THE number of polar bears in eastern Alaska and western Canada has declined by 40 percent, according to a study that raises questions about the impact of global warming on the creature.

    The study, published in Ecological Applications, was carried out by scientists from the U.S. Geological Survey and Environment Canada who 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. Just two of 80 cubs the team tracked between 2003 and 2007 survived. Normally half live.

    “Climate change is not some future threat,” said Sarah Uhlemann, senior lawyer for the Center for Biological Diversity. “Global warming is 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 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 680 km; the mother lost 22 percent 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 U.S. Endangered Species Act. (SD-Agencies)