The vast expanses of ice floating in the Arctic Sea are melting in winter as well as in the summer, likely because of global warming, NASA scientists said Wednesday.
"This is the strongest evidence yet of global warming in the Arctic,'' said Josefino Comiso, a research scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
And if the ice continued to melt at the current rate, Comiso said, it could have profound effects on all life in the Arctic and other consequences around the world.
Particularly hard hit would be the polar bears, which live on the ice, he said. Sea ice also provides oxygen-rich cold water needed for the growth of phytoplankton. A decline in the number of the tiny plants could have a cascading effect on the food supply of fish and crustaceans, seals and the other marine mammals.
The size of this summer's Arctic ice won't be known for a few weeks because it usually reaches its smallest size the third week of September. Last year, scientists found that polar ice twice the size of Texas had melted since NASA started compiling satellite data 27 years ago. Scientists said there could be no ice left in the Arctic in the summer by the end of the century.
Until 2005, the wintertime sea ice -- which is thick and multilayered -- had been relatively stable. In the summer, the ice is thinner, more mobile and melts at the edges every spring before freezing up again in the autumn.
In the last two winters -- 2005 and 2006 -- the size of the sea ice was 6 percent smaller than average, the data show. The sea ice in the Northern Hemisphere covers nearly 10 million square miles in the winter. The melting -- most of it occurring in the eastern Arctic near the North Pole -- correlates with a rise in the ocean's surface water temperature.
The melting period is growing by 15 days each decade, meaning less time for ice to grow back, experts said.
When Comiso saw the decline of winter sea ice in 2005, he said, "it was only one year, and I didn't think it was so serious.''
However, based on NASA data, his computer simulations and two years of melting ice, "this has a very large chance of continuing," he said.
Already a greater number of polar bears have been showing up in Inuit communities in the Arctic, apparently searching for food, said NASA researcher Claire Parkinson.
The bears use the sea ice to hunt seals and other marine mammals. "When the ice retreats, they have to come on the land. Normally, when they're on the land, they're not eating,'' she said.
The bears come on land more often now, she said, because they're probably hungrier and afraid of being stranded on a retreating floe, she said.
Parkinson and Ian Stirling, a biologist in the Canadian Wildlife Service, published a study in the journal Arctic this month showing that the polar bear population is shrinking, even though there have been more sightings. Instead, the Hudson Bay population has declined from 1,200 bears in 1989 to 950 bears in 2004, and the weight of adult females has dropped. None of the 18 other populations in the Arctic has grown, either, she said.
It's not impossible that the sea ice could recover in coming years, Parkinson said.
"The possibility is there that the Arctic will recover, but that is not as likely as that it will continue to decrease,'' she said.
The U.S. Fish and Wildlife Service is considering listing the polar bear as a threatened species under the Endangered Species Act and is conducting studies in the North Slope of Alaska and elsewhere in the Arctic.
The loss of Arctic sea ice has global effects, scientists say.
Sea ice is made of frozen ocean water, and when it melts, it doesn't raise the ocean's level as do melting glaciers and ice sheets. But less sea ice means a smaller area of ice to reflect radiation away from Earth, and the dark, open water absorbs heat. Both phenomena could accelerate the world's warming, scientists say.
"We're seeing an overall pattern of global warming,'' said Mark Serreze, senior research scientist at the National Snow and Ice Data Center in Boulder, Colo., which joined NASA scientists in a telephone news conference Wednesday.
Ice core borings in Antarctica have produced a record of historic carbon dioxide concentrations over the last 600,000 years. The borings show that the levels of carbon dioxide, a greenhouse gas, are at their highest ever because of the burning of fossil fuels, Serreze said.
Serreze said he was surprised to see a new lake, or polynya, the size of Maryland, opening up in the sea ice north of the Beaufort Sea.
In 20 years of looking at sea ice, he has never seen anything like it.
"If you asked me five years ago if it was human activity (causing global warming) versus natural variability, I was a fence-sitter,'' Serreze said.
"The magnitude of the changes is starting to rise above the noise of natural variability. There is a continuing trend. What we see in the Arctic is part of a much larger picture. We hate to say, 'We told you so.' But we told you so.''