ANCHORAGE, Alaska -- An analysis of 20 years' worth of real-life observations supports recent UN computer predictions that by 2050, summer sea ice off Alaska's north coast will probably shrink to nearly half the area it covered in the 1980s, federal scientists say.

A mother polar bear and her cub sleep near the ice outside Churchill, Canada Nov. 4, 2006. [AP]

Such a loss could have profound effects on mammals dependent on the sea ice, such as polar bears, now being considered for threatened species status because of changes in habitat due to global warming. It could also threaten the catch of fishermen.

In the 1980s, sea ice receded 30 to 50 miles each summer off the north coast, said James 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 highly productive waters over the relatively shallow continental shelf, considered important habitat for polar bears and their main prey, ringed seals, as well as other ice-dependent mammals, such as walrus.

The NOAA researchers reviewed 20 computer scenarios of the effects of warming on sea ice, used by the United Nations' Intergovernmental Panel on Climate Change in its assessment report released this year.

The researchers compared those models with observations from 1979 through 1999, Overland said, and concluded that the summer ice in the Beaufort Sea likely will have diminished by 40 percent, compared with its 1980s area.

The same is likely for the East Siberian-Chukchi Sea region off northwest Alaska and Russia. In contrast, Canada's Baffin Bay and Labrador showed little predicted change.

There was less confidence for winter ice, but the models also predict a sea ice loss of more than 40 percent for the Bering Sea off Alaska's west coast, the Sea of Okhotsk east of Siberia and the Barents Sea north of Norway.

The research paper by Overland and Muyin Wang, a NOAA meteorologist, will be published Saturday in Geophysical Research Letters, a publication of the American Geophysical Union.

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.