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In August 2012, in the icy desert of West Greenland, the Jakobshavn Glacier was flowing and throwing into the sea at a record speed, three times faster than in previous years. A calving underwater had lost sight of the huge glacier. But the movement was not linear like a train on the run (as suggested by other studies), but dynamic: rapid acceleration then slowdown after a few days.
Now, a new assessment by a multi-institutional team led by CIRES has harnessed an extremely detailed new dataset to identify the drivers behind the acceleration and slowdown. As the glacier flowed faster, it became increasingly thin and unstable – then, in a jiffy, a stack of thick ice restored the glacier's terminus, slowing it down again. The work, published today in the Glaciology Journalcould help scientists better predict the contribution of tidal glaciers to sea level rise.
"As tidal glaciers, like Jakobshavn Isbræ, they are becoming more and more sensitive to small variations in the thickness of the ice," said Ryan Cassotto, researcher at CIRES and lead author of the new study conducted during his Ph.D. at the University of New Hampshire. "This is because the water pressure at the base of the glacier compensates for the pressure exerted by the weight of the ice above it, which affects the rate at which the glacier is ice-free. ; flows ". For ice tides trapped below sea level, thicker, heavier ice moves more slowly and ice is thinner and lighter, faster. According to Cassotto, this looks like the way hydroplanes of cars of different sizes are: large heavy trucks tend to be very stable and slip resistant, while light and compact cars slip easily. "
Video of automatic reading here of the calving 2012
Jakobshavn Isbræ, the subject of James Balog's 2012 "Chasing Ice" documentary, produces some of the largest icebergs and fastest speeds in the Arctic. And since iceberg calving contributes significantly to sea level rise, it is essential to understand the dynamics of glaciers and the calving events that produce them, the researchers said.
"Over the past two decades, Jakobshavn Isbræ has dumped more ice than any other glacier in Greenland," Cassotto said. "It alone contributes to about three percent of the current sea level rise in the world each year."
The research team, which included co-authors from the University of Alaska Fairbanks, the University of Alaska Southeast, the University of New Hampshire and the University of New Hampshire. from Ohio State University, has used new techniques to observe the glacier at a level of detail never seen before. They used instruments called ground-based radar interferometers to observe the deformation of the ice surface every three minutes. Calving occurs in a few minutes and therefore can not often be captured by satellite instruments that repeat measurements every 11 days.
Cassotto and his team found that the bed geometry of the fjord was of crucial importance to understand the speed of the glaciers, as others have proposed. The new work shows that even small changes at the ends of the glaciers, these last hundred feet heading towards the ocean, can profoundly affect the speed.
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