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By Keith Wagstaff

As the Arctic ice continues to melt, greenhouse gas emissions remain the "primary threat" to polar bears, according to a new government report.

In a study released Tuesday by the U.S. Geological Survey, polar bear populations will decline even if greenhouse gas emissions are stabilized by the end of the century.

Melting sea ice and disappearing prey are the two main threats to polar bears, which have been labeled a "globally threatened species" under the U.S. Endangered Species Act since 2008.

The study looked at two scenarios: one where carbon emissions continue to grow at their current pace, and one where they peak in 2040 and then decline through the end of the century. In both scenarios, polar bears suffer.

The problem is that polar bears live on the sea ice, where they hunt for ringed and bearded seals. When the ice melts, they are forced onto land, where they have trouble hunting and have trouble reproducing.

USGS research ecologist Mike Runge noted that there would "be a lag, likely on the order of several decades," between any action taken on reducing carbon emissions and the stabilization of sea ice levels. The only optimistic scenario the researchers found involved "immediate and aggressive" cuts in greenhouse gas emissions.