Global warming could cause polar bears to go extinct by the end of the century by eroding the sea ice that sustains them, according to the most comprehensive international assessment ever done of Arctic climate change.
The thinning of sea ice -- which is projected to shrink by at least half by the end of the century and could disappear altogether, according to some computer models -- could determine the fate of many other key Arctic species, said the Arctic Climate Impact Assessment, the product of four years of work by more than 300 scientists.
Bears are dependent on sea ice because they use it to hunt for seals, which periodically pop up through breathing holes in the ice. Because the ice has broken up earlier and earlier in the year over the past few decades, polar bears are deprived of crucial hunting opportunities.
The uncertain fate of the world's largest non-aquatic carnivores -- as well as the future of other animals and humans who live in the Arctic -- was sketched in stark relief yesterday by the 139-page document.
The report offered a broad picture of the evidence that climate change has disproportionately affected far northern latitudes.
The researchers concluded that some areas in the Arctic have warmed 10 times as fast as the world as a whole, which has warmed an average of 1 degree Fahrenheit over the past century.
"The Arctic is really warming now," said Robert Corell, a senior fellow at the American Meteorological Society who chaired the assessment. "These areas provide a bellwether of what's coming to planet Earth."
In Alaska, western Canada and eastern Russia, average winter temperatures have risen as much as four to seven degrees Fahrenheit within the past 50 years, according to the report and are projected to increase an additional seven to 13 degrees over the next century. Winter temperatures have risen faster than summer temperatures, according to Michael MacCracken, chief scientist for climate change programs at the Washington-based Climate Institute, because thin sea ice releases more energy from the ocean into the atmosphere.
The sea ice in Hudson Bay, Canada, now breaks up 2 1/2 weeks earlier than it did 30 years ago, said Canadian Wildlife Service research scientist Ian Stirling, and as a result female polar bears there weigh 55 pounds less than they did then. Assuming the current rate of ice shrinkage and accompanying weight loss in the Hudson Bay region, bears there could become so thin by 2012 they may no longer be able to reproduce, said Lara Hansen, chief scientist for the World Wildlife Fund.
"Once the population stops reproducing, that's pretty much the end of it," Hansen said.
Arctic residents have already detected changes in polar bears' behavior. Jose Kusugak, president of the Canadian Inuit political association, said at a news conference that within the past two years he witnessed a polar bear "stock up on caribou" because it was deprived of seals. Hudson Bay residents now complain the bears are coming onto land more often, forced to seek sustenance in a habitat where they are less well adapted.
Polar bears are not the only Arctic animals in trouble. The ringed seals that bears eat, and that humans hunt, are also dependent on the sea ice to rest, give birth, nurse and feed.
"You have organisms that have been pushed beyond their limits," said James McCarthy, director of the Harvard University Museum of Comparative Zoology.
While some questioned the report -- Los Alamos Laboratory atmospheric scientist Petr Chylek said he has charted declining temperatures at the summit of Greenland's ice sheet between 1986 and 2003 -- environmentalists said it shows the need for stricter curbs on greenhouse gas emissions linked to global warming.
"This study is the smoking gun. Skeptics, polluting industries and President Bush can't run away from this one," said Philip E. Clapp, president of the National Environmental Trust. He added the study showed "concrete evidence that global warming pollution is already having serious impacts."
Administration officials, who oppose mandatory curbs on carbon emissions on the grounds that it will cost U.S. jobs, said yesterday that they consider Arctic climate change an important issue and will work to draft policy recommendations for the region. Some European negotiators have complained that the U.S. State Department is resisting issuing policy guidelines based on the scientific study, a charge Bush officials deny.
"The United States is committed to working within the United Nations framework and elsewhere to develop an effective and science-based global approach to climate change that ensures continued economic growth and prosperity for our citizens and for citizens throughout the world," said State Department spokesman Richard Boucher.