The bears are losing weight as their hunting grounds melt away, making it harder for them to hunt seals, experts said.

Polar bears "are on the way toward extinction in the next 100 years or so," Robert Corell, senior fellow at the American Meteorological Society, told a conference in Alaska this week.

In northern Canada, females that once averaged 620 pounds (280 kg) are down to 485 pounds (220 kg), said Corell, who is also chairman of the Arctic Climate Impact Assessment.

The population of polar bears in Canada's Western Hudson Bay fell 14 percent in the 10 years to 2004 to 950, according to the Center for Biological Diversity, the Natural Resources Defense Council and Greenpeace. In December, the groups sued Fish and Wildlife and the Department of Interior for failing to respond to their petition to list polar bears under the law.

Last month, NASA scientists said 2005 was the warmest year since modern record-keeping began in the late 1890s and that five of the hottest years were within the last decade.

In line with most scientists, NASA's Drew Shindell attributed the warming to a buildup of greenhouse gases, with the burning of fossil fuels as the primary source.

To combat global warming, the Bush administration favors voluntary caps on greenhouse gases, rather than mandatory caps required by all other developed nations, excluding Australia.

Late last year, NASA and National Ice Data Center scientists said total Arctic ice cover had melted back to the lowest level in modern records. They added that if melting rates continue, the summertime Arctic could be ice-free before the end of the century.

If the government lists the bears, all US government agencies would have to consider whether their actions are adding to global warming, said Kassie Siegel, a spokeswoman for the Center for Biological Diversity.

Fish and Wildlife will review the health of the polar bear population for 12 months and then decide whether to propose giving them protected status.