ow on sea ice. From 1998 to 2004, just 37 percent gave birth on sea ice. The rest instead dug snow dens on land, according to the study by three U.S. Geological Survey researchers.

Bears that continued to den on ice moved east in the Beaufort Sea off Alaska's northern coast, away from ice that was thinner or unstable.

"We hypothesized that the sea ice changes may have reduced the availability or degraded the quality of offshore denning habits and altered the spatial distribution of denning," said wildlife biologist Anthony Fischbach, lead author of the study. "In recent years, Arctic pack ice has formed progressively later, melted earlier, and lost much of its older and thicker multiyear component."

The study makes no predictions of harm in the short term but suggests the Beaufort Sea bear population could be harmed if warming continues. Though bears are powerful swimmers, at some point they might face daunting distances of open water to reach denning habitat on shore.

"If Arctic sea ice continues to decline, we predict that the proportion of coastal denning will continue to increase until the autumn ice conditions prevent pregnant bears foraging offshore from reaching the coast in advance of denning," Fischbach said.

The study is under USGS review. Fischbach spoke about the study at the Alaska Marine Science Symposium, which continues through today in Anchorage. The co-authors are research wildlife biologists Steven Amstrup and David Douglas.

The study is likely to give ammunition to conservation groups calling for polar bears to be listed as threatened under the Endangered Species Act.

Three conservation groups sued the federal government in late 2005 seeking protections for polar bears under the law, blaming global warming for the melting of sea ice, the primary habitat of the animals.

Department of Interior Secretary Dirk Kempthorne in December proposed listing polar bears as a "threatened" species. A public comment period on the proposal is open through April 9.

"Threatened" under the law means a species is likely to become endangered in the foreseeable future. "Endangered" means it is in danger of extinction throughout all or a significant portion of its range.

The listing is opposed by Alaska Republican Gov. Sarah Palin, who told Kempthorne by letter that listing polar bears has the potential to damage the economy of Alaska and the nation without any benefit to polar bear numbers or their habitat, and that there are no human activities that can be regulated to effect change.

Conservationists disagree.

"The dire impacts from global warming on America's polar bears continue to mount: drownings, cannibalism, starvation, reduced cub survival and now denning dislocation," said Deborah Williams, president of Alaska Conservation Solutions, an Anchorage-based group aimed at halting climate change. "Clearly, we need to demand that Congress and the administration protect polar bears, and our future, by reducing greenhouse gas emissions and listing polar bears under the Endangered Species Act."

Kassie Siegel of the Center for Biological Diversity, the lead author of the petition seeking to list polar bears as threatened, said the study underscores the scope of changes in the Arctic.

"It's such the canary in the coal mine," Siegel said. "If you want to know what's going to be happening in the rest of the world in 25 years, all you have to know is what's happening in the Arctic. Everything is changing, and not for the better."

Alaska polar bears spend most of their lives on sea ice, a marine habitat from which they hunt for their main prey, ringed seals, plus bearded seals and other animals.

Typically in November or December, after sea ice has reconnected to Alaska's coast, pregnant polar bears dig dens where snow has piled into drifts.

Sea ice pushed into shore becomes jumbled into pressure ridges that capture snow used for dens. However, with new ice, that can happen after bears want to make dens. On shore, terrestrial features catch snow.

"They're generally using coastal bluffs and the river bluffs," said study co-author David Douglas. "Along the big rivers, they have bluffs that catch snow."

The USGS estimates the Beaufort Sea polar bear population at 1,526. In the study, researchers determined that denning distribution had shifted, based on satellite radio tracking of 89 bears in northern Alaska that led them to 124 dens between 1985 and 2004.

They believe pregnant bears shifted onto shore because the sea froze later, creating few pressure ridges. Also, more old ice that may have had pressure ridges had melted.

"The first-year ice would just be forming," Douglas said. "It's very flat, unless there's been an early winter that allows it to thicken enough and actually ridge up and then catch snow."

They did not speculate whether bears might be harmed in the short term.

"The big issue is, the long term may be coming sooner that we thought it was," Fischbach said.

"If the foraging areas get so far off shore they (bears) cannot reach the coastal areas in advance of denning, and at the same time they'll be facing deterioration of the offshore denning habitat, then we would expect there would be reproductive consequences to the population."

Researchers rejected two alternative hypotheses for the shift to land -- hunting and the presence of more whale carcasses on beaches. Canada and the United States minimized hunting about 30 years ago but the denning shift occurred less than a decade ago. They also observed that just 5 percent of "pre-denning" females passed within 5 kilometers of a carcass site, compared to 30 percent of females that did not den that year.