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New data confirm the Greenland ice sheet is losing mass at an accelerating rate. The new calculations, based on state-of-the-art satellite observations combined with models of Greenland's changing icescape, are further evidence, scientists say, of the impact of global warming.
Greenland has lost about 1,500 gigatons of ice mass between 2000 and 2008, according to a new report, resulting in an average sea level rise of 0.46 millimeters per year. A gigaton is one billion tons.
Between 2006 and 2008, the authors say the rate at which Greenland's ice sheet is shrinking due to global warming accelerated, causing ocean levels to rise 0.75 millimeters per year.
The calculations are considered the most reliable to date because they combine data from the twin Gravity Recovery and Climate Experiment satellites, known as GRACE, with computer models of Greenland's changing icescape.
GRACE detects alterations in gravity caused by reductions in the ice sheet. But the calculations do not tell scientists what is causing the ice cap to shrink, says Michiel van den Broeke, a professor of polar meteorology at Utrecht University in the Netherlands.
Van den Broeke says some observations indicate sea levels are rising as the ice mass that makes up 80 percent of Greenland melts, resulting in a run-off of liquid water into the sea. Other studies suggest that rising ocean levels are caused by glacial ice that breaks off into the sea along Greenland's coast and forms icebergs.
Van den Broeke and colleagues created a model he says indicates the formation of icebergs and melting ice play equal roles in reducing the size of the Greenland Ice Sheet.
"It turns out that both increased iceberg production, because these glaciers have started to flow faster in the last 10 years, and increased melting," said Michiel van den Broeke. "They have both contributed about equally to the recent mass loss."
The study by van den Broeke and colleagues traces the beginning of Greenland's ice loss to 1996. Some experts believe if current trends continue, global sea levels will rise by a meter or more by the end of the century.
Steve Nerem, a professor of aerospace engineering sciences at the University of Colorado in Boulder says that is why it is important to refine predictions of what the Greenland Ice Sheet is going to do.
"And that is still very uncertain, which is why a paper like this is very interesting," said Steve Nerem. "Because we are really just now starting to get a handle of what the dynamics of these ice sheets are and the question is, they are almost certainly going to melt, but how quickly are they going to melt; you know, how much is going to melt in 100 years, in 1,000 years or 2,000 years."
If the entire Greenland ice sheet were to melt, Nerem says it contains enough water to cause a global sea-level rise of seven meters.
For low-lying countries to prepare, Nerem says, scientists need to know how quickly the Greenland ice mass is melting.
"If the meter in sea level rise were to happen very rapidly, say in 50 years, it will be very hard to build the infrastructure, you know the dykes - and the other things to hold back the water - quickly enough to prevent the inundation that would occur with a meter of sea-level rise," he said. "If it were to take hundreds of years, then that would probably be enough time for populated areas to build the protections that they need to combat this."
New measurements of sea level rise and ice loss in Greenland are published in the journal Science.