WASHINGTON, Dec 10 (Reuters) - The Greenland ice sheet melted at a record rate this year, the largest ever since satellite measurements began in 1979, a top climate scientist reported on Monday.

"The amount of ice lost by Greenland over the last year is the equivalent of two times all the ice in the Alps, or a layer of water more than one-half mile (800 meters) deep covering Washington DC," said Konrad Steffen of the University of Colorado at Boulder.

Using data from military and weather satellites to see where the ice is melting, Steffen and his colleagues were able to monitor the rapid thinning and acceleration of ice as it moved into the ocean at the edge of the big arctic island.

The extent of the melt area was 10 percent greater than the last record year, 2005, the scientists found.

Greenland is about one-fourth the size of the United States and about 80 percent of it is covered by the ice sheet. One-twentieth of the world's ice is in Greenland; if it all melted it would be equivalent to a 21-foot (6.4 metre) global sea level rise, the scientists said.

One factor in the speed-up of Greenland's ice melt is an increase in cylindrical shafts in the ice called moulins.

These huge tunnels in the ice act like drains and appear to let the ice sheet respond more rapidly than researchers expected to spikes in temperature at the beginning of the annual warm season, Steffen said.

In recent years, melting has started earlier in the year than normal. Air temperatures on the ice sheet have risen by about 7 degrees F (3.9 degrees C) since 1991, mostly because of the build-up of greenhouse gases in the atmosphere, the scientists said in research presented at a meeting of the American Geophysical Union in San Francisco.

This is in keeping with persistently gloomy news about the state of the Arctic this year. In October, a U.S. government "report card" found less ice, hotter air and dying wildlife.

In May, a U.S. expert at the National Snow and Ice Center in Colorado found that Arctic ice cap is melting much faster than expected and is now about 30 years ahead of predictions made by the Intergovernmental Panel on Climate Change.

(Reporting by Deborah Zabarenko, Editing by Sandra Maler)

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