The floating cap of sea ice on the Arctic Ocean shrank this summer to what is probably its smallest size in at least a century, continuing a trend toward less summer ice, a team of climate experts reported yesterday.
That shift is hard to explain without attributing it in part to human-caused global warming, they and other experts on the region said.
The change also appears to be becoming self-sustaining: the increased open water absorbs solar energy that would otherwise be reflected back into space by bright white ice, said Ted A. Scambos, a scientist at the National Snow and Ice Data Center in Boulder, Colo., which compiled the data along with the National Aeronautics and Space Administration.
The data were released on the center's Web site, nsidc.org.
The findings are consistent with recent computer simulations showing that a buildup of smokestack and tailpipe emissions of greenhouse gases could lead to a profoundly transformed Arctic later this century, when much of the once ice-locked ocean would routinely become open water in summer.
Expanding areas of open water in summer could be a boon to whales and cod stocks, and the ice retreat could create summertime shipping shortcuts between the Atlantic and Pacific.
But a host of troubles lie ahead as well. One of the most important consequences of Arctic warming will be increased flows of meltwater and icebergs from glaciers and ice sheets, and thus an accelerated rise in sea levels, threatening coastal areas. The loss of sea ice could also hurt both polar bears and Inuit seal hunters.
The North Pole icecap always grows in winter and shrinks in summer. The average minimum area from 1979, when precise satellite mapping began, until 2000 was 2.69 million square miles, similar in size to the contiguous United States. The new summer low, measured Sept. 19, was 20 percent less than that.
Before 1979, scientists estimated the size of the icecap based on reports from ships and airplanes.
The difference between the average ice area and the area that persisted this summer was about 500,000 square miles, an area about twice the size of Texas, the scientists said.
This summer was the fourth in a row with icecap areas sharply below the long-term average, said Mark C. Serreze, a senior scientist at the snow and ice center and a professor at the University of Colorado, Boulder.
A natural cycle in the polar atmosphere called the Arctic oscillation, which contributed to the reduction in Arctic ice in the past, did not appear to be a factor in the past several years, he said.
He said the role of accumulating greenhouse gas emissions had become increasingly apparent with rising air and sea temperatures. Still, many scientists say that it is not yet possible to determine what portion of Arctic change is being caused by rising levels of carbon dioxide and other emissions from human sources and how much is just climate's usual variations.
Serreze and other scientists said that more variability could lie ahead and that the area of sea ice might increase some years. But the scientists have found few hints that other factors, such as more Arctic cloudiness in a warming world, will reverse the trend.
"With all that dark open water, you start to see an increase in Arctic Ocean heat storage," Serreze said. "Come autumn and winter, that makes it a lot harder to grow ice, and the next spring you're left with less and thinner ice. And it's easier to lose even more the next year."
The result, he said, is that the Arctic is "becoming a profoundly different place than we grew up thinking about."