Of late, the enormous glaciers that flow down to the sea from the interior of Greenland have been picking up speed. In the last few years, enough ice has come off the northern landmass to sustain the average flow of the Colorado River for six years or fill Lake Mead three times over or cover the state of Maryland in 10 feet of water, assuming it were perfectly flat. And whether it is the glaciers' weight, speed or volume that is measured, a quickening of the their movement can be detected. In fact, the latest gravity-based measurements show that the glaciers lost roughly 101 gigatons of ice annually between 2003 and 2005, according to a paper published online in Science.
Geophysicist Scott Luthcke and glaciologist Jay Zwally of NASA's Goddard Space Flight Center as well as several colleagues analyzed new mass measurements from the tandem Gravity Recovery and Climate Experiment (GRACE) satellites. The twin satellites, in polar orbit, one roughly 200 kilometers in front of the other, continuously monitor the distance between them. As they pass over Earth¿s varying mass, such as mountains compared with plains, the lead satellite speeds up thanks to its attraction to the mountains, increasing the distance between the two orbiters. The trailing satellite only catches up as it passes over the same landform. By measuring this distance change, scientists can derive a picture of mass concentrations on the planet's surface. "We were able to do that every 10 days for Greenland," Luthcke notes. "What we see is a massive amount of mass shedding that far outweighs an interior growth."
The team concentrated their measurements only on Greenland, in contrast with earlier studies that used global measurements and applied them to Greenland, and were able to refine these readings down to individual drainage systems on the landmass. In perfect agreement with previous results, these new GRACE measurements revealed that the same three glacial systems--including Kangerdlugssuaq, now one of the fastest moving glaciers in the world--are racing to the sea. "The glaciers are accelerating," Zwally says. "We are now losing 20 percent more coming out each year than goes in as snowfall."
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More data will be needed to confirm this trend, and a third year of GRACE observations should be available in the next few months. But the melting signal is clear. "There has been a significant change in a short period of time," Zwally says. "We believe that we are seeing the effects of climate warming in Greenland." And although the various estimates of the amount of loss sometimes conflict, there is an overall consensus. "This study and other GRACE studies published by [others] all agree that there is significant mass wastage of Greenland into the ocean," notes glaciologist Eric Rignot of NASA's Jet Propulsion Laboratory. "None of this has been predicted by numerical models, and therefore all projections of the contribution of Greenland to sea level [rise] are way below reality."
Further, preliminary data from yet another satellite analyzing the problem--ICESat, which uses laser altimetry to precisely measure the volume of specific glaciers--shows the same thing, Zwally notes. "The change is taking place faster than we thought it would be," he adds. The consequences of climate change are arriving ahead of schedule in the form of speeding northern glaciers.
