Scientists: Arctic sea ice tells of warming

Rocky Mountain News
September 14, 2006

Wintertime declines seen as strong sign of climate change

Recent sharp declines in the extent of wintertime Arctic sea ice provide strong new evidence that global warming is already at work there, shrinking the habitat available to polar bears and potentially threatening rich fisheries, scientists reported Wednesday.

Boulder researchers have observed dramatic drops in the end- of-summer size of the floating polar ice cap for five straight years. Those changes have been widely viewed as bellwethers of human-caused climate warming.

Climate models predicted that similar ice-cap losses should be seen during the Arctic winter. But until now, they had not been detected. Two new NASA-sponsored studies report the first observations of sharp winter reductions. "It may be the strongest evidence of global warming in the Arctic," NASA scientist Josefino Comiso, author of one of the studies, said during a Wednesday news briefing.

Comiso and his colleagues used satellite data to study changes in the maximum wintertime extent of Arctic sea ice. The cap shrinks each summer to roughly the size of the 48 contiguous U.S. states, then grows each winter.

Until two years ago, the winter maximum had been declining at a rate of about 1.5 percent per decade, dating back to the advent of detailed satellite observations in the late 1970s. But in the winter of 2004-2005 and again last winter, the cap's maximum size dipped 6 percent below the long-term average each winter.

"(Climate) modelers have predicted that warming due to increases in greenhouse gases would be most pronounced in winter months," said Comiso, whose study will be published this month in the journal Geophysical Research Letters. "A winter warming signal is finally coming out," he said. Boulder climate modeler Gerald Meehl cautioned that more data are needed before scientists can conclude a long-term wintertime trend has been observed in Arctic sea ice.

"It's hard to extract a trend from two years of data," said Meehl, who was not involved in the study. "But I think what we're seeing in a decrease of winter sea ice is consistent with what the models have been projecting and also consistent with the general features of a warming planet," said Meehl, who works at the National Center for Atmospheric Research.

In a second study, researchers found that the wintertime loss of sea ice in the East Arctic Ocean, above Europe and Asia, neared 50 percent during 2004 and 2005. But that change was due to shifting winds rather than melting. The overall winter ice decrease totaled 280,000 square miles, an area the size of Texas, according to a team led by Son Nghiem of NASA's Jet Propulsion Laboratory.

Their study was published in the Sept. 7 edition of Geophysical Research Letters. As for the end-of-summer measurements of the Arctic sea-ice minimum, it looks like 2006 won't match last year's record-setting season, said Mark Serreze, a senior research scientist at the National Snow and Ice Data Center in Boulder. The summer sea-ice minimum marks the date at the end of the summer melting season when the ice retreats to its lowest extent. Last September, it shrank to what the Boulder team said was its smallest size in more than a century.

Arctic air temperatures were 4 to 7 degrees above average last year. Due largely to stormy, cool Arctic weather last month, 2006 probably won't set a record, though the final numbers aren't in yet, Serreze said. Even so, 2006 marks the fifth straight year of dramatic summer sea-ice declines in the Arctic.

"I think that if we put all the evidence that we have together, the best explanation of what we're seeing is that we are starting to see the emerging signs of a greenhouse warming," he said. But some researchers contend that shifting Arctic wind patterns play a greater role in the summer sea-ice retreats than warming Arctic temperatures. "I think the changes in wind circulation and the way the ice is moving are the driving factors, and temperature helps but it's not the primary driver," said Ignatius Rigor of the University of Washington.

Either way, the loss of ice is forcing polar bears to spend more time on land, scrounging for garbage in native villages instead of hunting, said NASA's Claire Parkinson.