WASHINGTON — Interior Secretary Dirk Kempthorne listed polar bears as "threatened" under the Endangered Species Act on Wednesday, saying the loss of Arctic sea ice in a warming climate could drive them to the brink of extinction in less than four decades.

Although the Bush administration handed environmentalists a victory they had sought for more than three years, Kempthorne said he would ensure his decision did not "open the door" for activists to force the adoption of limits on greenhouse gas emissions linked to global warming.

The act "is not the right tool to set U.S. climate-change policy," he said in a news conference. "This has been a difficult decision. But in light of the scientific record and the restraints of the inflexible law that guides me, I believe it was the only decision I could make."

The decision to list polar bears, which have become the iconic symbol of global warming's impact, highlights how an administration opposed to mandatory cuts in emissions has begun to acknowledge the growing evidence of their effects. Kempthorne pointed to satellite images of shrinking Arctic sea ice that has outpaced scientists' most dire projections. Polar bears use sea ice as a platform to hunt ringed seals and other prey.

"The fact is that sea ice is receding in the Arctic," he said. "As you can see, when we have looked at what is actually happening in the Arctic, we have found considerably less sea ice than the models are projecting. Because polar bears are vulnerable to this loss of habitat, they are, in my judgment, likely to become endangered in the foreseeable future — in this case, 45 years."

Under the law, the federal government is now required to draft a recovery plan for the species, which entails assessing the population and its habitat. The ruling also compels federal agencies to consult with the Interior Department when considering decisions that could further imperil the polar bears.

Administration officials, however, sought to minimize the policy consequences of the decision — the first time the Endangered Species Act has been invoked to protect an animal principally threatened by global warming. Kempthorne made clear that the decision would not justify regulating emissions from power plants, vehicles or other human activities.

Dale Hall, who directs the Fish and Wildlife Service, which decides how to protect listed species, said such regulations would be justified only if the administration could prove a direct connection between the emissions and the polar bears' predicament.

"We have to be able to connect the dots," Hall said. "We don't have the science today to be able to do that."

But environmentalists, who by and large praised the decision, said the administration would have no choice but to curb greenhouse gases.

"The law says what it says, not what the administration wishes it says," said Kassie Siegel, climate program director at the Arizona-based Center for Biological Diversity. "This is great news for polar bears. ... It's also a watershed moment, the strongest statement we've had to date from this administration about global warming."

Conservative and business groups, however, hailed Kempthorne's intention to limit the regulatory fallout.

U.S. Sen. James Inhofe, R-Okla., a leading congressional skeptic on climate change, said that "the decision to list the polar bear as 'threatened' appears to be based more on politics than science. With the number of polar bears substantially up over the past 40 years, the decision announced today appears to be based entirely on unproven computer models."

Part of the uncertainty surrounding the polar bears' fate stems from the fact that there are 19 sub-populations in five different countries — Norway, Russia, Canada, Denmark and the United States — and these groups are faring differently.

Researchers estimate that the world population of the bears ranges from 20,000 and 25,000, although the exact figure remains unknown. In Canada's western Hudson Bay, their numbers are declining, but in Norway they are on the rise.

Still, climate scientists are increasingly concerned that melting sea ice could lead to the polar bears' demise within decades. Northern latitudes are warming twice as rapidly as the rest of the world, according to a 2004 assessment, and some computer projections forecast that ocean temperatures in the Arctic may rise 13 degrees Fahrenheit by the end of the century.

In September, reports by the U.S. Geological Survey suggested that polar bears living in two of the four regions under analysis would be extinct by 2050 and by 2075 in a third.