3.1.8 Glacial Periods and Ice Sheets in the Northern Hemisphere
The ice age that began 350,000 years ago has completed four cycles of alternating glacial and interglacial periods. We are currently in the midst of the last interglacial, which began several thousand years ago and is known as the "postglacial period. " During the glacial periods the ice sheets expanded over the earth's continents. Because the ice sheets absorbed water that would otherwise have been returned to the oceans, the sea level dropped and the continental shelf jutted above the ocean's surface. Because the ice sheets were not able to melt completely during the interglacials before the onset of the next glacial, the sea level dropped with each repetition of the glacial phase of the cycle. In the final glacial period, when the sea level reached its lowest point, the sea level is estimated to have been fully 120m below its present level.
Some 20,000 years ago, during the last glacial period, the ice sheet that covered Scandinavia extended north across the entire continental shelf from the Svalbard Islands to Frantsa Josifa and Severnaya Zemlya, west across all of the British Isles and east almost to the Taymyr Peninsula. In North America, the Laurentide Ice Sheet covered almost all of Canada and extended from Greenland in the north to the Great Lakes on its southern fringe. The ice sheet west of the Canadian Rockies, known as the Cordilleran Ice Sheet, followed the southern Alaskan coastline to the Aleutian Islands. With the drop in sea level that attended the glacial period, the continental shelf under the Kara, Laptev and East Siberian Seas was exposed to become dry land. Moreover, a wide swath of ocean from the Chukchi Sea to the eastern part of the Bering Sea rose above sea level, linking Eurasia and the Americas in a vast land region called Beringia. It is believed that the prehistoric Mongoloid crossed this land bridge and reached South America.
The region from the Taymyr Peninsula across Beringia to northern Alaska was not covered by an ice sheet but was exposed to bitterly cold air during the glacial periods, causing the formation of vast permafrost in this area.
With the advent of the late glacial stage, the Scandinavian Ice Sheet became extinct, leaving behind ice caps and glaciers in some limited areas. With the release of the immense weight of the ice sheets, the Scandinavian Peninsula began to rise. Parts of this peninsula are still slowly rising today at a rate of about 1cm per year. In modern times a number of ice sheets and glaciers remain in the Arctic Circle. These include the Greenland Ice Sheet and numerous smaller ice sheets in Canada's Arctic Archipelago, Svalbard, Novaya Zemlya, Severnaya Zemlya and Iceland, as well as ice caps and alpine glaciers in Alaska, Canada and Scandinavia.
