Greenhouse gas emissions remain the "primary threat" to polar bears, according to a study released Tuesday by the U.S. Geological Survey. Polar bear populations will decline even if emissions are stabilized by the end of the century, the study said. Polar bears have been categorized as a "globally threatened species" under the U.S. Endangered Species Act since 2008. The two main threats to polar bears are melting sea ice and disappearing prey. The study concluded that polar bears would suffer whether carbon emissions grew at their current pace or peaked in 2040 and then declined. The only optimistic scenario would involve "immediate and aggressive" cuts in greenhouse gas emissions, researchers said.