The evolution of climate

by C. E. P. Brooks

BIBLIOGRAPHY

Wright, W. B. “The Quaternary Ice-age.” London, Macmillan, 1914.

Brooks, C. E. P. “The correlation of the Quaternary deposits of the British Isles with those of the continent of Europe.” Ann. Rep. Smithsonian Inst., 1917, pp. 277-375.

de Geer, G. “A thermographical record of the late Quaternary climate.” Ber. Internat. Geologenkongr., Stockholm, 1910. “Die Veränderungen des Klimas,” p. 303.

——. “A geochronology of the last 12,000 years.” Ber. Internat. Geologenkongr., Stockholm, 1910, Vol. 1, p. 241.

Penck, A., and Brückner, E. “Die Alpen in Eiszeitalter.” Leipzig, 3 Vols., 1901-9.

Ahlmann, H. W., son. “Geomorphological studies in Norway.” Stockholm, Geografiska Annaler, 1, 1919, pp. 1-157, 193-252.

Richthofen, F. “On the mode of origin of the loess.” Geol. Mag., 1882, p. 293.

CHAPTER V
THE GLACIAL HISTORY OF NORTHERN AND CENTRAL EUROPE

The literature of the glacial period in Europe is stupendous and is, further, of a highly contradictory nature. Space does not permit of any summary of the great conflict between the monoglacialists and the polyglacialists; it is sufficient to say that the latter often went to extremes and so laid themselves open to defeat, but the twofold nature of the glaciation is now widely accepted. It must be understood, however, that the following summary represents the views of a certain section of geologists only, views which are not universally held. In the British Isles especially, where the remains of the maximum glaciation completely dominate those of all the others, the theory of a single glaciation still largely prevails.

When ice began to accumulate on the rising Scandinavian plateau it naturally formed at first on the Norwegian mountains near the Atlantic, which was the chief source of snowfall. These mountain glaciers spread rapidly down the steep seaward slopes to the west and more slowly down the gentler landward slopes to the east. At this stage the centre of the ice-sheet, and consequently the centre of the glacial anticyclone, as soon as the latter developed a definite existence, lay quite near the Norwegian coast. Under anticyclonic conditions the circulation of the winds round the centre is in the same direction as the motion of the hands of a watch, combined with an outward inclination at an angle of about thirty to forty-five degrees. Consequently, while the centre lay in Norway, due north of the Alps, the prevailing winds in the latter must have been from north-east, and therefore very cold. Accordingly, this stage is probably contemporaneous with the Gunz glaciation of the Alps. In the same way, over the North Sea area the winds must have been easterly, causing the currents which piled up the great shell-banks of the East Anglian coast, already referred to as marking the end of the Tertiary and beginning of the Quaternary period.

But the ice which reached the northern North Sea broke up into icebergs not far from the coast, and floated away, while that which moved east into the north of Sweden could only be dissipated by melting and ablation, processes which we have reason to believe went on very slowly. Hence ice began to accumulate and spread over a wide area east of the main Scandinavian mountain chain. Fresh snow was deposited directly on this ice-surface, until it gradually overtopped the mountains which originally gave rise to it, and reversed the flow, so that the ice actually moved uphill across the mountain chain. As the center of the ice-sheet moved eastward the glacial anticyclone moved with it, and this new position to the eastward caused an alteration in the direction of the prevailing winds over the rest of Europe. The Alps were now south-west of the anticyclonic centre, and the winds in that district accordingly became easterly instead of north-easterly. Of course, the glacial anticyclone was now more intense, but in summer in central Europe easterly winds are naturally so much warmer than north-easterly winds that at first this increase in intensity was not enough to counterbalance the change in direction, and there was a slight improvement in the Alpine climate. In the same way, over the North Sea district the prevailing winds had now become south-easterly instead of easterly, which would make for a slight rise of temperature, as also would the occasional depressions which would be able to make their way in from the westward, bringing warm moist air from the Atlantic and occasional rainfall. By this time the process of elevation had converted the North Sea floor into an extensive plain.

From Sweden and the Gulf of Bothnia the ice spread out in all directions, extending in the east to the foot of the Ural Mountains, which formed an independent centre of glaciation; in the south-east over a large part of European Russia, where it reached as far south as latitude 40° in the Dnieper valley; in the south over almost the whole of Germany as far as the Riesengebirge and Harz Mountains; and in the south-east over the whole of Holland and the North Sea basin. It should be noted that Holland and Denmark were glaciated, not by Norwegian ice, but by ice from the Baltic sheet which had crossed southern Sweden. The North Sea glacier extended across East Anglia as far as Cambridge, while a northern branch of it swept across Caithness and the Orkney and Shetland Islands, but most of the British Isles were glaciated from independent centres—the Scottish Highlands, the Pennines, Cumberland, Wales and northern Ireland.

With the growth of the glaciated area, and particularly with its extension south-westward across the North Sea, the Alpine climate again became very severe, and the local glaciers and Piedmont ice-sheets of the Alps reached their maximum development in the Mindelian. At the same time the central plateau of France developed a local plateau glacier of its own, and the Pyrenees underwent their first and greatest glaciation, no traces of the Gunzian having been found in this range.

The British Isles show an interesting outward migration of the local centres of maximum ice-development. The Scandinavian glacier which invaded East Anglia extended arctic anticyclonic conditions across the North Sea, and induced a heavy snowfall over the high lands of Great Britain. These, in consequence, developed independent glaciers, which on their eastern sides fused with the Scandinavian glacier and, partly by deflecting its flow, partly by intercepting some of its snowfall, pushed it back into the North Sea plain. The Scottish glaciers became strong enough to encroach on Ireland, partly in the north-east, and partly by way of the Irish Sea and St. George’s Channel (then a valley) on to the south-east. This further extension of the cold area enabled the Irish glaciers to develop, and these in turn pushed back the Scottish glaciers until Ireland was solely glaciated by Irish ice.

The southern margin of the ice-sheet did not extend beyond the Thames valley, but at some stage the English Channel carried floating ice, which formed the deposits of ice-borne boulders, of which that at Selsey is a well-known example.

This great ice-sheet nowhere formed marked terminal moraines, but its deposits fade away in thin beds of stiff boulder-clay. This absence of moraines is probably connected with the great thickness of the ice-sheets, which did not leave any appreciable nunataks or rocky “islands” exposed in its path, so that there was nothing to give rise to detritus on the surface of the ice. All the transportation had to be carried on beneath the ice-sheets, and these, penetrating into comparatively low latitudes where the sun is powerful in summer, would suffer gradual melting and ablation for some distance from their margins. Near the actual ice-limit the motion must have been slow and the thickness of the ice small, so that conditions were against the accumulation of thick beds of detritus.

On the borders of the ice-sheet the climate cannot have been over-rigorous, for pre-Chellean man was able to live almost up to the ice-edge. The air must have been extremely cold, and there was a belt of high arctic climate round the ice, but in the south and south-west this appears to have been very narrow, and sub-arctic conditions, no worse than those in which many races live to-day, prevailed not very far from the ice. The configuration of the ice-surface largely explains this. A high steeply sloping wall of ice causes intensely violent winds, carrying dense clouds of drift-snow—blizzards, in fact, similar to those now experienced in parts of Antarctica under similar circumstances, which sweep the land bare of all life for a considerable distance. But a low and gradually sloping surface, such as seems to have existed near the borders of the maximum glaciation, favours instead comparatively gentle winds without much drift snow. It is only on the north-west ice-ridge, where ice-cliffs fronted the sea and where severe storms from the Atlantic were frequent in winter, that blizzards occurred.

When the land in Scandinavia began to sink under the ice-load more rapidly than the supply of snow could build up the surface of the ice-sheet the force which pushed out the ice in all directions from the centre gradually died away, and the ice-masses over the North Sea area—now probably again below sea-level—and the low grounds of Europe were left derelict, with no resources but the snowfall on their own surfaces. Under these conditions they melted away more or less rapidly. While these derelict ice-masses were still large, the auxiliary peripheral centres in the Alps, Pyrenees and British Isles maintained an independent existence for a while, probably with fluctuations similar to those which marked the close of the last glaciation in the Alps, though the evidence of these has now been wiped away. It is even likely that the beginnings of the weakening of the central source of supply helped the British ice to divert the Scandinavian ice into the North Sea. Had there been any powerful rivers bearing great masses of detritus from the south, as there are in Siberia, some of these derelict ice-sheets might have been preserved for a time, at least, as “fossil ice,” but in western Europe conditions were not favourable for this.

With the disappearance of the ice-sheets the general climate of Europe must have passed through a series of stages of amelioration, of which traces can be found here and there, though the details are lost to us. Ultimately temperate conditions again prevailed; and for a very long time, approaching a quarter of a million years, Europe cannot have differed greatly from present climatic conditions. In Scandinavia the mammoth roamed in forests of birch, pine and spruce; further south the mammoth is absent, and we find instead more southern forms—Elephas antiquus, resembling the Indian elephant, Rhinoceros merckii, a southern form, the sabre-toothed tiger, cave-lion, cave-bear and cave-hyæna, wolf, beaver, horse and various forms of deer, while the flora included even such warmth-loving trees as the fig. Obviously, during part of this interglacial period, the climate must have been even warmer than the present.

Let us glance for a moment at the probable conditions. One of the dominant features in the present weather of Europe is the accumulation of floating ice in the Arctic basin. This keeps the temperature low and the pressure high—forms in fact during the spring and summer months a temporary glacial anticyclone similar in kind to, though of less intensity than, that which has been described as covering the Scandinavian ice-sheet. This anticyclone maintains on its southern edges a belt of easterly winds, and these winds enter into the general circulation of the earth. Their effect is to push southward the permanent storm-centres normally situated near Iceland and the Aleutian Islands, and it is these storm-centres which play a large part in causing the rainy weather of northern and central Europe. But occasionally—as in the remarkable spring and summer of 1921—these conditions break down. The Arctic Ocean becomes unusually ice-free and warm, the pressure falls, and in consequence the storm-centres move northward. Europe comes under the influence of the permanent anticyclones of sub-tropical latitudes, rain-bearing storms pass far to the northward, and we have a dry warm summer of the Mediterranean type.

This is presumably what happened during the long warm Mindel-Riss interglacial. For some reason, possibly connected with a temporary widening and deepening of the Bering Strait, the waters of the Arctic Ocean became warmer and the amount of floating ice less. Pressure became lower in the polar basin and therefore higher over the Atlantic and Europe, and fine warm conditions prevailed in Europe as the normal climate instead of only as an occasional event.

This warm interval was finally brought to a close by the renewed elevation of Scandinavia, and the ice-sheets began to develop again, heralded by a period of dry steppe climate. This time, however, the conditions were different; the elevation was not so great, and was more local. Hence the resulting glaciation was less intense; it filled the Baltic basin and extended some distance on to the North German plain and into Holland. It failed to reach the coast of Britain, but that it extended some way across the North Sea plain is indicated by the peculiar distribution of the Newer Drift of Britain, to be referred to later. In the north of Norway the slope of the ice towards the sea was very steep, so that many of the coastal hills extended above it as nunataks. The ice extended into the channel between the mainland and the Lofoten Islands (then a peninsula), but according to Ahlmann these islands were an independent centre of local glaciation, as the British Isles had been during the preceding period, and the local ice met the main ice-sheet in the fiords. On the coast of Nordland sufficient land lay bare to harbour a small Arctic flora, and Vaero, the southernmost island of Lofoten, had only small hanging snow-banks.

The interpretation of the British glacial deposits is still very much under discussion, but it seems probable that the Scottish highlands formed a subsidiary centre which glaciated the whole of Scotland and north-east England, sending a stream south-eastward, which was prevented from spreading across the North Sea plain by the presence of Scandinavian ice to the east and impinged on the coast of Yorkshire and Lincolnshire, just reaching the northern extremity of Norfolk. Many British geologists regard this development as the concluding phase of a single glaciation of Britain, but the differences in the amount of weathering undergone are against such an interpretation. At the same time there were local glaciers in Cumberland, Wales and Ireland.

In England limits of this glaciation are characterized by a well-marked series of end-moraines, which indicate that the ice carried much surface detritus, and probably ended in a steep cliff. In Scandinavia, on the other hand, the centre of glaciation again lay over the low ground well to the east of the mountains, and the ice which reached Germany and Denmark was still largely free of surface detritus, and so did not form marked end-moraines. There was a difference, however. On this occasion, owing to the local nature of the elevation in Scandinavia, the ice-sheet did not extend its borders so far to the eastward, and the glaciation of Asia, as described in [Chapter VII], was slight. Europe came more under the influence of cold north-easterly and northerly winds, and life on the ice-borders was not so easy as during the preceding glaciation. Man could still live near the ice, but he took to making his home in caves, and to clothing himself in skins for warmth.

After the ice had reached its Rissian maximum of glaciation the climate improved somewhat. The ice-edge retreated, leaving Denmark and the German coast, and vacating the Baltic basin, but not disappearing altogether from Scandinavia. At Rixdorf, near Berlin, there is a bed of gravel deposited in this “interglacial,” containing numerous and well-preserved bones of the mammoth, woolly rhinoceros, aurochs, bison, horse, reindeer, red deer and other species of Cervus, musk ox and wolf—a cold temperate to sub-arctic fauna. In south Germany fresh-water mollusca indicate that the summers in that district were almost as warm as at present, but the winters were probably severe. As described in the preceding chapter this “interglacial” was the time of loess formation par excellence, with a continental climate and steppe conditions over much of central Europe.

Investigations at Skærumhede in Denmark show that this recession of the ice was accompanied by, and presumably due to, a fall in the level of the land relatively to that of the sea, for at the beginning of the oscillation the land lay about 350 feet above its present level, sinking gradually to only 30 feet above present. Even at its best during this interglacial the climate was almost sub-arctic in Denmark. In northern Finland, on the eastern edge of the ice-sheet, there was also an “interglacial,” with a slight improvement in the climate accompanying a temporary submergence. But in Scandinavia there are no traces of any interglacial deposits of this period, and considering the cold climates which prevailed in Denmark and North Germany, it seems probable that Scandinavia continued to be glaciated during the whole period.

The mode of life among Mousterian men, who lived during this “interglacial,” also points to a severe climate. For at this time man did not live in the open, but in caves and rock-shelters, and the practice of wearing the fur skins of animals as a protection against the cold, begun in the preceding Rissian glacial period, was not discontinued.

After the temporary subsidence had ceased, elevation again set in, causing a readvance of the ice-sheets and glaciers. The limits fell short of those of the preceding maximum, and the climate was not so severe, but in its general character it resembled that of the preceding maximum, but was much stormier, and there were probably frequent blizzards of the Antarctic type, carrying drift-snow. The new ice-sheet carried more surface detritus than its predecessors, presumably because all the high ground was not covered, and it formed high terminal moraines. The close association of cold ice and irregular masses of bare sand and stones, strongly heated by the summer sun, set up a belt of powerful convection very favourable for the development of blizzards; possibly there was something in the nature of an ice-cliff down which the cold winds could blow with great strength. At any rate, man found the near neighbourhood of the ice unpleasant, and went, so that there are no contemporaneous human implements near the moraines. The limits of the Scandinavian ice-sheet ran from the Norwegian coast across Denmark from north to south, through North Germany and northern Russia, and included Finland. The ice probably did not extend far across the North Sea plain, and in the British Isles there was no ice-sheet, but the high mountains of Scotland, Ireland, Wales and Cumberland bore small local glaciers, which were long enough to reach the sea in the Scottish highlands. The Alps bore considerable glaciers, indicating a depression of the snow-line of about 3500 feet, corresponding to a temperature 11° F. lower than the present.

After this ice-development had reached its maximum limits and remained there for perhaps a thousand years, retreat set in, and the Scandinavian ice once more withdrew from Germany and Denmark to the Baltic basin. But its edge was never far from the German coast, and occasionally readvanced across it, for numerous fossiliferous deposits are intercalated in boulder-clay. The fauna and flora, which are well known, point to an arctic climate. At its best the mean temperature of July rose to about 50° F., and there was a vegetation period of three or four months with an average temperature of about 40° F., but these relatively mild conditions lasted at most for a few decades or perhaps a century at a time, and the winters were severe throughout. The duration of the whole of this “Baltic Interstadial” was from one to two thousand years.

Next followed the final readvance of the ice forming the great “Baltic” moraines which fringe the Baltic coast of Germany, turning northward in the west into Denmark and in the east into Finland. There was a corresponding re-development of glaciers in the Alps (Bühl stage) and in the mountains of Ireland and Scotland, though these probably failed to reach the sea even in Scotland. This period gave us a repetition of the climate of the preceding maxima. In this case we have definite evidence of the presence of a belt of easterly winds on the southern side of the ice-sheet, in a series of “barkans” or fossil dunes in Holland, Germany and Galicia. These dunes were formed of fine ice-deposited material, and they are crescent-shaped, with their convexities to the east, indicating that they were built by strong easterly winds. A moment’s consideration will show the truth of the latter statement. Suppose there is an isolated round hillock of sand exposed to strong easterly winds. The sand grains will travel up the easterly windward slope of the hillock and roll down the westerly leeward side. In this way the whole hillock will advance very slowly westwards. But in the centre, where the hillock reaches its greatest height, the grains will take longer to reach the highest point than near the edges, where they have not to rise so high. At the edges a strong gust will carry some of the heavier grains right over the dune, while nearer the centre they will be left half-way, and when the gust ceases will perhaps roll back to their original position. In this way the margins of the dune will advance westward more rapidly than the centre, producing the crescent shape with the convex side to the east. At the time of their formation these dunes must have had their steepest side to the westward, but the westerly winds which have prevailed during the last few thousand years have succeeded in modifying that detail, without destroying the general shape of the dunes, and the steepest slopes are now on the eastern side. The preservation of the original shape, in spite of the subsequent development of westerly winds, is due in part to the coating of vegetation, which protected the dunes as soon as more favourable conditions occurred, and probably in part to the lesser velocity of the westerlies. If the period of east winds and dune formation had been long enough, we might have had another deposit of loess, but it was short, and vegetation, which is necessary to the genesis of true loess, had no time to establish itself before the climate changed again with the final retreat of the ice. The climate of this period in Rumania has been ably described by G. Murgoci: “In general the prevailing climate of the time of the formation of loessoid soils and blown sands must have been that which is named by E. de Martonne the aralian climate, a dry climate with some rain in spring to call forth a poor and transient vegetation and to maintain the flowing water in rivers and lakes. The temperature with great extremes, in summer up to 120° F. and in winter below 20° F., was the characteristic of this climate; the atmosphere was very dry in the hot season, but in the rest of the year there was some humidity in the air and moisture in the soil, the water of the subsoil being not very deep. The atmospheric precipitation in this region was caused by the south-west wind just as at present; but the dominant wind giving the character of a dry continental climate was the north-east wind (Crivat) which has left its traces in the fossil dunes of the Baragan.”

A history of the changes of climate in Europe which followed the maximum of the last readvance of the ice-sheet must be left to later chapters.