U.S. environmental groups sue to protect polar bears

Three environmental groups sued the federal government Thursday, seeking to protect polar bears from extinction because of disappearing Arctic sea ice.

The lawsuit, filed in federal court in San Francisco, demands that the government take action on a petition environmentalists filed earlier to have polar bears listed as "threatened" under the Endangered Species Act.

Once a species is listed as threatened, the government is barred from doing anything to jeopardize the animal's existence or its habitat. In the case of the , the environmentalists hope to force the government to curb U.S. emissions of greenhouse gases such as carbon dioxide.