No Dice for Greenland Ice

By Phil Berardelli
ScienceNOW Daily News
10 August 2006

Something ominous may be happening beneath Greenland's vast ice sheet. For nearly 50 years, the world's second largest ice cap has inched inexorably downhill toward the ocean, but at a stable rate. Now, the sheet seems to be melting and sliding seaward much faster, and the rate seems to be accelerating--a condition that could eventually endanger coastal populations and affect Earth's climate.

When glaciers begin to melt, water works its way down to the bottom of the ice. There it lubricates the glacier, which will pick up its downhill pace. Why worry? Greenland holds about 10% of the world's ice, so if it melts completely--though an unlikely prospect--it would raise global sea level about 6.5 meters. That's enough to flood all of the planet's coastal cities and displace billions of people.

To check in on the sheet's status, University of Texas at Austin research scientist Jianli Chen and his team measured its mass with NASA's Gravity Recovery and Climate Experiment (GRACE) satellite data. The twin GRACE satellites have been sharing a polar orbit since 2002, recording gravity variations over time, which can be used to compute the size and behavior of large ice masses. When the researchers compared monthly data over Greenland between April 2002 and November 2005, they discovered the ice melt rate had tripled compared to the rate other scientists had calculated from data collected by Earth-observing satellites between 1997 and 2003. This acceleration is consistent with an increase in global temperatures, says Chen, whose team reports its results online today in Science.

Josefino Comiso, a senior research scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, says he agrees with the analysis but cautions about jumping to conclusions. "The data record of a little more than three years is really not robust enough to enable an accurate assessment of a trend or an acceleration," he says. Comiso, says the last 4 years have been anomalously warm, so it is possible that a return to more normal temperatures will slow the rush of Greenland's ice to the sea.