Researchers Dr. Eric Rignot of NASA's Jet Propulsion Laboratory, Pasadena, Calif.; Andres Rivera of Universidad de Chile, Santiago, Chile; and Dr. Gino Casassa of Centro de Estudios Cientificos, Valdivia, Chile, compared conventional topographic data from the 1970s and 1990s with data from NASA's Shuttle Radar Topography Mission, flown in February 2000. Their objective was to measure changes over time in the volumes of the 63 largest glaciers in the region.

Results of the study, published this week in the journal Science, conclude the Patagonia Icefields lost ice at a rate equivalent to a sea level rise of 0.04 millimeters (0.0016 inches) per year during the period 1975 through 2000.

This is equal to nine percent of the total annual global sea-level rise from mountain glaciers, according to the 2001 Intergovernmental Panel on Climate Change Scientific Assessment. From 1995 through 2000, however, that rate of ice loss from the icefields more than doubled, to an equivalent sea level rise of 0.1 millimeters (0.004 inches) per year.

In comparison, Alaska's glaciers, which cover an area five times larger, account for about 30 percent of total annual global sea-level rise from mountain glaciers. So what's causing the increased Patagonia thinning?

Rignot and his colleagues concluded the answer is climate change, as evidenced by increased air temperatures and decreased precipitation over time. Still, those factors alone are not sufficient to explain the rapid thinning. The rest of the story appears to lie primarily in the unique dynamic response of the region's glaciers to climate change.

"The Patagonia Icefields are dominated by so-called 'calving' glaciers," Rignot said. "Such glaciers spawn icebergs into the ocean or lakes and have different dynamics from glaciers that end on land and melt at their front ends. Calving glaciers are more sensitive to climate change once pushed out of equilibrium, and make this region the fastest area of glacial retreat on Earth."

Rignot said the study underscores NASA's unique contributions to understanding changes in Earth's cryosphere. "From the unique vantage point of space, the Shuttle Radar Topography Mission provided the first complete topographic coverage of the Patagonia Icefields," he explained.

"Researchers can now access data on this remote Earth region in its totality, allowing them to draw conclusions about the whole system, rather than just focusing on changes on a few glaciers studied from the ground or by aircraft."

Rignot said scientists are particularly interested in studying how climate interacts with glaciers because it may be a good barometer of how the large ice sheets of Greenland and Antarctica will respond to future climate change.

"We know the Antarctic peninsula has been warming for the past four decades, with ice shelves disappearing rapidly and glaciers behind them speeding up and raising sea level," he noted. "Our Patagonia research is providing unique insights into how these larger ice masses may evolve over time in a warmer climate," he said.

The Northern Patagonia Icefield in Chile and the Southern Patagonia Icefield in Chile and Argentina, cover 13,000 and 4,200 square kilometers (5,019 and 1,622 square miles), respectively.

The region, spanning the Andes mountain range, is sparsely inhabited, with rough terrain and poor weather, restricting ground access by scientists. Precipitation in the region ranges from 2 to 11 meters (6.6 to 36 feet) of water equivalent per year, a snow equivalent of up to 30 meters (98.4 feet) a year.

The icefields discharge ice and meltwater to the ocean on the west side and to lakes on the east side, via rapidly flowing glaciers. The fronts of most of these glaciers have been retreating over the past half- century or more.

The study benefited from ground experiments led jointly by Centro de Estudios Cientificos; Universidad de Chile; University of Washington, Seattle; and University of Alaska, Fairbanks, with funding by NASA, Fondecyt (Chilean National Science Foundation) and the National Science Foundation International Program.

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