Ian Joughin/UW These icebergs recently calved from the front of the north branch of Jakobshavn Isbrae, a large outlet glacier that drains 6.5 percent of the Greenland ice sheet. The fact that they are upright, indicated by their dirty and crevassed surfaces, suggests they calved from the floating end of a glacier.
"So far, on average we’re seeing about a 30 percent speedup in 10 years," said Twila Moon, a University of Washington doctoral student in Earth and space sciences and lead author of a paper documenting the observations published May 4 in Science.
The faster the glaciers move, the more ice and meltwater they release into the ocean. In a previous study, scientists trying to understand the contribution of melting ice to rising sea level in a warming world considered a scenario in which the Greenland glaciers would double their velocity between 2000 and 2010 and then stabilize at the higher speed, and another scenario in which the speeds would increase tenfold and then stabilize.
At the lower rate, Greenland ice would contribute about four inches to rising sea level by 2100 and at the higher rate the contribution would be nearly 19 inches by the end of this century. But the researchers who conducted that study had little precise data available for how major ice regions, primarily in Greenland and Antarctica, were behaving in the face of climate change.
In the new study, the scientists created a decadelong record of changes in Greenland outlet glaciers by producing velocity maps using data from the Canadian Space Agency’s Radarsat-1 satellite, Germany’s TerraSar-X satellite and Japan’s Advanced Land Observation Satellite. They started with the winter of 2000-01 and then repeated the process for each winter from 2005-06 through 2010-11, and found that the outlet glaciers had not increased in velocity as much as had been speculated.
"In some sense, this raises as many questions as it answers. It shows there’s a lot of variability," said Ian Joughin, a glaciologist in the UW’s Applied Physics Laboratory who is a coauthor and is Moon’s doctoral adviser.
Other coauthors are Benjamin Smith of the UW Applied Physics Laboratory and Ian Howat, an assistant professor of earth sciences at Ohio State University.
The scientists saw no clear indication in the new research that the glaciers will stop gaining speed during the rest of the century, and so by 2100 they could reach or exceed the scenario in which they contribute four inches to sea level rise.
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