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

The Arctic sea ice will retreat hundreds of miles further from the coast of Alaska in the summer, the scientists at the National Oceanic and Atmospheric Administration concluded. That will open up vast waters for fishermen and give easier access to new areas for oil and gas exploration. It also likely will mean an upheaval in species, bringing new predators to warmer waters and endangering those that depend on ice.

The study, by NOAA oceanographer James Overland and meteorologist Muyin Wang, adds to the increasingly urgent predictions of major ice loss in the Arctic. Six years ago, the Intergovernmental Panel on Climate Change predicted major ice loss by 2100. An update by that U.N.-sponsored panel in February said that without drastic changes in greenhouse gas emissions, Arctic sea ice will “almost entirely” disappear by the end of the century.

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

“The amount of emissions we have already put out in the last 20 years will stay around for 40 to 50 years,” Mr. 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.”

The researchers compared 20 climate change computer models with actual satellite observations of Arctic ice cover. They discarded models that did not accurately track the ice cover for 20 years in the past, 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,” Mr. 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 east 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.”