The number of polar bears in eastern Alaska and western Canada has declined by 40%, 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 their 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 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. One of the marathon aquatic excursions in 2011 lasted nine days and 426 miles; the mother lost 22% of her body weight and her cub died.

Groups have been fighting for years in the courts to protect the bears and have won a threatened status for them under the federal Endangered Species Act. Groups have been seeking even more protection, and the latest data could be part of a push to get the bears placed on the endangered species list, Uhlemann said.

“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.

The bear population in the southern Beaufort Sea appears to have stabilized between 2008 and 2010, according to scientists. They said the stabilization appeared to be due to unusual oceanographic conditions, less competition or behavioral changes. Some polar bears stayed on land during the summer, feeding on subsistence-hunted bowhead whale carcasses.

“Given projections for continued climate warming,” these changes are “unlikely to counterbalance the extensive habitat degradation projected to occur over the long term,” the report found.

Conservationists have predicted that more than two-thirds of the world’s polar bear subpopulations could be extinct by 2050.

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