Greenland Ice Sheet Is Melting Faster, Study Says
for National Geographic News
|August 10, 2006|
The Greenland ice sheet is melting three times faster today than it was
five years ago, according to a new study.
The finding adds to evidence of increased global warming in recent years and indicates that melting polar ice sheets are pushing sea levels higher, the authors report.
According to the study, Greenland ice loss now amounts to more than 48 cubic miles (200 cubic kilometers) each year.
"Significant melting has a significant impact on sea level rise," said Jianli Chen, a research scientist at the University of Texas at Austin who led the study.
The finding, reported today by the online edition of the journal Science, closely agrees with another study on the rapid wasting of Greenland's glaciers published in the journal in February.
Both studies suggest the shrinking ice sheet now contributes about 0.02 inch (0.5 millimeter) a year to global sea level rise.
"That's a very big number," Chen said.
Losses and Gains
Global sea levels have risen by about 0.1 inch (2.8 millimeters) a year over the past decade.
If all the ice on Greenland were to melt into the North Atlantic Ocean, global sea levels would rise by about 21.3 feet (6.5 meters).
Thus scientists are keen to understand if the Danish-owned Arctic island (Greenland map) is losing more ice mass through melting and discharge of glaciers than it is gaining from fresh snowfall.
Richard Alley is a glaciologist at Pennsylvania State University in University Park who was not involved with the study.
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.
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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