"We looked at the sea ice conditions in five Canadian Arctic polar bear regions, and in each of those we found that the sea ice breakup is occurring, overall, earlier and earlier in the year," Parkinson said.
The climatologist says the summer breakup of Arctic sea ice has crept an average of 0.75 days earlier a year, or 7 to 8 days a decade.
"I would say that the strongest likelihood for why the Arctic ice is decreasing is because the Arctic is getting warmer," Parkinson said.
"I'll leave it to other people to say whether or not the Arctic getting warmer is a global [phenomenon]."
Such changes directly impact the region's polar bears, since the Arctic predators hunt seals on the winter sea ice and must fast on land during the summer melt.
"In western Hudson Bay the ice is breaking up there now about three weeks earlier than it did only 30 years ago," Stirling, the polar bear biologist, said.
Most polar bears can handle a single season of early ice breakup, the authors write. But they add that "as the number of consecutive short ice seasons increases, the cumulative stress on the polar bears is bound to increase as well."
Ringed seals, the polar bears' main food source, also appear to have experienced either lower birth rates or lower cub-survival rates in the 1990s, perhaps as a result of poor ice conditions and other environmental changes, Stirling says.
The average weight of adult female polar bears in western Hudson Bay has dropped by nearly 150 pounds (65 kilograms) in just 25 years, from 650 pounds (295 kilograms) in 1980 to 507 pounds (230 kilograms) in 2004, the scientists say.
The UN-sponsored Intergovernmental Panel on Climate Change predicts that global warming will cause Earth's average surface temperatures to warm between 2.5 to 10.4 degrees Fahrenheit (1.4 to 5.8 degrees Celsius) within the next century.
If that forecast proves true, the five Canadian Arctic polar bear populations will experience significant declines, Stirling says.
He adds that within 20 to 30 years, adult females in western Hudson Bay may become too skinny to produce cubs.
"If that happens, then it's going to be hard on polar bears [everywhere]," he said.
"Obviously some bears will probably survive for a long time. But we won't have the kinds of numbers we have today."
In light of the study findings, the biologist has called on regional governments to use a precautionary approach to managing polar bears, reduce hunting quotas as needed, and include climate trends in developing conservation plans for the species.
He adds that countries and consumers must also reduce greenhouse gas emissions that cause global warming.
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