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

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 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 feet--and 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 percent--to 935 from 1,194 between 1987 and 2004--according 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 habitat--one 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."

LESSON PLAN 3: INTERNATIONAL

POLAR BEARS: ON THIN ICE

DEBATE

* Have students debate global warming. Do they believe those who warn that action needs to be taken immediately to ward off catastrophe? Or are they skeptical that humans have a direct impact on Earth's temperature?

CRITICAL THINKING

* Next, ask students to discuss the endangered-species issue. Why are scientists so concerned about endangered or threatened species?

* [Scientists say there is a direct link between humans and animal and plant life. The health of one species can affect other species in the food chain. In the case of global warming, polar bears are one sign of a threat that could potentially affect all species, including humans.]

DISCUSSION QUESTIONS

* Would you support efforts to curb global warming if that meant you'd have to pay more for your electricity or gas, or had to drive a more expensive, less efficient electric car?

* If the polar bear were a less-majestic creature, would its threatened status attract as much attention?

WRITING PROMPT

* Have students write five paragraph essays arguing for or against taking immediate action to curb global warming.

FAST FACTS

** The U.S. Fish and Wildlife Services lists a total of 1,311 endangered animal and plant species in the U.S. Another 568 animals and plants in foreign countries are identified.

WEB WATCH

www.fws.gov The U.S. Fish and Wildlife Service provides background on polar bears and the threats to their survival. Scroll down to "Polar Bears," click on Fact Sheet.

Felicity Barringer is a national correspondent, and Andrew C. Revkin reports on the environment, for The New York Times. Additional reporting by John Collins Rudolf for The Times and Suzanne Bilyeu.