Interior Secretary Dirk Kempthorne cited dramatic declines in sea ice over the last three decades and projections of continued losses, meaning, he said, that the polar bear is a species likely to be in danger of extinction in the near future.
But Kempthorne said it would be "wholly inappropriate" to use the protection of the bear to reduce greenhouse gases, or to broadly address climate change.
The Endangered Species Act "is not the right tool to set U.S. climate policy," said Kempthorne, reflecting a view recently expressed by President Bush.
The department outlined a set of administrative actions and limits to how it planned to protect the bear with its new status so that it would not have wide-ranging adverse impact on economic activities from building power plants to oil and gas exploration.
"This listing will not stop global climate change or prevent any sea ice from melting," said Kempthorne. He said he had consulted with the White House on the decision, but "at no time was there ever a suggestion that this was not my decision."
Kempthorne, at a news conference, was armed with slides and charts showing the dramatic decline in sea ice over the last 30 years and projections that the melting of ice -- a key habitat for the bear -- would continue and may even quicken.
He cited conclusions by department scientists that sea ice loss will likely result in two-thirds of the polar bears disappearing by mid-century. The bear population across the Arctic from Alaska to Greenland doubled from about 12,000 to 25,000 since 1960, but he noted that scientists now predict a significant population decline.
It is the first time that the Endangered Species Act has been used to protect a species threatened by the impacts of global warming.
Environmentalists and a number of Democrats in Congress welcomed the decision to bring the bear under the protective umbrella of the Endangered Species Act, but expressed concern that its protection would be blunted by the provisions and exceptions outlined by Kempthorne.
The Bush administration "is forcing the polar bear to sink or swim," said Rep. Edward Markey, D-Mass., chairman of a House committee on global warming.
Sen. John Kerry, D-Mass., called it "a lifeline for our last remaining polar bears" but said the bear's survival won't be assured without limits on oil development in the same Arctic waters where the bears are found.
Despite the new listing, the announcement underscores the need to approve climate legislation that would limit the release of greenhouse gases and avert the future effects on climate change, said Sen. Barbara Boxer, D-Calif., chairwoman of the Environment Committee.
Environmentalists questioned how the bear would be adequately protected given the limits outlined by Kempthorne.
"It remains to be seen how much this belated listing decision will improve protection for polar bears and their rapidly shrinking habitat," said Clayton Jernigan, an attorney for Earthjustice. He said the Interior's announcement made clear steps would be taken to avoid interfering with offshore oil development in waters where bears and oil drilling are expected to coexist.
Kempthorne proposed 15 months ago to investigate whether the polar bear should be declared threatened under the Endangered Species Act.
That triggered a year of studies into the threats facing the bear and its survival prospects at a time when scientists predict a continuing warming and loss of Arctic sea ice. The Arctic sea ice serves as a primary habitat for the bear and is critical to its survival, scientists say.
"The science is absolutely clear that polar bear needs protection under the Endangered Species Act," said Andrew Wetzler, director of the endangered species program at the Natural Resources Defense Council.
A decision had been expected early this year, but the Interior Department said it needed more time to work out many of the details, prompting criticism from members of Congress and environmentalists. Environmentalists filed a lawsuit aimed at forcing a decision and a federal court on April 29 set a May 15 deadline for a decision.