The annual summer retreat of the sea ice cloaking the Ocean appears to have ended with the ice not quite matching last year’s extraordinary recession, polar scientists said Tuesday.

Still, the scientists, at the National Snow and Ice Data Center in Boulder, Colo., said that the ice in the Arctic this summer was 33 percent below the average extent tracked since satellites started monitoring the region in 1979 and that the trend continued toward an ice-free Arctic Ocean within a few decades.

This summer also was notable because scientists confirmed that two fabled shipping routes, the Northwest Passage over Canada and the Northern Sea Route over Russia, were briefly open simultaneously.

The ice, which is now expanding as the sun dips toward the horizon for the winter, hit a minimum extent of about 1.74 million square miles on Sept. 12, said Walter Meier, a research scientist at the center. In 2007, the minimum extent was 1.59 million square miles, he said.

Federal biologists have said that this long-term ice retreat is the main reason they had concluded that polar bears, which hunt seals from the ice, deserved protection under the Endangered Species Act. The Bush administration listed the species in May as threatened with extinction.

Global warming from the buildup of human-generated greenhouse gases almost certainly contributes to the Arctic ice retreats, according to a host of Arctic specialists. But many say natural variations in Arctic winds and cloud cover probably had a role in shaping the particularly large ice losses in the past two summers.

Dr. Meier said that small variations from one year to the next were less significant than the long-term trajectory, which remained toward progressively more open water. “It’s hard to see the summer ice coming back in any substantial way,” he said.


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