The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Coleman, A. P. “Climates and physical conditions of the early Pre-Cambrian.” Geol. Mag. (6), Vol. 1, 1914, p. 466.

Eckardt, W. R. “Paläoklimatologie.” 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 seq.

Neumayr, M. “Ueber klimatischen Zonen während der Jura- und Kreidezeit.” Wien, Denkschr. Ak. Sci., 47, 1883, p. 211.

Ramsay, W. “Orogenesis und Klima.” Ofvers af Finska Vetenskaps. Soc. Forb., 52, 1909-10, A, No. 11. Helsingfors, 1910.

Schuchert, C. “Climates of Geologic Time, In: The climatic factor as illustrated in arid America,” by Ellsworth Huntington. Washington, 1914, Pt. 2.

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 characterized by remarkably mild and equable climatic conditions 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 considerable 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 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 accounted for by the presence of the great ice-bearing Antarctic continent. Its 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 continent and the floating ice in its neighbourhood extending 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 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 nearly 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 nearly 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 conditions 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.