9Įvidence of accelerating shrinkage of the Greenland ice sheet continues to increase. 7, 8 By 2007 the melt season at elevations above 6,500 feet (2,000 meters) was a month longer than the average from 1988 to 2006. 5 The melt area set a new record in 2007: it was about 60 percent larger than the previous record in 1998, and extended farther inland. In 2005 the Greenland ice sheet lost around 53 cubic miles (220 cubic kilometers) of mass-more than two times the amount it lost in 1996 (22 cubic miles, or 90 cubic kilometers). Melting reaches its maximum in late summer. 6 Satellites measure the extent of melting by differentiating between areas of the ice mass that are fully frozen and those with surface meltwater. Over the past quarter-century, both the extent of melting and the length of the melt season on the Greenland ice sheet have been growing, as local temperatures have risen. (See Helheim and Jakobshavn Isbræ hotspots for more information on Greenland''s rapidly retreating glaciers.) The ice sheet loses most of its mass on the perimeter, through a dozen relatively fast-moving glaciers that have recently become thinner, significantly increased their rates of retreat, and broken up at the ocean end (the terminus). The Greenland ice sheet accumulates snow in its northernmost area and in the central region at high elevations. 2, 3, 4, 5 Ice sheets grow through snowfall, and shrink through surface melting, water runoff, breakup into the ocean (calving), and direct transformation into water vapor (sublimation). But unlike glaciers, ice sheets are thick enough to cover most of the terrain beneath them, including mountains. Like glaciers, ice sheets are large masses of slow-moving ice formed from layers of compacted snow. Glaciers can be found on every continent on Earth, but only three massive ice sheets exist in the world today: the Greenland ice sheet, the West Antarctic ice sheet, and the East Antarctic ice sheet.
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