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ANCHORAGE, Alaska -- Melting faster than scientists had expected, the Arctic ice cap will shrink 40% in most regions by 2050, with grim consequences for polar bears, walruses and other marine animals, according to government researchers.

In the 1980s, sea ice receded 30 to 50 miles each summer off the north coast, said James E. Overland, a Seattle-based oceanographer for the National Oceanic and Atmospheric Administration. "Now we're talking about 300 to 500 miles north of Alaska," he said of projections for 2050.

That's far past the edge of the relatively shallow continental shelf, considered important habitat for polar bears and their main prey, ringed seals, as well as walrus and other ice-dependent mammals.

Six years ago, the Intergovernmental Panel on Climate Change predicted major ice loss by 2100. A February update by that United Nations-sponsored panel said that without drastic changes in greenhouse gas emissions, Arctic sea ice will "almost entirely" disappear by century's end.

But Overland's calculations are largely based on the carbon dioxide that already has been pumped into the atmosphere. That pollution will help greatly diminish the ice by 2050 regardless of future emissions curbs, he said Thursday.

"The amount of emissions we have already put out in the last 20 years will stay around for 40 to 50 years," Overland said. "I'm afraid to say that a lot of impacts we will see in the next 30 to 40 years are pretty much already established."

Overland and meteorologist Muyin Wang compared 20 climate change computer models with satellite observations of Arctic ice cover. They discarded models that did not accurately track the ice cover for the last 20 years, and extended the accurate models to predict the ice melt by 2050. The research will be published Saturday in the journal Geophysical Research Letters.

"The rate of ice loss now is faster than what was depicted," Overland said. "This moves the threshold up."

The two researchers also narrowed their focus to specific parts of the Arctic. Because of wind and water currents, the sea ice off Canada's northeast coast in Baffin Bay will change little, he said, but most other regions, including Alaska's Bering Sea, will shrink dramatically.

"This will have a profound effect on the animals that use sea ice all the time, including walrus and polar bears and ringed seals," he said. "You will actually have a change in the whole ecosystem. You will have winners and losers. Crabs, clams, walrus and bears will not do well. Salmon, pollock and other fish that live higher up in the water column will extend their range."

"We really don't have a clue how that will look," Overland said of the species changes. Pollock fish already moving into the Bering Sea were expected to thrive, for example, but there has also been an unexpected loss in species on which the pollock feed and an unexpected increase in predator species, he said.

Ice in Alaska's Bering Sea has shrunk significantly this decade, due in large part to an unusual wind blowing warm air toward the Arctic, Overland said. But that natural effect has been aggravated by global warming, which is heating Arctic areas faster than more southerly regions.

"There is a whole lot of uncertainty about what will happen next," he said. "Are we approaching a tipping point where we have lost so much ice already that, when the winds change, we have already gone too far?"

The situation is dire for polar bears, said Kassie Siegel of the Center for Biological Diversity, who wrote the petition seeking federal protection for the animals. "They're going to drown, they're going to starve, they're going to resort to cannibalism, they're going to become extinct," she said.

As ice recedes, many bears will be stuck on land in summer, where they have virtually no sustainable food source, Siegel said. Some will try and fail to swim to sea ice, she said.

Bears that stay on sea ice will find water beyond the continental shelf to be less productive, she said, and females trying to den on land in the fall will face a long swim.

"It's absolutely horrifying from the polar bear perspective," she said.

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