Pregnant polar bears in Alaska, which spend most of their lives on sea ice, are increasingly giving birth on land, according to researchers who say global warming is probably to blame.
The study by three scientists for the U.S. Geological Survey suggests the state’s bear population could be harmed if the climate continues to grow warmer. Though bears are powerful swimmers, at some point they might have to cross vast stretches of open water to reach habitat on shore suitable for building dens in which to give birth.
From 1985 to 1994, 62 percent of the female polar bears studied dug dens in snow on sea ice in the Beaufort Sea. From 1998 to 2004, just 37 percent made dens on ice. The rest dug snow dens on land, according to the study.
Researchers “hypothesized that the sea ice changes may have reduced the availability or degraded the quality of offshore denning habits,” said wildlife biologist Anthony Fischbach, lead author of the study. In recent years, Arctic pack ice has formed progressively later and melted earlier each season, he said.
"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 review by the U.S. Geological Survey. Fischbach spoke about the findings Monday at the Alaska Marine Science Symposium.
Scientists estimate the Beaufort Sea polar bear population at 1,526. In the study, researchers used satellite technology to track 89 bears in northern Alaska that led them to 124 dens between 1985 and 2004.
Endangered Species Act protection?
After conservation groups sued to get Endangered Species Act protection for polar bears, 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.
Kassie Siegel of the Center for Biological Diversity, the lead author of the petition seeking to list polar bears as threatened, said the new 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."
Dens and pressure ridges
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 researchers 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."