The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Bericht Internat. Geologenkongress, Stockholm, 1910. “Die Veränderungen des Klimas seit der Maximum des letzten Eiszeits.” Numerous papers ranging from the Arctic to the Antarctic.

Brooks, C. E. P. “The evolution of climate in north-west Europe.” London, Q. J. R. Meteor. Soc., 47, 1921, p. 173.

Praeger, R. Ll. “Report on the estuarine clays of the north-east of Ireland.” Proc. R. Irish Acad., ser. 3, Vol. 2, 1892, pp. 212-89.

Goldthwait, J. W. “The twenty-foot terrace and sea-cliff of the lower St. Lawrence.” Amer. J. Science, ser. 4, Vol. 32, 1911, pp. 291-317.

Cowles, H. C. “A remarkable colony of northern plants along the Apalachicola River, Florida, and its significance.” Rep. 8 Internal. Geogr. Congress, 1904, p. 599.

Shimer, H. W. “Post-glacial history of Boston.” Amer. J. Science, ser. 4, Vol. 40, 1915, pp. 437-42.

Halle, T. G. “On Quaternary deposits and changes of level in Patagonia and Tierra del Fuego.” Bull. Geol. Inst., Upsala, 9, 1908-9, pp. 93-117.

Süssmilch, C. A. “An introduction to the geology of New South Wales,” Sydney, 1914.

Marshall, P. “New Zealand.” Handbuch regional Geologie, Heft 5, 1911.

CHAPTER XV
THE FOREST PERIOD OF WESTERN EUROPE

Hitherto we have been dealing with climatic changes which can be recognized with more or less certainty over most of the polar and temperate regions of the world, but we have now to describe a stage which appears to have been peculiar to Europe and possibly Asia—the Forest period. By 3000 B.C., or towards the close of the Neolithic period, considerable elevation had again taken place over the central latitudes of western Europe (the northern parts of Norway and Sweden were still several hundred feet below their present level). The southern part of the British Isles, which had remained slightly elevated since the last Glacial period, had now emerged to a height of nearly ninety feet above its present level; the area of Ireland had increased appreciably and part of the North Sea was land. The geographical changes were not great, but they were sufficient to turn the scale in the direction of a continental climate in the British Isles. The more or less complete closing of the Straits of Dover, and the consequent bar to the free circulation of the Gulf Drift, must have had an appreciable effect on the climate in the direction of continentality. At the same time the low level of northern Norway, and possibly the persistence of warm conditions in the Arctic basin, more and more attracted depressions to the northernmost track, so that the British Isles especially, and to a lesser extent Holland, Germany, southern Scandinavia and Russia, came more persistently under the influence of anticyclonic conditions. The rainfall of these countries diminished, and the surface of the bogs dried sufficiently to enable forests to grow in the western countries; in Germany heath-plants took the place of bog-plants, while in Russia steppe conditions supervened. The normal meteorological conditions at this time in fact resembled those of the memorable drought of 1921, which was characterized by low pressure and stormy conditions in the Arctic Ocean and a belt of high pressure and persistently fine weather across central Europe.

During this phase the winters may have been severe, but the summers were warmer than the present, for in the peat-bogs of Ireland and Scotland are the remains of trees larger than any now found in the neighbourhood. The Irish bogs dried so completely that they were extensively inhabited; corded oak roads have been found at this horizon, while in 1883 a two-story log house, surrounded by an enclosure, was found in Drumkelin Bog, Co. Donegal; it was twelve feet square and nine feet in height, and a roadway led to it across the bog. Both house and roadway were entirely constructed of oak. With the hut were found a stone chisel and a flint arrowhead. Beneath the floor were fourteen feet of bog, and above the floor twenty-six feet. This time was also one of relatively little wind movement, for stools occur even in exposed positions on the mountain slopes of western Ireland, where trees will only grow now in sheltered positions near sea-level.

Further evidence of the very dry climate of this phase is the frequent occurrence of trees apparently in situ beneath the surface of fresh-water lakes, both in Ireland and Scandinavia. I was able to examine one very good example near Lough Toome in north-west Ireland. An unusually dry spring had lowered the surface of the water and a large number of tree-stools were exposed; when these trees were growing the water-surface must have been at least two feet below the level of the present outlet. Most of the lakes in which these stools are found are shallow upland basins with a small drainage area, and if the present climate became drier they would more or less completely disappear.

Mr. Fairgrieve has noted the action of blown sand on the westward side of broken-off tree-stumps in a submerged forest on the shore in south Wales, which, though not conclusive, suggests dry conditions. Mr. Fairgrieve also noted the direction of fall of twenty-one trees, and found that in the great majority of cases they were blown down by westerly winds.

The forest phase was short; according to the late C. Reid the land again began to subside shortly after 3000 B.C., and by 1600 B.C., in Britain at least, had reached its present level; this carries us to the beginning of the Bronze Age. In connexion with Ellsworth Huntington’s theory that the dampness of Ireland lowers the energy of its inhabitants, it is interesting to note that this dry period apparently corresponds to the legendary Heroic Age, when the vigour of the Irish reached a level never since attained. Civilization in Scandinavia also seems to have benefited by the drier conditions, for Scandinavian technique advanced rapidly to a high level about 1800 B.C. But though there is evidence of a considerable sea-borne commerce with Britain and Ireland, there appears to have been comparatively little land traffic between different parts of Scandinavia at this time. In fact, to primitive man dense forest with thick undergrowth was almost impenetrable. But at the close of the forest phase and the beginning of the peat-bog phase the trees were weakening under conditions becoming unfavourable. Such dying forests are marked by the absence of undergrowth and young trees, and afford safe and easy land communication. Accordingly we find that by 1500 B.C. a considerable traffic had developed across Scandinavia by land.

Although we have no direct evidence, the meteorological conditions suggest very strongly that the dry belt extended across Russia into Siberia as a marked period of desiccation, possibly worse than any droughts of the historic period. At present Siberia receives its rainfall mainly from depressions which cross Russia from the Baltic or Black Seas, and follow a well-marked track north of the central Asiatic mountains. But during the forest period these tracks were abandoned, and the majority of the depressions passed north-eastward off the coast of Norway into the Arctic Ocean. The result must have been a great diminution of rainfall over the continent. We shall see later ([Chapter XIX]) that this period of drought was of extraordinary importance in human history. For during the moist maritime phase central and eastern Europe, and probably also Asia, had become extensively peopled by neolithic nomads of Aryan and Semitic races, while the great river valleys of the south were in the possession of dense agricultural populations in a more advanced state of civilization. As the climate became progressively drier and the pasture diminished, the land was unable to support such a large nomadic population, and there was a great outburst of raiding and conquering expeditions directed southwards and westwards, resulting in a succession of empires in the rich Mesopotamian regions and neighbouring countries, which form the beginnings of our history. The beginnings of history in China also, about 2500 B.C., show that at this time the settled peoples of that country were in trouble with the nomads of the interior.