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The evolution of climate 1925 climatehistory
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THE EVOLUTION OF CLIMATE
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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 juxta-
position 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
sufficiënt.
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.
VI
THE EVOLUTION OF CLIMATE
During the last few years much study has been given
to “ palseoclimatology,” 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 deter-
mined by the fossils it contains. Wherever fossils of a
the strata are given the same
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 teil us that the chmate
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 sufficiënt 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 approxi-
mately 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 saké of argu-
ment 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
correlated by their fossils are
PREFACE
vii
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 indi-
cated 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 geolo-
gical 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 ex-
tremely difïicult to arrivé at the truth from such material,
viiï THE EVOLUTION OF CLIMATE
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
ehanges 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 condse way, and that meteorologists
(and may I add geologists ?) owe to him a deep debt of
gratitude.
G. C. Simpson
CONTENTS
Prefaci ....... y
Introduction to thi Second Edition . . .4
I. Factors of Climate and the Causes of Cumatic Fluctuations i 5
II. The Cumatic Record as a Whole . . . 32
III. CONDITIONS BEFORE THE QüATERNARV IcE AgE . . 4*
IV. The Great Ice Age . . . . • 47
V. The Glacial History of Northern and Central Europe 55
VI. The Mediterranean Regions ddring the Glacial Period 68
VII. Asia ddring 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 Edrope . . .136
XVI. The “ Classical ” Rainfall Maximum, 1800 b.c. to a.d. 500 140
XVII. The Climatic Fluctuations since a.d. 500 . . 149
XVIII. Cumatic Fluctuations and the Evolution of Man . . 159
XIX. CUMATE AND HlSTORY . . . . .102
Appendix—The Factors of Temperature . . . i6fl
169
INTRODUCTION TO THE
SECOND EDITION
On the whole, the first edition of “ The Evolution of
Climate ” met with a good reception. The meteoro-
logical interpretation of the succession of climatic
stages during the Quaternary Ice Age and subsequently
was especially welcomed, and it appears that with the
spread of our knowledge of the climatic conditions of
different parts of the world during the various geological
periods there will be increasing scope for work of this
kind. An important beginning has already been made
by F. Kerner-Marilaun (see later). The climatic
sequence should be a valuable guide to the complicated
stratigraphy of the Quaternary, and mainly on climatic
grounds it appeared to me most probable that the
Chellean industry, with its warm fauna, occupied the
Mindel-Riss interglacial. This conclusion was severely
criticized by several British archaeologists, on the ground
that work in France, especially by H. Obermaier, showed
that the Chellean industry probably feil in the Riss-
Wurm interglacial. The age of the Chellean is likely
to remain controversial for some time, but it may be
noted that the French archaeologist L. Mayet (i)1 places
the Chellean in the Mindel-Riss interglacial and at the
beginning of the Riss glaciation. A similar view is now
adopted by H. F. Osbom and C. A. Reeds (2) in a
valuable synthesis of the standards of Pleistocene classi-
fication; this is a reversal of the view which they
expressed in 1914. On the other hand, J. Reid Moir (3)
on the basis of his researches in East Anglia, and L.
Palmer (4) from work in south-east England, place the
1 These numbers refer to the Bibliography on page 12.
4
INTRODUCTION 5
Chellean in the Gunz-Mindel interglacial. There are
thus three views to choose from, andfuture researches
alone can show which is correct. The question is of
climatic importance, because the greater part of the
Chellean is admitted to have been warm.
With regard to the climatic effect of volcanic dust,
Dr. W. J. Humphreys informs me that his suggestion
was that volcanic dust may act in conjunction with
mountain building and increased elevation of the
continents to produce glaciation. On page 18 the
figure for the maximum eccentricity should of course
have been 0.07775. H. Gams and R. Nordhagen have
made a number of helpful criticisms and suggestions.
Most of these are referred to in the summary of their
recent book (17); they will be introduced into the main
text when opportunity offers.
The past two or three years have seen great activity
in the study of past climates, and only a few of
these researches can be alluded to here. Ellsworth
Huntington and S. S. Visher (5) have published a new
hypothesis of the main cause of climatic variations.
According to their view the climate of the earth is
largely governed by changes in solar activity, acting on
the position and intensity of the storm beits. An
increase in solar activity, represented by an increase in
the relative sunspot numbers, is considered to result in
an increase of storminess, together with some displace-
ment of the storm tracks. When such a period of
increased solar activity occurs with extensive and high
continents, and perhaps with other favourable con-
ditions, such as a paucity of C02, a glaciation results.
This is considered to account for the Quaternary glacia-
tion and probably also for that of the Permo-Carbon-
iferous period, in which the storm tracks lay very far
south, and higher latitudes remained unglaciated because
they were occupied by deserts. Periods of slight solar
activity and few sunspots had slight storminess and
steady winds from the equator towards the poles, hence
6 THE EVOLUTION OF CLIMATE
they were periods of mild and equable climate over the
whole earth. The variations of solar activity are con-
nected with changes in the distance of the nearest fixed
stars. The theory is attractive, but it presents several
very great difficulties. In particular the relationship,
if any, between sunspots and storminess at the present
day is still very obscure, and does not provide an adequate
basis for the enormous superstructure. In this country
at least it has not been well received.
A valuable summary of the palseoclimatological
evidence from the Antarctic has been presented by
C. S. Wright and R. E. Priestley (6). According to
this summary, the pre-Cambrian climate of Antarctica
was mainly warm temperate, with, however, indications
of frost action. In the Cambrian warm temperate to
tropical conditions prevailed ; in the Devonian possibly
temperate. In the Permo-Carboniferous period, during
the glaciation of the tropics, it appears that the high land
of Antarctica was an arid windswept desert, but in
sheltered lowlands a rich Glossopteris flora flourished.
There was a considerable seasonal range, but there is
no definite tracé of glacial conditions. In the Jurassic
a sub-tropical to warm temperate climate prevailed,
growing cooler through the Cretaceous, until in the
Eocene moraine-like deposits doubtfully suggest the
first Antarctic glaciation. In the Oligocene sub-
tropical to temperate conditions reappeared, followed
by the first undoubted glacial evidence. The Miocene
may have been a temperate interglacial period, but in
the Pliocene glacial conditions again appeared, and
persisted until the present, though with diminishing
intensity in recent times. This evidence must be taken
into account in future discussions of the causes of
climatic change.
F. Kerner-Marilaun (7) has studied the influence of
Permo-Carboniferous geography on the temperature
distribution, assuming a supply of solar energy similar
to that of to-day and the present position of the poles.
INTRODUCTION 7
He finds that under these conditions a high Coastal
range of hills or plateau in northern India would pro-
bably be glaciated. His assumptions include a cold
Arctic ocean, and it is doubtful if this is vahd, but the
paper is a useful indication of the extent to which geo-
graphical changes might modify the present more or less
Zonal distribution of climates. The climatic conditions
of Permo-Carboniferous time are peculiar and now
appear to be well defined. There was a large expanse
of ocean in the northern hemisphere, with several large
islands or small continents, in the Coastal regions of
which the climate of the Coal Measures prevailed, moist
and probably rather warm. Isolated mountain areas
in the northern hemisphere, however, bore glaciers.
In the southem hemisphere, in which the equatorial
continent extended much farther south, the hardier
Glossopteris flora developed in high latitudes, and the
climate was probably equable but cool. Thus there was
a considerable temperature difference between the two
hemispheres, and this would lead to winds Crossing the
equatorial continent from south to north, similar to the
south-west monsoon of India. These winds would
deposit great quantities of moisture on the hills, which
at altitudes of about ten thousand feet would fall as
snow, originating the great ice-sheets of this period.
An investigation along these lines appears to present
the only possibility of accounting for the inversion of
zones in the Permo-Carboniferous period, apart from
displacements of the poles or Continental drift.
The theory of mild polar climates has also been
investigated by F. Kerner-Marilaun (8). He found that
the land and sea distribution prevailing in the Upper
Jurassic and Middle Eocene periods would lead to
winter temperatures in the Arctic many degrees above
the present ones. He also found that the cooling effect
of the floating ice in the Arctic Ocean is so great that if
it could be cleared away the temperature over an open
ocean near the pole in January would be only a few
8 THE EVOLUTION OF CLIMATE
degrees below freezing point. For some reason he did
not put these two results together, and apparently he
failed to realize that his researches showed that during
the two periods chosen the Arctic Ocean must have
been free of ice. A recalculation of his figures on this
basis (9) gave for the Upper Jurassic a January tempera-
ture in 750 N., approximatdy equal to that now found
in the Scilly Isles, while in the Middle Eocene it was
only a few degrees lower. The probable winter tem-
peratures calculated on climatological grounds thus fall
into very good agreement with those required by
palaeobotanists from the evidence of fossil floras.
The views of M. Depéret on the correlation of the
various Quaternary stages by means of changes of level
have attracted a great deal of attention. According
to Depéret the various changes of level which he
traced in the Mediterranean during the Quaternary
were due mainly to movements of the sea and only
locally to movements of the land, and he tracés the
Mediterranean raised beaches round the Atlantic coast
to the Baltic and also up the river valleys to the glaciated
regions, where they pass into glacial moraines. I
accepted Depéret’s system as applied to the Mediter-
ranean, but did not take seriously his extension of it
to the glaciated regions. Osborn and Reed (2), after
a careful examination, also find difficulty in accepting
Depéret’s correlation of the northern drifts. On the
other hand, it has been widely accepted in Europe as
a great advance. An objection to die scheme is that
each stage except the last includes both a glacial and an
interglacial phase; thus the Sicilian includes the Gun-
zian or Scanian glaciation and the Gunz-Mindel inter-
glacial, the Milazzian includes the Mindelian and the
Mindel-Riss, the Tyrrhenian includes the Rissian and
the Riss-Wurm, ^and the Monastirian includes the
Wurmian.
A. R. Dwerryhouse (10) has reinvestigated the glacia-
tion of north-eastern Ireland. He finds that this area
INTRODUCTION 9
was covered first by Scottish ice from the Firth of Clyde,
and later by Irish ice from the hills of Donegal. The
two glaciations form part of a single maximum, and the
ice-sheets from the two centres were probably in contact
during part of the retreat of the Scottish ice. The
earlier work of Kilroe is mainly confirmed, with some
corrections of detail.
Prometheus:
The late-glacial and post-glacial history of the Baltic
continues to be actively studied, and a number of papers
on the subject have appeared in the past two years.
E. Antevs (11) has contendëd that the Ancylus elevation
in the south-west Baltic region has been over-estimated.
He considers that during Ancylus time the Baltic was
never a true lake, but was an inland sea connected with
the Atlantic by a narrow channel, and kept fresh by the
enormous volume of water supplied by the melting
Scandinavian ice-sheet. This view is accepted by
G. de Geer, but is denied by H. Munthe. It is admitted
that the water was fresh, and if there was free com-
munication with the Atlantic it seems improbable that
the amount of thaw water during the cold dry winter
would be sufficiënt to keep out the sea water. From
the climatological point of view, however, the important
point is that die inflow of sea water at a higher tempera-
ture was interrupted, and it does not seem to matter
greatly which view is correct.
I. Hogbom (12) has reinvestigated “ fossil dunes ” of
northern Europe, and concludes that they were formed
by dry winds from west-north-west during Finiglacial
time (ca. 7000-6000 b.c.) and not to periglacial easterly
winds, as formerly supposed. The type of pressure
distribution reconstructed from the dunes and other
evidence resembles that prevailing during the cold
spell of spring.
G. de Geer (13) has been investigating the annual
clay-varves of the late-glacial period in North America.
It will be remembered that by an examination of similar
annual layers in Sweden he arrived at an absolute
10 THE EV0LUTI0N OF CLIMATE
measure of the age of various stages of the retreat. He
considers that the succession of different thicknesses
in certain groups of annual layers in North America
bears so close a resemblance to parts of the Swedish
succession that they must refer to the same groups of
years, and on these grounds he has dated parts of the
final stages of the glacial period in North America.
The ice left the eastern end of Lake Champlain about
I,loo years before the end of the Ice Age in Sweden
(ca. 5000 b.c.). In Timiskaming (northern Ontario)
the recession was traced for over 600 years, the ice
leaving the district 297 years after the close of the Ice
Age in Sweden. This indicates that the melting of the
inland ice lasted somewhat longer in Canada than in
Sweden ; but de Geer considers that there can be no
more doubt as to the exact agreement between the
climatic conditions in the two regions. It is greatly
to be hoped that de Geer will publish a table showing
the relative thicknesses of each of his annual layers,
similar to that published by A. E. Douglass of the
width of annual tree-rings. Sir T. W. Edgeworth
David (14) has discovered similar banded clays associated
with the pre-Cambrian and Carboniferous tillites of
Australia, indicating a duration of 12,000 years in the
former case and about 4,000 years in the latter.
The fourfold division of the Quaternary Ice Age
adopted by Penck and Bruckner for the Alps is graduafly
being extended beyond the limits of Europe. Sir T. W.
Edgeworth David (15) accepts it for the glaciation of
Australia and Tasmania; he States that Tasmanian
man is now considered to date back probably to the
Rissian. The Australian type came later, but the
Talgai skull from near Warwick, Queensland, which is
placed in the Riss-Wurm interglacial, has Australian
affinities. As a result of Dainelli’s researches in the
Himalayan region, F. Loewe (16) has delineated a four-
fold glaciation of the western Himalayas. The second
ice-extension was the greatest, the positions of the
INTRODUCTION n
snow-line being: Glaciation I, unknown; II, 11,500
feet; III, 12,300 feet; IV, 12,550 feet. The fourth
glaciation was followed by retreat stadia as in the Alps.
No fossiliferous interglacial deposits are known, so that
the correlation with the Alpine stages is problematical.
Finally, I have to mention an important publication
by H. Gams and R. Nordhagen (17), deahng primarily
with the post-glacial climatic changes in central Europe,
but summarizing also the results of recent researches
in other parts of the Continent. Their summary
commences with the “ Great Interglacial ” (following
the Mindelian glaciation), in which they place the
Chellean industry. After this they intercalate a new
glacial stage, the Mühlbergian, followed by the short
Rabutz interglacial, in which they place the Acheulian.
This additional glaciation certainly clears up some
difficulties, and facilitates correlation with the (possibly)
five-fold American series (H. F. Osborn and C. A. Reeds
ignore the Iowan and so make the American series four-
fold); but much field-work will be required before
geologists will consent to such a modification of Penck
and Bruckner’s classic scheme. Gams and Nordhagen
consider the Rissian glaciation to have been the greatest,
instead of the Mindelian; it was followed by the
Rixdorf interglacial, also short. The Würrn glaciation
is divided into a number of stages—Schaffhauser Advance,
Laufen Oscillation, Mecklenburgian End Moraine,
Alleröd Oscillation, Fennoscandian End Moraine, fol-
lowed by the other familiar retreat stages. For the
post-glacial period the pioneer work of Axel Blytt is
regarded as thoroughly confirmed, and his terminology
is accepted. The temperature is considered to have
risen steadily through the dry Boreal Period (Continental
Phase, Azilian-Tardenoisian), the moist Atlantic Period
(Maritime Phase), and the dry sub-boreal Period (Later
Forest Phase), reaching a maximum 40 F. above the
present near 1000 b.c. It was at this period that the
hazel reached its greatest extension in northern Scan-
2
12 THE EVOLUTION OF CLIMATE
dinavia, and not during the boreal period, as formerly
believed. About B.c. 850 occurred a sudden deteriora-
tion of climate, which in the Alps had almost the appear-
ance of a catastrophe. This begins the sub-Atlantic
Period (Later Peat-bog Phase) which in the opinion of
the authors corresponds with the Daun readvance of
the Alpine glaciers; after this the climate of Europe
passed by a series of oscillations to its present level.
If the results of all the remaining papers published in
the past two or three years were discussed, this preface
would grow to the size of another book. In the face of
such an outpouring of material one’s views require
constant adjustment, and the most urgent need at the
moment, as pointed out by Osbom and Reeds (2), is a
stable framework of classification for the Quaternary
period, which shall embody at once the glacial advances
and retreats, the river terraces and raised beaches, the
succession of faunas, both land and marine, and of
floras, the human industries and the waves of climate.
Unfortunately we seem now to be farther than ever
from such a framework. Let us hope that this is the
darkest hour which precedes the dawn, and that some
generally accepted framework will soon emerge.
C. C. P. B.
January, 1925.
BIBLIOGRAPHY
(1) Mayet, Lucien. “ Corrélations géologiques et archéologiques des tempi
quaternaires.” Paris, C.-R, Ass. frattf. avanc. sci., 44 Session, Stras-
bourg, 1920, pp. 481-490.
(2) Osbom, Henry Fairfield, and Chester A. Reeds. “ Old and new stan-
dards of Pleistocene diyision in relation to the prehistory of man in
Europe.” Buil. Geol. Soc. America, 33, 1922, pp. 411-490.
(3) Moir, J. Reid. “ The Ice-age and Man.” Man, 1922, p. [52].
. “ The antiquity of man in East Anglia.” Science
Progress, July, 1924, p. 129. See also The Times, August 22, 1924.
BIBLÏOGRAPHY 13
(4) Palm er, L. S. “The Ice-age and man in Hampshire.” Man, 1922,
p. 106.
-------------, and J. H. Cooke. “The Pleistocene deposits of the
Portsmouth district and their relation to man.” London, Proc.
Geol. Ass., 34, 1923, p. 253.
(3) Huntington, Ellsworth, and S. S. Visher. “ Climatic changes, their
nature and cause.” New Haven, 1922.
(6) British (Terra Nova) Antarctic Expedition 1910-1913. “ Glaciology,”
by C. S. Wright and R. E. Priestley. London (Harrison & Sons), 1922.
(7) Kerner-Marilaun, F. “ Untersuchungen über die morphogene Klima-
komponente der permischen Eiszeit Indiens.” Wien, Sitzungsber.
Akai. Wiss., Matb.-nat. KL, Abt. x, 126 Bd., 1917, pp. 177-228.
(8) ------------------. “ Das akryogene Seeklima und seine Bedeutung
für die geologischen Probleme des Arktis.” Wien, Sitzungsber. Akai.
Wiss., 131, 1922, p. 133.
(9) Brooks, C. E. P. “ The problem of mild polar climates.” London,
Q. J. R. Meteor. Soc., 31, 1923.
(10) Dwerryhouse, A. R. “ The glaciation of north-eastern Ireland.”
London Q. J. G. S., 79, 1923, p. 332.
(11) Antevs, Ernst. “On the late-glacial and post-glacial history of the
Baltic.” New York, N. Y., Geogr. Ren., 12, 1922, pp. 602-612.
(12) Hogbom, I. “ Ancient inland dunes of northern and middle Europe.”
Stockholm, Geogr. Ann., 3, 1923, pp. 113-243.
(13) Geer, G. de. “ Correlation of late-glacial annual day-varves in North
America with the Swedish time scale.” Stockholm, Geol. Foren.
Forb., 43, 1921, p. 70.
(14) David, Sir T. W. Edgeworth. “The ' Varve Shales’ of Australia.”
Amer.J. Sd. (5), 3, 1922, p. 115.
(15) -------------------------. “ Geological evidence of the antiquity
of man in the Commonwealth, with especial reference to the Tas-
manian aborigines.” Hobart, Papers and Proc. R. Soc. F asmania,
1923, pp. 109-150.
(16) Loewe, F. “ Die Eiszeit in Kaschmir, Baltistan and Ladakh.” Berlin,
Zs. Ges. Erik., 1924, p. 42.
(17) Gams, H., and R. Nordhagen. “ Postglaziale Klimaanderungen
und Erdkrustenb ewegungen in Mitteleuropa.” München, Geogr.
Geseüscb., Landesk. Forscbungen, H.25, 1923.
THE EVOLUTION OF CLIMATE
CHAPTER I
FACTORS OF CLIMATE AND THE CAUSES OF CLIMATIC
FLU CTUATI ONS
T he climate of any point on the earth’s surf ace 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 intro-
ducing cosmic dust into the atmosphere; but it is highly
improbable that this is of appreciable effect.
The amount of solar radiation1 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 quanti-
ties 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 surf ace by about 70 C., or between 12°
1 By this term we shall in future understand only that part of it which
is responsible for thermal effects.
15
16 THE EVOLUTION OF CLIMATE
and 130 F., with, of course, a corresponding fall for a
decrease.
After the sun’s radiation reach.es 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 Soufriére (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 varia-
tions 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 t;he 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
FACTORS OF CLIMATE 17
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 palaeoclimatology, 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 espe-
cially 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 fluctua-
tions 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
18 THE EVOLUTION OF OLIMATE
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 eccen-
tricity 6^7775) and maximum obliquity (270 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.
Aphelior i December. Aphelion June.
Winter. Summer. Year. Winter. Summer. Year.
°F. °F. °F. °F. "F. °F.
N. 600 - 9 + 15 -1 -s -4 -1
30° -13 + 13 -2 + 1 -8 -2
Equator - 8 + 4 -2 + 1 -6 -2
S. 30° - 6 + 1 -2 +3 -s -2
6o° “ 2 - 1 -1 + 1 — 2 —1
FACTORS OF CLIMATE 19
Spitaler claims that these differences are sufficiënt to
cause a glacial period in the hemisphere with winter in
aphelion, but from. this point his theory départs 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 sufficiënt 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
altemating 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 sup-
posed 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
20 THE EV0LUTI0N OF CLIMATE
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 sufficiënt present, and variations of carbon
dioxide cannot have any appreciable effect.
Prometheus:
Brief mention may be made here of a theory put •
forward by Humphreys, who attributed gladation 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 mam 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 un-
changed, and the geographical poles swing backwards
and forwards along die 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 regres-
sions of the sea and the elevation and subsidence of
the land.
An alternative form put forward by P. Krdchgauer,
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 assump-
tion that the firm crust has moved over the earth’s core
FACTORS OF CLIMATE 21
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 sym-
metricaUy .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 treat-
ment 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 doublé 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
22 THE EVOLUTION OF CLIMATE
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 suffidently 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 glacia-
tion 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 geolo-
gical research has demonstrated. In these circumstances
we may tentatively explain the pre-Tertiary gladal
periods by combining Wegener’s theory of the move-
ments 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
FACTORS OF CLIMATE
23
never seriously challenged, has sufïered 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 6o° 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 semi-
circle 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 Meteoro-
logical 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 sufficiënt here to give the following general con-
clusions :
(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
24 THE EVOLUTION OF CLIMATE
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 6o° N. at sea-level:
January — 30° F.; July 720 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 230 F.
Supposing that the belt were entirely oceanic, the
mean temperature in 6o° N. would be :
January 290 F.; July 410 F.
These figures show how enormously effective the
land and sea distribution really is. From Appendix
it is easy to calculate the probable temperature distribu-
tion resulting from any arrangement of land and water
masses. Since the geography of the more recent geolo-
gical 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
FACTORS OF CLIMATE 25
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 pro-
cedure 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 distri-
bution of land, sea and ice with the actual temperatures
of the Ice Age as estimated by various authorities
(inferred fall) :
Locality. Author. Inferred Fall. Calculated Fall.
Jan. July. Mean.
«F. °F. •F. •F.
Scandinaria J. Geikie More than 20 36 18 27
East Anglia C. Reid 20 18 »3 IS
AIpi Penck and Brückner 11 13 9
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,
26 THE EVOLUTION OF CLIMATE
perhaps 10,000 or 20,000 feet, and these high lands
have a very different climate to the neighbouring low-
lands. Meteorologists have measured this difference
in the case of temperature and found that the average
fall with height is at the rate of i° 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 tempera te 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 tem-
perate countries is associated with the passage of “ depres-
sions.” 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,
FACTORS OF CLIMATE 27
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 rain-
fall 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 afïected by them.
Since the total energy in the atmosphere is increased by
the presence of an ice-sheet, which afïords 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 southem 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 northem 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
3
28 THE EVOLUTION OF CLIMATE
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 condensa-
tion 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 sufficiënt 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.
FACTORS OF CLIMATE
29
Penck and Brückner have demonstrated that in the
Alps the increase of glaciation was due to a fall of tem-
perature 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 Scandi-
navian 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.
Prometheus:
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 inter-
tropical mountains—the Andes and Kenya and Kiliman-
jaro 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
30 THE EV0LUTI0N OF CLIMATE
which now fall short of the snow-line by a few hundred
feet were still unglaciated even then. The only alter-
native 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 evapora-
tion 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 gradiënt 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 gradiënt, 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 sub-
sidiary ice-sheets on their Southern and western borders.
(4) The lowering of temperature in high latitudes
increased the thermal gradiënt 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.
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FACTORS OF CLIMATE
3i
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Spitaler, R. “ Das Klima des Eiszeitalters.” Prag, 1921. Lithographed.
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Simroth, H. “ Die Pendvdationstheorie.” Leipzig, 1908.
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Koppen, W. “ Ueber Aenderungen der geographischen Breiten und des
Klimas in geologischer Zeit.” Stockholm, Geografiska Annaler, 2,
1920, 1 p. 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 geographis-
chen Konsequenzen derselbe.” Buil. 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
Quaternary
Recent
Pleistocene
Pliocene
Tertiary or
Cajnozoic
Mesozoic or
Secomdary
Permian
Carboniferou8
Palasozoic
Devonian
Silurian
Ordovician
Cambrian
Proterozoic
Pre-Cambrian
CHAPTER II
THE CLIMATIC RECORD AS A WHOLE
It is a remarkable fact that one of the oldest known
sedimentary rocks is glacial in origin, and indïcated the
presence of an ice-sheet at a very early stage in the earth’s
history. This is a “ tilKte,” or boulder-clay, discovered
1 ""af. Coleman at the base of the Lower Huronian
Proterozoic) of Canada. It extends in an east
and west direction for 1000 miles across northern
Ontario, and northward from the northern shore of
Lake Huron for 750 miles. It rests on a scratched or
polished surface of various rocks, and the included
boulders are not always local, but some have been brought
from a considerable distance. AH these characters point
to a large ice-sheet.
Tracés of Proterozoic glaciations have been discovered
in various other parts of the world, and some of these
may be of the same age as the Canadian ice-sheet, but
they cannot yet be dated exactly. An interesting
example is western Scotland, which J. Geikie considered
to have been glaciated by ice from the north-west which
has since sunk into the North Atlantic. Other glacial
remains have no doubt been destroyed or deeply buried,
while some may stiH await discovery, and at present we
must be content to note the occurrence of a glacial
period at this time without attempting any description
of the distribution of climates over the globe. ‘
FoHowed a long period of milder climate indicated in
America by thick deposits of limestone with the remains
of reef-buüding organisms and other marine Hfe. This
32
CLIMATIC RECORD AS A WHOLE 33
period may have been interrupted at least once by the
recurrence of gladal conditions, but the evidence for this
is doubtful. It must be remembered that the duration
of the Proterozoic was very great, at least as long as all
subsequent time, while the relics of it which are now
known to us are few and scattered, so that much which
happened during that time is a closed book. It is not
until the very close of the Proterozoic that we again
find indisputable evidence of widespread gladal
action.
This second great glaciation was placed originally in
the earliest Cambrian (see table of geological formations
at the end of Chapter I), but later evidence shows that
it is slightly older than the oldest deposit which can be
referred to this series, and it may be designated the Pre-
Cambrian glaciation. Tillites of this age have been
found in the middle Yangtse region of China and in South
Australia (where they extend from 20 miles south of
Adelaide to 440 miles north, with an east-west extension
of 200 miles). Gladal deposits which probably refer to
this period have been found also in India, both in the
Deccan and near Simla, over a wide area in South Africa,
and in the extreme north of Norway. This distribution
suggests the presence of two centres of gladation, one
between China, India and Australia, and the other north-
west of Europe. The south-eastem of these was the
most extensive, and probably indicates a ring of gladated
continents surrounding the pole, rather than a single
enormous ice-sheet.
During the Cambrian all evidence of gladal action
ceases, and we have, instead, evidence of a warm, fairly
uniform climate in the abundant marine life. This
continued during the Ordovidan and became accentuated
during the Silurian period, when reef corals lived in the
seas of all parts of the world. Terrestrial deposits are
curiously lacking in all this series, and this suggests that
in the absence of any great mountaïn building and
elevation shallow seas extended over almost the whole
34 THE EVOLUTION OF CLIMATE
of the surface, accompaniè'd by mild oceanic chmates
extending to high latitudes.
At the close of the Silurian there was a period of
mountain building and the formation of continents.
The extinction of numerous species of marine organisms
and the rapid evolution of others point to the seas
becoming cooler and the stress of life more acute. In
the succeeding Devonian period there is evidence of
glacial conditions in South Africa in the form of a thick
series of quartzites with striated pebbles up to fifteen
inches long, but no typical boulder-clay has been dis-
covered. There are also some doubtful tracés from
England. The most noteworthy development of the
Devonian in the British Isles is, however, a thick deposit
of red sandstone (Old Red Sandstone) of the type that
is formed in shallow lagoons or inclosed basins, and
suggesting desert conditions, so that the rainfall of the
British Isles was probably slight.
These Continental conditions passed away towards the
close of the Devonian period, and once again extensive
warm oceans appear to have spread over a large part of
the globe, associated with the development of reef-
building corals. Climate continued warm and equable
throughout the greater part of the Carboniferous.
The important feature of this period is the great System
of coal-beds which extends through North America and
Europe to China, with northem and Southern limits
in 8o° N. (north-east Greenland and Spitzbergen) and
150 S. (Zambesi River). Wegener, summing up the
evidence, and considering especially the absence of
annual rings in the trees, concludes that the coal-beds are
the remains of the tropical rain-forest when the equator
lay across Europe some 30 degrees north of its present
position.
Towards the close of the Carboniferous period great
mountain-building set in, resulting in the formation of
the famous Gondwanaland, including south and central
Africa, Southern Asia, part of Australia and possibly
CLIMATIC RECORD AS A WHOLE 35
Brazil. From a consideration of the glacial evidence,
however, it appears, as will be seen later, that this was
probably a ring of neighbouring and partly adjoining
land areas rather than a single enormous continent.
At the same time the climate became cooler, and a hardier
vegetation, known as the Glossopteris flora, developed in
the Southern hemisphere, including woody trees with
annual rings indicating seasons. The large insects of
the coal forests which did not undergo a metamorphosis
were replaced by smaller types which did pass through
such a stage ; this change of habit is considered to be
due to the winters having become severe, so that the
insects learnt to hibernate through them. In the early
Permian, Gondwanaland was occupied by great ice-
sheets, remains of which in the form of tiUites of great
thickness, ice-wom surfaces and striated boulders have
been found in South Africa, Belgian Congo, and Togo-
land, Tasmania and widely separated parts of Australia,
peninsular and north-westem India, and probably also
Afghanistan. In India the glacial striae show that the
ice-sheet was moving north, while in South Africa it was
moving south, i.e. away from the present equator in
both cases. Widespread glacial remains have been found
also in Brazil, northem Argentine and the Falkland
Islands, and there are probable tracés near Boston in
North America, in Armenia, the Urals and the Alps, and
possibly also in England.
Wegener’s reconstruction of the geography of this
period places the south pole a little to the south-east of
South Africa, surrounded by a great continent composed
of the junction of Africa, South America, Antarctica,
Australia, and an extended Deccan added to by smooth-
ing out the folds of the Himalayas. This great circum-
polar continent he considers to have been the site of an
immense ice-cap. The North Pole lay in the Pacific
Ocean, so that almost all the remaining land areas enjoyed
temperate or tropical climates.
It is admitted that this peculiar distribution of glacial
36 THE EVOLUTION OF CLIMATE
remains apparently necessitates a position of the pole
somewhere near that described by Wegener, but the
theory of a single polar ice-cap extending beyond 50°
latitude on nearly all sides presents difficulties. From
the mechanism of the supply of snow to an ice-sheet
described in the precedlng chapter it follows that,
except close to the edges, all the moisture precipitated
must be brought in by upper currents. But even if we
take into account the increase in the strength of the
atmospheric circulation due to the introduction of an
ice-cap, there is a limit to the supply of moisture by this
process. All such mbisture has to cross the periphery,
and with increasing radius; the number of square miles
of area to each mile of periphery becomes greater, slowly
at first, then more and more rapidly. We shall see in
Chapter VIII that even the North American Quaternary
ice-sheet became unwieldy from this cause, and sufïered
several changes of centre.
Hence it seems that the rapprochement of the continents
in Permo-Carboniferous times need not have been so
complete as Wegener supposes, the glacial phenomena
being more readily explicable by a ring of continents
surrounding a polar sea, as in the case of the Quaternary
glaciation of the northern hemisphere. The local
Permian glaciations of Europe and North America, some
of which feil close to Wegener’s equator, are easily ex-
plicable as due to mountain glaciers similar to those of
Ruwenzori and other tropical mountains during the
Quaternary. There were interglacial periods in South
Africa, Brazil and New South Wales, which increase the
resemblance between the Permian and Quaternary Ice
Ages.
In Upper Permian times there was a widespread
development of arid climates, especially in the present
temperate parts of North America and Europe.
Wegener attributes this to the northerly position of the
equator bringing the sub-trppical desert belt (Sahara,
Arizona) to these regions. In the Trias these conditions
CLIMATIC RECORD AS A WHOLE 37
gradually gave place to another period of widespread
warm shallow seas, with abundant marine life and corals
extending over a large part of the world, even to Arctic
Alaska. In the United States there are the remains of
the forests of this period, in which the tree-trunks show
very little evidence of annual rings, indicating that the
seasonal changes were slight, so that the elfmate had
again become mild and oceanic.
In the Lias (Lower Jurassic), there was crustal move-
ment and volcanic action accompanied by land-formation
and a gradual lowering of temperature. There yvas a
great reduction in the abundance and geographical range
of corals, and most of the species of insects are of dwarf
types. There is, however, no evidence of glacial action.
The Upper Jurassic period appears to have been
warmer than the Lias. Insects of a large size and corals
again attained a very wide distribution, but there is
enough difference in the marine faunas of different
regions to indicate a greater development of climatic
zones than in the extremely oceanic periods such as the
late Triassic. Schuchert points out that the plants of
Louis Philippe Land in 63° S. are the same, even to
species, as those of Yorkshire.
In the Cretaceous period the elfmate was at first
similar to that of the Jurassic, and trees grew in Alaska,
Greenland and Spitzbergen. These trees, however, show
marked annual rings, indicating a considerable differentia-
tion of seasons, while trees of this age found in Egypt are
devoid of rings. Towards the close of the Cretaceous
there were many crustal movements and great volcanic
outbursts, accompanied by a considerable reduction of
temperature, which led to the extinction of many forms
of life and the rapid evolution of others. There is no
evidence of glacial action during the Cretaceous, however,
though at the beginning of the Eocéne there was a local
glaciation of the San Juan Mountains of Colorado.
According to W. W. Atwood this glaciation was doublé,
the first stage being of the Alpine mountain glacier type,
38 THE EVOLUTION OF CLIMATE
separated by an interglacial from the second stage, which
was of the Piedmont type (mountain glaciers spreading
out on the plain at the foot of the mountain). This
Eocene gladation has been found nowhere else, however,
and the climate of the Tertiary, which is discussed more
fully in the next chapter, was in general warm and
oceanic, becoming rigorous towards its close.
Summing up, we find that in the geological history of
the earth two main types of climate seem to have alter-
nated. Following on periods of great crustal movement,
and the formation of large land areas, the general climate
was cool, with a marked zonal distribution of temperature,
culminating during at least four periods in the develop-
ment of great sheets of inland ice. It is in such a period,
though, fortunately, not at its worst, that we are living
at present. During quiescent periods, on the other hand,
when these continents largely disappeared beneath the
sea, climate became mild and equable, and approached
uniformity over a great part of the world. At these
times, as soon as the surface water of the sea in high
latitudes began to cool, it sank to the bottom, and its
place was taken by warmer water from lower latitudes.
The oceanic circulation was very complete, but there
were practically no cold surface currents. Instead,
there was probably a general drift of the surface waters
from low to high latitudes (with an easterly trend owing
to rotation of the earth), and a return drift of cooled
water along the floor of the ocean. The formation of
sea-ice near the poles became impossible, while the wide-
spread distribution of marine life was facilitated.
The alternation of periods of crustal deformation with
periods of quiescence has frequently been noticed, and
has been termed the “ geological rhythm.” It may be
attributed to the gradual accumulation of small strains
during a quiescent period until the breaking point is
reached, when earth-movements take place until equi-
librium is restored, when the process is repeated.
The gradual erosion of the land by river and wave-
CLIMATIC RECORD AS A WHOLE 39
.
Prometheus:
action and the consequent shifting of the load provides
a certain amount of stress; but this is local, and calls for
local readjustments only. A more generally effective
agency may be the gradual slowing down of the earth’s
rotation under the influence of tidal friction. The
mechanism of this process was described by A. E. H.
Love (“ Nature,” 94, 1914, p. 254) : “ The surface of
the ocean, apart from waves and tides, is at any time a
figure of equilibrium answering to the speed of rotation
at the time, more oblate when the speed is greater, less
oblate when it is slower. Let us imagine that the litho-
sphere also is at some time a figure of equilibrium
answering to the speed of rotation at that time. If the
speed remained constant, the lithosphere would retain
this figure, and the matter within it would remain
always in the same configuration without having to
support any internal tangential stress. Now suppose
that the speed of rotation gradually diminishes. The
surface of the ocean will gradually become less and less
oblate. The lithosphere also will gradually become less
oblate, but not to such an extent as to make it a figure
of equilibrium answering to the diminished speed of
rotation, while the matter within it will get into a state
of gradually increasing internal tangential stress. The
effect on the distribution of land and water will be that
the depth of the ocean will gradually diminish in lower
latitudes and increase in higher latitudes, the latitudes
of no change being 350 ió' N. and S.
“ The internal tangential stress in the matter within the
lithosphere may increase so much that it can no longer
be supported. If this happens a series of local fractures
will take place, continuing until the lithosphere is again
adjusted much more nearly to a figure of equilibrium,
which will be less oblate than the original figure. The
effect on the distribution of land and water will be that
the depth of the ocean will increase rather rapidly and
spasmodically in lower latitudes and diminish in higher
latitudes.
40
THE EV0LUTI0N OF CLIMATE
“ Accordingly, the kind of geological change which the
theory of tidal friction would lead us to expect is a sort
of rhythmic sequence, involving long periods of com-
parative quiescence, marked by what Suess calls ‘ positive
movements of the strand,’ in the higher latitudes, and
‘ negative movements ’ in the lower, alternating with
comparatively short periods of greater activity, marked
by rise of the land around the poles and subsidences in
the equatorial regions.”
The main periods of adjustment under this scheme
fall at the beginning and end of the Proterozoic, in the
Permo-Carboniferous and in the Quaternary. The two
latter at least were periods of great earth-movement,
while the two former were also Continental periods, since
the land masses were large and high enough to develop
ice-sheets.
The diflicult question raised by the low latitudes
in which the Pre-Cambrian and Permo-Carboniferous
glaciations were chiefly developed cannot yet be regarded
as solved, but the geological facts speak strongly in favour
of ice-sheets rather than mountain glaciers, and practi-
cally speaking it is meteorologically impossible for large
ice-sheets to extend to sea-level in the Tropics while the
rest of the world enjoys a temperate climate. The only
escape seems to be to assume a position of the South Pole
somewhere between Africa, India and Australia through-
out the whole of the Proterozoic and Palaeozoic periods.
On the other hand, from the Jurassic onwards, there is
no real support to the hypothesis that the positions of
the poles were other than they are now. Wegener’s
explanation of the Quaternary Ice Age we have seen to
be untenable. The period of transition appears to lie
in the later Permian and Triassic. The Proterozoic
and Permo-Carboniferous glacial periods were much less
defimte in the north than in the south-east; but such as
they were they appear to have been most severe in the
east of North America, where the ice was coming from
the north ; there are also some glacial tracés in Europe.
CLIMATIC RECORD AS A WHOLE 41
This indicates that the position of the North Pole cannot
have been in the North Pacific Ocean, which is antipodal
to the South Indian Ocean. Hence it seems that what
we have to consider is not so much the wanderings of
the poles at large among the continents as the break-off
at the close of the Palaeozoic period of portions of the
Antarctic continent and their drift northwards towards
the equator. Without going into the mathematics of
the question, it seems just possible that the periodic
overloading of circumpolar continents by large ice-
masses could have this effect in the course of time,1 but
the suggestion is put forward tentatively for consideration
rather than as a definite hypothesis. We must be thank-
ful that in the next chapter we are on safer ground.
BIBLIOGRAPHY
Coleman, A. P. “ Climates and physical condition» of the early Pre-
Cambrian.” Geol. Mag. (6), Vol. t, 1914, p. 466.
Eckardt, W. R. “ Palaoklimatologie.” Sammlung Goschen, Leipzig, 1910.
Geikie, J. “ The Evolution of Climate.” Edinburgh, Scot. Geogr. Mag. (6),
1890, p. 57.
Grabau, A. W. “ Principles of Stratigraphy.” New York, 1913, pp. 74, et ieq.
Neumayr, M. “ Ueber klimatischen Zonen wahrend der Jura- und Kreide-
zeit.” Wien, Denkscbr. Ak. Sci., 47, 1883, p. 211.
Ramsay, W. “ Orogenesis und Klima.” Ofven af Fins ka Vetert! kaps. Soe.
Forb., 52, 1909-10, A, No. 11. Helsingfors, 1910.
Schuchert, C. “ Climates of Geologie Time, In: The climatic factor as
illustrated in arid America,” by Ellsworth Huntington. Washington,
1914, Pt. 2.
1 If the figure of the earth is adjusted to its speed of rotation before the
development of ice-sheets, the latter renders it too prolate, and there will
be a tendency for readjustment by the transference of mass towards the
equator.
CHAPTER III
CONDITIONS BEFORE THE QUATERNARY ICE AGE
The third of the great periods into which the geological
record is divided is known as the Tertiary. Throughout
most of its length it appears to have been chafacterized
by remarkably mild and equable climatic condïtions
extending into comparatively high latitudes, so that the
west coast of Greenland, for instance, had a flora of
almost sub-tropical aspect. Since the plants in question
—chiefly palms and cycads—are not of identical species
with their present-day representatives, it is unsafe to
base numerical estimates of the temperature upon them,
but it is at least obvious that these regions were warmer
than they are at present.
Let us glance for'a moment at the geography of the
Tertiary period. The most noticeable point is a great
expanse of sea over south-eastern Europe, including the
Mediterranean countries, extending away over the
Black Sea and Caspian, and stretching in a great arm to
the Arctic Ocean, south of Novaya Zemlya. The
geology of the archipelago north of Canada is not yet
well known, but it seems probable that there was a con-
siderable area of Tertiary ocean there also. The sea
further encroached on the present boundaries of North
America, both east and west, and on the north-eastern
coast of Asia. Bearing in mind the principles set out in
the first chapter, we can infer from these changes a great
increase in the winter temperature of-the regions along
the Arctic circle. The increase reached a maximum
on the west of Greenland and in western Siberia, but
42
BEFORE THE QUATERNARY ICE AGE 43
the west coast of Alaska also had a decidedly warm
climate in the late Miocene and Pliocene.
The basin of the Arctic Ocean, which already existed
at that stage, was raised to a temperature considerably
higher than the present by three great streams of warm
water flowing into it. If, as seems probable, the Bering
Strait was deeper, and the submarine ridge across the
North Atlantic less pronounced, the obstacles to the
outflow of cold water along the ocean floor were much
less than now. Finally, the winter temperatures of the
land masses to the south, and especially Siberia, being
already very much less severe owing to the sea over
Europe, the temperature of the water of the great rivers
flowing into the Arctic was not so low. For these reasons
the development of ice in the Arctic Ocean was very
much diminished, and possibly entirely absent, allowing
a great amelioration of the climate of Greenland, the
rigor of which is at present much enhanced by the ice
which flows down the Greenland Sea and round Cape
Farewell.
The cumulative effect of all these changes—greater
water area, greater inflow of warm surface water, less
inflow of cold river water, less ice-development—must
have been a mild equable rainy climate, entirely suitable
to a rich vegetation. The sub-tropical aspect of that
vegetation should not be stressed, for it was probably as
much an expression of the geological age of the period in
question as of its climate.
The objection may be raised that at the present time
the sub-antarctic islands in the great Southern Ocean
have the most maritime climate in the world, but are
not by any means places of opulent vegetation. The
difference is entirely accountea for by the presence of
the great ice-bearing Antarctic continent. lts effect
is twofold. Firstly, the glaciers shed into the Southern
Ocean an immense quantity of ice and ice-cold water
annually, which must have an appreciable effect on
temperature. Secondly, the presence of this ice-covered
4
44 THE EVOLUTION OF CLIMATE
continent and the floating ice in its neighbourhood
ex ten ding as far as the sixtieth parallel, by forming a
marked contrast with the warmer waters further north,
greatly intensifies the strength of the atmospheric
circulation in these regions, resulting in the development
of a great succession of severe storms which sweep the
sub-antarctic islands. There are no great land-masses
to break the force of the wind, and these latitudes are
among the stormiest, windiest regions of the earth—gale
succeeding gale, winter and summer alike; and it is
largely to the extraordinary power of the wind that we
must attribute the desolate appearance of the islands.
The picture we have drawn of the high northern
latitudes in early Tertiary times is vastly different. A
great warm ocean occupied the Arctic regions, fed by
three ocean currents analogous to the Gulf Drift, and
the fall of temperature was gradual from the tropic to
the pole. The return colder currents were mainly along
the ocean floor and with little ice-formation the storms
were few and not severe. On the western shores of the
continents mild rain-bearing south-west winds prevailed,
and a quiet moist warm atmosphere existed which was
especially favourable to plant life. This favourable
state of affairs lasted until well on in the Miocene, and
then changes set in. The land and sea distribution
underwent essential modifications. The great Tertiary
continent or archipelago which is believed to have
existed in the western Pacific, and whose last remaining
summits now form the scattered islands of that ocean,
gradually subsided, and in its place elevation began in
higher latitudes. Bering Strait became narrow and
shallow, and was probably for a time entirely closed, while
the connexion between the Arctic and Indian Oceans
was closed permanently, leaving in its lowest areas a chain
of great inland seas and lakes, of which the Caspian and
Aral Seas are now the greatest representatives. The
Canadian Archipelago was probably raised above its
present level, and formed a great northern extension of
BEFORE THE QUATERNARY ICE AGE 45
the American Continental area. The changes in the
Atlantic also were very extensive. The West Indies
were the site of a large and lofty Antillean continent;
further north a considerable land-mass existed east of
Newfoundland; Greenland was joined on the west to the
extended American continent, and considerably enlarged
to the south-east. Iceland, though it remained an
island, was elevated and probably neariy doubled in area,
and between Iceland and the north of Scotland was
developed a great submarine ridge, which may or may
not have risen above the sea in places. The British
Isles became a solid block of land, united with Continental
Europe across the English Channel and the great plain
which is now the North Sea. Scandinavia was elevated
by more than a thousand feet, and the elevation extended
at least as far as Spitzbergen. The Murman area had a
considerable extension. In eastern Asia the Sea of
Okhotsk was land and Japan was united to the mainland.
In the Southern hemisphere our knowledge is not
neariy so detailed. The presence of marine Middle-
Tertiary beds with temperate mollusca in Graham Land
and of plant-bearing beds in Seymour Island point to a
smaller Antarctic continent and very much warmer con-
ditions at this time in the South as well as in the North
Polar regions. For the close of the Tertiary, however,
we have strong grounds in the distribution of animals
and plants for assuming that the Antarctic continent was
greatly increased in size, with promontories uniting it to
Australia on the one hand and to South America on the
other. New Zealand was largely increased in area, and
South Africa probably extended further polewards.
The sub-antarctic islands attained a much greater area.
Conditions were ripe for the Ice Age in the Southern as
well as the northern hemisphere.
BIBLIO GRAPHY
Kerner von Marilaun, F. “ Synthese der morphogenen Winterklimate
Europas zur Tertiarzeit.” Wien, SitxBer, K. Akad. H tss, 12,2, 191 j,
pp. 233-98-
46 THE EVOLUTION OF CLIMATE
Oibom, H. F. “ The age of mammals in Enrope, Aria and North America.”
8vo. New York, 1910.
Nathorst, A. G. “ On the fossil floras of the Arctic regions as eridence of
geological climates.” London, Bot. J. 2, 1913, pp. 197-202; and
Washington, Report Smithsonian Inst., 1911.
Dall, W. H. “ On climatic conditions at Nome, Alaska, during the Pliocene.”
Amer. J. Sci., ser. 4, Vol. 23, 1907, p. 457.
Nansen, F. “ The bathymetrical features of the North Polar seas, with a
discussion of the Continental shelves and previous oscillations of the
shore-line.” Norviegian N. Polar Exped., 1893-96. Scientific Results,
Vol. 4.
Spencer, J. W. “ Reconstruction of the Antillean continent.” Buil. Amer.
Geol. Soc., 6, 1895, pp. 103-40.
Wilckens, D. “ Die Mollusken der antarktischen Tertiarformation.” Wiss.
Ergebn. der Scbteed. Sudpolar Exped., 1901-3, Bd. 3, 1911.
Hedley, C. “ The palaeographical relations of Antarctica.” London, Proc.
Ltnneean Soc., 124, 1911-2, p. 80.
CHAPTER IV
THE GREAT ICE AGE
As the land began to rise, the first effect was an increased
snowfall on the higher summits, and increased rainfall
on the rising coast lands. The rivers had an increasing
fall towards the sea, and rapidly carved out deep narrow
valleys, which were later developed by the ice into the
great fiords of Norway and other heavily glaciated
regions. But on the whole the first beginnings of the
Ice Age occurring towards the close of the Pliocene
period are obscure, and are likely always to remain so,
for the simple reason that the advancing and retreating
ice-sheets have wiped out most of the evidence of the
conditions which immediately preceded their advent.
The deteriorations of the climate had begun long before
the geographical changes outlined at the close of the
last chapter were complete, for mollusca of the Pliocene
beds in East Anglia indicate progressive refrigeration of
the North Sea at the same time as it became increasingly
shallow. At the close we have great shell-banks with
northern species which must have been piled up by
powerful easterly winds; these easterly winds show that
the storm tracks had been driven south of their present
course and suggest that the glacial anticyclone already
existed over Scandinavia. At the present day similar
shell-banks are forming on the coast of Holland under
the influence of the prevailing westerly winds. The
next series of deposits in this region are fresh-water
“ forest beds,” attributed to a greatly extended Rhine,
47
48 THE EVOLUTION OF CLIMATE
and belong to the period when the North Sea had become
a plain.
It is no part of the plan of this work to go over the
geological ground of the Quaternary Ice Age, which has
already been so frequently and so efficiently covered.
All I can hope to do is to give a brief general account of
the succession of climatic changes involved, necessarily
incomplete since so much of the world is at present
insufficiently explored for glacial tracés. But a certain
amount of explanatory introduction is necessary.
In Europe and North America there are distinct
tracés of several separate glaciations with “ interglacial ”
periods when the climate approached or became even
warmer than the present. The time-relations of these
glaciations are not yet fully worked out, but there seems
Httle doubt that they were contemporaneous in the two
continents. The correlation is not perfect, however,
siricF the United States geologists recognized altogether
five glaciations. The explanation appears to belfhat the
equivalent of the Rissian glaciation in Americais doublé;
two stages, the'Illinoian md.Iowan, being recognizable,
separated by a retreatoFthe ice. The series is as follows :
Alp.. Northern Europe. North America. Date. B.C.
I Gunz ? Jerseyan or Nebraskan i
II Mindel Lower Diluvium Kansan 430,000-370,000
III Rits Middle Diluvium (Illinoian) jlowan ) 130,000-100,000
IV Wurm Upper Diluvium Wi.consin 40,000- 18,000
The correlation is based on the amount of weathering
and erosion which the various deposits have undergone.
The time which has elapsed since the glaciers of the last
or Wurm stage were in active retreat has been estimated
by comparing the growth of peat-bogs, river-deltas, etc.,
THE GREAT ICE AGE 49
during historical times with that since the last
retreat of the ice. But the most conclusive method is
due to the Swedish geologist Baron de Geer, who has
actually counted the years since the ice in its final retreat
left any particular point between Ragunda and the south
of Sweden. The work is based on the idea that the
lamination observed in certain marine and lacustrine
clays in Sweden is seasonal, the thick coarse layers being
due to the floods produced by the rapid melting of the
ice in summer, and the thinner and finer layers being
due to the partial cessation of melting in winter. By
correlating one section with another it is possible to date
any particular layer with great exactness, and further
to prove the existence of several great climatic fluctua-
tions during the retreat. The topmost limit of the
section is given by the surface of the old floor of Lake
Ragunda in Jemtland, which received its waters from
one of the permanent glaciers and was accidentally
drained in 1796. De Geer finds that the edge of the last
great^ce-sheets lay over Southern Scania about 12,000
years ago, and further estimates 8000 years for the retreat
across the Baltic. These results are in general agreement
with those obtained by other methods, and we may
accordingly, with some confidence, put the date when
the ice-sheet of the Wurm glaciation finally left the coast
of Germany at about 18000 b.c.
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