The largest mass of ice in the Northern Hemisphere resides atop Greenland, which is home to 10 percent of the world's ice. This ice has been melting at an accelerating rate, especially in coastal areas.

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. The amount of ice lost by Greenland during 2007 was equivalent to two times all the ice in the Alps, or a layer of water more than 0.8 kilometer deep covering Washington, D.C., according to Konrad Steffen, a professor of geography at the University of Colorado, who also directs the Cooperative Institute for Research in Environmental Sciences.

Philippe Huybrechts, a glaciologist and ice-sheet modeler at the Free University of Brussels, has modeled the behavior of Greenland's ice sheet, finding that, with an anticipated annual temperature increase of 8œ Celsius, the ice sheet would shrink to a small glaciated area far inland and the worldwide sea level would rise by 6 meters. Erosion of the Greenland ice sheet due to prolonged warming would be irreversible.

Upon close examination of paleoclimatic proxies, scientists examining ice cores from Greenland have found that its climate can change very quickly. The island's climate altered to a different state within one to three years at the onset of the present interglacial period, for example. Other scientists have found evidence that Greenland's surface has been covered with pine forests within a few hundred thousand years of the present, with the ice mass a fraction of today's size.

Ice melt in Greenland has accelerated significantly since 1990, according to a report in the Journal of Climate coauthored by Steffen. A scientific team surveyed the rate of summer melting there between 1958 and 2006, and found that the five largest melting years occurred after 1995. The year 1998 was the biggest (454 cubic kilometers), followed by 2003, 2006, 1995, and 2002. Melting in 2007 exceeded that of all previous years. Scientists have been keeping close watch on many glaciers, one of the most notable being the Jakobshavn glacier (the probable source of the iceberg that sank the Titanic), which has been accelerating toward the ocean.

Greenland is largely an artifact of the last ice age, held in place by the albedo (reflectivity) of its massive ice sheet. Once the ice melts, present-day climatic conditions will not allow its re-creation. Greenland's coastal ice often melts with a boost from the sea, warm water upwelling, flowing from below. Scientists have been debating the amount of additional warming that could cause Greenland's ice cap to cross a "tipping point," beyond which continued melting will be inevitable. During the winters between 2003 and 2007, Greenland lost two to three times as much ice in summer melt as it regained during winter snows.

Greenland's ice loss is accelerating irregularly year by year as well. During the 2007 melting season, with temperatures 4œ to 6œ Celsius higher than the previous thirty years' average, 450 billion metric tons of ice vanished, 30 percent more than the previous year and 4 percent more than the previous record, in 2005. During years of record melting, such as 2005 and 2007, high-pressure systems over Greenland keep storms away, clearing skies, allowing the Sun to shine for extended hours. Greenland's ice-mass loss between 2004 and 2006 was two and a half times the loss between 2002 and 2004.

While increases in the rate of ice loss to date have been greatest in southern Greenland, melting has been spreading northward, especially along the coastline. By the summer of 2007, the ice cap was studded by more than one thousand shallow meltwater lakes, some as wide as 5 kilometers. Tens of millions of cubic meters of water swirl from these lakes to the base of the ice sheet within a matter of days, opening huge waterfalls where none had previously existed.

Melting of the Greenland ice sheet's western reaches increased by about 30 percent from 1979 to 2006. Ice is melting most rapidly at the edges of the ice sheet. Although Greenland's ice sheet has been thickening at higher elevations due to increases in snowfall (warmer air holds more moisture, thus more snow), the gain is more than offset by an accelerating mass loss, primarily from rapidly thinning and accelerating outlet glaciers.

Scientists using pollen records from marine sediment off southwest Greenland have deduced that much of the Greenland ice sheet has melted relatively quickly during sharp natural warming in Earth's climate, some warming as recent as 400,000 years ago, when carbon dioxide levels were lower than they are today. During these spells, boreal coniferous forest covered much of the island. These spells of abrupt warming (abrupt, that is, in geologic time) reduced the ice sheet to about onequarter of its present size and by themselves have raised world sea levels 4 to 6 meters.

Bibliography:

1) Alley, Richard B. The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Princeton, N.J.: Princeton University Press, 2000.

2) Appenzeller, Tim. "The Big Thaw." National Geographic, June, 2007, 56-71.

3) Hanna, Edward, et al. "Increased Runoff from Melt from the Greenland Ice Sheet: A Response to Global Warming." Journal of Climate 21, no. 2 (January, 2008): 331-341.

4) Murray, Tavi. "Greenland's Ice on the Scales." Nature 443 (September 21, 2006): 277-278.

5) Paterson, W. S. B., and N. Reeh. "Thinning of the Ice Sheet in Northwest Greenland over the Past Forty Years." Nature 414 (November 1, 2001): 60-62.

6) Steffensen, Jurgen Peder, et al. "High-Resolution Greenland Ice Core Data Show Abrupt Climate Change Happens in Few Years." Science 321 (August 1, 2008): 654-657.

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