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THE EVOLUTION OF CLIMATE
THE EVOLUTION
OF CLIMATE
BY
C. E. P. BROOKS,
M.Sc., F.R.A.I., F.R.Met.Soc.
WITH A PREFACE BY
G. C. SIMPSON, D.Sc., F.R.S.,
DIRECTOR OF THE METEOROLOGICAL OFFICE
LONDON: BENN BROTHERS, LIMITED
8 BOUVERIE STREET, E.C.4
1922
PREFACE
Geologists very early in the history of their science, in fact as
soon as fossils began to be examined, found indisputable evidence
of great variations in climate. The vegetation which resulted in
the coal measures could have grown only in a sub-tropical climate,
while over these are vast remains of ice-worn boulders and scratched
rocks which obviously have been left by ice existing under polar
conditions. Such records were not found only in one region, but
cropped up in juxtaposition in many parts of the world. Remains of
sub-tropical vegetation were found in Spitzbergen, and remains of an
extensive ice-sheet moving at sea-level from the south were clearly
recognized in central and northern India. At first it was simply
noticed that the older fossils generally indicated a warmer climate,
and it was considered that the early climate of a globe cooling from
the molten state would be warm and moist, and so account for the
observed conditions. It was recognized that the ice remains were
relatively recent, and so far as a cause for the Ice Age was sought
it was considered that astronomical changes would be sufficient.
It was only when geologists began to find records of ice ages
far anterior to the Carboniferous Age, and astronomers proved by
incontrovertible observations and calculations that changes in the
earth’s orbit, or its inclination to that orbit, could not account
for the ice ages, that the importance and inexplicability of the
geological evidence for changes of climate came to be clearly
recognized.
During the last few years much study has been given to
“palæoclimatology,” but such a study is extremely difficult.
Only a very small fraction of the total surface of the earth can
be geologically examined, and of that fraction a still smaller
proportion has up to the present been studied in detail. There has
been a great tendency to study intently a small region and then to
generalize. The method of study which has to be employed is extremely
dangerous. A geological horizon is determined by the fossils it
contains. Wherever fossils of a certain type are found the strata are
given the same label. Isolated patches correlated by their fossils
are found in different parts of the world, and it is frequently
assumed not only that these rocks were laid down at the same time,
but that the conditions which they indicate existed over the whole of
the earth’s surface simultaneously. Thus geologists tell us that the
climate of the Carboniferous Age was warm and damp; of the Devonian
Age cool and dry; of the Eocene Age very warm; of the Ice Age very
cold.
But has the geologist given sufficient attention to the climatic
zones during the various geological climates? It is true that the
geologist has definitely expressed the view that in certain ages
climatic zones did not exist; but from a meteorological point of
view it is difficult to see how the climate could have been even
approximately the same in all parts of the world if solar radiation
determined in the past as in the present the temperature of the
surface of the earth.
The climatic zones of the various geological periods will need much
closer study in the future; the data hardly exist at present, and the
great area covered by the ocean will always make the study difficult
and the conclusions doubtful. Admitting, for the sake of argument
only, large changes in average conditions, but with zonal variations
of the same order of magnitude as those existing to-day, the slow
changes from period to period will cause any given climatic state
to travel slowly over the surface of the earth, and this will so
complicate the problem as to make it doubtful whether any conclusions
can be reached so long as the same criteria are used to determine
both the geological epoch and the climatic conditions.
These considerations apply more particularly to the earlier records,
while Mr. Brooks has confined his work chiefly to the later records,
beginning with those of the Great Ice Age, in which climatic zones
are clearly indicated by the limits of the ice; but in this problem
one cannot confine one’s attention to a portion of the record, for
the test of any explanation must be its sufficiency to explain all
the past changes of climate. One will not be satisfied with an
explanation of the Great Ice Age which does not explain at the same
time the records of earlier ice ages, of which there is indubitable
evidence in the Permo-Carboniferous and Pre-Cambrian periods, and
the records of widespread tropical or sub-tropical conditions in the
Carboniferous and Eocene Ages. Whether Mr. Brooks’ theory for the
cause of the recent changes of climate satisfies this criterion must
be left to each reader to decide.
As Mr. Brooks says, the literature on this subject is now immense,
and it is most unsatisfactory literature to digest and summarize. In
the first place, many of the original observations which can be used
in the study of past climates are hidden away in masses of purely
geological descriptions, and a great deal of mining has to be done
to extract the climatic ore. Then, again, most of the writers who
have made a special study of climatic changes have had their own
theoretical ideas and most of their evidence has been _ex parte_. To
take a single example, for one paper discussing dispassionately the
evidence for changes in climate during the historical period, there
have been ten to prove either that the climate has steadily improved,
steadily deteriorated, changed in cycles or remained unchanged. It is
extremely difficult to arrive at the truth from such material, and
still more difficult to summarize the present state of opinion on the
subject.
It may be complained that Mr. Brooks has himself adopted this same
method and has written his book around his own theory. But was
there any alternative? There are so many theories and radically
different points of view that no writer could confine himself to the
observations and say what these indicate, for the indications are so
very different according to each theory in turn. And new theories are
always being propounded; since Mr. Brooks commenced to write this
book, Wegener has put forward his revolutionary theory according
to which the polar axis has no stability, and the continents are
travelling over the face of the globe like debris on a flood. Where
is there solid ground from which to discuss climatic changes if the
continents themselves can travel from the equator to the pole and
back again in the short period of one or two geological epochs?
Mr. Brooks has studied deeply geology, anthropology, and meteorology,
and he has considerable mathematical ability. By applying the latter
to the results of his studies he has developed a theory for the cause
of climatic changes based on changes of land and sea area, and on
changes of elevation of land surfaces, and naturally he has made this
theory the basis of his work.
That there will be some who are not able to agree with him as to
the sufficiency of the causes he invokes, or who may even question
whether he also has not taken for granted what others dispute, goes
without saying; but all will agree that he has presented a difficult
subject in a clear and concise way, and that meteorologists (and may
I add geologists?) owe to him a deep debt of gratitude.
G. C. SIMPSON.
CONTENTS
PAGE
PREFACE 5
INTRODUCTION 11
I. FACTORS OF CLIMATE AND THE CAUSES OF CLIMATIC FLUCTUATIONS 15
II. THE CLIMATIC RECORD AS A WHOLE 32
III. CONDITIONS BEFORE THE QUATERNARY ICE AGE 42
IV. THE GREAT ICE AGE 47
V. THE GLACIAL HISTORY OF NORTHERN AND CENTRAL EUROPE 55
VI. THE MEDITERRANEAN REGIONS DURING THE GLACIAL PERIOD 68
VII. ASIA DURING THE GLACIAL PERIOD 76
VIII. THE GLACIAL HISTORY OF NORTH AMERICA 86
IX. CENTRAL AND SOUTH AMERICA 97
X. AFRICA 103
XI. AUSTRALIA AND NEW ZEALAND 109
XII. THE GLACIATION OF ANTARCTICA 114
XIII. THE CLOSE OF THE ICE AGE—THE CONTINENTAL PHASE 118
XIV. THE POST-GLACIAL OPTIMUM OF CLIMATE 127
XV. THE FOREST PERIOD OF WESTERN EUROPE 136
XVI. THE “CLASSICAL” RAINFALL MAXIMUM, 1800 B.C. TO A.D. 500 140
XVII. THE CLIMATIC FLUCTUATIONS SINCE A.D. 500 149
XVIII. CLIMATIC FLUCTUATIONS AND THE EVOLUTION OF MAN 159
XIX. CLIMATE AND HISTORY 162
APPENDIX—THE FACTORS OF TEMPERATURE 166
INDEX 169
INTRODUCTION
The following study is an attempt to reconstruct in some detail
the sequence of climatic changes through which the world passed
during that important stage of its geological history which is
variously known as the Ice Age or Glacial Period, the Pleistocene,
the Quaternary, or the Human Period. That time saw the growth of
humanity from a primitive stage but little removed from the higher
animals to the beginnings of a complicated civilization, and it saw
that human life spread from its cradle or cradles to the ends of the
earth; it saw the configuration of the globe passing through a series
of modifications which ended by establishing the physical geography
of the present day. Finally, it saw a series of startling changes
of climate which almost merit the term “Revolutions” of the old
catastrophic geologists, at the conclusion of which we can trace the
gradual development of the climatic conditions of the present day.
In short, it is a period of immense interest which has a personal
application lacking in the remoter parts of geological time, and for
that reason it is worthy of the fullest study.
On the geological side the literature of the Ice Age is immense, and
is beyond the power of any one man to master. Volumes might be, and
not infrequently have been, written on the glacial geology of areas
limited to a few square miles, or even on the deposits of a single
section. On the archæological side the literature is not yet so
voluminous, but is technical and conflicting in a high degree. It is
only when we seek the contributions of competent meteorologists that
we find a serious gap in the literature. Nor is this surprising, for
meteorologists are still so much occupied with the present vagaries
of the weather, that few of them have time to extend their researches
into the geological past. Yet this is eminently a case where the past
is the key to the present, and it may be that the solution of many
problems which meteorologists have hitherto faced in vain will yet be
suggested by studies of the climatic changes of the Ice Age.
The writer’s excuse for setting down his views is that he is
intensely interested in all three sciences—geology, anthropology,
and meteorology. The combination of these three subjects naturally
ended in specialization on their common meeting place, and led him to
hope that he could assist his fellow geologists and anthropologists
by acquainting them with some of the bearings of meteorology on
their subject, and could open out to his fellow meteorologists a
fascinating branch of their science.
The Quaternary, however, was not the only geological period to
exhibit the phenomena of an Ice Age, and in order that we may more
fully understand the status of the Quaternary Ice Age in the long
succession of geological climates, and also to avoid the charge of
presenting part of the evidence only, a brief discussion of the
climates of the earlier periods has been attempted. The plan of
the work is as follows: Chapter I deals generally with the causes
of climatic fluctuations and with the meteorology of an Ice Age.
Chapter II gives a brief account of the climatic record as a whole,
and Chapter III deals with the Tertiary period considered as leading
up to the Quaternary Ice Age. Chapter IV discusses the subdivisions
of the Glacial period, and the conflict between advocates of one
and of repeated glaciations. Chapters V to XII give brief accounts,
_from the standpoint of a meteorologist_, of the glacial history of
Northern Europe, the Mediterranean Region, Asia, North and South
America, Africa, Australia, and the South Polar regions. In Chapters
XIII to XV post-Glacial climatology is considered. Chapters XVI and
XVII deal with the major climatic fluctuations of the “historic”
period, and finally, in Chapters XVIII and XIX, is a short discussion
of the influence of climate on the evolution and history of man. A
brief bibliography concludes each chapter.
THE EVOLUTION OF CLIMATE
CHAPTER I
FACTORS OF CLIMATE AND THE CAUSES OF CLIMATIC FLUCTUATIONS
The climate of any point on the earth’s surface depends on a complex
of factors, some of them due to influences arriving from outside the
earth, and others purely terrestrial. Since any variations of climate
must be due to a change in one or more of these, it is necessary,
before we can discuss changes of climate, to consider briefly what
the factors are.
The only important extra-terrestrial factor of climate is the amount
of radiant energy which reaches the borders of the earth’s atmosphere
from the heavenly bodies—that is, from the sun, for the moon and
stars can be ignored in this connexion. The only other conceivable
factor is the arrival of meteorites, bringing kinetic energy which is
converted into heat, and introducing cosmic dust into the atmosphere;
but it is highly improbable that this is of appreciable effect.
The amount of solar radiation[1] which reaches the earth depends in
the first place on the total radiation emitted by the sun, and in
the second place on the distance of the earth from the sun, both of
which quantities are variable. It has been calculated that if other
factors remained unchanged an increase of ten per cent. in the solar
radiation would raise the mean temperature of the earth’s surface
by about 7° C., or between 12° and 13° F., with, of course, a
corresponding fall for a decrease.
After the sun’s radiation reaches the outer limits of the earth’s
atmosphere its nature and intensity are modified by the composition
of the air through which it passes. In general the air itself is
very transparent to the small wave-lengths which make up the solar
rays, but the presence of fine dust, whether of volcanic or of cosmic
origin, has been shown by Humphreys to be a distinct hindrance to
their passage, so that volcanic eruptions of an explosive nature,
such as that of Krakatoa in 1883, La Soufrière (St. Vincent) in 1902,
or Katmai (Alaska) in 1912, may result in a fall of temperature over
the world as a whole.
The temperature of the earth is determined by the balance between
the radiation received from the sun and the terrestrial radiation to
space, and a decrease in the latter would be as effective in raising
the mean temperature as an increase in the former. The use of glass
for greenhouses depends on this principle; for glass is transparent
to heat rays of small wave-length, but is largely opaque to the rays
of greater wave-length which make up terrestrial radiation. Certain
constituents of the atmosphere, especially water-vapour, carbon
dioxide and ozone, are effective in this way, and variations in the
amount of these gases present may affect the temperature.
The angle at which the sun’s rays strike the earth’s surface is a
highly important factor. Within the Tropics the sun at midday is
nearly vertical throughout the year, and the mean temperature in
these regions is correspondingly high; on the other hand, during
the long polar night the sun is not seen for half the year, and
very low temperatures prevail. There is thus a seasonal variation
of the heat received from the sun in middle and high latitudes, the
extent of which depends on the “obliquity of the ecliptic,” i.e. the
inclination of the earth’s axis to the plane of its orbit round the
sun, and any changes in this factor must alter the seasonal variation
of climate.
Further, since the climate of any place depends so closely on its
latitude, it follows that if the latitude changes the climate will
change. A ship can change its latitude at will, but we are accustomed
to regard the position of the “firm ground beneath our feet”
relatively to the poles as fixed within narrow limits. This stability
has, however, been questioned from time to time, mainly on evidence
derived from palæoclimatology, and theories of climatic change have
been based on the wanderings of continents and oceans. Finally, local
climate is intimately bound up with the distribution of land and sea,
and the marine and atmospheric currents resulting therefrom, and on
elevation above sea-level, both of which factors, as we shall see,
have suffered very wide variations in the geological past.
Nearly all the theories which have been put forward to account for
geological changes of climate, and especially the occurrence of the
last or Quaternary Ice Age, are based on the abnormal variation of
one or other of the above factors, and we may consider them briefly
in turn. Very few have ever been taken seriously. In the first place,
we can at once dismiss fluctuations in the radiation emitted by
the sun as a cause of _great_ changes of climate. It is true that
many small fluctuations are traceable directly to this cause, such
as the eleven-year periodicity of temperature and rainfall; but
these fluctuations are, and must be, greater at the equator than at
the poles, while the fall of temperature during the Glacial period
reached its maximum near the poles and was least at the equator.
Moreover, there is not the slightest direct evidence in support of
such a theory, and it can only be admitted when all other hypotheses
have failed.
The “astronomical” theory of the cause of climatic fluctuations is
associated chiefly with the name of James Croll. Croll’s theory
connects abnormal variations of climate with variations, firstly of
the eccentricity of the earth’s orbit, and secondly of the ecliptic.
In periods of high eccentricity the hemisphere with winter in
aphelion is cold because the long severe winter is far from being
balanced by the short hot summer; at the same time the opposite
hemisphere enjoys a mild equable climate. This theory commanded
instant respect, and still finds a place in the text-books, but
difficulties soon began to appear. The evidence strongly suggests
that glacial periods did not alternate in the two hemispheres,
but were simultaneous over the whole earth; even on the equator
the snow-line was brought low down. Moreover, on Mars the largest
snow-cap appears on the hemisphere with its winter in perihelion.
Although Croll’s reasoning was beautifully ingenious he gave very few
figures; while the date which he gives for the conclusion of the Ice
Age, 80,000 years ago, has been shown by recent research to be far
too remote, 15,000 years being nearer the mark.
Croll’s theory has recently been revived in an altered form by R.
Spitaler, a Czecho-Slovakian meteorologist, who calculated the
probable alteration in the mean temperature of each latitude under
maximum eccentricity (0.7775) and maximum obliquity (27° 48′), the
distribution of land and water remaining unchanged. The results are
shown in the attached table, where - means that the temperature was
so much below the present mean, and + that it was so much above.
-------+-------------------------+--------------------------
| Aphelion December. | Aphelion June.
+-------------------------+--------------------------
| Winter. Summer. Year. | Winter. Summer. Year.
+-------------------------+--------------------------
| °F. °F. °F. | °F. °F. °F.
N. 60° | -9 +15 -1 | -5 -4 -1
30° | -13 +13 -2 | +1 -8 -2
Equator| -8 +4 -2 | +1 -6 -2
S. 30° | -6 +1 -2 | +3 -5 -2
60° | -2 -1 -1 | +1 -2 -1
-------+-------------------------+--------------------------
Spitaler claims that these differences are sufficient to cause
a glacial period in the hemisphere with winter in aphelion, but
from this point his theory departs widely from Croll’s. During the
long severe winter great volumes of sea water are brought to a low
temperature, and, owing to their greater weight, sink to the bottom
of the ocean, where they remain cold and accumulate from year to
year. But the water warmed during the short hot summer remains on the
surface, where its heat is dissipated by evaporation and radiation.
Thus, throughout the cold period, lasting about 10,000 years, the
ocean in that hemisphere is steadily growing colder, and this mass
of cold water is sufficient to maintain a low temperature through
the whole of the following period of 10,000 years with winter in
perihelion, which would otherwise be a genial interval. In this
way a period of great eccentricity becomes a glacial period over
the whole earth, but with crests of maximum intensity alternating
in the two hemispheres. Unfortunately the numerical basis of this
theory is not presented, and it seems incredible that a deficiency
of temperature could be thus maintained through so long a period.
Further, the difficulty about chronology remains, and the work brings
the astronomical theory no nearer to being a solution of the Ice Age
problem than was Croll’s.
The theory which connects fluctuations of climate on a geological
scale with changes in the composition of the earth’s atmosphere is
due to Tyndall and Arrhenius, and was elaborated by Chamberlin. The
theory supposed that the earth’s temperature is maintained by the
“blanketing” effect of the carbon dioxide in the atmosphere. This
acts like the glass of a greenhouse, allowing the sun’s rays to
enter unhindered, but absorbing the heat radiated from the earth’s
surface and returning some of it to the earth instead of letting it
pass through to be lost in space. Consequently, any diminution in the
amount of carbon dioxide present would cause the earth to radiate
away its heat more freely, so reducing its temperature. But it is
now known that the terrestrial radiation which this gas is capable of
absorbing is taken up equally readily by water-vapour, of which there
is always sufficient present, and variations of carbon dioxide cannot
have any appreciable effect.
Brief mention may be made here of a theory put forward by Humphreys,
who attributed glaciation to the presence of great quantities of
volcanic dust in the atmosphere. It would require an enormous output
of volcanic dust to reduce the temperature sufficiently; but in
any case the relation, if any, between vulcanicity and temperature
during the geological ages is rather the reverse of that supposed
by Humphreys, periods of maximum volcanic action coinciding more
frequently with high temperatures than with low. Perhaps the best
comment on Humphreys’ theory is that in 1902 F. Frech produced its
exact opposite, warm periods being associated with an excess of
vulcanicity and cold periods with a diminution.
The theory which attributes climatic changes in various countries
to variations in the position of the poles has been adduced in
two main forms. The first is known as the Pendulation Theory, and
supposes the existence of two “oscillation poles” in Ecuador and
Sumatra. The latitude of these points remains unchanged, and the
geographical poles swing backwards and forwards along the meridian
of 10 E. midway between them. Varying distances from the pole cause
changes of climate, and the movements of the ocean, which adjusts
itself to the change of pole more rapidly than the land, causes the
great transgressions and regressions of the sea and the elevation and
subsidence of the land.
An alternative form put forward by P. Kreichgauer, and recently
brought up again by Wegener, explains the apparent variations in
the position of the pole, not through a motion of the earth’s
axis, but by the assumption that the firm crust has moved over the
earth’s core so that the axis, remaining firm in its position,
passes through different points of the earth’s crust. The cause of
these movements is the centrifugal force of the great masses of the
continents, which are distributed symmetrically about the earth.
Imagine a single large continent resting on a sub-fluid magma in
temperate latitudes. Centrifugal force acting on this continent
tends to drive it towards the equator. There is thus a tendency for
the latitude of Europe to decrease. Similar forces acting through
geological ages have caused the poles and equator to wander at large
over the earth’s surface, and also caused the continents to shift
their positions relatively to one another. According to Wegener, in
the Oligocene there was only a single enormous continent, America
being united to Europe and Africa on the one hand, and through
Antarctica to Australia on the other; while the Deccan stretched
south-westwards nearly to Africa. The poles were in Alaska and north
of the Falkland Islands. The treatment in Kreichgauer’s original book
is speculative and at times fanciful; Wegener’s treatise appears
to demand more respectful attention, but is open to some vital
objections. In the first place, theories of this class demand that
the glaciation occurred in different regions at widely different
times, whereas we shall see in the following pages that the evidence
points very strongly to a double glaciation during the Quaternary
occurring simultaneously over the whole earth. This objection, which
was fatal to Croll’s theory in its original form, is equally fatal
to theories of pole-wandering as an explanation of the Quaternary
Ice Age. Secondly, we know that the last phase of this glaciation,
known as the Wisconsin stage in America and the Wurmian in Europe,
was highly developed only 20,000 years ago, and probably reached its
maximum not more than 30,000 years ago. In the last 5000 years there
has been no appreciable change of latitude, at least in Eurasia; and
it seems impossible for the extensive alterations required in the
geography of the world by Wegener’s theory to have taken place in so
short a time.
The great glaciation of the Permian period, referred to in the
next chapter, is a totally different matter. During this time the
ice-sheets appear to have reached their maximum area, and to have
extended to sea-level, in countries which are at present close to
the equator, while lands now in high latitudes remained unglaciated.
It is true that at the present day glaciers exist at high latitudes
under the equator itself, and given a ridge sufficiently steep and
a snowfall sufficiently heavy such glaciers would possibly extend
to sea-level; but even these conditions would not give rise to the
enormous deposits of true boulder-clay which have been discovered,
and there seems no way of avoiding the supposition of an enormous
difference in the position of the pole relatively to the continents
at this time.
Wegener’s theory alone, however, requires that glaciation should
always have been proceeding in some part of the globe (unless both
poles were surrounded by wide expanses of ocean), which is hard to
reconcile with the extremely definite and limited glaciations which
geological research has demonstrated. In these circumstances we may
tentatively explain the pre-Tertiary glacial periods by combining
Wegener’s theory of the movements of continents and oceans as a
whole with the theory of changes of elevation and of land and sea
distribution which is outlined below. That is to say, we may suppose
that the positions of the continents and oceans have changed,
relatively both to each other and to the poles, slowly but more
or less continuously throughout geological time; while at certain
periods the land and sea distribution became favourable for extensive
glaciation of the regions which at that time were in high latitudes.
The geographical theory, which states that the Ice Age was brought
about by elevation in high latitudes, and by changes in the land and
sea distribution, though never seriously challenged, has suffered
until recently from a lack of precision. The present author attempted
to remedy this by a close mathematical study of the relation of
temperature to land and sea distribution at the present day. The
method at attack was as follows: from the best available isothermal
charts of all countries the mean temperature reduced to sea-level was
read off for each intersection of a ten-degree square of latitude and
longitude, for January and July, from 70° N. to 60° S. latitude; this
gave 504 values of temperature for each of these months. Round each
point was next drawn a circle with an angular radius of ten degrees,
divided into east and west semicircles. The area of each semicircle
was taken as 100, and by means of squared paper the percentage of
land to the east and land to the west were calculated; finally, in
each month the percentage of the whole circle occupied by land, ice,
or frozen sea was calculated, this figure naturally being greater in
winter than in summer. The projection used was that of the “octagonal
globe,” published by the Meteorological Office, which shows the world
in five sections, the error nowhere exceeding six per cent.
These figures were then analysed mathematically, and from them the
effects on temperature of land to the east, land to the west, and
ice were calculated. The detailed numerical results are set out in
an Appendix; it is sufficient here to give the following general
conclusions:
(1) In winter the effect of land to the west is always to lower
temperature.
(2) In winter the effect of land to the east is almost negligible,
that is to say, the eastern shore of a continent is almost as cold
as the centre of the continent. The only important exception to this
rule is 70° N., which may be considered as coming within a belt of
polar east winds.
(3) In summer the general effect of land, whether to the east or
west, is to raise temperature, but the effect is nowhere anything
like so marked as the opposite effect in winter.
(4) The effect of ice is always to lower temperature.
(5) For every latitude a “basal temperature” can be found. This is
the temperature found near the centre of an ocean in that latitude.
This “basal temperature” is a function of the amount of land in the
belt of latitude. Poleward of latitude 20° an increase of land in the
belt lowers the winter basal temperatures very rapidly and raises the
summer basal temperature to a less extent. The “basal temperature”
is important, since it is the datum line from which we set out to
calculate the winter and summer temperatures of any point, by the
addition or subtraction of figures representing the local effect of
land in a neighbouring 10° circle.
As an illustration of the scale of the temperature variations which
may be due to geographical changes, suppose that the belt between
50° and 70° N. is entirely above the sea. Then we have the following
theoretical temperatures; for a point on 60° N. at sea-level:
January -30° F.; July 72° F.
Data for calculating the effect of ice are rather scanty, but the
following probable figures can be given, supposing that the belt in
question were entirely ice-covered:
January -30° F. (as for land); July 23° F.
Supposing that the belt were entirely oceanic, the mean temperature
in 60° N. would be:
January 29° F.; July 41° F.
These figures show how enormously effective the land and sea
distribution really is. From Appendix it is easy to calculate the
probable temperature distribution resulting from any arrangement
of land and water masses. Since the geography of the more recent
geological periods is now known in some detail, we have thus a means
of restoring past climates and discussing the distribution of animals
and plants in the light of this knowledge. Of course it is not
pretended that no other possible causes of great climatic variation
exist, but no others capable of seriously modifying temperature
over long periods are known to have been in operation. As we shall
see later, there are solar and other astronomical causes capable of
modifying climate slightly for a few decades or even centuries, but
these are insignificant compared with the mighty fluctuations of
geological time.
In applying the results of this “continentality” study to former
geological periods the method adopted is that of differences. The
present climate is taken as a standard, and the temperatures of,
for instance, the Glacial period are calculated by adding to or
subtracting from the present temperatures amounts calculated from
the change in the land and sea distribution. This has the advantage
of conserving the present local peculiarities, such as those due
to the presence of the Gulf Drift, but such a procedure would be
inapplicable for a totally different land and sea distribution, such
as prevailed during the Carboniferous period. That it is applicable
for the Quaternary is perhaps best shown by the following comparison
of temperatures calculated from the distribution of land, sea and ice
with the actual temperatures of the Ice Age as estimated by various
authorities (inferred fall):
------------+--------------------+--------------+-----------------
| | | Calculated Fall.
Locality. | Author. |Inferred Fall.+-----+-----+-----
| | | Jan.|July.|Mean.
------------+--------------------+--------------+-----+-----+-----
| | °F. | °F. | °F. | °F.
Scandinavia | J. Geikie | More than 20 | 36 | 18 | 27
East Anglia | C. Reid | 20 | 18 | 13 | 15
Alps | Penck and Brückner | 11 | 13 | 9 | 11
Japan | Simotomai | 7 | 9 | 5 | 7
------------+--------------------+--------------+-----+-----+-----
It is seen that the agreement is quite good.
There is one other point to consider, the effect of height. The
existence of a great land-mass generally implies that part of it
at least has a considerable elevation, perhaps 10,000 or 20,000
feet, and these high lands lave a very different climate to the
neighbouring lowlands. Meteorologists have measured this difference
in the case of temperature and found that the average fall with
height is at the rate of 1° F. in 300 feet. In the lower levels the
fall is usually greater in summer than in winter, but at 3000 feet it
is fairly uniform throughout the year. Consequently, quite apart from
any change in climate due to the increased land area, an elevation of
3000 feet would result in a fall of temperature of 10° F., winter and
summer alike. This reinforces the effect of increased land area and
aids in the development of ice-sheets or glaciers.
The effect of geographical changes on the distribution of rainfall
are much more complicated. The open sea is the great source of
the water-vapour in the atmosphere, and since evaporation is very
much greater in the hot than in the cold parts of the globe, for
considerable precipitation over the world as a whole there must
be large water areas in the Tropics. In temperate latitudes the
water-vapour is carried over the land by onshore winds, and some of
it is precipitated where the air is forced to rise along the slopes
of hills or mountains. Some rain falls in thunderstorms and similar
local showers, but the greater part of the rain in most temperate
countries is associated with the passage of “depressions.” These are
our familiar wind- and rain-storms; a depression consists essentially
of winds blowing in an anti-clockwise direction round an area of low
pressure.
These centres of low pressure move about more or less irregularly,
but almost invariably from west to east in the temperate regions.
They are usually generated over seas or oceans, and, since a supply
of moist air is essential for their continued existence, they tend to
keep to the neighbourhood of water masses or, failing that, of large
river valleys. In a large dry area depressions weaken or disappear.
Their tracks are also very largely governed by the positions of areas
of high pressure or anticyclones, which they tend to avoid, moving
from west to east on the polar side of a large anticyclone and from
east to west on the equatorial side. Since anticyclones are developed
over the great land areas in winter, this further restricts the paths
of depressions to the neighbourhood of the oceans at that season.
For all these reasons the tracks of depressions, and therefore the
rainfall, are intimately connected with the distribution of land and
sea. In winter there is little rainfall in the interior of a great
land-mass, except where it is penetrated by an arm of the sea like
the Mediterranean; on the other hand, the coasts receive a great deal
of rain or snow. The interior receives its rain mostly in spring or
summer; if the coastal lands are of no great elevation this will be
plentiful, but if the coasts are mountainous the interior will be
arid, like the central basins of Asia.
The development of an ice-sheet is equivalent to introducing
perpetual winter in the area occupied by the ice. The low temperature
maintains high pressure, and storm-tracks are unable to cross the
ice. At the present day depressions rarely penetrate beyond the
outer fringe of the Antarctic continent, and only the southern
extremity of Greenland is affected by them. Since the total energy
in the atmosphere is increased by the presence of an ice-sheet,
which affords a greater contrast of temperature between cold pole
and equator, storms will increase in frequency and their tracks
must be crowded together on the equatorial side of the ice-sheet.
In the southern hemisphere we have great storminess in the “roaring
forties”; south of Greenland the Newfoundland banks are a region
of great storminess. Hence, when an ice-sheet covered northern and
central Europe the Mediterranean region must have had a marked
increase of storminess with probably rain in summer as well as in
winter.
But if snow-bearing depressions cannot penetrate an ice-sheet, it may
be asked how the ice-sheet can live. The answer depends on the nature
of the underlying country. A land of high relief such as Antarctica
is, and as Greenland probably is, rising to a maximum elevation of
many thousand feet near its centre, draws its nourishment chiefly
from the upper currents which flow inward on all sides to replace
the cooled air which flows outwards near the surface. These upper
currents carry a certain amount of moisture, partly in the form of
vapour, but partly condensed as cirrus and even cumulus cloud.
At low temperatures air is able to hold only a negligible amount of
water-vapour, and this current, coming in contact with the extremely
cold surface of the ice, is sucked dry, and its moisture added to
the ice-sheet. Probably there is little true snowfall, but the
condensation takes place chiefly close to the surface, forming a
frozen mist resembling the “ice-mist” of Siberia. Even if the central
land is not high enough to reach into the upper current at its normal
level, the surface outflow of cold air will draw the current down
to the level of the ice. This will warm it by compression, but the
ice-surface is so cold that such warming makes little difference in
the end. This process of condensation ensures that after the ice
reaches a certain thickness it becomes independent of topography, and
in fact the centre of the Scandinavian ice-sheet lay not along the
mountain axis, but some distance to the east of it.
It is probably only on the edges of the ice-sheet, and especially in
areas of considerable local relief, that snowfall of the ordinary
type takes place, associated with moist winds blowing in the front
section of depressions which skirt the ice-edge. But when conditions
are favourable this source of supply is sufficient to enable these
local ice-sheets to maintain an independent life, merely fusing with
the edges of the larger sheet where they meet. Examples of such
local centres in Europe were the Irish and Scottish glaciers, and
at a later stage the Lofoten glaciers of the west of Norway, and in
America the Cordilleran glaciers of Columbia.
Penck and Brückner have demonstrated that in the Alps the increase of
glaciation was due to a fall of temperature and not to an increase
of snowfall. The argument is threefold: firstly, the lowering of the
snow-line was uniform over the whole Alpine area, instead of being
irregular as it would be if it depended on variations of snowfall;
secondly, the area and depth of the parent snow-fields which fed
the glaciers remained unchanged, hence the increased length of the
glaciers must have been due to decreased melting below the snow-line,
i.e. to lower temperatures; thirdly, the upper limit of tree-growth
in Europe sank by about the same amount as the snow-line. The same
conclusion holds for the great Scandinavian and North American
ice-sheets, the extension of which was undoubtedly due to a great
fall of temperature. In the case of the Alps the interesting point
has come to light that the fall of temperature, though due in part
to increased elevation, is mainly accounted for by the presence of
the Scandinavian ice-sheet, which extended its influence for many
miles beyond the actual limits of glaciation, so that its waxings and
wanings are faithfully reproduced in those of the Alpine glaciers,
even to the details of the final retreat after the last maximum.
It is only when we turn to tropical and sub-tropical regions that
we find variations of temperature unable to account for increased
glaciation. Not only were the changes of land and sea distribution
on a very much smaller scale than further north, but the Appendix
shows that the temperature value of a corresponding change of land
area is also very much less. But the high intertropical mountains—the
Andes and Kenya and Kilimanjaro in central Africa—which to-day bear
glaciers, in Quaternary times carried much greater ones. We cannot
call in a fall of temperature, for the reason above stated, and also
because at lower levels there is no evidence of colder conditions.
In the Glacial period the marine fauna was the same as to-day, and
mountains which now fall short of the snow-line by a few hundred
feet were still unglaciated even then. The only alternative is
increased snowfall on the higher mountains. Fortunately this fits in
well with meteorological theory. The rain and snowfall of tropical
regions depends, first of all, on the evaporation over the oceans.
But evaporation is profoundly influenced by the velocity of the wind;
and the wind, which in the Tropics represents the strength of the
atmospheric circulation, depends on the thermal gradient between the
equator and the poles; since there is no evidence of any appreciable
change of temperature over the Tropics as a whole, while there was
a very great fall in cold temperate and polar regions, the thermal
gradient, and therefore, ultimately, the tropical and sub-tropical,
rain and snowfall must have been very greatly increased. Hence the
increased glaciation of high mountains near the equator, and hence
also the evidence of “Pluvial periods” in the sub-tropical arid
regions on either side of the equator.
Thus during Glacial periods we have:
(1) Elevation in high latitudes caused a great increase of land areas
there.
(2) Both elevation and increase of land area resulted in a lowering
of temperature, materially increased by the gradual development of
great ice-sheets.
(3) These ice-sheets caused the development of subsidiary ice-sheets
on their southern and western borders.
(4) The lowering of temperature in high latitudes increased the
thermal gradient between equator and poles, resulting in:
(_a_) Increased snowfall, and hence increased glaciation on
high mountains near the equator.
(_b_) Pluvial periods in the sub-tropical arid regions.
BIBLIOGRAPHY
Humphreys, W. J. “Physics of the air.” Philadelphia, 1920. [Pt. 4,
pp. 556-629.]
Chamberlin, T. C. “An attempt to frame a working hypothesis of
the cause of glacial periods on an atmospheric basis.” _Journal
of Geology_ (American), Vol. 7, 1899, pp. 545-84, 667-85, 751-87.
[Carbon dioxide theory.]
Croll, J. “Climate and time in their geological relations.” London,
1875. “Discussions on climate and cosmology.” London, 1889.
[Eccentricity of earth’s orbit.]
Spitaler, R. “Das Klima des Eiszeitalters.” Prag, 1921.
Lithographed. [Eccentricity of earth’s orbit. Reviewed in the
_Meteorological Magazine_, London, September, 1921.]
Simroth, H. “Die Pendulationstheorie.” Leipzig, 1908.
Kreichgauer, P. “Die Aequatorfrage in der Geologie.” Steyr, 1902.
Wegener, A. “Die Entstehung der Continente und Ozeane.” _Die
Wissenschaft_, Bd. 66, Braunschweig, 1920.
Köppen, W. “Ueber Aenderungen der geographischen Breiten und des
Klimas in geologischer Zeit.” Stockholm, _Geografiska Annaler_, 2,
1920, pp. 285-99.
Brooks, C. E. P. “Continentality and temperature.” _Quarterly
Journal of Royal Meteorological Society_, Vol. 43, 1917, p. 169;
and 44, 1918, p. 253. [Influence of land and sea distribution.]
Enquist, F. “Eine Theorie über die Ursache der Eiszeit und die
geographischen Konsequenzen derselbe.” _Bull. Geol. Inst._, Upsala,
13, 1915, No. 2. [Influence of land and sea distribution.]
Hobbs, W. H. “Characteristics of existing glaciers.” New York,
1911. [Glacial anticyclone.]
TABLE OF GEOLOGICAL FORMATIONS
{ Recent
QUATERNARY { Pleistocene
{ Pliocene
TERTIARY or { Miocene
CAINOZOIC { Oligocene
{ Eocene
{ Cretaceous
MESOZOIC or { Jurassic
SECONDARY { Triassic
{ Permian
{ Carboniferous
{ Devonian
PALÆOZOIC { Silurian
{ Ordovician
{ Cambrian
PROTEROZOIC Pre-Cambrian