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Taking Stock of a Defrosting Big Chill

Lucia Glacier, Southern Patagonia Icefield
Lucia Glacier, Southern Patagonia Icefield. The denuded flanks of the glaciers are another illustration of the rapid retreat of glaciers in this region. Image credit: Andres Rivera.
It's a region of breathtaking beauty. Saddling the peaks of South America's Andes mountain range in Chile and Argentina, the Northern and Southern Patagonia Icefields defy description. Remote. Rugged. Wild. Sparsely inhabited. Whipped by high winds, drenched by rain and up to 100 feet of snow each year. Within these icy confines lie the largest temperate ice masses in Earth's Southern Hemisphere, and perhaps more importantly, clues to how Earth's climate may change in the future.

Scientists have long known that most of Patagonia's glaciers have been retreating for the past half-century or more. Until recently, scientific studies of this region were limited to ground and aircraft studies of a few glaciers. With the flight of the joint NASA/National Imagery and Mapping Agency Shuttle Radar Topography Mission aboard Space Shuttle Endeavour in February 2000, scientists found themselves with a new tool to conduct a complete study of this entire 17,200 square kilometer (6,641 square mile) region. Now, a joint research effort by NASA and Chile's Centro de Estudios Cientificos has measured the changes in mass in these vast icefields over a 25-year period. This undertaking used data from the Shuttle mission and conventional topographic data from the 1970s and 1990s. Using this unique "big picture" vantage point of space from NASA, scientists took a complete stock of Earth's cryospheric refrigerator in Patagonia. And what the scientists found was that the icebox is defrosting, quickly.

Jorge Montt Glacier in the northern part of the Southern Patagonia Icefield
This animation depicts the thinning of Jorge Montt Glacier in the northern part of the Southern Patagonia Icefield, South America between 1975 and 2000. It was created using data from NASA's Shuttle Radar Topography Mission and the Landsat Thematic Mapper. Animation credit: Vince Realmuto and Zareh Gorjian, Digital Image Animation Laboratory, JPL.
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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, found that the Patagonia Icefields are a significant contributor to global sea level rise, losing ice at a rate equivalent to 0.04 millimeters (0.0016 inches) per year between 1975 and 2000. That amounts to nearly 10 percent of global sea-level change from mountain glaciers. What was most surprising, however, was that the rate of thinning more than doubled between 1995 and 2000, making this region the fastest area of glacial retreat on Earth. 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."

Mt. San Valentin, the summit of the Northern Patagonia Icefield
Steep glaciers can be seen running down Mt. San Valentin, the summit of the Northern Patagonia Icefield. The glacial retreat has already cut the direct supply of ice from smaller glaciers. Image credit: Andres Rivera
Rignot said studying the interactions of climate with glaciers is important because it may be a good barometer of how the large ice sheets of Greenland and Antarctica will respond to future climate change. "Total global sea level rise from all known sources is about 1.8 millimeters, or 0.07 inches, per year, of which mountain glaciers like those in Patagonia and Alaska contribute about one fourth," he noted. "Our Patagonia research is providing unique insights into how the larger polar ice masses may evolve over time in a warmer climate. We already know the Antarctic Peninsula, for example, has been warming for the past four decades. Will this trend continue? If so, how quickly? We don't know yet. That's why studies like this one are so important. This isn't something we need to do just once. We need to see how things change over time."

Information about the Shuttle Radar Topography Mission is available at: http://www.jpl.nasa.gov/srtm/. Information on NASA's Cryospheric Science program is available at: http://oceans.nasa.gov/csp/index.html.

NASA's Jet Propulsion Laboratory