The Greenland ice sheet is a vast body of ice covering 1.71 million km², roughly 80% of the surface of Greenland. It is the second largest ice body in the World, after the Antarctic Ice Sheet. The ice sheet is almost 2,400 kilometers long in a north-south direction, and its greatest width is 1,100 kilometers at a latitude of 77°N, near its northern margin. The mean altitude of the ice is 2,135 meters. The thickness is generally more than 2 km (see picture) and over 3 km at its thickest point. It is not the only ice mass of Greenland – isolated glaciers and small ice caps cover between 76,000 and 100,000 square kilometers around the periphery. Some scientists believe that global warming may be about to push the ice sheet over a threshold where the entire ice sheet will melt in less than a few hundred years. If the entire 2.85 million km³ of ice were to melt, it would lead to a global sea level rise of 7.2 m (23.6 ft). This would inundate most coastal cities in the World and remove several small island countries from the face of Earth, since island nations such as Tuvalu and Maldives have a maximum altitude below or just above this number.

The Greenland Ice Sheet is also sometimes referred to under the term inland ice, or its Danish equivalent, indlandsis. It is also sometimes referred to as an ice cap. Ice sheet, however, is considered the more correct term as ice cap generally refers to less extensive ice masses.

The ice in the current ice sheet is as old as 110,000 years. However, it is generally thought that the Greenland Ice Sheet formed in the late Pliocene or early Pleistocene by coalescence of ice caps and glaciers. It did not develop at all until the late Pliocene, but apparently developed very rapidly with the first continental glaciation.

The massive weight of the ice has depressed the central area of Greenland; the bedrock surface is near sea level over most of the interior of Greenland, but mountains occur around the periphery, confining the sheet along its margins. If the ice were to disappear, Greenland would most probably appear as an archipelago, at least until isostasy would lift the land surface above sea level once again. The ice surface reaches its greatest altitude on two north-south elongated domes, or ridges. The southern dome reaches almost 3,000 metres at latitudes 63°–65°N; the northern dome reaches about 3,290 metres 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, so that 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 occur in North Atlantic shipping lanes. The best known of these outlet glaciers is Jakobshavn Isbræ (Kalaallisut: Sermeq Kujalleq), which, at its terminus, flows at speeds of 20 to 22 metres per day.

On the ice sheet, temperatures are generally substantially lower than elsewhere in Greenland. The lowest mean annual temperatures, about -31°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).

During winter, the ice sheet takes on a strikingly clear blue/green color. During summer, the top layer of ice melts leaving pockets of air in the ice that makes it look white.

The ice sheet, consisting of layers of compressed snow from more than a hundred thousand years, contains in its ice today’s most valuable record of past climates. In the past decades, scientists have drilled ice cores up to four kilometres deep. Scientists have, using those ice cores, obtained information on (proxies for) temperature, ocean volume, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity, desert extent and forest fires. This variety of climatic proxies is greater than in any other natural recorder of climate, such as tree rings or sediment layers.

Positioned in the Arctic, the Greenland ice sheet is especially vulnerable to global warming. Arctic climate is now rapidly warming and much larger Arctic shrinkage changes are projected. The Greenland Ice Sheet has experienced record melting in recent years and is likely to contribute substantially to sea level rise as well as to possible changes in ocean circulation in the future. The area of the sheet that experiences melting has increased about 16% from 1979 (when measurements started) to 2002 (most recent data). The area of melting in 2002 broke all previous records. The number of glacial earthquakes at the Helheim Glacier and the northwest Greenland glaciers increased substantially between 1993 and 2005. In 2006, estimated monthly changes in the mass of Greenland’s ice sheet suggest that it is melting at a rate of about 239 cubic kilometres (57.3 cubic miles) per year. A more recent study, based on reprocessed and improved data between 2003 and 2008, reports an average trend of 195 cubic kilometres (46.7 cubic miles) per year. These measurements came from the US space agency’s GRACE (Gravity Recovery and Climate Experiment) satellite, launched in 2002, as reported by BBC. Using data from two ground-observing satellites, ICESAT and ASTER, a study published in Geophysical Research Letters (September 2008) shows that nearly 75 percent of the loss of Greenland’s ice can be traced back to small coastal glaciers

If the entire 2.85 million km³ of ice were to melt, global sea levels would rise 7.2 m (23.6 ft.). Recently, fears have grown that continued global warming will make the Greenland Ice Sheet cross a threshold where long-term melting of the ice sheet is inevitable. Climate models project that local warming in Greenland will exceed 3 degrees Celsius during this century. Ice sheet models project that such a warming would initiate the long-term melting of the ice sheet, leading to a complete melting of the ice sheet (over centuries), resulting in a global sea level rise of about seven meters. Such a rise would inundate almost every major coastal city in the World. How fast the melt would eventually occur is a matter of discussion. According to IPCC, the expected 3 degrees warming at the end of the century would, if kept from rising further, result in about 1 meter sea level rise over the next millennium

Some scientists have cautioned that these rates of melting as overly optimistic as they assume a linear, rather than erratic, progression. James Hansen has argued that multiple positive feedbacks could lead to nonlinear ice sheet disintegration much faster than claimed by the IPCC. According to a 2007 paper, “we find no evidence of millennial lags between forcing and ice sheet response in paleoclimate data. An ice sheet response time of centuries seems probable, and we cannot rule out large changes on decadal time-scales once wide-scale surface melt is underway

The melt zone, where summer warmth turns snow and ice into slush and melt ponds of meltwater, has been expanding at an accelerating rate in recent years. When the meltwater seeps down through cracks in the sheet, it accelerates the melting and, in some areas, allow the ice to slide more easily over the bedrock below, speeding its movement to the sea. Besides contributing to global sea level rise, the process adds freshwater to the ocean, which may disturb ocean circulation and thus regional climate.

Researchers monitoring daily satellite images have discovered that a massive 11-square-mile (29-square-kilometer) piece of the Petermann glacier in northern Greenland broke away between July 10 and July 24, 2008. The last major ice loss to Petermann occurred when the glacier lost 33 square miles (86 square kilometers) of floating ice between 2000 and 2001. Between 2001 and 2005, a massive breakup of Sermeq Kujalleq erased 36 square miles (94 square kilometers) from the ice field and raised the awareness of worldwide of glacial response to global climate change.

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