The largest mass of ice in the Northern Hemisphere covers Greenland, which is about 10 percent of the world's ice. This ice is being measured and monitored as never before, by satellites, aircraft, and by dozens of scientists who are enduring —30°F temperatures and deadly snow-cloaked crevasses at the slumping edges of the ice cap (Revkin, 2004). Greenland's southern tip is no further north than Juneau or Stockholm. The persistence of the ice cap is due to its mass, the fact that
ice makes its own climate. The ice reflects sunlight and heat and deflects weather systems from the south. The elevation of the ice sheet helps to keep it cold. As it erodes, these advantages diminish (Appenzeller, 2007, 68). Western Greenland is losing ice mass most quickly, as the east gains some mass due to increased precipitation.
Greenland's ice is only a fraction of Antarctica's, but it is melting more rapidly, in part because summers are warmer, allowing for more rapid runoff. During the last few years, Greenland's "melt zone," where summer warmth turns snow on the edge of the ice cap into slush and ponds of water, has expanded inland, reaching elevations more than a mile high in some places, said Konrad Steffen, a glaciologist at the University of Colorado.
"The higher elevation appears to be stable, but in a lot of areas around the coast the ice is thinning," said Waleed Abdalati, a manager in the Earth Sciences Department of NASA's Goddard Space Flight Center, "There is a net loss of ice, particularly in the south" (Brown, 2002, A-30). While some studies suggest that Greenland's ice is melting at increasing rates, one study indicates that temperatures at the summit of the ice sheet have declined at the rate of 2.2°C per decade since 1987
(Chylek et al., 2004, 201). In some places, however, coastal thinning of ice increased to as much as three feet a year during the 1990s.
Modeling by Jonathan Gregory of the Centre for Global Atmospheric Modeling in Reading, UK, "suggests that as ice is lost, portions of the surface of Greenland's interior will heat and up at lower elevations where the air is warmer. Less snowfall and more rain would cause the ice to disappear at a faster rate than it is being replaced, leading in turn to further drops in elevation" (Schiermeier, 2004, 114—115). Greenland's largest glacier, the Jakobshavn, has doubled its speed into the ocean in a few years, to 120 feet per day on average, delivering 11 cubic miles of ice to the sea each year. The glacier's ice tongue, the point at which the glacier meets the ocean, has also retreated 4 miles since 2000 (Appen-zeller, 2007, 61).
In Greenland (and elsewhere), summer melt collects in deep-blue lakes sometimes several hundred yards across. The lakes then find fissures in the ice (called moulins), which conduct the water to the glacier's base, forming a slick surface that speeds the glacier's movement into the sea. The more the ice melts, the faster the glacier moves. Sometimes surface lakes disappear down moulins nearly instantly (Appenzeller, 2007, 68).
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