 was drawn in 1910.
[Illustration: _Fig. 5. Changes in California climate for 2000 years,
as measured by growth of Sequoia trees._
Fig. 5 is the same as the later portion of Fig. 4, except that the
vertical scale has been magnified threefold. It seems probable that the
dotted line at the right is more nearly correct than the solid line.
During the thirty years since the end of the curve the general tendency
appears in general to have been somewhat upward.]
Since the curve of the California trees is the only continuous 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
represents (1) the local rainfall and (2) the rainfall of remote regions
such as Palestine. Perhaps the best way to determine 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 positive 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
automobile 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 importance of the various factors. A
correlation coefficient four times the probable error, which can always
be determined 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 regarded 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 California. But the growth of the trees also depends on their
position, the amount 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 correlation coefficient high enough to be convincing, but
decidedly 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 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 preceding rainy seasons
as the table indicates.
+-------------------------------------------------------------------+
| TABLE 3 |
| |
| CORRELATION COEFFICIENTS BETWEEN RAINFALL AND |
| GROWTH OF SEQUOIAS IN CALIFORNIA[26] |
| |
| (_r_) = _Correlation coefficient_ |
| (_e_) = _Probable error_ |
| (_r_/_e_) = _Ratio of coefficient to probable error_ |
| |
| A. SACRAMENTO RAINFALL AND GROWTH OF 18 SEQUOIAS IN DRY |
| LOCATIONS, 1861-1910 |
| |
| (_r_) (_e_) (_r_/_e_) |
| ------ ------ ----- |
| 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 RAINFALL 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 4.5 |
| 5 years of rainfall +0.398 +-0.082 4.9 |
| 6 years of rainfall +0.418 +-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 RAINFALL OF PRECEDING 5 YEARS |
| AT STATIONS ON SOUTHERN PACIFIC RAILROAD |
| |
| 1 = _Years_ |
| 2 = _Altitude_ (_feet_) |
| 3 = _Rainfall_ (_inches_) |
| 4 = _Approximate distance from sequoias_ (_miles_) |
| |
| 1 2 3 4 (_r_) (_e_) (_r_/_e_) |
| --------- ---- ----- --- ------ ------ --------- |
| Sacramento, 1861-1910 70 19.40 200 +0.398 +-0.081 4.9 |
| Colfax, 1871-1909 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 |
| Winnemucca, 1871-1909 4300 8.65 300 +0.492 +-0.089 5.5 |
| |
+-------------------------------------------------------------------+
In the first line of Section A a correlation coefficient of only -0.056,
which is scarcely six-tenths of the probable error, means that there is
no appreciable relation between the rainfall of a given season and the
growth during 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 preceding
seasons, however, has some effect on the trees, as appears in the second
line of Section A, where the correlation 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 coefficient begins to fall
off. Thus the growth of these eighteen sequoias on relatively dry slopes
appears to have depended 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, growing 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 growing in moist locations are employed, the
coefficient between 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 well 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
allowance 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 conclusion so far as
it goes. But as Fresno lies at a low altitude and its rainfall is of
essentially the Sacramento type, its short record is of less value than
that of Sacramento. The only rainfall records among the Sierras at high
levels, where the rainfall and temperature are approximately 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 coefficients 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 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 deg.N versus 37 deg.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 climatic 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.
+-------------------------------------------------------------------+
| TABLE 4 |
| |
| CORRELATION COEFFICIENTS BETWEEN |
| RAINFALL RECORDS IN CALIFORNIA |
| AND JERUSALEM |
| |
| (_r_) = _Correlation coefficient_ |
| (_e_) = _Probable error_ |
| (_r_/_e_) = _Ratio of coefficient to probable error_ |
| |
| A. JERUSALEM RAINFALL FOR 3 YEARS AND VARIOUS GROUPS OF |
| SEQUOIAS[27] |
| |
| (_r_) (_e_) (_r_/_e_)|
| ------ ----- ---------|
| 11 trees measured by Douglass +0.453 +-0.078 5.8 |
| 80 trees, moist locations, Groups IA, |
| IIA, IIIA, VA +0.500 +-0.073 6.8 |
| 101 trees, 69 in moist locations, 32 in |
| dry, I, II, III +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. RAINFALL AT JERUSALEM AND AT STATIONS IN CALIFORNIA AND NEVADA |
| |
| 1 = _Altitude_ (_feet_) |
| 2 = _Years_ |
| |
| -- 3 years -- -- 5 years -- |
| 1 2 (_r_) (_r_/_e_) (_r_) (_r_/_e_) |
| ---- --------- ------ ------- ------ ------- |
| 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 |
|[A]Boca, 5500 1871-1909 +0.482 6.4 +0.617 8.6 |
| Winnemucca, 4300 1871-1909 +0.235 2.2 +0.260 2.4 |
| San Bernardino, 1050 1871-1909 +0.275 2.7 +0.177 1.8 |
| |
| C. RAINFALL FOR 3 YEARS AT CALIFORNIA AND NEVADA STATIONS, |
| 1871-1909 |
| |
| (_r_) (_r_/_e_) |
| ------ ------- |
| Sacramento and San Bernardino +0.663 10.7 |
| San Bernardino and Winnemucca +0.291 2.8 |
| |
+-------------------------------------------------------------------+
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 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 Sacramento 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 remembered 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 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
Jerusalem are positive. If 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
Mediterranean 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 coefficient 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 California 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 followed
nearly the same course as those of California and Palestine. This
apparent discrepancy seems to be explained by our previous conclusion
that changes of climate 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 conclusion 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 occurred at periods which seem on other evidence to have been
moister than the present. This argument has been much used, but it seems
to have little force. If the rainfall of a given region averages thirty
inches and varies from fifteen to forty-five, a famine will ensue if the
rainfall 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 mistaken use of ancient dryness
as proof of climatic uniformity is illustrated by the march of Alexander
from India to Mesopotamia. Hedin gives an excellent presentation 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 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 especially wet year. In a desert region like
Makran, in southern Persia and Beluchistan, where the chief difficulties
occurred, the rainfall varies greatly from year to year. We have no
records from Makran, but the conditions 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 rainfall |
| during period |
| _1885_ _1905_ since |
| observations |
| began_ |
| Yuma, 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 Mexico, 3.99 19.50 8.62 |
| El Paso, Texas (on New |
| Mexico border), 7.31 17.80 9.06 |
| ---- ----- ----- |
| Average, 4.61 18.52 7.95 |
| |
+------------------------------------------------------------+
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 difficulty 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 southeastern 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 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 estimate 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 accomplished 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 almost 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 conjunction 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 Alexander'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,[28] 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 past. After stating various less
important reasons for believing that the climate of Palestine has not
changed, he discusses vegetation. The following quotation indicates 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 conclusions
which can be based upon the historical and geographical evidence of
the Bible. In the absence of rain gauge and thermometric 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 determined by temperature;
thus it does not reach full maturity or produce ripe fruit of good
quality below the mean annual temperature of 69 deg.F. The isothermal of
69 deg. crosses southern Algeria near Biskra; it touches the northern
coasts of Cyrenaica near Derna 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 deg.. Between the isotherms of 68 deg. and 64 deg.,
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 deg., 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.
The date palm therefore affords a test of variations in mean annual
temperature of three grades between 62 deg. and 69 deg..
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 highlands 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 xxiv: 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.[29]
In the main Gregory's conclusions seem to be well grounded, although
even according to his data a change of 2 deg. or 3 deg. in mean temperature
would be perfectly feasible. It will be noticed, however, that they
apply to temperature and not to rainfall. They merely prove that two
thousand years ago the mean temperature of Palestine and the neighboring
regions was not appreciably different from what it is today. This,
however, is in no sense out of harmony with the hypothesis of climatic
pulsations. Students of glaciation believe that during the last glacial
epoch the mean temperature of the earth as a whole was only 5 deg. or 6 deg.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 culmination of the
last glacial epoch, the change in two thousand years has been of large
dimensions. Yet this would require 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 compared 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 deg.C. On the other hand, the ten years of least
rainfall had an average of 483 mm. and a mean temperature of 9.35 deg.. In
other words, a difference of 137 mm., or 39 per cent, in rainfall was
accompanied by a difference of only 0.2 deg.C. in temperature. Such
contrasts 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, Mississippi,
and Shreveport, Louisiana, and the two maritime 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,[30] 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 deg.F. during the more rainy decade and 66.94 deg.F. during
the less rainy decade--a difference of only 0.42 deg.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
component. Similar relationships are apparent in other places. We select
Jerusalem because we have been discussing Palestine. At the time of
writing, the data available in the _Quarterly Journal of the Palestine
Exploration 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 temperature of 62.04 deg.F. The twelve with least rain
had 24.4 inches and a temperature of 62.44 deg.. A difference of 40 per cent
in rainfall was accompanied by a difference of only 0.4 deg.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
persist 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 hypotheses of uniformity and of progressive change--that 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 unhealthful
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 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 credible that such a
people would have lived in the worst possible 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 storminess 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 climate, 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 atmosphere,
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 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.
Volcanic dust, however, cannot account for such a condition, 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 permits in one direction, but there have been greater
extremes in the same direction. The thermal solar hypothesis is likewise
unable to explain the observed phenomena, for neither it nor the
volcanic hypothesis offers any explanation of why the climate varies in
one way in Mediterranean 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 chapter we shall
study this matter more fully, for it can best be appreciated by
examining the course of events in a specific century.
FOOTNOTES:
[Footnote 16: Much of this chapter is taken from The Solar Hypothesis of
Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]
[Footnote 17: 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.]
[Footnote 18: J. Hann: Klimatologie, Vol. 1, 1908, p. 352.]
[Footnote 19: H. C. Butler: Desert Syria, the Land of a Lost
Civilization; Geographical Review, Feb., 1920, pp. 77-108.]
[Footnote 20: 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.]
[Footnote 21: H. Leiter: Die Frage der Klimaanderung waherend
geschichtlicher Zeit in Nordafrika. Abhandl. K. K. Geographischen
Gesellschaft, Wien, 1909, p. 143.]
[Footnote 22: A most careful and convincing study of this problem is
embodied in an article by J. W. Smith: The Effects of Weather upon the
Yield of Corn; Monthly Weather Review, Vol. 42, 1914, pp. 78-92. On the
basis of the yield of corn in Ohio for 60 years and in other states for
shorter periods, he shows 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.]
[Footnote 23: 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 Relation of Precipitation to Tree Growth,
in the Monthly Weather Review, Vol. 41, 1913.]
[Footnote 24: The dotted line is taken from Palestine and Its
Transformation, pp. 327 and 403.]
[Footnote 25: M. A. Stein: Ruins of Desert Cathay, London, 1912.]
[Footnote 26: In the preparation and interpretation of this table the
help of Mr. G. B. Cressey is gratefully acknowledged.]
[Footnote 27: 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.]
[Footnote A: One year interpolated.]
[Footnote 28: J. W. Gregory: Is the Earth Drying Up? Geog. Jour., Vol.
43, 1914, pp. 148-172 and 293-318.]
[Footnote 29: Geog. Jour., Vol. 43, pp. 159-161.]
[Footnote 30: See A. J. Henry: Secular Variation of Precipitation in the
United States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.]
CHAPTER VI
THE CLIMATIC STRESS OF THE FOURTEENTH CENTURY
In order to give concreteness to our picture of the climatic 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 Christian 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 intensified, 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[31]
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 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 to note that
of the nineteen great floods on record eighteen occurred in the cold
season between the autumnal and vernal equinoxes.
The second remarkable phenomenon mentioned by the chronicles is the
freezing of the entire Baltic, which occurred many times during the
cold winters of these centuries. On such occasions it was possible
to travel with carriages over the ice from Sweden to Bornholm and
from Denmark to the German coast (Lubeck), and in some cases even
from Gotland to the coast of Estland.
Norlind[32] 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
Germany on the ice.
Not only central Europe and the shores of the North Sea were marked by
climatic stress during the fourteenth century, but Scandinavia also
suffered. As Pettersson puts it:
On examining the historic (data) from the last centuries of the
Middle Ages, Dr. Bull of Christiania has come to the conclusion 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 corn [wheat] for bread had to be imported
from Luebeck, Rostock, Wismar and so forth. The Hansa Union undertook
the importation and obtained 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. In 1367 corn was imported from Luebeck to a value of
one-half million kroner. The trade balance inclined to the
disadvantage of Norway whose sole article of export at that time
was dried fish. (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
Norway.) Dr. Bull draws a comparison with the conditions described
in the Sagas when Nordland [at the Arctic Circle] produced enough
corn to feed the inhabitants of the country. At the time of
Asbjoern Selsbane the chieftains in Trondhenaes [still farther north
in latitude 69 deg.] grew so much corn that they did not need to go
southward to buy corn 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 misgovernment and political strife as
has hitherto been taken for granted.
During this same unfortunate first half of the fourteenth century
England also suffered from conditions which, if sufficiently
intensified, might be those of a glacial period. According to Thorwald
Rogers[33] the severest famine ever experienced in England was that of
1315-1316, and the next worst was in 1321. In fact, 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 Rogers, "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, which 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 level. The severity of the
famine may be judged from the fact that previous to the Great War the
most notable scarcity of wheat in modern England and the highest
relative price was in December, 1800. At that time wheat cost nearly
three times the usual amount, instead of five as in 1316. During the
famine of the early fourteenth century "it 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 continental 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 conditions which prevail at the present time
during periods of many sunspots, as indicated by the growth of trees at
Eberswalde in Germany and by the number of storms in winter as compared
with summer. The prevalence of droughts, especially 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 season
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 certain times the cold area of high pressure, which is
the predominating characteristic of a continent during the winter,
apparently spread out over the neighboring ocea
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