eanographer Mike Steele. The cooler conditions, which resulted largely from cloudy skies during late summer, slowed ice loss compared to the past two years. In addition, atmospheric patterns in August and September helped to spread out the ice pack, keeping extent higher.
The September 2009 ice cover remained thin, leaving it vulnerable to melt in coming summers, according to the CU-Boulder report. At the end of the summer, younger, thinner ice less than one year in age accounted for 49 percent of the ice cover. Second- year ice made up 32 percent of the ice cover, compared to 21 percent in 2007 and 9 percent in 2008.
Only 19 percent of the ice cover was over two years old -- the least ever recorded in the satellite record and far below the 1981-2000 summer average of 48 percent, according to the CU-Boulder report. Measurements of sea ice thickness by satellites are used to determine the age of the ice.
Earlier this summer, NASA researcher Ron Kwok and colleagues from the University of Washington in Seattle published satellite data showing that ice thickness declined by 2.2 feet between 2004 and 2008.
"We've preserved a fair amount of first-year ice and second-year ice after this summer compared to the past couple of years," said NSIDC scientist Walt Meier of CU-Boulder's Cooperative Institute for Research in Environmental Sciences. "If this ice remains in the Arctic thorough the winter, it will thicken, which gives some hope of stabilizing the ice cover over the next few years. However, the ice is still much younger and thinner than it was in the 1980s, leaving it vulnerable to melt during the summer."
Arctic sea ice follows an annual cycle of melting through the warm summer months and refreezing in the winter. Sea ice reflects sunlight, keeping the Arctic region cool and moderating global climate temperatures.
While Arctic sea ice extent varies from year to year because of changing atmospheric conditions, ice extent has shown a dramatic overall decline over the past 30 years.
"A lot of people are going to look at the graph of ice extent and think that we've turned the corner on climate change," said NSIDC Lead Scientist Ted Scambos of CU-Boulder's CIRES. "But the underlying conditions are still very worrisome."
NSIDC is part of CIRES and is funded primarily by NASA.
Image Caption: This graphics show multi-year Arctic sea ice changes. Credit: Courtesy C. Fowler and J. Maslanik, University of Colorado at Boulder
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