Aug. 10, 2012
Greenland's ice sheet thinning at uneven pace, scientists find
By NIKA SOON-SHIONG | Los Angeles Times
LOS ANGELES -- ]
The Greenland ice sheet, the second-largest ice body in the world, has lost its ice in fits and starts - with short bursts of dramatic ice loss separated by periods of stability, a study has found.
Scientists predict that if Greenland's entire ice sheet were to melt, sea levels would rise more than 20 feet. But though recent observations have shown dramatic losses, they cannot easily predict what the pace of future loss is likely to be.
One of the issues is how far the data go back. Scientists track ice mass patterns via satellites in space, but such images date only to 2000, making predictions tentative. The new study, published Aug. 3 in Science, extended the time period for which Greenland's ice can be observed by using photographs taken from airplanes dating back to the 1980s.
"When people took those photographs way back then, they weren't looking for changes in anything. They didn't realize things were changing," said study coauthor John Wahr, a physicist at the University of Colorado in Boulder.
Kurt Kjaer, science director at the Natural History Museum of Denmark, and colleagues used the aerial data to create a map of ice sheet elevations in Northwest Greeland. This was done by matching pairs of photographs of the same area of ice taken from slightly different angles to create a three-dimensional image. The scientists then used satellite data to extend the record up to the present day.
Using these digitally produced models of the area's elevation, the scientists found high amounts of ice thinning - up to 150 meters (about 490 feet) in some places - had occurred on the ice sheet during the last 25 years.
But the thinning had occurred in starts and stops, they noticed, with one significant period of loss occurring from 1985-1993 and another event of even more loss of ice from 2005-2010. In between the two events there was no significant ice loss, although the ice sheet did not grow back to pre-1985 levels.
The results suggest that the ice sheet can stabilize itself after periods of ice loss, the authors said.
"We are able to show that the recent dramatic ice loss observed after 2003 is not unique, and that similar mass loss also happened prior to the recent warming," Kjaer wrote in an email. Scientists need to learn more about the causes of these dramatic events to create more accurate long-term Greenland ice-loss predictions, he said.