The Great Tce Sheets

Two great ice masses, the Antarctic and Greenland ice sheets, stand out in the world today and may be similar in many respects to the large Pleistocene ice sheets. About 99 percent of the world's glacier ice is in these two ice masses— 91 percent in Antarctica alone.

THe Antarctic Ice SHeeT

The bedrock of the continent of Antarctica is almost completely buried under ice. Mountain ranges and isolated nunataks (a term derived from Greenland's Inuit language, used for individual mountains surrounded by ice) locally protrude through the ice. Extensive in area are the ice shelves, where the ice sheet extends beyond the land margin and spreads out to sea. The ice sheet, with its associated ice shelves, covers an area of 13,829,000 square km (5,340,000 square miles); exposed rock areas total less than 200,000 square km (77,000 square miles). The mean thickness of the ice is about 1,829 metres (6,000 feet) and the volume of ice more than 25.4 million cubic km (6 million cubic miles).

The land surface beneath the ice is below sea level in many places, but this surface is depressed because of the weight of the ice. If the ice sheet were melted, uplift of the land surface would eventually leave only a few deep troughs and basins below sea level—even though the sea level itself also would rise about 80 metres (260 feet) from the addition of such a large amount of water. Because of the thick ice cover, Antarctica has by far the highest mean altitude of the continents (2 km [1.3 miles]); all other continents have mean altitudes less than 1 km (0.6 mile).

Antarctica can be divided into three main parts: the smallest and the mildest in climate is the Antarctic Peninsula, extending from latitude 63° S off the tip of South America to a juncture with the main body of West Antarctica at a latitude of about 74° S. The ice cover of the Antarctic Peninsula is a complex of ice caps, piedmont and mountain glaciers, and small ice shelves.

The part of the main continent lying south of the Americas, between longitudes 45° W and 165° E, is characterized by irregular bedrock and ice-surface topography and numerous nunataks and deep troughs. Two large ice shelves occur in West Antarctica: the Filchner-Ronne Ice Shelf (often considered to be two separate ice shelves), south of the Weddell Sea, and the Ross Ice Shelf, south of the Ross Sea. Each has an area exceeding 500,000 square km (193,000 square miles).

The huge ice mass ofEast Antarctica, about 10,200,000 square km (about 4,000,000 square miles), is separated from West Antarctica by the Transantarctic Mountains. This major mountain range extends from the eastern margin of the Ross Ice Shelf almost to the Ronne-Filchner Ice Shelf. The bedrock of East Antarctica is approximately at sea level, but the ice surface locally exceeds 4,000 metres (13,100 feet) above sea level on the highest parts of the polar plateau.

At the South Pole the snow surface is 2,800 metres (9,200 feet) in altitude, and the mean annual temperature is about -50°C (-58T), but at the Russian Vostok Station (78^7' S, i06°52' E), 3,500 metres (about 11,500 feet) above sea level, the mean annual temperature is -58°C (-73°F), and in August i960 (the winter season) the temperature reached a low of-88.3°C (-i27°F).The temperatures on the polar plateau of East Antarctica are by far the coldest on Earth; the climate of the Arctic is quite mild by comparison. Along the coast of East or West Antarctica, where the climate is milder, mean annual temperatures range from -20 to -9°C (-4 to i6°F), but temperatures exceed the melting point only for brief periods in summer, and then only slightly. Katabatic (drainage) winds, however, are very strong along the coast; the mean annual wind speed at Commonwealth Bay is 20 metres per second (45 miles per hour).

The Greenland Ice Sheet

The Greenland Ice Sheet, though subcontinental in size, is huge compared with other glaciers in the world except that of Antarctica. Greenland is mostly covered by this single large ice sheet (1,730,000 square km [668,000 square miles]), while isolated glaciers and small ice caps totaling between 76,000 and 100,000 square km (29,000 and 39,000 square miles) occur around the periphery. The ice sheet is almost 2,400 km (about 1,500 miles) long in a north-south direction, and its greatest width is 1,100 km (680 miles) at a latitude of 77° N, near its northern margin. The mean altitude of the ice surface is 2,135 metres (7,000 feet). The term Inland Ice, or, in Danish, Indlandsis, is often used for this ice sheet.

The bedrock surface is near sea level over most of the interior of Greenland, but mountains occur around the periphery. Thus, this ice sheet, in contrast to the Antarctic Ice Sheet, is confined along most of its margin. The ice

Map of Greenland highlighting the major geographic regions and the locations of human settlement.

surface reaches its greatest altitude on two north-south elongated domes, or ridges. The southern dome reaches almost 3,000 metres (about 9,800 feet) at latitudes 63°-65° N; the northern dome reaches about 3,290 metres (10,800 feet) at about latitude 72° N. The crests of both domes are displaced east of the centre line of Greenland.

The unconfined ice sheet does not reach the sea along a broad front anywhere in Greenland, and no large ice shelves occur. The ice margin just reaches the sea, however, in a region of irregular topography in the area of Melville Bay southeast of Thule. Large outlet glaciers, which are restricted tongues of the ice sheet, move through bordering valleys around the periphery of Greenland to calve off into the ocean, producing the numerous icebergs that sometimes penetrate North Atlantic shipping lanes. The best known of these is the Jakobshavn Glacier, which, at its terminus, flows at speeds of 20 to 22 metres (66 to 72 feet) per day

The climate of the Greenland Ice Sheet, though cold, is not as extreme as that of central Antarctica. The lowest mean annual temperatures, about -3i°C (-24°F), occur on the north-central part of the north dome, and temperatures at the crest of the south dome are about -20°C (-4°F).

The ice sheet is the largest and possibly the only relict of the Pleistocene glaciations in the Northern Hemisphere. In volume it contains 12 percent of the world's glacier ice, and, if it melted, sea level would rise 20 feet (6 metres). In the 1970s and early 1980s the Greenland Ice Sheet Program was organized by scientists from the United States, Denmark, and Switzerland. Deep ice cores from the Greenland Ice Sheet were obtained for comparison with deep cores from the Antarctic ice mass to gain a better understanding of the factors controlling present and past ice mass dynamics, atmospheric processes, and the response of ice sheets to climatic change and to determine whether the past changes in climate were global or regional in character.

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