SEATTLE, July 6 — As the pack ice that is the bedrock of their existence melts because of global warming, polar bears are facing unprecedented environmental stress that will cause their numbers to plummet, according to a report by a panel of the world’s leading experts on the species.

In a closed meeting here late last month, 40 members of the polar bear specialist group of the World Conservation Union concluded that the imposing white carnivores — the world’s largest bear — should now be classified as a “vulnerable” species based on a likely 30 percent decline in their worldwide population over the next 35 to 50 years. There are now 20,000 to 25,000 polar bears across the Arctic.

“The principal cause of this decline is climatic warming and its consequent negative affects on the sea ice habitat of polar bears,” according to a statement released after the meeting. Scientists from five countries, including the United States, attended the meeting.

“All of the evidence is heading in the same direction, and the trend is dramatic,” said Scott Schliebe, who led the Seattle meeting and is polar bear project leader in Alaska for the U.S. Fish and Wildlife Service. “In a shrinking ice environment, the ability of the bears to find food, to reproduce and to survive will all be reduced.”

A 30 percent drop in the number of polar bears is expected as diminishing ice packs affect the bears’ ability to find food and to reproduce. (By Subhankar Banerjee — Associated Press)

Schliebe emphasized that he was speaking for the panel and not for the U.S. government.

The panel’s conclusions became public this week as President Bush traveled to a Group of Eight meeting in Scotland, where U.S. officials have lobbied to prevent any specific targets for reducing greenhouse gases from being included in the meeting’s final communique. The United States is the only member of the G-8 that has refused to ratify the Kyoto Protocol, which calls for reducing emissions that many scientists say are causing Earth to warm up.

The best longitudinal information on the effect of global warming on polar bears comes from the western coast of Hudson Bay, in the Canadian province of Manitoba. It shows a 17 percent decline in the polar bear population in the past 10 years, from 1,200 to fewer than 1,000. The panel here in Seattle used the Canadian research as the primary basis for its warning about the future of polar bears around the world.

“We have seen with our own eyes that climatic warming is causing the ice to break up earlier, and that is affecting the survival of the bears,” said Ian Stirling, a research scientist for the Canadian Wildlife Service.

Ice is melting there about three weeks earlier than it did 30 years ago, said Stirling, who has been studying polar bears for 35 years.

“For a polar bear, not all weeks are created equal,” he said. “They are losing three weeks at the best time of the year for feeding on the ice, when seal pups are abundant and bears put on fat that they store for the four months that they have to live onshore.”

Having lost this critical hunting opportunity, polar bears in western Hudson Bay weigh about 15 percent less (about 150 pounds less for an adult male) than they did 30 years ago, Stirling said.

“The bears are losing their physical condition,” he said. “It is a cumulative process that is causing a steady decline in survival, particularly for cubs and sub-adults. It is causing the population to decline.”

In Alaska, the ice situation appears to be equally “grim” for polar bears, Schliebe said. He said that in three of the past four years, there have been record low ice packs in Alaska’s Beaufort Sea region, pushing more and more polar bears on land for protracted periods. Hungry bears are drawn to village dumps and other settled areas where they come into conflict with people and are sometimes shot.

Polar bears evolved from brown bears about a quarter-million years ago to become specialist carnivores, marine mammals that can thrive on ice packs and feast on seals. Climate change, though, is happening too fast for the bears to adapt, experts say.

“They don’t have time to evolve backwards,” Stirling said.

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