Pollution and overhunting also threaten their existence. Greenland and Norway have the most polar bears, but almost 5,000 live mainly in Alaska and travel to Canada and Russia.
Interior Secretary Dirk Kempthorne plans to announce later Wednesday that polar bears should be listed as a "threatened" species on the government list of imperiled species, a department official confirmed Wednesday. The "endangered" category is reserved for species more likely to become extinct.
Such a decision would require all federal agencies to ensure that anything they authorize that might affect polar bears will not jeopardize their survival or the sea ice where they live. That could include oil and gas exploration, commercial shipping or even releases of toxic contaminants or climate-affecting pollution.
Environmentalists hope that invoking the Endangered Species Act protections eventually might provide impetus for the government to cut back on its emissions of carbon dioxide and other heat-trapping "greenhouse" gases that are warming the atmosphere.
The proposed listing also marks a potentially significant departure for the administration from its cautious rhetoric about the effects of global warming.
President Bush's steadfast refusal to go along with United Nations-brokered mandatory controls on carbon dioxide, the chief global warming gas, has contributed to international tension between the United States and other nations.
Polar bears, an iconic and cold-dependent animal, are dropping in numbers and weight in the Arctic. In July, the House approved a U.S.-Russia treaty to help protect polar bears from overhunting and other threats to their survival.
That vote put into effect a 2000 treaty that sets quotas on polar bear hunting by native populations in the two countries and establishes a bilateral commission to analyze how best to sustain sea ice. It also approved spending $2 million a year through 2010 for the polar bear program.
The Polar Bear Specialist Group of the World Conservation Union, based in Gland, Switzerland, has estimated that the polar bear population in the Arctic has dwindled to 20,000 to 25,000.
The group lists the polar bear among more than 16,000 species threatened for survival worldwide, and projects a 30 percent decline in their numbers over the next 45 years. It says sea ice is expected to decrease 50 percent to 100 percent over the next 50 to 100 years."
The Interior Department plans to allow up to 90 days of public comment on its proposal, which was first reported by The Washington Post on its Web site on Tuesday night.
A little over a year ago, three environmental groups — the Center for Biological Diversity, Natural Resources Defense Council and Greenpeace — filed suit to force such a proposal from Interior's Fish and Wildlife Service, which oversees endangered species. Fish and Wildlife officials have been reviewing the status of polar bears more than two years.
They were pleased by the decision Wednesday.
"This is a victory for the polar bear, and all wildlife threatened by global warming," Kassie Siegel, a lawyer for the Center for Biological Diversity, said Wednesday. "There is still time to save polar bears but we must reduce greenhouse gas pollution immediately."