"As slight as that may seem, it's enough to make a difference," said Armstrong, who is affiliated with the National Snow and Ice Data Center headquartered at CU-Boulder. "Now, long-term monitoring of a series of cold region, or cryospheric, parameters shows that for several decades the amounts of snow and ice around the world have been decreasing."

To assemble the big picture, the NSIDC, commemorating 25 years of service, has organized a special session at the 2001 Fall Meeting of AGU, "Monitoring an Evolving Cryosphere." The session begins Tuesday, Dec. 11, and extends through Thursday afternoon, with 75 contributions from all areas of cryospheric study.

Papers and posters include examinations of lake and river ice, glacier dynamics and mass ice balance studies in polar and continental glaciers, regional and polar snow cover trends and variations of Canadian ice cap elevation changes.

In the world of climate change, trends are most readily observed in the Earth's cold regions, where the sensitivity of ice and snow to temperature changes serves as an early indicator of even relatively small differences, he said. Today's receding and thinning sea ice, mountain glacier mass losses, decreasing snow extent, melting permafrost and rising sea level are all consistent with warming.

Although Arctic sea ice extent is decreasing by about 3 percent per decade, the trends are not uniform. While recent studies have indicated that the ice thickness also had decreased over several decades, new information shows that the ice may have thinned rapidly, Armstrong said.

Examination of springtime ice thickness in the Arctic Ocean indicates that the mean ice thickness decreased 1.5 meters between the mid-1980s and early 1990s.

"We attribute at least some of the thinning to changes in Arctic atmosphere and ice circulation patterns. While no similar trend was evident in ice thickness near the North Pole, the data unquestionably indicate a decrease in total ice volume in the western Arctic Ocean," said Walter B. Tucker, of the Army Cold Regions Research and Engineering Laboratory in Hanover, N.H.

"At low latitudes, glacial changes are pronounced, uncontested and solid evidence of climate warming," said Eric Rignot, a researcher at the Radar Science and Engineering Section of NASA's Jet Propulsion Laboratory. "But what is happening in the polar ice sheets is less clear.

"My study shows that a number of areas previously believed to be gaining mass in the Antarctic are in fact close to being balanced or even losing mass. The only area which stands out as clearly out of balance is the Amundsen Sea sector of Antarctica drained by the Pine Island and Thwaites glaciers."

There are other lines of evidence -- besides the mass budget calculations -- that the region is undergoing rapid changes. These include where the ice leaves the continent and begins to float, as well as ice thinning and flow acceleration. In the remainder of Antarctica, it is too soon to say, said Rignot.

"We now know that the retreat of the Pine Island, Thwaites and Smith glaciers was due to a widespread thinning of ice that extended from their termini to over 200 kilometers inland," said Andrew Shepherd of the Centre for Polar Observation and Modeling at University College London. These glaciers are the principal ice drainage channels for the Amundsen Sea sector of the West Antarctic Ice Sheet.

According to Shepherd, between 1991 and 2001 the Pine Island, Thwaites and Smith glaciers thinned by more than 15, 25 and 45 meters respectively where they leave the continent and begin to float, losing a total of 157 cubic kilometers of ice to the ocean. At that rate, Shepherd projects that the glaciers could begin to float within 150 years.

Besides these observations, changes within other cryospheric areas will be reported on at the meeting. They include papers on mountain glacier monitoring, Canadian snow cover, results of river ice monitoring, changes in onset of Arctic snow melt dates, and variations in snow accumulation over northern Eurasia and their connections to the Atlantic and Pacific Oceans.

Related Links
State of the Cryosphere
University of Colorado-Boulder
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