Greenland ice sheet melting away
by Cat Lazaroff Greenland's ice sheet, which holds almost 10 percent of the world's frozen water, is melting at a rate of more than three feet a year in places, a new official survey reveals. The study by the National Aeronautics and Space Administration (NASA) provides evidence that the melting is contributing to a rise in sea level. Greenland spans over two million square kilometres, 85 percent of which is covered by ice. A new aerial survey shows that more than 51 cubic kilometres of ice is vanishing from the Greenland ice sheet each year. Melted, that ice amounts to about 1.25 trillion gallons (4.74 trillion litres) of water. "A conservative estimate, based on our data, indicates a net loss of approximately 51 cubic kilometres of ice per year from the entire ice sheet, sufficient to raise global sea level by 0.005 inches per year, or approximately seven percent of the observed rise"' said Bill Krabill, project scientist at the NASA Goddard Space Flight Center's Wallops Flight Facility. "This amount of sea level rise does not threaten coastal regions, but these results provide evidence that the margins of the ice sheet are in a process of change", Krabill said. "The thinning cannot be accounted for by increased melting alone. It appears that ice must be flowing more quickly into the sea through glaciers." Krabill said the changes in the ice sheet could be caused by a warming climate as well as glacial movement. The southern tip of Greenland protrudes into temperate latitudes, and scientists say monitoring this portion of the ice sheet may be one of the best ways to measure changes in our climate, at least in the Northern Hemisphere. The ice mapping was completed by NASA, which has been surveying the Greenland ice sheet for almost seven years. In 1993 and 1994, NASA researchers surveyed the ice sheet using an airborne laser altimetre and precision global positioning satellite receivers. Those same areas were surveyed again in 1998 and 1999. Now, for the first time, portions of the entire ice sheet covering Greenland have been mapped with sufficient accuracy to detect significant changes in elevation. The centre of the island is "amazingly stable", said Krabill. At its highest elevations, the Greenland ice sheet has kept its mass or even gained a small amount of ice. But Krabill noted that while some internal areas of Greenland show slight ice thickening, most areas along the coast show significant thinning. "Why the ice margins are thinning so rapidly warrants additional study", Krabill said. "It may indicate that the coastal margins of ice sheets are capable of responding more rapidly than we thought to external changes, such as a warming climate." Krabill and his team warned that the measurements can not yet be used to make long-term projections; more information must be gathered before the team can make accurate predictions of future trends. In fact, some of the team's observations raise more questions than answers. Although the North Atlantic region is currently in a cold phase, temperatures on the coast of Greenland have risen over the past decade. The team also cannot explain the thickening of ice at high altitudes, where "both ice cores and model predictions show reduced snowfall during the 1990s", the team reports in an article in the journal Science. The team is sure that continued melting will lead to higher sea levels in the future. A rise of even a few inches could inundate low lying coastal areas.
* * *Environment News Service (ENS) 2000 (abridged)