In December 2006, Kempthorne announced that there was sufficient scientific evidence of the bear's melting habitat to propose protecting it under the Endangered Species Act. Since then, the future for the top predator at the top of the world has looked increasingly bleak.
In September, scientists at the U.S. Geological Survey released a nine-volume analysis of how a warming Arctic is affecting sea ice and reached a dire forecast: Two-thirds of the polar bear's habitat would disappear by 2050.
Polar bears are experts at hunting ringed seals and other prey from sea ice. But they are so unsuccessful on land that they spend their summers fasting, losing more than 2 pounds a day.
This forced fast is now about three weeks longer than it was 30 years ago, according to studies in Canada's western Hudson Bay. This gives the bears less time to hunt and build up the fat reserves they need to survive until ice re-forms in the fall and they can resume hunting.
As bears have become thinner, the reproductive rates of females and survival rates of cubs have declined. Overall, the western Hudson Bay population has dropped by 22% since 1987, according to studies.
These bears in Hudson Bay are among the best studied. Scientists don't know if similar trends exist elsewhere in the Arctic, a vast and forbidding place to conduct field studies. Surveys have shown other problems, including bears drowning in open waters and cannibalism among hungry bears.
Scientists think the global population of 20,000 to 25,000 bears remains robust, rebounding from the 1960s, when hunting had driven down the population to about 12,000. But virtually all polar bear experts predict rapid population declines in the Arctic, which is warming faster than anywhere else in the world and changing too rapidly for the bears to adapt and find other food sources.
A group of Canadian scientists last month declared the polar bear a "species of concern" but stopped short of saying it was threatened with extinction -- a designation that could have restricted hunting by Canada's Inuit people.
Canada has about two-thirds of the world's polar bear population. Kempthorne and Canada's environmental minister last week signed an agreement with tribal government to consider "the best available scientific information and aboriginal traditional knowledge."
The Nunavut Wildlife Management Board, which operates in Canada's far north, recently proposed reducing the quota of polar bears hunted in Baffin Bay. But Inuit trappers and hunters opposed the move, saying their traditional knowledge revealed too many bears in the area.
The new protections under U.S. law will not affect the subsistence hunt in Canada, nor preclude native hunters from selling permits for as much as $30,000 to trophy hunters from the United States.
But it will prohibit trophy hunters from importing polar bear skins or heads into the United States. About 70 such trophies have been brought back each year.
"Native hunters claim they'll kill these bears anyway," said Paul Todd of the International Fund for Animal Welfare. "But if they don't need them for subsistence hunting, then, in reality, fewer bears will be killed each year."