FILE - In this undated file photo, a polar bear is shown in the Arctic National Wildlife Refuge in Alaska.  If the world dramatically changes its steadily increasing emissions of greenhouse gases, global warming can be slowed enough to prevent a total loss of critical summer sea ice for the polar bears, according to a new study in the journal Nature released Wednesday, Dec. 15, 2010. (AP Photo/Subhankar Banerjee, File)

Photo by: SUBHANKAR BANERJEE

FILE - In this undated file photo, a polar bear is shown in the Arctic National Wildlife Refuge in Alaska. If the world dramatically changes its steadily increasing emissions of greenhouse gases, global warming can be slowed enough to prevent a total loss of critical summer sea ice for the polar bears, according to a new study in the journal Nature released Wednesday, Dec. 15, 2010. (AP Photo/Subhankar Banerjee, File)