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PAGE PREFACE 5

INTRODUCTION 11

INDEX 169

INTRODUCTION

The following study is an attempt to reconstruct in some detail the sequence of climatic changes through which the world passed during that important stage of its geological history which is variously known as the Ice Age or Glacial Period, the Pleistocene, the Quaternary, or the Human Period. That time saw the growth of humanity from a primitive stage but little removed from the higher animals to the beginnings of a complicated civilization, and it saw that human life spread from its cradle or cradles to the ends of the earth; it saw the configuration of the globe passing through a series of modifications which ended by establishing the physical geography of the present day. Finally, it saw a series of startling changes of climate which almost merit the term "Revolutions" of the old catastrophic geologists, at the conclusion of which we can trace the gradual development of the climatic conditions of the present day. In short, it is a period of immense interest which has a personal application lacking in the remoter parts of geological time, and for that reason it is worthy of the fullest study.

On the geological side the literature of the Ice Age is immense, and is beyond the power of any one man to master. Volumes might be, and not infrequently have been, written on the glacial geology of areas limited to a few square miles, or even on the deposits of a single section. On the archaeological side the literature is not yet so voluminous, but is technical and conflicting in a high degree. It is only when we seek the contributions of competent meteorologists that we find a serious gap in the literature. Nor is this surprising, for meteorologists are still so much occupied with the present vagaries of the weather, that few of them have time to extend their researches into the geological past. Yet this is eminently a case where the past is the key to the present, and it may be that the solution of many problems which meteorologists have hitherto faced in vain will yet be suggested by studies of the climatic changes of the Ice Age.

The writer's excuse for setting down his views is that he is intensely interested in all three sciences--geology, anthropology, and meteorology. The combination of these three subjects naturally ended in specialization on their common meeting place, and led him to hope that he could assist his fellow geologists and anthropologists by acquainting them with some of the bearings of meteorology on their subject, and could open out to his fellow meteorologists a fascinating branch of their science.

THE EVOLUTION OF CLIMATE

FACTORS OF CLIMATE AND THE CAUSES OF CLIMATIC FLUCTUATIONS

The climate of any point on the earth's surface depends on a complex of factors, some of them due to influences arriving from outside the earth, and others purely terrestrial. Since any variations of climate must be due to a change in one or more of these, it is necessary, before we can discuss changes of climate, to consider briefly what the factors are.

The only important extra-terrestrial factor of climate is the amount of radiant energy which reaches the borders of the earth's atmosphere from the heavenly bodies--that is, from the sun, for the moon and stars can be ignored in this connexion. The only other conceivable factor is the arrival of meteorites, bringing kinetic energy which is converted into heat, and introducing cosmic dust into the atmosphere; but it is highly improbable that this is of appreciable effect.

The amount of solar radiation which reaches the earth depends in the first place on the total radiation emitted by the sun, and in the second place on the distance of the earth from the sun, both of which quantities are variable. It has been calculated that if other factors remained unchanged an increase of ten per cent. in the solar radiation would raise the mean temperature of the earth's surface by about 7? C., or between 12? and 13? F., with, of course, a corresponding fall for a decrease.

After the sun's radiation reaches the outer limits of the earth's atmosphere its nature and intensity are modified by the composition of the air through which it passes. In general the air itself is very transparent to the small wave-lengths which make up the solar rays, but the presence of fine dust, whether of volcanic or of cosmic origin, has been shown by Humphreys to be a distinct hindrance to their passage, so that volcanic eruptions of an explosive nature, such as that of Krakatoa in 1883, La Soufri?re in 1902, or Katmai in 1912, may result in a fall of temperature over the world as a whole.

The temperature of the earth is determined by the balance between the radiation received from the sun and the terrestrial radiation to space, and a decrease in the latter would be as effective in raising the mean temperature as an increase in the former. The use of glass for greenhouses depends on this principle; for glass is transparent to heat rays of small wave-length, but is largely opaque to the rays of greater wave-length which make up terrestrial radiation. Certain constituents of the atmosphere, especially water-vapour, carbon dioxide and ozone, are effective in this way, and variations in the amount of these gases present may affect the temperature.

The angle at which the sun's rays strike the earth's surface is a highly important factor. Within the Tropics the sun at midday is nearly vertical throughout the year, and the mean temperature in these regions is correspondingly high; on the other hand, during the long polar night the sun is not seen for half the year, and very low temperatures prevail. There is thus a seasonal variation of the heat received from the sun in middle and high latitudes, the extent of which depends on the "obliquity of the ecliptic," i.e. the inclination of the earth's axis to the plane of its orbit round the sun, and any changes in this factor must alter the seasonal variation of climate.

Further, since the climate of any place depends so closely on its latitude, it follows that if the latitude changes the climate will change. A ship can change its latitude at will, but we are accustomed to regard the position of the "firm ground beneath our feet" relatively to the poles as fixed within narrow limits. This stability has, however, been questioned from time to time, mainly on evidence derived from 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.

The "astronomical" theory of the cause of climatic fluctuations is associated chiefly with the name of James Croll. Croll's theory connects abnormal variations of climate with variations, firstly of the eccentricity of the earth's orbit, and secondly of the ecliptic. In periods of high eccentricity the hemisphere with winter in aphelion is cold because the long severe winter is far from being balanced by the short hot summer; at the same time the opposite hemisphere enjoys a mild equable climate. This theory commanded instant respect, and still finds a place in the text-books, but difficulties soon began to appear. The evidence strongly suggests that glacial periods did not alternate in the two hemispheres, but were simultaneous over the whole earth; even on the equator the snow-line was brought low down. Moreover, on Mars the largest snow-cap appears on the hemisphere with its winter in perihelion. Although Croll's reasoning was beautifully ingenious he gave very few figures; while the date which he gives for the conclusion of the Ice Age, 80,000 years ago, has been shown by recent research to be far too remote, 15,000 years being nearer the mark.

Croll's theory has recently been revived in an altered form by R. Spitaler, a Czecho-Slovakian meteorologist, who calculated the probable alteration in the mean temperature of each latitude under maximum eccentricity and maximum obliquity , 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.

Spitaler claims that these differences are sufficient to cause a glacial period in the hemisphere with winter in aphelion, but from this point his theory departs widely from Croll's. During the long severe winter great volumes of sea water are brought to a low temperature, and, owing to their greater weight, sink to the bottom of the ocean, where they remain cold and accumulate from year to year. But the water warmed during the short hot summer remains on the surface, where its heat is dissipated by evaporation and radiation. Thus, throughout the cold period, lasting about 10,000 years, the ocean in that hemisphere is steadily growing colder, and this mass of cold water is sufficient to maintain a low temperature through the whole of the following period of 10,000 years with winter in perihelion, which would otherwise be a genial interval. In this way a period of great eccentricity becomes a glacial period over the whole earth, but with crests of maximum intensity alternating in the two hemispheres. Unfortunately the numerical basis of this theory is not presented, and it seems incredible that a deficiency of temperature could be thus maintained through so long a period. Further, the difficulty about chronology remains, and the work brings the astronomical theory no nearer to being a solution of the Ice Age problem than was Croll's.

The theory which connects fluctuations of climate on a geological scale with changes in the composition of the earth's atmosphere is due to Tyndall and Arrhenius, and was elaborated by Chamberlin. The theory supposed that the earth's temperature is maintained by the "blanketing" effect of the carbon dioxide in the atmosphere. This acts like the glass of a greenhouse, allowing the sun's rays to enter unhindered, but absorbing the heat radiated from the earth's surface and returning some of it to the earth instead of letting it pass through to be lost in space. Consequently, any diminution in the amount of carbon dioxide present would cause the earth to radiate away its heat more freely, so reducing its temperature. But it is now known that the terrestrial radiation which this gas is capable of absorbing is taken up equally readily by water-vapour, of which there is always sufficient present, and variations of carbon dioxide cannot have any appreciable effect.

Brief mention may be made here of a theory put forward by Humphreys, who attributed glaciation to the presence of great quantities of volcanic dust in the atmosphere. It would require an enormous output of volcanic dust to reduce the temperature sufficiently; but in any case the relation, if any, between vulcanicity and temperature during the geological ages is rather the reverse of that supposed by Humphreys, periods of maximum volcanic action coinciding more frequently with high temperatures than with low. Perhaps the best comment on Humphreys' theory is that in 1902 F. Frech produced its exact opposite, warm periods being associated with an excess of vulcanicity and cold periods with a diminution.

The theory which attributes climatic changes in various countries to variations in the position of the poles has been adduced in two main forms. The first is known as the Pendulation Theory, and supposes the existence of two "oscillation poles" in Ecuador and Sumatra. The latitude of these points remains unchanged, and the geographical poles swing backwards and forwards along the meridian of 10 E. midway between them. Varying distances from the pole cause changes of climate, and the movements of the ocean, which adjusts itself to the change of pole more rapidly than the land, causes the great transgressions and regressions of the sea and the elevation and subsidence of the land.

An alternative form put forward by P. Kreichgauer, and recently brought up again by Wegener, explains the apparent variations in the position of the pole, not through a motion of the earth's axis, but by the assumption that the firm crust has moved over the earth's core so that the axis, remaining firm in its position, passes through different points of the earth's crust. The cause of these movements is the centrifugal force of the great masses of the continents, which are distributed symmetrically about the earth. Imagine a single large continent resting on a sub-fluid magma in temperate latitudes. Centrifugal force acting on this continent tends to drive it towards the equator. There is thus a tendency for the latitude of Europe to decrease. Similar forces acting through geological ages have caused the poles and equator to wander at large over the earth's surface, and also caused the continents to shift their positions relatively to one another. According to Wegener, in the Oligocene there was only a single enormous continent, America being united to Europe and Africa on the one hand, and through Antarctica to Australia on the other; while the Deccan stretched south-westwards nearly to Africa. The poles were in Alaska and north of the Falkland Islands. The treatment in Kreichgauer's original book is speculative and at times fanciful; Wegener's treatise appears to demand more respectful attention, but is open to some vital objections. In the first place, theories of this class demand that the glaciation occurred in different regions at widely different times, whereas we shall see in the following pages that the evidence points very strongly to a double glaciation during the Quaternary occurring simultaneously over the whole earth. This objection, which was fatal to Croll's theory in its original form, is equally fatal to theories of pole-wandering as an explanation of the Quaternary Ice Age. Secondly, we know that the last phase of this glaciation, known as the Wisconsin stage in America and the Wurmian in Europe, was highly developed only 20,000 years ago, and probably reached its maximum not more than 30,000 years ago. In the last 5000 years there has been no appreciable change of latitude, at least in Eurasia; and it seems impossible for the extensive alterations required in the geography of the world by Wegener's theory to have taken place in so short a time.

The great glaciation of the Permian period, referred to in the next chapter, is a totally different matter. During this time the ice-sheets appear to have reached their maximum area, and to have extended to sea-level, in countries which are at present close to the equator, while lands now in high latitudes remained unglaciated. It is true that at the present day glaciers exist at high latitudes under the equator itself, and given a ridge sufficiently steep and a snowfall sufficiently heavy such glaciers would possibly extend to sea-level; but even these conditions would not give rise to the enormous deposits of true boulder-clay which have been discovered, and there seems no way of avoiding the supposition of an enormous difference in the position of the pole relatively to the continents at this time.

Wegener's theory alone, however, requires that glaciation should always have been proceeding in some part of the globe , which is hard to reconcile with the extremely definite and limited glaciations which geological research has demonstrated. In these circumstances we may tentatively explain the pre-Tertiary glacial periods by combining Wegener's theory of the movements of continents and oceans as a whole with the theory of changes of elevation and of land and sea distribution which is outlined below. That is to say, we may suppose that the positions of the continents and oceans have changed, relatively both to each other and to the poles, slowly but more or less continuously throughout geological time; while at certain periods the land and sea distribution became favourable for extensive glaciation of the regions which at that time were in high latitudes.

The geographical theory, which states that the Ice Age was brought about by elevation in high latitudes, and by changes in the land and sea distribution, though never seriously challenged, has suffered until recently from a lack of precision. The present author attempted to remedy this by a close mathematical study of the relation of temperature to land and sea distribution at the present day. The method at attack was as follows: from the best available isothermal charts of all countries the mean temperature reduced to sea-level was read off for each intersection of a ten-degree square of latitude and longitude, for January and July, from 70? N. to 60? S. latitude; this gave 504 values of temperature for each of these months. Round each point was next drawn a circle with an angular radius of ten degrees, divided into east and west semicircles. The area of each semicircle was taken as 100, and by means of squared paper the percentage of land to the east and land to the west were calculated; finally, in each month the percentage of the whole circle occupied by land, ice, or frozen sea was calculated, this figure naturally being greater in winter than in summer. The projection used was that of the "octagonal globe," published by the Meteorological Office, which shows the world in five sections, the error nowhere exceeding six per cent.

These figures were then analysed mathematically, and from them the effects on temperature of land to the east, land to the west, and ice were calculated. The detailed numerical results are set out in an Appendix; it is sufficient here to give the following general conclusions:

In winter the effect of land to the west is always to lower temperature.

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.

In summer the general effect of land, whether to the east or west, is to raise temperature, but the effect is nowhere anything like so marked as the opposite effect in winter.

The effect of ice is always to lower temperature.

For every latitude a "basal temperature" can be found. This is the temperature found near the centre of an ocean in that latitude. This "basal temperature" is a function of the amount of land in the belt of latitude. Poleward of latitude 20? an increase of land in the belt lowers the winter basal temperatures very rapidly and raises the summer basal temperature to a less extent. The "basal temperature" is important, since it is the datum line from which we set out to calculate the winter and summer temperatures of any point, by the addition or subtraction of figures representing the local effect of land in a neighbouring 10? circle.

As an illustration of the scale of the temperature variations which may be due to geographical changes, suppose that the belt between 50? and 70? N. is entirely above the sea. Then we have the following theoretical temperatures; for a point on 60? N. at sea-level:

January -30? F.; July 72? F.

Data for calculating the effect of ice are rather scanty, but the following probable figures can be given, supposing that the belt in question were entirely ice-covered:

January -30? F. ; July 23? F.

Supposing that the belt were entirely oceanic, the mean temperature in 60? N. would be:

January 29? F.; July 41? F.

These figures show how enormously effective the land and sea distribution really is. From Appendix it is easy to calculate the probable temperature distribution resulting from any arrangement of land and water masses. Since the geography of the more recent geological periods is now known in some detail, we have thus a means of restoring past climates and discussing the distribution of animals and plants in the light of this knowledge. Of course it is not pretended that no other possible causes of great climatic variation exist, but no others capable of seriously modifying temperature over long periods are known to have been in operation. As we shall see later, there are solar and other astronomical causes capable of modifying climate slightly for a few decades or even centuries, but these are insignificant compared with the mighty fluctuations of geological time.

In applying the results of this "continentality" study to former geological periods the method adopted is that of differences. The present climate is taken as a standard, and the temperatures of, for instance, the Glacial period are calculated by adding to or subtracting from the present temperatures amounts calculated from the change in the land and sea distribution. This has the advantage of conserving the present local peculiarities, such as those due to the presence of the Gulf Drift, but such a procedure would be inapplicable for a totally different land and sea distribution, such as prevailed during the Carboniferous period. That it is applicable for the Quaternary is perhaps best shown by the following comparison of temperatures calculated from the distribution of land, sea and ice with the actual temperatures of the Ice Age as estimated by various authorities :

It is seen that the agreement is quite good.

There is one other point to consider, the effect of height. The existence of a great land-mass generally implies that part of it at least has a considerable elevation, perhaps 10,000 or 20,000 feet, and these high lands lave a very different climate to the neighbouring lowlands. Meteorologists have measured this difference in the case of temperature and found that the average fall with height is at the rate of 1? F. in 300 feet. In the lower levels the fall is usually greater in summer than in winter, but at 3000 feet it is fairly uniform throughout the year. Consequently, quite apart from any change in climate due to the increased land area, an elevation of 3000 feet would result in a fall of temperature of 10? F., winter and summer alike. This reinforces the effect of increased land area and aids in the development of ice-sheets or glaciers.

The effect of geographical changes on the distribution of rainfall are much more complicated. The open sea is the great source of the water-vapour in the atmosphere, and since evaporation is very much greater in the hot than in the cold parts of the globe, for considerable precipitation over the world as a whole there must be large water areas in the Tropics. In temperate latitudes the water-vapour is carried over the land by onshore winds, and some of it is precipitated where the air is forced to rise along the slopes of hills or mountains. Some rain falls in thunderstorms and similar local showers, but the greater part of the rain in most temperate countries is associated with the passage of "depressions." These are our familiar wind- and rain-storms; a depression consists essentially of winds blowing in an anti-clockwise direction round an area of low pressure.

These centres of low pressure move about more or less irregularly, but almost invariably from west to east in the temperate regions. They are usually generated over seas or oceans, and, since a supply of moist air is essential for their continued existence, they tend to keep to the neighbourhood of water masses or, failing that, of large river valleys. In a large dry area depressions weaken or disappear. Their tracks are also very largely governed by the positions of areas of high pressure or anticyclones, which they tend to avoid, moving from west to east on the polar side of a large anticyclone and from east to west on the equatorial side. Since anticyclones are developed over the great land areas in winter, this further restricts the paths of depressions to the neighbourhood of the oceans at that season.

For all these reasons the tracks of depressions, and therefore the rainfall, are intimately connected with the distribution of land and sea. In winter there is little rainfall in the interior of a great land-mass, except where it is penetrated by an arm of the sea like the Mediterranean; on the other hand, the coasts receive a great deal of rain or snow. The interior receives its rain mostly in spring or summer; if the coastal lands are of no great elevation this will be plentiful, but if the coasts are mountainous the interior will be arid, like the central basins of Asia.

The development of an ice-sheet is equivalent to introducing perpetual winter in the area occupied by the ice. The low temperature maintains high pressure, and storm-tracks are unable to cross the ice. At the present day depressions rarely penetrate beyond the outer fringe of the Antarctic continent, and only the southern extremity of Greenland is affected by them. Since the total energy in the atmosphere is increased by the presence of an ice-sheet, which affords a greater contrast of temperature between cold pole and equator, storms will increase in frequency and their tracks must be crowded together on the equatorial side of the ice-sheet. In the southern hemisphere we have great storminess in the "roaring forties"; south of Greenland the Newfoundland banks are a region of great storminess. Hence, when an ice-sheet covered northern and central Europe the Mediterranean region must have had a marked increase of storminess with probably rain in summer as well as in winter.

But if snow-bearing depressions cannot penetrate an ice-sheet, it may be asked how the ice-sheet can live. The answer depends on the nature of the underlying country. A land of high relief such as Antarctica is, and as Greenland probably is, rising to a maximum elevation of many thousand feet near its centre, draws its nourishment chiefly from the upper currents which flow inward on all sides to replace the cooled air which flows outwards near the surface. These upper currents carry a certain amount of moisture, partly in the form of vapour, but partly condensed as cirrus and even cumulus cloud.

At low temperatures air is able to hold only a negligible amount of water-vapour, and this current, coming in contact with the extremely cold surface of the ice, is sucked dry, and its moisture added to the ice-sheet. Probably there is little true snowfall, but the condensation takes place chiefly close to the surface, forming a frozen mist resembling the "ice-mist" of Siberia. Even if the central land is not high enough to reach into the upper current at its normal level, the surface outflow of cold air will draw the current down to the level of the ice. This will warm it by compression, but the ice-surface is so cold that such warming makes little difference in the end. This process of condensation ensures that after the ice reaches a certain thickness it becomes independent of topography, and in fact the centre of the Scandinavian ice-sheet lay not along the mountain axis, but some distance to the east of it.

It is probably only on the edges of the ice-sheet, and especially in areas of considerable local relief, that snowfall of the ordinary type takes place, associated with moist winds blowing in the front section of depressions which skirt the ice-edge. But when conditions are favourable this source of supply is sufficient to enable these local ice-sheets to maintain an independent life, merely fusing with the edges of the larger sheet where they meet. Examples of such local centres in Europe were the Irish and Scottish glaciers, and at a later stage the Lofoten glaciers of the west of Norway, and in America the Cordilleran glaciers of Columbia.

Penck and Br?ckner have demonstrated that in the Alps the increase of glaciation was due to a fall of temperature and not to an increase of snowfall. The argument is threefold: firstly, the lowering of the snow-line was uniform over the whole Alpine area, instead of being irregular as it would be if it depended on variations of snowfall; secondly, the area and depth of the parent snow-fields which fed the glaciers remained unchanged, hence the increased length of the glaciers must have been due to decreased melting below the snow-line, i.e. to lower temperatures; thirdly, the upper limit of tree-growth in Europe sank by about the same amount as the snow-line. The same conclusion holds for the great Scandinavian and North American ice-sheets, the extension of which was undoubtedly due to a great fall of temperature. In the case of the Alps the interesting point has come to light that the fall of temperature, though due in part to increased elevation, is mainly accounted for by the presence of the Scandinavian ice-sheet, which extended its influence for many miles beyond the actual limits of glaciation, so that its waxings and wanings are faithfully reproduced in those of the Alpine glaciers, even to the details of the final retreat after the last maximum.

It is only when we turn to tropical and sub-tropical regions that we find variations of temperature unable to account for increased glaciation. Not only were the changes of land and sea distribution on a very much smaller scale than further north, but the Appendix shows that the temperature value of a corresponding change of land area is also very much less. But the high intertropical mountains--the Andes and Kenya and Kilimanjaro in central Africa--which to-day bear glaciers, in Quaternary times carried much greater ones. We cannot call in a fall of temperature, for the reason above stated, and also because at lower levels there is no evidence of colder conditions. In the Glacial period the marine fauna was the same as to-day, and mountains which now fall short of the snow-line by a few hundred feet were still unglaciated even then. The only alternative is increased snowfall on the higher mountains. Fortunately this fits in well with meteorological theory. The rain and snowfall of tropical regions depends, first of all, on the evaporation over the oceans. But evaporation is profoundly influenced by the velocity of the wind; and the wind, which in the Tropics represents the strength of the atmospheric circulation, depends on the thermal gradient between the equator and the poles; since there is no evidence of any appreciable change of temperature over the Tropics as a whole, while there was a very great fall in cold temperate and polar regions, the thermal gradient, and therefore, ultimately, the tropical and sub-tropical, rain and snowfall must have been very greatly increased. Hence the increased glaciation of high mountains near the equator, and hence also the evidence of "Pluvial periods" in the sub-tropical arid regions on either side of the equator.

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