They depend on sea ice as a platform for hunting seals, the mainstay of the polar bears' diet. This ice platform is shrinking, which means that the bears must cover longer distances between ice and land. Some bears have drowned while trying to swim from one area of solid ice to another.
Many experts on the Arctic say that global warming is causing the ice to melt and that the warming is at least partly the result of the atmospheric buildup of greenhouse, or heat-trapping, gases from tailpipes and smokestacks. The plight of the polar bear has been held up by environmentalists as a symbol of global warming caused by humans.
The Interior Department proposed in December to designate polar bears as a threatened species. Many biologists believe that the accelerating loss of the Arctic ice will cause the polar-bear populations to decline, perhaps sharply, in the coming decades.
(The Bush administration has been skeptical about the causes of global warming. Interior Secretary Dirk Kempthorne says that although his decision to seek protection for polar bears acknowledged the melting of the Arctic ice, his department was not taking a position on why the ice was melting or what to do about it.)
In January, the National Oceanic and Atmospheric Administration announced that 2006 was the warmest year on record for the 48 contiguous states. It acknowledged that a contributing factor is "the long-term warming trend, which has been linked to increases in greenhouse gases."
Rising global temperatures are not simply melting ice in the Arctic, they are changing the geography of its coastlines. For example, chains of islands that were buried under Greenland's ice sheets are being exposed as the ice melts. This sudden appearance of islands is a symptom of an ice sheet going into retreat, say scientists.
Greenland is covered by 630,000 cubic miles of ice enough water to raise global sea levels by 23 feetand it is melting faster than scientists initially thought. Some experts say that a sea-level rise of a foot or more over the next few decades is possible. Even a one-foot rise is potentially disastrous for some island nations and people who live near coastlines.
"The general thinking until very recently was that ice sheets don't react very quickly to climate," says Martin Truffer, a glaciologist at the University of Alaska in Fairbanks. "But that thinking is changing right now, because we're seeing things that people have thought are impossible."
Polar bears have survived previous Arctic warming periods, including the last warm stretch between ice ages some 130,000 years ago, but some experts say that nothing in the species' history is likely to match the extent of warming and ice retreats projected in this century and beyond, should emissions of heat-trapping gases continue unabated.
The worldwide population of polar bears currently stands at 20,000 to 25,000, broken into 19 groups in Russia, Denmark, Norway, Canada, and the United States. As many as 5,000 of those bears live in areas off Alaska or the nearby coastlines.
The most-studied polar-bear population is that of the Western Hudson Bay in Canada. It dropped 22 percentto 935 from 1,194 between 1987 and 2004according to the U.S. Fish and Wildlife Service.
The World Conservation Union, an international network of government and private scientists and organizations, gave polar bears threatened status in May, projecting a decline of 30 percent by mid-century.
Scott Schliebe, a biologist and the polar-bear project leader for the Fish and Wildlife Service, notes in a recent essay that polar bears as a species have long had the advantage of having a relatively pristine habitatone almost devoid of human presence. But that is changing.
"Today," Schliebe writes, "polar-bear populations are facing threats unprecedented during recorded history in the Arctic."