The evolution of climate

by C. E. P. Brooks

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.