On thin ice: the Arctic territory where polar bears roam is literally melting out from under them.

• Author: Barringer, Felicity

BACKGROUND

Polar bears, which live in relative isolation in the Arctic, are not often front-page news. But many scientists see their melting habitat as a warning sign of global warming's potential impact. Melting sea ice will raise ocean levels, which could prove disastrous for island nations and anyone living near coastlines.

Polar bears are at the top of the Arctic food chain: A single bear can devour 100 pounds of seal blubber at one meal. With a double layer of fur, the bears are well insulated against frigid temperatures. They are excellent swimmers, and their wide, thickly padded paws are ideal for moving on ice. Most hunting of polar bears has been outlawed, and the species has no enemies in the wild. But something has placed these creatures in grave danger: climate change.

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 tip 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.)

RISING TEMPERATURES

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...