(05-14) 12:59 PDT WASHINGTON -- The U.S. government will list the polar bear as threatened under the Endangered Species Act, making it the first animal to win protections because of global warming.

The Arctic sea ice is vital to the bear's survival, its habitat has dramatically melted, and computer models show that the pack ice is likely to continue shrinking in the foreseeable future, Interior Secretary Dirk Kempthorne said Wednesday in announcing the move.

Environmental groups had pressed the Bush administration to list the polar bear in hopes that the U.S. government would then pass restrictions on carbon dioxide emissions from factories, power plants and vehicles, saying that strategy is the only way to avoid the harshest effects of climate change. They also wanted to curb oil drilling in the Arctic.

But Kempthorne said the Bush administration wouldn't use the act to limit emissions from industrial sources or otherwise regulate greenhouse gases. Law and science "wouldn't allow it" because direct connections couldn't be made between specific sources of gas emissions and harm to polar bears, he said. In addition, he announced a special rule allowing petroleum exploration and extraction to continue, saying it didn't harm polar bears.

"We know Earth is warming. We know man is a factor in that. But we cannot tell you to what extent," Kempthorne said.

"The habitat for the polar bear is declining. That's what triggered this. But the Endangered Species Act is not the vehicle to deal with global climate change," Kempthorne said. Such policy changes would have to come from elected leaders and nations worldwide, he said.

An estimated 20,000 to 25,000 polar bears live in the Arctic in the United States, Canada, Norway, Russia and Greenland, a territory of Denmark.

Kempthorne is asking the U.S. Fish and Wildlife Service to provide guidance on what new protections would be put in place for the bear, including making sure bears that come to land do not come into contact with humans in situations that could lead to the animals being shot and safeguarding dens of pregnant bears that come to land to prevent human interference.

Dale Hall, director of the Fish and Wildlife Service, said listing the bear would put an end to the importation into the United States from Canada of polar bear heads and hides by sport hunters. According to Interior Department records, since 1997, the U.S. government has issued 967 permits to hunters bringing in trophies of polar bears killed in Canada. Hunters had been able to buy hunting rights from indigenous Canadians.

Environmental groups issued statements showing disappointment that the Bush administration wouldn't limit greenhouse gases or curb Arctic oil and gas development activities.

In Juneau, Clayton Jernigan, a lawyer for Earthjustice, said he found it ironic that the interior secretary affirmed the danger of global warming to the polar bear yet at the same time announced plans to limit protections against oil and gas activities. He also pointed out that the Interior Department is leaving the definition of what would injure the bear or its habitat to the Marine Mammal Protection Act instead of the stronger Endangered Species Act. The bear already comes under the auspices of the mammal protection act.

Jernigan's group already is challenging the Interior Department for failing to fully evaluate the effects of oil and gas activities in the Beaufort Sea combined with effects of global warming on polar bears under the Marine Mammal Protection Act.

Carl Pope, Sierra Club executive director, said, "Drilling would inundate polar bear habitat with pipelines, well pads, boat traffic, ice-breaking vessels and seismic blasting, not to mention the ever-present threat of oil spills."

Sen. Barbara Boxer, D-Calif., chair of the Senate Committee on Environment and Public Works, called the listing "long overdue" and said it "underscores how important it is for the Senate to pass national legislation to cut global warming pollution and avert the dangerous effects of climate change." Rep. Edward Markey, D-Mass., chairman of the Select Committee on Energy Independence and Global Warming, called the rule allowing oil development to continue as "a gift to Big Oil."

In Alaska, a state that had opposed the listing, Republican Rep. Don Young called the decision "an assault on sound science and common sense." He praised Kempthorne for allowing exploration and drilling to continue but feared that the listing would open the door to "frivolous lawsuits" by environmental groups.

Reed Hopper, an attorney with Pacific Legal Foundation, said the Sacramento nonprofit group intends to challenge the listing in court. Polar bear counts are the highest in history, and the "government has relied on speculative computer modeling to justify its listing decision," he said. "Never before has a thriving species been listed under the Endangered Species Act, or should it be."

In 30 years of satellite monitoring by NASA, the pack ice has declined in the Arctic summer months as temperatures in the atmosphere have risen. September shattered all records with the lowest levels of sea ice since satellite measurements began in 1979. The average September 2007 sea ice was 39 percent below the long-term average from 1979 to 2000.

But some of the subpopulations are on the rise, and some are on the decline.

Several agencies, including the U.S. Geological Survey, have dire predictions for the polar bear. One study said two-thirds of the bears would be gone in 45 years. Their sea ice habitat could be completely gone during the summertime by midcentury, if not sooner.

Polar bears split from an ancestor shared with the grizzly about 200,000 years ago. Over thousands of years, they've adapted to certain seasons and the search for food - walrus, seals and some whales.

Most polar bears don't come to the land at all if the sea ice is strong, even choosing to bear young on the ice. Unlike black and brown bears, polar bears don't hibernate through the winter but continue to feed on the sea ice.

Studies by U.S. and Canadian scientists show that as the area of open ocean grows between the floating ice and the land - and sea ice thins and becomes less stable - bears swim to land and stay for longer periods of time. On land, they have a harder time finding food and staying out of danger than they do on the sea ice, where they hunt for walrus, seals and other marine mammals.

The Canada bears already have been shown to give birth to less well-nourished and fewer offspring on land. U.S. scientists conducting overflights with the Minerals Management Service have reported seeing floating dead polar bears, which they attributed to drowning when the animals tried to swim the great distances to reach shore.

E-mail Jane Kay at jkay@sfchronicle.com.