Global warming has driven a 40 percent decline in the number of polar bears in eastern Alaska and western Canada, a new study finds.

The Southern Beaufort Sea polar bear population has dropped to just 900 bears from the last estimate in 2006 that documented more than 1,500 bears.

These polar bears live in eastern Alaska and western Canada and are one of only two polar bear populations in the United States.

“Global warming has put Alaska’s polar bears in a deadly downward spiral,” said Sarah Uhlemann, the Center for Biological Diversity’s international program director. “It’s happening now, it’s killing polar bears now, and if we don’t act now, we will lose polar bears in Alaska.”

The study was published Monday in the journal “Ecological Applications.”

Between 2004 and 2006, “unfavorable ice conditions” limited the polar bears’ access to ice seals, their favored prey. Cubs were hit especially hard; only two of the 80 cubs observed in Alaska between 2003 and 2007 are known to have survived.

Polar bears depend on Arctic sea ice for their survival, but climate change is rapidly melting the Arctic sea ice. In September 2012 Arctic summer sea ice reached a new record low minimum extent, losing an area about the size of Texas since the previous record low in 2007.

The Southern Beaufort Sea population appears to have stabilized between 2008 and 2010, possibly due to unusual oceanographic conditions, less competition, or because some polar bears stayed on land during the summer, feeding on subsistence-hunted bowhead whale carcasses, the study’s authors say.

But they warn that “given projections for continued climate warming” these changes are “unlikely to counterbalance the extensive habitat degradation projected to occur over the long term.”

In 2008 the United States protected polar bears as “threatened” under the Endangered Species Act, predicting that more than two-thirds of the world’s polar bears would be extinct by 2050. Conservationists say the bears deserve the stronger “endangered” species designation.

Nevertheless, polar bear hunting remains legal.

While the United States permits only native subsistence hunting, Canada allows both sport hunting and hunting for the rug trade. Canadian polar bear hunting is on the rise, as fur prices have tripled since 2007.

The Center for Biological Diversity is working to strengthen polar bear protections in the United States and worldwide. Nations will be considering a total ban on commercial trade in polar bears in 2016.

“We’re very worried that eastern Alaska’s polar bears may be among the first to go,” said Uhlemann. “The United States and the world have to get serious about reducing greenhouse gases if we want polar bears to survive.”