The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Hedley, C. “The palæographical relations of Antarctica.” London, Proc. Linnæan Soc., 124, 1911-2, p. 80.

Lydekker, R. “A geographical history of mammals.” Cambridge University Press, 1896, pp. 125 ff.

David, T. W. E. “Antarctica and some of its problems.” London, Geogr. J. 43, 1914, pp. 605-30.

Nordenskjold, O. “Antarktis.” Handbuch Regional Geologie, Heft 15, 1913.

CHAPTER XIII
THE CLOSE OF THE ICE AGE—THE CONTINENTAL PHASE

In [Chapter V] we left the climatic history of northern Europe at the point where the ice in its final readvance had once more reached the German coast. But Scandinavia was now sinking, and the margin of the ice soon began to retreat again. At the same time the Alpine glaciers diminished in size, while the Irish and Scottish glaciers disappeared. This is the critical period in the change from glacial to temperate conditions, and, thanks to the researches of the Swedish geologists, and especially G. de Geer, H. Munthe and Gunnar Andersson, we are very well acquainted with it. The change was not uniform; at first the recession was very slow, and there were periods when for scores of years the ice-edge remained stationary or even readvanced, but on the whole the time was one of persistent amelioration. The following description is based chiefly on W. B. Wright’s summary of de Geer’s work.

After leaving the coast of Germany the ice-edge appears to have remained in the western Baltic, retreating slowly for some 8000 years. About 10,000 B.C. it lay along the southern coast of Sweden, and during the next 2000 years it withdrew to about 59° N. This was the Gotiglacial stage. Here came a pause, when, for 200 years, about 8000 B.C., owing presumably to a change for the worse in the climate, the ice-edge remained in one position, forming a great moraine. Then came another period of very rapid retreat, the Finiglacial occupying nearly 3000 years, followed by a further halt of some duration near Ragunda, about 5000 B.C. After this the ice-sheet split into two portions, and the Glacial period is regarded as over.

In the Alps there were similar periods of regression and of halting or readvance. The first, known as the Bühlstadium, corresponded to the Baltic readvance ([Chapter V]). The second, the Gschnitz-stadium, with a snow-line 2000 feet below the present (i.e. mean temperature about 6° F. lower than now), has not been dated, but probably occurred about 8000 B.C. This was followed by a warmer period, probably as warm as and drier than the present, after which the glaciers readvanced about 5000 B.C. in the third or Daun-stadium, when the snow-line was depressed 1000 feet (temperature 3° F. lower than now).

In the lower Nile valley the deposition of gravel ceased, and that of mud began about 8000 B.C., indicating that at this time the climate of north-east Africa reached its present state of dryness.

It is at present difficult to give more than a tentative explanation of these oscillations of climate during the Retreat Phase. Northern Europe was at the time passing through a complicated series of geographical changes. As the ice left the Baltic basin the latter became the site of a cold ice-lake, with narrow outlets to the Atlantic by way of the Sound and the Belts. At this time the recession was slow. Then the retreat of the ice opened a connexion with the White Sea, and elevation closed the outlet to the west. This probably made the waters still colder, and the Fennoscandian pause occurred. Elevation now closed the connexion with the White Sea, and an entirely closed-in ice-lake resulted. During this stage the retreat was slow, until between 7000 and 6000 B.C., when the ice-sheet vacated Scania, and direct communication between the Baltic and the Atlantic was opened across Lakes Wener and Wetter, and the climate, though still arctic at first, became appreciably warmer by 6000 B.C.

For more than 10,000 years of the retreat, or until 6000 B.C., the ice-sheet was still sufficiently large and powerful to maintain a border of Arctic anticyclonic conditions on its southern edge. During the retreat the mean annual temperature of southern Sweden increased from 17° F. to 35° F., equivalent to a change from North-east Greenland to South Greenland. The July temperature rose to about 43° F. On the North German Plain still lived the reindeer and the fauna and flora of the sub-Arctic tundras; the mean annual temperature rose to 45° F. by the close of the period. The land flora in Sweden was entirely xerophilous, indicating a slight rainfall. There is also geological evidence of a small annual rainfall on the south-west coast of Norway. This period covers the transition from Palæolithic to Neolithic culture.

It seems probable that the continental character of the climate of the final stages of the retreat phase was slightly increased by astronomical causes, the obliquity of the ecliptic being probably nearly one degree greater about 7500 B.C. than it is now. In Germany and Sweden this would have the effect of lowering the winter temperature and raising the summer temperature by rather more than 1° F.

While the land was still falling rapidly in the north of Scandinavia and the Gulf of Bothnia, the coasts of Germany and Denmark began to rise, and about 6000 B.C. again closed[5] the outlet of the Baltic, converting it into a large fresh-water lake, the Ancylus lake. A similar lake was formed farther east in central Finland. At this time the south-west Baltic lands stood more than 100 feet higher than at present. The land was probably still largely under the influence of dry easterly winds, and the shutting out of the Atlantic accentuated the continental conditions, and this stage in the climatic history of Europe is known as the “Continental Phase.” The winter climate was severe; at first the summers were not especially warm (July temperature about 54° F. in southern Sweden). This is probably the period of formation of the Ragunda moraines, and of a readvance of the glaciers on the Norwegian side of the divide, when the snow-line lay 200-300 metres lower than at present; it was also the time of the Daun readvance in the Alps. But as the land sank in the north and rose in the south, the waters of the Ancylus lake retreated farther and farther north, and the summers became hot and dusty, with a mean July temperature of about 60° F. Everywhere in the Baltic regions the older Ancylus beds show a monotonous pine-wood, but in the upper Ancylus these are followed closely by a number of plants and shrubs of southern type—black alder, curled birch, linden, etc. The temperature continued to rise, and oak, Norway maple, ash, and finally, in the southernmost parts of Sweden, the common maple appeared. The last-named plant has been found below the present level of the sea in Ystad Harbour.

Under the influence of these conditions the remnant of the Scandinavian ice-sheet again decreased in size, until it split into two portions, the break occurring at Ragunda, and this is considered by Scandinavian geologists to mark the end of the Ice Age in Europe. Gunnar Andersson compares the climate of southern Sweden at this time to the Baraba Steppes in western Siberia, with an annual rainfall of 12 to 16 inches, but this seems an extreme estimate. The “Karst” flora of the limestone areas of south-east Europe immigrated into eastern Sweden during this period, and south-east Europe probably gives a better idea of the climate of Sweden during the continental phase. Farther east, in Finland, Kupffer describes the climate as resembling that of central Russia. In central Germany the climate was dry, with a mean temperature in the four summer months of 63° F.; it resembled that of south-west Russia. This period of warm summers began earlier in Germany than in Sweden, and throughout this phase Scandinavia was occupied by a rich forest flora. The hazel extended several degrees north of its present position, and to higher levels, indicating a July temperature about 7° F. higher than the present. In southern Norway the pine extended to much greater heights. But the ivy and yew, whose limits depend on the winter rather than on the summer temperature, showed no such extension, indicating that the winters remained severe. In Denmark there was a dry climate, fairly warm at the close, with fir forests, though western Denmark is now too wet for this tree. On the coast of Norway the seas were still cold, so that there is a contrast between the animal life of the sea and the plant life of the land. The Alps also became warm and dry, and were occupied by a xerophilous flora.

As the glacial anticyclone decreased in intensity, depressions from the Atlantic began to take a more northerly course, but were held up near the British Isles and materially increased the rainfall. This is the first peat-bog period of these islands, when the birch and pine forests which had covered the non-glaciated lands during the cold dry period gave way to extensive growths of peat-bogs. Southern and eastern England, however, largely escaped this damp period, sharing in the dry climate of the Continent.

The absence of storms off the north-west coast of Norway is shown by the forests which at this period covered all the outermost islands of Norway as far as Ingo Island, off North Cape. These islands are now barren, and their afforestation indicates a drier and especially a less stormy climate than the present, with a decreased frequency of winds from the sea. These conditions were well developed about 5000 B.C. This is the Early Neolithic period. Owing to the great development of forests, this period is sometimes called the Early Forest period.

The late-glacial history of North America was equally complicated. Consider first the region of the St. Lawrence Estuary and the Great Lakes. As the Wisconsin ice-sheet retreated across the present site of the Lakes, the latter underwent a remarkable series of fluctuations of area and outflow, which have been made the subject of brilliant studies by several American geologists. The opening stage began when the ice abandoned the high ground south of the lakes, leaving depressions bounded on the south by the hills and on the north by the ice. The earliest of these in the basins of Lakes Erie and Huron are known as the first and second Lake Maumee. These gradually grew in size and coalesced, forming several series of connected lakes, to which various names have been given; thus Lake Warren extended well outside the present limits of Lake Erie and southern Huron, and was held up by ice over Lake Ontario and northern Huron. At a later stage an enormous Lake Algonquin extended beyond the combined limits of Lakes Superior, Michigan and Huron, and communicated by broad channels with an enlarged Lake Ontario known as Lake Iroquois, and with Lake Erie. But even before this time the northern shores of the lakes, relieved of the major portion of their ice-load, had begun to rise rapidly, and ultimately reduced the lakes to their present size.

These great areas of ice-cold water, bathing the southern edges of the ice-sheet, must have had an unfavourable influence on the climate, keeping it cold and damp, and preventing dry continental conditions from becoming established. They probably retarded the ice-retreat in these regions quite considerably, so that a lobe of ice was left here long after the edge had retreated northwards on either side. At the same time the climate further south was dry, with æolian deposits; but as the anticyclonic winds blew off the Atlantic the evidence of drought is not so marked as in Europe.

After this slow retreat had been in progress for a considerable time a submergence, known as the “Champlain stage” set in, reaching a depth of at least 600 feet and opening the St. Lawrence regions wide to the Atlantic, which penetrated into Lake Ontario. The ice now retreated rapidly under the influence of a maritime climate little colder than the present. In phase this period corresponds to the second Yoldia Sea stage of Scandinavia; in point of time it was probably somewhat earlier. This was followed by elevation, the first result of which was to cut off the warm water and cause a sharp fall of temperature exactly analogous to that of the Ragunda moraines, but a few thousand years earlier and probably more marked. The continuance of elevation brought on a long continental period of extreme aridity, when trees grew on the peat-bogs of the eastern States, while the lakes of the Great Basin further west were almost or wholly dried up. At the maximum of the continental conditions the summers at least were warmer than at present, as indicated by the northward extension of various species of plants and fresh-water mollusca. The winters were probably more severe. Possibly the great aridity of this period was partly due to a sub-glacial continental anticyclone obstructing the path of depressions across America from west to east. The drainage area of the Great Basin received hardly any rainfall and was a hopeless desert, but the Atlantic States were able to grow trees on the old peat-bogs, probably with rainfall derived from the Atlantic. By reference to the cutting of Niagara gorge, we can infer that the warm dry period began about 6000 B.C., so that it corresponds exactly with the continental phase (Ancylus stage) of Europe. This period of aridity was finally ended by a fresh submergence, the “Micmac,” which carried the land about twenty feet below its present level.

In Yukon and Alaska, where the glaciation was not nearly so severe as further to the south-east, the depression of the land by the ice-load and consequently the subsequent rise on its removal were not great. There were no complicated geographical changes, and correspondingly there appear to have been no fluctuations of climate, but only a gradual passage to present conditions.

Even in Iceland there are indications of a dry period following the last glacial maximum, for tree-trunks, buried in the peat-bogs, show that the birch formerly had a much greater extension. It is also quite possible that there was an accentuation of desert conditions in Asia during the retreat of the glaciers in Europe and North America, which may have played a part in the wave of Neolithic migration that appears to have overwhelmed the artistic Palæolithic races of western Europe; but of this we have as yet no direct evidence. The Neolithic invasion of Europe took place along two main routes, the Nordics passing from the centre of Asia north of the Caspian, across Russia to the Baltic shores, where they became the Kitchen-midden people; and the Alpine race passing from Transbaikalia, south of the Caspian and Black Sea, into southern Europe. The Nordics drove before them an older race, characterized by the transitional Maglemose culture, which passed from east of Russia to the shores of the Baltic and ultimately to England, where harpoons of Maglemose type have been found beneath the peat of Holderness.

In the southern hemisphere the continental phase does not appear to have been so well developed. The uppermost part of the Pampean loess is possibly post-glacial; more certainly so are the sand-dunes on the coast near Buenos Aires, in which human remains have been found in association with the bones of some extinct animals. In New South Wales, after the retreat of the glaciers, there was a period with land a little above its present level, so that the stools of Eucalyptus trees are now found ten feet below sea-level; but there is no evidence as to the climate of this stage. In New Zealand we have no definite post-glacial beds of continental type. The occurrence of xerophilous plants, such as Aciphylla, still living in a climate which is now decidedly moist, may be a remnant of a continental phase in New Zealand, or may date back to the steppe conditions of the loess. As to Antarctica, we have, of course, no evidence.