for National Geographic News
The Greenland ice sheet is rapidly wasting away—but not as quickly as some recent studies have found, according to a new analysis.
Several recent studies have suggested that the island has been losing ice at a rate sufficient to push global sea levels up by 0.02 inch (0.5 millimeter) a year.
(Related news: "Greenland's Ice Melt Grew by 250 Percent, Satellites Show" [September 20, 2006].)
The new analysis cuts that rate in half, but the ice-mass loss is still happening fast enough to alarm scientists.
"Greenland is losing each year 20 percent more mass than goes into the ice sheet as snowfall," said Jay Zwally, a glacier expert at the NASA Goddard Space Flight Center in Greenbelt, Maryland.
Zwally is a co-author of the new study, which is reported tomorrow in the journal Science.
The new finding suggests that the Danish-owned island (Greenland map) lost about 110 billion tons (100 billion metric tons) of ice a year between 2003 and 2005.
That's enough water to keep the U.S.'s Colorado River flowing for six years.
The new study and several previous studies are based on analyses of data collected by a pair of orbiting satellites that measure tiny variations in gravity caused by changes in Earth's mass.
Because ice melt changes Greenland's mass, the satellites can measure changes in mass over time to determine how much ice is lost or gained.
In the most recent study, Zwally and colleagues applied a new technique to determine ice-mass changes along specific drainage basins on Greenland.
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