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Himalayan ice melt estimates get a major downsizing


This view of the Himalayas was taken from the International Space Station.

Are global warming skeptics being armed with a new weapon? Estimates from satellite monitoring suggest the melt rate from the Himalayas and other high-altitude Asian mountains in recent years was much less than what scientists on the ground had estimated, but those monitoring the satellite data warn not to jump to the skeptical conclusion.

The region's ice melt from 2003-2010 was estimated at 4 billion tons a year, far less than earlier estimates of around 50 billion tons, according to the study published Wednesday in the peer-reviewed journal Nature.

But study co-author John Wahr, a physics professor at the University of Colorado Boulder, emphasized that it's important to note that the region is a small contributor to overall ice melt and that the satellite estimates for the largest contributors, Antarctica and Greenland, are in line with ground-based estimates: about 385 billion tons a year.

"It's Greenland and Antarctica that pose by far the greatest threat to rising sea levels in the future," he told msnbc.com. "That's, basically, where all the ice is."

In the study itself, the authors also noted that the Asian mountain region has seen a lot of variability in ice melt and that the time period might be too short to be of much use. "These results suggest that care should be taken in extending the 2003–2010 results presented in this paper to longer time periods," they wrote in the study published online.

One potential reason for the huge difference, Wahr noted, is that scientists on the ground are limited to where they can sample the ice.

The NASA satellite used for monitoring, on the other hand, can cover the entire globe and its 200,000 glaciers.

Dubbed GRACE, the satellite "does this by mapping out the Earth's gravity field, all over the globe, every month," Wahr said.  

Satellite tracks where ice is melting

"One way to think of this is that as GRACE passes over Alaska, say, it feels the gravitational pull of all the Alaskan glaciers," he added. When it passes over Alaska later on, it also feels the pull of all those glaciers, but now that pull is smaller because there is less mass in those glaciers to do the pulling. So you end up learning about the change in the cumulative mass of all those glaciers. You don't miss any of the glaciers; you see the combined effects of all of them."

"The price you pay with GRACE," he said, "is that because you're 500 kilometers above the Earth, you can't distinguish the effects of an individual glacier from the effects of its neighbor. Individual glaciers are usually just too close to one another for you to separate them in the GRACE data. If you want to know what individual glaciers are doing, you need to rely heavily on those traditional, ground-based methods."

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