Polar bears in the Arctic suffered sharp declines in the first decade of this century, losing about 40 percent of their population, according to US and Canadian scientists.

The worst years for polar bears in the southern Beaufort Sea were 2004 to 2007, when scientists believe thinning ice made it harder for the bears to find seals for food.

"Of the 80 cubs observed in Alaska from 2004 to 2007, only two are known to have survived," said Jeff Bromaghin, US Geological Survey research statistician and lead author of the study in the journal Ecological Applications.

Survival improved after 2007 and the population had stabilized at 900 bears by the year 2010, said the study.

However, when looking just at juveniles, the research showed their numbers declined throughout the 10-year-period, suggesting that "conditions remained unfavorable for young bears newly separated from their mothers."

Polar bears are considered a globally threatened species, due to concerns about how ice loss affects their ability to survive.

Co-authors on the study came from Environment Canada, University of Alberta, the US Fish and Wildlife Service, Polar Bears International and Western Ecosystems Technology.