Scientists estimate that between 20,000 and 25,000 polar bears live in 19 distinct populations scattered throughout the Arctic.
About 4,700 live in Alaska and spend part of the year in Canada and Russia. Other bear populations are found in Greenland and Norway.
The most studied population resides in western Canada's Hudson Bay. The population there has declined 22 percent due to weight loss and low cub survival stemming from sea ice loss since 1987.
While the Alaska population has not experienced such a steep decline, biologists are concerned that the bears' numbers may drop in the future, as the populations face similar challenges.
Scientists have linked emissions of carbon dioxide and other greenhouse gases to warmer temperatures that are rapidly melting glaciers and polar ice caps.
Earlier this month, for instance, a study based on computer models predicted the Arctic could have a completely ice-free summer by 2040, decades earlier than previously expected.
Some scientists and environmental activists say that reduced greenhouse gas emissions could slow the Arctic ice retreat, allowing polar bears to recover.
The U.S. Fish and Wildlife Service (FWS), the agency responsible for the Endangered Species Act, has ruled out oil and gas development and subsistence hunting as factors in the polar bear decline.
"This is directly tied to the sea ice loss and the ultimate dependence of the polar bear on drift ice," Dale Hall, the FWS director, said in today's teleconference.
But Interior Secretary Kempthorne said his department's scientists lack the authority to address the causes of the receding sea ice and therefore stopped short of blaming ice loss on global warming.
Causation, he said, is beyond the scope of the department's responsibility under the endangered species law.
"However, climate change science initiatives of causation are discussed in other analyses undertaken by the [Bush] Administration," he said. "The administration treats climate change very seriously and recognizes the role of greenhouse gases in climate change."
According to Siegel, of the Center for Biological Diversity, a formal listing could affect the federal approval process for facilities such as industrial coal-fired power plants and lead to stricter fuel economy standards for automobiles.
"Federal agencies will have to ensure that their greenhouse gas emissions do not adversely modify the critical habitat of polar bears or the continued existence of polar bears," she said.
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