During the glacial advances, two enormous ice-covered areas were created, one in North America and the other in northern Eurasia. These two areas are known as the Laurentide and Fennoscandian ice sheets (respectively). During the coldest periods, the areas in which ice accumulated extended far south and the northern ice sheets covered not only the higher latitudes, but also spread to mid-level regions also.

One question that has yet to be answered is that of where the moisture required to form such an enormous volume of ice so quickly in the continental sheets (especially the Laurentide one) actually came from. Up until recently, the most widely accepted theory has been that the moisture came from the North Atlantic. For this to have occurred, the surface of the ocean must have remained warm for a fairly long period of time, thanks to the ongoing influence of the Gulf Stream. However, in order for the huge Laurentide ice sheet to have formed, snow storms much greater and more frequent than those which occur today in Québec and the north-eastern United States would have been required. These snow storms, which must have been ten times as intense as modern-day ones, would have been associated with extremely active fronts generated by the contrast between the polar masses of cold air from the American continent and the warm, moisture-laden air masses which formed over the Atlantic Ocean (Duplessy, 1993).

A second, more recent, theory holds that the moisture came from more southerly, even perhaps tropical, latitudes. A detailed analysis of foraminifera indicates that in the higher latitudes, the ocean water cooled extremely quickly. If this occurred right from the beginning of the glaciation, then the northern ocean could not have been the source of the moisture required to form the ice sheets. However, the surface waters in the tropical latitudes remained warm, and even increased in temperature during the initial phase of the glaciation. This would have increased the meridional temperature gradient, which would in turn have transported a greater amount of moisture from the tropics to the Arctic (Khodri, 2001).

As the very cold surface airmasses (white dots) were expelled towards the south from the Laurentide ice sheet and the Arctic region, return flows were created along their eastern edges (red arrows), which carried warm, moisture-laden air back towards the north, thus supplying the ice sheets with abundant snowfall. Along the North American Pacific coast, the high peaks of the Rockies (marked in brown) facilitated this back flow (Leroux, 2005). When the cold, dense, polar air masses moved south along the surface, higher up, a current of subtropical air was formed along the edges moving in the opposite direction (i.e. south-north) and carrying moisture evaporated from milder waters in more southerly regions. During the glaciation, as the huge Laurentide and Fennoscandian ice caps were formed, the meridional exchange of very different air masses became increasingly intense. In the Pacific, the north-south line of the Rockies intensified the warm back currents which flowed over and along the eastern edges of the cold surface air mass expelled from the Arctic. The moisture carried by these back currents supplied both the western part of the Laurentide sheet and the Cordilleran ice sheet with abundant snowfall (Leroux, 1998).

The Laurentide ice sheet

During the Last Glacial Maximum, the largest North American sheet, the Laurentide sheet, covered an area stretching over 16 million km2 (32 times the surface area of Spain), and had a volume of around 30 million km3, much larger than the ice sheet that currently covers Antarctica. As a result, the North American ice encompassed (in volume at least) one third of all the land-based continental ice on the planet. Moving south along the east coast of North America, the Laurentide ice sheet eventually arrived at a latitude of 36ºN, the modern-day location of New York (in Europe, this would have been equivalent to the Fennoscandian ice sheet reaching the Mediterranean coast).

Thanks to the moisture provided by the Atlantic, more ice accumulated on the eastern side of the Laurentide ice sheet than on the western side. The highest point of the dome was located above what is today Hudson Bay and at this point, the ice measured between 3,000 and 4,000 metres in thickness. This whole region was fed by ocean moisture supplied by deep winter depressions which formed along the Atlantic coast of the north-eastern United States and Canada.  It is likely that another major dome existed in the west, over Keewatin. On its western side, where it reached the Rocky Mountains, the Laurentide ice sheet merged with the western ice sheet that covered the Pacific Coast Ranges, known as the Cordilleran sheet. Between these two sheets, a meridional trough existed which thawed before the rest of the ice and which was perhaps used by the primitive peoples of America who arrived from Asia and gradually moved south.

This western region of North America received much less moisture. Indeed, in much of Alaska no major ice sheets formed at all at sea level, and evidence exists which indicates that the region served as a refuge for some species of conifers which somehow managed to survive there despite the intense cold. Furthermore, since the surface of the ocean was located around 120 metres below its current level, the Bering Strait which nowadays separates Alaska and Siberia did not exist, thus enabling both animals and humans to move freely between Asia and America.

The Fennoscandian ice sheet

In Europe, during the Last Glacial Maximum, the Fennoscandian ice sheet had a volume of around 7 million km3, less than one quarter of the volume of the Laurentide ice sheet. The Fennoscandian sheet basically covered what is today Scandinavia and Finland. The highest part of the dome (where the ice was around 2,000 metres thick) was located in the northern part of the Baltic Sea. To the south, the ice covered the whole of the Baltic right down to Denmark and the northern plains of Germany and Poland, where Berlin stands today, leaving many clear indications of glacial occupation in its wake.

To the west, across a vast tundra plain that today lies under the shallow waters of the North Sea, the Fennoscandian sheet merged with the British one, which covered almost all of Great Britain and most of Ireland. The English Channel was also located above water during this period and the Rhine probably took a more westerly course than at present and crossed the whole region before flowing into the Atlantic near where Brittany lies today. The drop in sea level also exposed a vast area of the Atlantic continental shelf in northwest France and southwest Cornwall.

The Barents-Kara ice sheet

Towards the east, the borders of the Fennoscandian ice sheet are still fairly blurred. The ice extended throughout northern Siberia, but it seems unlikely that the eastern regions located at a great distance from the Atlantic would have received the snowfall required for a major ice cap to form. Recent studies of glacial sediments found on the floor of the Barents and Kara Seas (Polyak, 2001; Polyak, 2002) and in the northern part of continental Russia, indicate that these regions probably marked the outermost boundaries of the ice sheet, reached not during the Last Glacial Maximum, but rather at the beginning of the glaciation, around 80,000 years ago, when ice sheets covered the whole northern region of Western Siberia, including the Kara Sea (Krinner et al, 2004). It is therefore possible that the Siberian rivers which today flow into the Arctic Ocean would have become blocked, forming large lakes on the southern edge of the sheet. They may also have diverted their course south, towards the Black Sea and the Caspian Sea.

Figure 48. Scandinavian and Barents-Kara ice sheets 80,000 years ago.

This sheet later shrunk, probably due to the growth of the Fennoscandian ice sheet, which spread to the west and absorbed much of the rain and snowfall. The amount of snowfall may also have diminished as the result of the cooling of the Atlantic waters and the Kara Sea (Svendsen, 1999). Traces of human habitation (perhaps Neanderthal) from around 40,000 years ago have been found just north of the Urals, in the Arctic Circle, indicating that, despite the cold, the Barents-Kara ice sheet had retreated and the Fennoscandian ice sheet remained confined to the western end of the Eurasian continent (Pavlov, 2001). By the time of the Last Glacial Maximum, 22,000 years ago, the Barents-Kara ice sheet is thought to have disappeared completely.

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