The National Snow and Ice Data Center is part of the Cooperative Institute for Research in Environmental Sciences, based at CU-Boulder. Serreze and Scambos presented the finding at a meeting of the American Association for the Advancement of Science in Boston Feb. 16th.

In summer 2000, an international team of scientists led by Serreze released results of a study documenting widespread environmental changes over the Arctic. As part of their study, they noted late 20th century Arctic temperatures were the warmest in 400 years.

"Recent data show more of the same," Serreze said. "We're seeing significant surface air temperature increases over the Arctic Ocean, accompanied not only by an 18-year downturn in ice cover over the Atlantic Ocean but by a record reduction in ice cover over the Beaufort and Chukchi seas in late summer 1998." Arctic sea ice cover also is thinning significantly.

Results for 2000 showed that global mean temperatures have risen 1 degree Fahrenheit over the past 100 years, while parts of northern North America and northern Eurasia warmed much more in the winter months over the past 30 years.

Funded by the National Science Foundation's Office of Polar Programs, the climate study made a sweeping examination of existing evidence for recent environmental change in the northern high latitudes. It compared findings to climate model predictions of human-induced greenhouse warming.

The researchers assessed a wide body of long-term data including temperatures, sea ice and ocean structure, snow and glacier cover, and atmospheric circulation.

The picture in Antarctica parallels northern ice reductions. "Ice shelves that have been stable for centuries are being lost over a spectacularly short period of time," said scientist Ted Scambos. "After hundreds of years in the making, it took only one decade of high summer temperatures to see the destruction of both the Larsen A and Larsen B ice shelves."

Scambos said that this process may indicate that other, larger ice shelves are more vulnerable than previously believed.

"After several years of gradual reductions in extent, Larsen A was lost in about a week at the end of January 1995," he said. "Over 1,700 square kilometers of ice shelf disintegrated in a single storm event. For weeks afterward, a plume of smaller icebergs was visible in satellite images, drifting away from the Antarctic Peninsula."

According to Scambos, starting in early 1998 and accelerating in 1999 and 2000, the Larsen B ice shelf also began to retreat, losing more than 2,400 square kilometers.

"The retreats and melting are due to a very strong climatic warming trend," Scambos said. "Mean temperatures in the peninsula have increased 2.5 degrees Celsius over the last 50 years."

After a study funded by NASA's Office of Earth Sciences, Scambos and colleagues Christina Hulbe of Portland State University and Mark Fahnestock of the University of Maryland, have proposed that Antarctic ice shelves are at risk when summer melting reaches the point at which melt-ponds form on the ice surface.

When ice shelves are not compressed between adjoining land masses, they are susceptible to surface cracking. Cracks admit water that wedges in and shatters the ice, rapidly making it weak.

To initiate the extensive melting needed to form ponds, a mean summer temperature of about minus 1 C is needed -- typical in January in the Southern Hemisphere.

This model of warming, ponding and disintegration means that several ice shelves are more at risk than previously believed, Scambos said. In particular, the giant Ross Ice Shelf, a region of floating ice about the size of Texas, has areas with mean January temperatures only a few degrees below the ponding threshold.

If a warming trend similar to that experienced in the Antarctic Peninsula were to occur for the Ross, the ponding and disintegration process could begin there. Closer to the threshold are the Wilkins and George VI ice shelves, where some ponding and retreat have already begun.

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