Scott Luthcke weighs Greenland -- every 10 days. And the island has been losing weight, an average of 183 gigatons (or 200 cubic kilometers) -- in ice -- annually during the past six years. That's one third the volume of water in Lake Erie every year. Greenland's shrinking ice sheet offers some of the most powerful evidence of global warming.

melting ice

Lutchke "weighs" Greenland by processing and interpreting data from one of the most sophisticated gravitational "scales" ever built: the U.S.-German satellite mission called GRACE -- the Gravity Recovery and Climate Experiment.

The two GRACE satellites fly over Greenland several times each day. As the first satellite approaches, the island's mass causes the satellite to accelerate and thereby move slightly away from its trailing companion. Over time, the Grace mission carefully records fluctuations in the distance between the two satellites each time they pass over Greenland.

By examining the GRACE data, Luthcke can monitor subtle changes in the gravitational pull that the land mass exerts on the satellites to get a reliable measure of Greenland's shrinking mass. The system, says Luthcke, is accurate enough to "detect the loss of just a centimeter of ice over an area the size of Delaware."

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