The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

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

Wright, W. B. “The Quaternary Ice Age.” London, 1914. (Ch. 17, Late-glacial changes of level in North America.)

Bericht Internat. Geologenkongress, Stockholm, 1910. “Die Veränderungen des Klimas seit der Maximum des letzten Eiszeits.” Numerous papers, dealing with Europe and North America.

Munthe, H. “Studies in the Late-Quaternary history of southern Sweden.” Stockholm, Geol. Foren. i Forh., 22, 1910, pp. 1197-1292.

Antevs, E. On the late-glacial and post-glacial history of the Baltic. Geogr. Rev., New York, 12, 1922, p. 521.

CHAPTER XIV
THE POST-GLACIAL OPTIMUM OF CLIMATE

In most of the polar and temperate regions of the world the Glacial period seems to have been separated from the present by a short interval of slightly more maritime climate. The existence of this phase was the chief point brought out in the great collection of papers communicated to the Stockholm meeting of the International Geological Congress, which has frequently been referred to in this volume. The pioneer work on the subject has been done by the Scandinavian geologists, and we may commence with a discussion of this period in the countries bordering on the Baltic.

About 4000 B.C., at the conclusion of the continental phase referred to in the preceding chapter, a rapid movement of submergence set in over the whole of the southern Baltic, and shortly afterwards the land-bar which had formerly separated the fresh waters of the Ancylus lake from the Atlantic gave place to a wide strait, through which the waters of the ocean flowed into the Baltic across southern Sweden. Ultimately this channel became wider than the present outlet between Sweden and Denmark, and maritime influences penetrated to all parts of the Baltic. In recognition of this influence the period was termed by Blytt the “Atlantic stage.” The much greater freedom with which the waters of the Atlantic were able to enter is shown by a comparison of the “isohalines” of this period with those of the present day. Isohalines indicate the degree of saltness of the water; those of to-day can, of course, be measured directly, and show that in the Gulf of Bothnia the water becomes continually less salt as we go northward, for which reason many species of marine mollusca are unable to live. By studying the distribution of the mollusca in the Littorina Sea the isohalines of that period have been reconstructed also, and show that the salt content was much greater than at the present day, indicating a greater influx of oceanic waters.

If we take a map showing a reconstruction of the geography of Littorina time, and apply to it the formulæ given in the Appendix, comparing our results with the inferences of Scandinavian and north German geologists as to the temperature, we find that there is a remarkably good agreement. Many of the palæo-botanists comment on the prolongation of the autumn into the present winter, which is especially characteristic of a more insular climate. The amounts of change in each case are also in good agreement, except perhaps in the Christiania region and in north Denmark, where the geologists require a greater change than that calculated from the land and sea distribution; this is probably accounted for by a higher temperature in the waters of the Atlantic. The maximum change as calculated is shown in south-west Finland (winter 6° F. warmer, summer 2° F. cooler). Finland is described as having at that time the climate of western Europe, which we may take as meaning winter 8° warmer, summer 3-4° cooler. There was thus a great change from the extreme climate of the continental phase with its hot summers and severe winters and little rain, to an extremely temperate climate with cool summers, mild winters and a heavy rainfall. The warmth-loving plants which had begun to immigrate during the later part of the continental phase continued to spread, and probably the highest average temperatures were reached at the time of maximum submergence, but now they were accompanied by plants for which a large rainfall is necessary, and it seems that the average rainfall of southern Sweden must have been about 40 inches a year. The oak began to dominate the forests in place of the hazel, and the peat-bogs, which during the preceding dry period had hardened into a firm surface on which birch and pine were able to take root, again became moist, so that the trees were choked by growths of bog-plants. On the shores lived men of the Transition and Early Neolithic. As the land rose again and the Littorina Sea decreased in area the climate again became drier and more rigorous. In Denmark the forests of the Ancylus period gave place to oak as the land sank, and there are also remains of two water plants, the water-nut (Trapa natans), which is no longer found in Denmark, and Najas marina, still living in one isolated locality. Northern Denmark was broken up into islands, among which marine deposits were formed, containing the remains of southern mollusca, many of which are found in the kitchen-middings. Most of the wood used by Neolithic man was oak; there is little fir and no beech.

In Norway the work of C. Brögger has made us familiar with the Tapes beds, which correspond in point of time to the Littorina stage of the Baltic. Tapes decussatus is itself a southern species of mollusc, and it is associated with a very rich warmth-loving fauna. In southern Norway the geographical conditions were different from those in Sweden, for the land reached its lowest level relatively to the sea about the close of the Glacial period, and has been rising throughout the post-glacial. The seas show a progressive rise of temperature from 8° F. below present at the close of the Glacial period to 4° F. above the present in the older Tapes beds. The littoral climate at this stage resembled that prevailing at present on the coast of northern England. After this, as the land approached its present level, the temperature fell again, and in the upper Tapes stage was only 2° F. above the present.

The warm period represented by the Tapes beds is found at intervals along the west coast of Norway, and we again find evidence of a submergence of the land contemporary with the maximum temperature. These conditions extend even as far north as Tromsö, within the Arctic circle. In Spitzbergen there are raised beaches 30 to 80 feet above the sea, containing remains of molluscs and a species of Fucus, none of which are now living so far north. On the land there are old peat-bogs of great thickness, though peat-mosses cannot now grow, since the ground never thaws below a depth of 6-10 inches. It has been pointed out that a great number of the plants now living in Spitzbergen are unable to ripen their seeds under present climatic conditions, though they must have done so in the past. Ripe seeds of some species, in fact, have been found in the peat-bogs, which are contemporaneous with the raised beach. There is thus evidence of a very well-marked warm period associated with submergence in Spitsbergen.

In Franz Josef Land, Nansen found raised beaches with mussels 10 to 20 feet above the present level; this shell does not now live so far north. In the White Sea and on the Murman coast there are also raised beaches with a southern fauna. The warm period shown in the beds of the New Siberian Islands has already been referred to (p. 79).

Returning to the British Isles we find that in the south the land was above its present level throughout the whole of the post-glacial period. On the other hand, a 25-foot beach is found in north-west England (Formby and Leasowe marine beds), but without any evidence as to climate; the same applies to the 25-foot beach of Scotland. It is only when we come to the north-east of Ireland that we find evidence of conditions appreciably warmer than the present, in the section of the Alexandra Dock, Belfast, where marine clays overlie beds of grey sand and peat. The lower estuarine clay is essentially a littoral clay, known as the Scrobicularia zone. It is brownish-blue and sandy, and contains in abundance the roots and leaves of the grass-wrack (Zostera marina), and a vast number of shells of a few species which live between tide-marks, indicating that the land stood 10 feet or so above its present level at first, while the climate cannot have differed greatly from that prevailing at the present day. It must have been formed during a period of gradual depression, for throughout its six feet or more of thickness it preserves identical littoral characters. After a time this depression became more rapid, and the upper estuarine clay began to form—a light blue clay, very pure and unctuous, with a very rich and well-preserved fauna, known as the Thracia zone. The fauna has a decidedly southern aspect, and indicates that the coasts of north-east Ireland had the present temperature of Bantry Bay—an increase of at least 3° F. in the mean annual temperature. The Thracia zone is followed by a bed of yellow shore sand, indicating re-elevation to about seven feet above the present level.

Corresponding to the upper estuarine clay are raised beaches at a height of 25 feet in north-east Ireland, falling to 15 feet at Dublin, and to only 6 or 7 feet in western Donegal and Sligo. The mollusca indicate a somewhat higher temperature than the present. In the beaches have been found flint scrapers and arrowheads of early Neolithic type.

Looking further westwards, we find that Iceland, which had undergone a slight elevation during the continental phase, so that peat was formed below present sea-level, again subsided, falling to 10 or 12 feet below its present level. During this subsidence the temperature rose, the greatest warmth coinciding with the lowest level of the land. Species from the south-west shores, where the temperature of the water is directly influenced by the Gulf Drift, extended to the cold northern coast. In some places the marine clays of this period have been ploughed up by a subsequent readvance of the glaciers.

From Greenland comes abundant evidence of a post-glacial warm period coincident with a subsidence of about 80 feet. Raised beaches all along the west coast contain mollusca, some species of which are not now living north of the St. Lawrence estuary. On the other hand, some northern species which lived off the west coast during the glacial maximum retreated northwards during this period, and have not re-established themselves, though the climate is now suitable. Further, K. Steenstrup describes the occurrence of “dead ice” at several places in North Greenland—masses of ice which have become separated from their parent glaciers owing to rapid recession, and are now buried in morainic matter. Subsequently the ice again advanced, and in some cases a new glacier has advanced over these masses of “dead ice.”

Passing to the mainland of North America, we find in eastern Canada colonies of southern mollusca, especially oysters and quohogs, separated from their main area of distribution south of Cape Cod by a wide area of cold seas—the Gulf of Maine and Bay of Fundy. At the beginning of the warm phase the land lay slightly below its present level, but subsequently rose above it. The climate became still warmer, until its temperature resembled that of the middle New England States. At the same time the rainfall diminished and the peat-bogs were replaced by forests of hardwood trees. In the basin of the Great Lakes the warm period is represented by gravel beds in the Niagara gorge, which from their position must, according to the most recent determinations, have been formed about 4000 to 3000 B.C. These gravels contain shells of fresh-water mollusca, especially species of Unio, which are not now living in the St. Lawrence system, but are found in tributaries of the Mississippi further south. Further south on the eastern coast of the United States there are marine deposits indicating a slight submergence, with a climate somewhat warmer than the present.

Passing to South America, we find in southern Patagonia and Tierra del Fuego exactly similar evidence of a post-glacial subsidence with a warmer climate than the present. Raised beaches at a height of 50 feet contain mollusca, some of which are now rare or extinct in that locality, and in sheltered situations plants are found still living whose nearest neighbours are some way to the north.

In the same way, in southern and eastern Australia there are beaches a few feet above present level, containing warmth-loving species of mollusca and indicating a post-glacial warm period. There is some evidence in the distribution of plants and marine mollusca that this warm period extended to New Zealand. Raised beaches at a height of 50 to 180 feet are also known from many places in Antarctica, and these contain mollusca, some of which are not now living south of the sub-antarctic islands. An interesting confirmation of this has been given by E. Philippi, from the results of an examination of the sea-floor at four points in about 63° S., 75-95° E., all within the present limit of pack-ice. The deposit at present forming is poor in pelagic foraminifera, and consequently contains little lime, but this deposit is very thin, and beneath it is a much more calcareous clay especially rich in Globigerina. The latter deposit is still forming north of the limit of pack-ice, and Philippi concludes that at no very distant date the limits of ice were further south, indicating warmer conditions. It is interesting to note that a similar sequence has been found in the Norwegian North Sea, the brown foraminiferous deposit (in this case containing Biloculina) being known to be underlain as well as overlain by an unfossiliferous grey clay attributed to the Glacial period. Finally, with regard to Cape Colony, A. W. Rodgers says: “It is possible that the presence of marine mollusca belonging to species that are only known in the living state from the coast north of Pondoland, in the raised beaches of Mossel and Algoa Bays, indicates that the sea on the south coast was formerly warmer than now.”

Thus we have evidence of a period of submergence and climates warmer than the present from a large number of places, including the Arctic Ocean and Greenland, the temperate coasts of North America and Europe, the Southern Ocean and Antarctica. The stage appears to be missing on the temperate coasts of the Pacific, on both the Asiatic and North American sides, and from the whole of the Tropics. It is fairly certain that the warm period occurred at the same time in eastern North America and western Europe; in the case of the southern hemisphere we have no direct proof of this, but in all cases the deposits are comparatively recent, and since they obviously refer to a similar state of affairs we may assume that they are of the same date.

In the Baltic area we know that the great change of level was due largely to a subsidence of the land and only to a small extent to a rise of the sea. But in other parts of the world the amount of submergence was remarkably uniform at places in the same latitude, and decreased steadily from the polar regions to about latitude 40-50°, where it became zero. Now such a general change suggests that it was the sea which rose rather than the land which sank, and points to some general cause which piled up the waters of the oceans in the higher latitudes. A possible cause of this nature has been adduced by O. Pettersson, which he terms the “tide-generating force,” which reached one of its maxima in an 1800-year cycle about 3500 B.C. This possibility will be dealt with more fully in [Chapter XVII].