Greenland ice sheet

The Greenland ice sheet (Danish: Grønlands indlandsis, Greenlandic: Sermersuaq) is a vast body of ice covering 1,710,000 square kilometres (660,000 sq mi), roughly 80% of the surface of Greenland.Until 2007, rate of decrease in ice sheet height in cm per year.Modelling results of the sea-level rise under different warming scenarios.Satellite image of dark melt ponds.Albedo change in GreenlandSatellite measurements of Greenland's ice cover from 1979 to 2009 reveals a trend of increased melting.NASA's MODIS and QuikSCAT satellite data from 2007 were compared to confirm the precision of different melt observations.This narrated animation shows the accumulated change in the elevation of the Greenland ice sheet between 2003 and 2012. The Greenland ice sheet (Danish: Grønlands indlandsis, Greenlandic: Sermersuaq) is a vast body of ice covering 1,710,000 square kilometres (660,000 sq mi), 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 kilometres (1,500 mi) long in a north-south direction, and its greatest width is 1,100 kilometres (680 mi) at a latitude of 77°N, near its northern margin. The mean altitude of the ice is 2,135 metres (7,005 ft). The thickness is generally more than 2 km (1.2 mi) and over 3 km (1.9 mi) at its thickest point. In addition to the large ice sheet, isolated glaciers and small ice caps cover between 76,000 and 100,000 square kilometres (29,000 and 39,000 sq mi) around the periphery. If the entire 2,850,000 cubic kilometres (684,000 cu mi) of ice were to melt, it would lead to a global sea level rise of 7.2 m (24 ft). The Greenland Ice Sheet is sometimes referred to under the term inland ice, or its Danish equivalent, indlandsis. It is also sometimes referred to as an ice cap. The presence of ice-rafted sediments in deep-sea cores recovered from northeast Greenland, in the Fram Strait, and south of Greenland indicated the more or less continuous presence of either an ice sheet or ice sheets covering significant parts of Greenland for the last 18 million years. From about 11 million years ago to 10 million years ago, the Greenland Ice Sheet was greatly reduced in size. The Greenland Ice Sheet formed in the middle Miocene by coalescence of ice caps and glaciers. There was an intensification of glaciation during the Late Pliocene. Ice sheet formation occurred in connection to uplift of the West Greenland and East Greenland uplands. The Western and Eastern Greenland mountains constitute passive continental margins that were uplifted in two phases, 10 and 5 million years ago, in the Miocene epoch. Computer modelling shows that the uplift would have enabled glaciation by producing increased orographic precipitation and cooling the surface temperatures. The oldest known ice in the current ice sheet is as old as 1,000,000 years old. The 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 suddenly disappeared, Greenland would most probably appear as an archipelago, at least until isostasy lifted 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 (10,000 ft) at latitudes 63°–65°N; the northern dome reaches about 3,290 metres (10,800 ft) at about latitude 72°N (the fourth highest 'summit' of Greenland). 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 Glacier (Greenlandic: Sermeq Kujalleq), which, at its terminus, flows at speeds of 20 to 22 metres or 66 to 72 feet 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). The ice sheet, consisting of layers of compressed snow from more than 100,000 years, contains in its ice today's most valuable record of past climates. In the past decades, scientists have drilled ice cores up to 4 kilometres (2.5 mi) 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. Many scientists who study the ice ablation in Greenland consider that a two or three °C temperature rise would result in a complete melting of Greenland's ice. Positioned in the Arctic, the Greenland ice sheet is especially vulnerable to climate change. Arctic climate is believed to be now rapidly warming and much larger Arctic shrinkage changes are projected. The Greenland Ice Sheet has experienced record melting in recent years since detailed records have been kept and is likely to contribute substantially to sea level rise as well as to possible changes in ocean circulation in the future if this is sustained. The area of the sheet that experiences melting has been argued to have increased by about 16% between 1979 (when measurements started) and 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 kilometers (57 cu mi) per year. A more recent study, based on reprocessed and improved data between 2003 and 2008, reports an average trend of 195 cubic kilometers (47 cu mi) 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 2,850,000 km3 (684,000 cu mi) of ice were to melt, global sea levels would rise 7.2 m (24 ft). Recently, fears have grown that continued climate change 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 be 3 °C (5 °F) to 9 °C (16 °F) 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 7 metres (23 ft). 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 the IPCC 2001 report, such warming would, if kept from rising further after the 21st Century, result in 1 to 5 meter sea level rise over the next millennium due to Greenland ice sheet melting. Some scientists have cautioned that these rates of melting are overly optimistic as they assume a linear, rather than erratic, progression. James E. 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, allows 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. In July 2012, this melt zone extended to 97 percent of the ice cover. Ice cores show that events such as this occur approximately every 150 years on average. The last time a melt this large happened was in 1889. This particular melt may be part of cyclical behavior; however, Lora Koenig, a Goddard glaciologist suggested that '...if we continue to observe melting events like this in upcoming years, it will be worrisome.' Global warming is increasing growth of algae on the ice sheet. This darkens the ice causing it to absorb more sunlight and potentially increasing the rate of melting.