U.S. weighs listing polar bear as threatened species

Hall said all signs point to global warming and the resulting disappearance of Arctic sea ice as the major threat facing polar bears.

"We are dealing with the fact of reduced sea ice," Hall said. "The only cause we can identify right now that merits further review is the ice melting."

Arctic ice coverage in recent years has been the lowest on record and studies have found polar bears to be smaller and suffering from lack of food. Some have drowned swimming vast distances of open water between ice floes and far fewer cubs are surviving the first months of life, government studies show.

International climate scientists have predicted unless global warming is stemmed, polar bears will be extinct by the end of the century. The bears number 20,000 to 25,000 worldwide, and about 4,700 in Alaska and the surrounding waters.

"Polar bears are the first species to be proposed for listing where the link is so clear, so direct even the Bush administration can't deny it," Siegel said. "That was a very big deal to hear those people say that global warming is causing the sea ice to melt."

President George W. Bush has been skeptical of scientific conclusions on global warming. He withdrew from the Kyoto Protocol on cutting greenhouse gases and his administration has consistently rejected capping greenhouse gas emissions as bad for business and U.S. workers.

Scientists have concluded that global warming is caused mainly by heat-trapping carbon dioxide emissions from power plants and motor vehicles. The United States is the largest producer of those pollutants worldwide.

(Additional reporting by David Alexander in Washington)