The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Humphreys, W. J. “Physics of the air.” Philadelphia, 1920. [Pt. 4, pp. 556-629.]

Chamberlin, T. C. “An attempt to frame a working hypothesis of the cause of glacial periods on an atmospheric basis.” Journal of Geology (American), Vol. 7, 1899, pp. 545-84, 667-85, 751-87. [Carbon dioxide theory.]

Croll, J. “Climate and time in their geological relations.” London, 1875. “Discussions on climate and cosmology.” London, 1889. [Eccentricity of earth’s orbit.]

Spitaler, R. “Das Klima des Eiszeitalters.” Prag, 1921. Lithographed. [Eccentricity of earth’s orbit. Reviewed in the Meteorological Magazine, London, September, 1921.]

Simroth, H. “Die Pendulationstheorie.” Leipzig, 1908.

Kreichgauer, P. “Die Aequatorfrage in der Geologie.” Steyr, 1902.

Wegener, A. “Die Entstehung der Continente und Ozeane.” Die Wissenschaft, Bd. 66, Braunschweig, 1920.

Köppen, W. “Ueber Aenderungen der geographischen Breiten und des Klimas in geologischer Zeit.” Stockholm, Geografiska Annaler, 2, 1920, pp. 285-99.

Brooks, C. E. P. “Continentality and temperature.” Quarterly Journal of Royal Meteorological Society, Vol. 43, 1917, p. 169; and 44, 1918, p. 253. [Influence of land and sea distribution.]

Enquist, F. “Eine Theorie über die Ursache der Eiszeit und die geographischen Konsequenzen derselbe.” Bull. Geol. Inst., Upsala, 13, 1915, No. 2. [Influence of land and sea distribution.]

Hobbs, W. H. “Characteristics of existing glaciers.” New York, 1911. [Glacial anticyclone.]

TABLE OF GEOLOGICAL FORMATIONS

Quaternary	{ Recent
	{ Pleistocene
Tertiary or Cainozoic	{ Pliocene
	{ Miocene
	{ Oligocene
	{ Eocene
Mesozoic or Secondary	{ Cretaceous
	{ Jurassic
	{ Triassic
Palæozoic	{ Permian
	{ Carboniferous
	{ 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 indicated the presence of an ice-sheet at a very early stage in the earth’s history. This is a “tillite,” or boulder-clay, discovered by Prof. Coleman at the base of the Lower Huronian (Early 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. All these characters point to a large ice-sheet.

Traces 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 still 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.

Followed a long period of milder climate indicated in America by thick deposits of limestone with the remains of reef-building organisms and other marine life. This period may have been interrupted at least once by the recurrence of glacial 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 glacial 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). Glacial 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 glaciation, one between China, India and Australia, and the other north-west of Europe. The south-eastern of these was the most extensive, and probably indicates a ring of glaciated continents surrounding the pole, rather than a single enormous ice-sheet.

During the Cambrian all evidence of glacial action ceases, and we have, instead, evidence of a warm, fairly uniform climate in the abundant marine life. This continued during the Ordovician 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 mountain-building and elevation shallow seas extended over almost the whole of the surface, accompanied by mild oceanic climates 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 discovered. There are also some doubtful traces 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 northern and southern limits in 80° N. (north-east Greenland and Spitzbergen) and 15° 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 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 tillites of great thickness, ice-worn surfaces and striated boulders have been found in South Africa, Belgian Congo, and Togoland, Tasmania and widely separated parts of Australia, peninsular and north-western India, and probably also Afghanistan. In India the glacial striæ 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, northern Argentine and the Falkland Islands, and there are probable traces 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 smoothing out the folds of the Himalayas. This great circumpolar 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 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 preceding 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 moisture 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 suffered 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 fell close to Wegener’s equator, are easily explicable 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-tropical desert belt (Sahara, Arizona) to these regions. In the Trias these conditions 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 climate had again become mild and oceanic.

In the Lias (Lower Jurassic), there was crustal movement and volcanic action accompanied by land-formation and a gradual lowering of temperature. There was 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 climate 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 differentiation 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 Eocene there was a local glaciation of the San Juan Mountains of Colorado. According to W. W. Atwood this glaciation was double, the first stage being of the Alpine mountain glacier type, 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 glaciation 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 alternated. 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 development 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 widespread 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 equilibrium is restored, when the process is repeated.

The gradual erosion of the land by river and wave-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 lithosphere 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 35° 16′ 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.

“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 comparative 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 difficult 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 practically 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 throughout the whole of the Proterozoic and Palæozoic 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 definite 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 traces in Europe. 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 Palæozoic 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,[2] but the suggestion is put forward tentatively for consideration rather than as a definite hypothesis. We must be thankful that in the next chapter we are on safer ground.