Greenland Ice Sheet Is Melting Faster, Study Says

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He says the new study fits well with other recent studies showing a Greenland meltdown.

"It really does appear that the ice sheet is losing mass," he said in an email.

"Looking at the history of these measurements, the ice sheet was probably near balance a couple of decades ago and has begun shrinking recently," he continued.

"This parallels recent warming."

Full of GRACE

The new study is based on an analysis of gravity measurements collected by a pair of twin wedge-shaped satellites that orbit the Earth in tandem.

The satellites are part of the Gravity Recovery and Climate Experiment (GRACE), which was launched in March 2002 and is run by a team of experts in the U.S. and Germany.

GRACE measures landmass based on its gravitational pull. The denser a region is, the stronger its pull and the faster the satellites will move above it.

The satellites are separated by a distance of 137 miles (220 kilometers) when they are in stable orbit. As the front satellite crosses over an area of strong gravity, it speeds up, increasing the distance between the two satellites.

"Any tiny change in the distance can be used to infer the surface mass change," Chen said.

Liquid water is generally denser than ice and so has a stronger gravitational pull.

Chen and his University of Texas colleagues analyzed the gravity measurements over Greenland between April 2002 and November 2005, separating the mass change from other signals.

The team found that Greenland is now losing between 52 and 63 cubic miles (216 and 262 cubic kilometers) of ice mass each year.

The current wasting is about three times the rate gleaned from an earlier study of the first two years of GRACE data.

Jay Zwally is a glaciologist with the NASA Goddard Space Flight Center in Greenbelt, Maryland.

He agrees that Greenland ice loss has accelerated in recent years.

But based on he and his colleagues' unpublished analysis of the latest GRACE data, he believes the current ice loss rate is less than half what Chen's team reports.

Nevertheless, he says, Greenland does appear to be losing more ice mass than it gains.

"I would say Greenland now is beginning to contribute significantly to sea level rise," Zwally said. "There's been a significant change in a relatively short period of time."

As methods for analyzing GRACE data are refined and combined with other techniques, scientists will reach agreement over just how quickly the continent is wasting away, Zwally adds.

Historical Perspective

GRACE has only been orbiting Earth for three and a half years, not long enough to determine if the increase in melting is due to global warming or natural variability, the University of Texas's Chen says.

Longer term trends, and confidence in data interpretation, must wait until several more years of data are collected, he says.

According to Alley, the Pennsylvania State glaciologist, increasing snowfall, increasing melting, and increasing flow of glaciers into the ocean are all expected to result from global warming.

Historical analyses indicate that Greenland shrank when changes in Earth's orbit gave more summer sunshine to the island a few thousand years ago and about 130,000 years ago, he says.

"History and physics and recent observations tie warming to ice shrinkage," he said.

And projections of future climate change indicate continued warming over Greenland if greenhouse gas emissions remain unchecked.

"So shrinkage seems likely," Alley said.

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