The number of polar bears in eastern Alaska and western Canada has declined by 40 percent, according to a scientific study that raises more questions about the impact of global warming on the creature that has become the symbol of some of its worst effects.

The study, published in the current issue of Ecological Applications, was carried out by scientists from several groups, including the U.S. Geological Survey and Environment Canada, that tagged and released polar bears in the southern Beaufort Sea from 2001 to 2010. The bear population in the area shrank to about 900 in 2010, down from about 1,600 in 2004, according to the findings.

Perhaps even more worrisome, just two of 80 polar bear cubs that the international team tracked between 2003 and 2007 survived, according to the study. Normally about half of the cubs live.

“Climate change is not some future threat,” Sarah Uhlemann, senior attorney for the Center for Biological Diversity, one of the groups that has been fighting to save polar bears, told the Los Angeles Times. “Global warming is happening now and killing polar bears now.”

Polar bears have long been followed as scientists watch for early signs of global warming. The bears are especially at risk as Arctic ice melts.

They spend much of their waking lives on Arctic sea ice floes, eating seals that are also dependent on sea ice. As the ice has dramatically shrunk, the bears have been forced into long, painful swims in search of new ice. “Global warming has put Alaska’s polar bears in a deadly downward spiral,” Uhlemann said. “If we don’t act now, we will lose polar bears in Alaska.”

Polar bears suffered particularly low survival rates between 2004 and 2006, when “unfavorable ice conditions” limited their access to ice seals, their favored prey, according to the study.