By LEE BOWMAN
Scripps Howard News Service
February 17, 2006
Using information from satellites to track glacier movement from space, scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., calculate that some of the island's glaciers have recently doubled the velocity of their flow to more than eight miles a year.
Taking the faster glacier speeds into account, the researchers calculate in the journal Science that Greenland contributes a half-millimeter a year to the annual global sea-level rise of 3 millimeters a year.
The Greenland Ice Sheet covers an area just slightly smaller than Mexico with a thickness of nearly 2 miles, enough to raise global sea levels by nearly 28 feet if it all melted.
Other satellite and airborne radar measurements analyzed by Norwegian researchers last year indicated that while some glaciers are dumping more ice near the sea, interior portions of the ice cap have been growing by an average of 2 inches a year, and increased snowfall is even thickening the ice cap in some coastal areas, too.
But many climate scientists argue that glaciers pushing into the sea are sentinels for what will eventually happen to the entire ice mass.
"The behavior of the glaciers that dump ice into the sea is the most important aspect of understanding how an ice sheet will evolve in a changing climate," said Eric Rignot, a researcher at the Jet Propulsion lab and lead author of the Science study.
"It takes a long time to build and melt an ice sheet, but glaciers can react quickly to temperature changes," he added.
Over the past 20 years, the air temperature in southeast Greenland has risen by more than 5 degrees Fahrenheit, although other research shows temperatures at the top of the ice sheet have fallen by more than 3 degrees per decade since the 1940s.
Rignot and his colleagues say that warmer temperatures around the coastal glaciers increase the amount of melt water reaching the glacier-rock interface at the bottom of the ice, where it serves as a lubricant to speed the march to the ocean.
"Climate warming can work in different ways, but generally speaking, if you warm up the ice sheet, the glacier will flow faster," he said.
So far, this trend toward acceleration hasn't extended to other glaciers flowing into the sea in northern Greenland, although some images do hint at more speed for some of them. "The southern half of Greenland is reacting to what we think is climate warming," Rignot said. "The northern half is waiting, but I don't think it's going to take long."
Julian Dowdeswell of the University of Cambridge, England, noted in an accompanying perspective article in Science that while the accelerating glaciers seem to point to the beginning of a meltdown, the complex dynamics of the ice sheets are still poorly understood and only now beginning to be regularly measured by satellites.
"In a warming world, it is likely that the contribution to sea-level rise from Greenland is set to grow further," Dowdeswell wrote, but added that increased snowfall over some areas could offset some of the loss of ice mass.
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