WASHINGTON – APRIL 02: U.S. Senate Environment and Public Works Committee Chairman Barbara Boxer (D-CA) questions witnesses during a hearing about the possible listing of the polar bear under the Endangered Species Act April 2, 2008 in Washington, DC. U.S. Interior Secretary Dirk Kempthorne was invited to testify but declined until he said he could provide more information regarding the listing of the polar bear as endangered. According to conservation groups, polar bears are threatened because their habitat, sea ice, is shrinking from global warming. (Photo by Chip Somodevilla/Getty Images)