"It takes a long time to build and melt an ice sheet, but glaciers can react quickly to temperature changes," notes Eric Rignot, a glaciologist at NASA's Jet Propulsion Laboratory. "Greenland is probably going to contribute more and faster to sea level rise than predicted by current models."
Rignot partnered with Pannir Kanagaratnam of the University of Kansas to look at satellite data on Greenland's glaciers. New satellites and new techniques allowed the two to figure out how fast the glaciers were moving, thinning and even what the bedrock beneath them looked like. Based on this data, the researchers found that the glaciers were traveling faster than anyone had predicted. They also determined that even more northerly glaciers were on the move and that in just 10 years the amount of fresh water lost by all the glaciers had more than doubled from 90 cubic kilometers of ice loss a year to 224 cubic kilometers. "The amount of water Los Angeles uses over one year is about one cubic kilometer," Rignot points out. "Two hundred cubic kilometers is a lot of fresh water."
Current climate models do not take into account glacial flow and therefore underestimate the impact of glacial melt and the calving of ice flows, the researchers argue in a paper detailing the findings in today's Science. According to climate records stretching back a century, southern Greenland has warmed three degrees Celsius in just the past 20 years, driving melting that may help lubricate glacial flow along the bedrock, the two speculate. With the higher glacier speeds in mind, they calculate that Greenland currently contributes 0.57 millimeter of ocean level rise every year out of a total of three millimeters.
But Greenland contains an ice sheet that covers 1.7 million square kilometers--an area nearly the size of Mexico--and is as much as three kilometers thick in places. If it all melted, it would raise the world's oceans by seven meters, though that is not likely to happen anytime soon. "The southern half of Greenland is reacting to what we think is climate warming," Rignot adds. "The northern half is waiting, but I don't think it's going to take long."