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Saturday, September 24, 2011

Increased crevasse extent in Greenland may dampen ice sheet sliding

Increased crevasse extent in Greenland may dampen ice sheet sliding
Chihiko Yamashita

The area covered by crevasses northeast of Ilulissat, West Greenland, has expanded by 13% over the last 24 years, according to scientists at the Cooperative Institute for Research in Environmental Sciences (CIRES)—a change that may impact the sliding of the Greenland Ice Sheet and subsequent sea level rise. 

“The area covered by crevasses is increasing,” said CIRES research associate William Colgan, lead author of the study published online today in Geophysical Research Letters

“Theoretically, this change may cause the ice sheet to slide more slowly toward the coast or into the ocean.” 

Colgan and his coworkers, a team led by CIRES Director Konrad Steffen at the University of Colorado Boulder, investigate the slide of the Greenland Ice Sheet, the second largest ice sheet on Earth. “People typically think of a block of ice as something really solid and inflexible,” Colgan said. 

“But when a block of ice is as big as the Greenland Ice Sheet, there is sufficient pressure from its weight to cause it to flow like a really, really slow river into the ocean.”

The weight of the ice causes the sheet to flow like a viscous fluid, and water between the ice sheet and its bed means the sheet slides as well as flows, Colgan said. The flow and sliding of the ice sheet can cause more ice than normal to flow into the ocean, which can lead to sea level rise, he said. “It is really important to understand how the Greenland Ice Sheet flows, slides and melts today, in order to be able to predict how it will contribute to sea level rise in the future.”

To investigate the impact of crevasses on ice sheet flow, the team first analyzed crevasse extent at Sermeq Avannarleq, northeast of Ilulissat, West Greenland. The scientists detected crevasses in high-resolution digital images taken in 1985 and 2009 and what they found surprised them. “We initially weren't looking for changes in crevasse area, we had thought it was stationary in time,” Colgan said. “But we found that the change in crevasse area was significant.”

Several factors influence the extent of crevasses, Colgan said. The weight of ice surrounding the crevasse acts as a “closing” force on crevasses whereas tensile stress, caused by local variations in surface slope, acts as an “opening” force. Warmer temperatures, however, result in increased surface melt and a thinning ice sheet reducing the closing force. Conversely, the subsequent change in ice flow speed steepens the ice sheet, which strengthens the tensile stress. As the ice sheet melts, a third factor also contributes to the crevasses expanding, Colgan said. “Twenty years ago crevasses might have been sitting open exposed to air, but now they are filled with water which forces them open.”

The team then went on to investigate how the crevasse area increase might impact ice sheet sliding. In this study, and a companion study published online September 13 in the Journal of Glaciology, the scientists compared the drainage of water by crevasses and moulins—near-vertical chutes in the ice—and found moulin drainage to be more efficient in moving water to the bed of the ice sheet and promoting sliding. But with the increase in crevasse area, the number of moulins had decreased, Colgan said. “The crevasse fields seem to be absorbing the moulins.”

Previously, scientists had believed that as more ice melts and water accumulates on the surface of the ice sheet, more water drains though the moulin network to the underside of the ice sheet enhancing its slide into the ocean. “For the last ten years popular opinion has been more melt equals more slide into the ocean,” he said. “Some recent papers have speculated that more melt might equal no change in sliding, but ours might be the first paper that says that despite more melt, changes in the way that the water is routed to the bed might equal less slide.”

The team now intends to investigate whether crevasse area has increased Greenland-wide and, if so, determine the impact on ice-sheet sliding. While the study may have identified one previously unrecognized influence on ice-sheet dynamics, Colgan cautions it is just one of many factors which determine the iceberg calving rate. “The recent trend of increasing iceberg calving rates is unlikely to be reversed by a potential decrease in sliding, as the majority of ice movement comes from flow rather than sliding,” he said.

The study “An increase in crevasse extent, West Greenland: Hydrologic implications” was funded by the National Aeronautics and Space Administration and the National Science Foundation. Coauthors include CIRES Director Konrad Steffen, CIRES student W. Scott McLamb and CIRES Fellow Waleed Abdalati. Collaborators on the project include the Aerospace Engineering and Sciences and Civil, Environmental, and Architectural Engineering departments at the University of Colorado Boulder, the Institute of Arctic and Alpine Research at the University of Colorado Boulder and the Geophysical Institute of the University of Alaska Fairbanks. 

CIRES graduate student, Daniel McGrath led the study published in the Journal of Glaciology “Assessing the summer water budget of a moulin basin in the Sermeq Avannarleq ablation region, Greenland Ice Sheet.” Coauthors of this study include William Colgan and Konrad Steffen. 

Collaborators on the project include the Aerospace Engineering Sciences Department at the University of Colorado Boulder and the Extreme Ice Survey, Boulder. 

William Colgan, CIRES, 303-735-3681,
Konrad Steffen, CIRES, 303-492-8773,
Jane Palmer, CIRES media communications, 303-492-6289, 

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