The breakup of Arctic sea ice because of global warming is depriving polar bears of weeks of valuable hunting time they need to survive the long ice-free months without food, according to a new study in the scientific journal Arctic.
As a result, hungry polar bears are being driven to shore weeks earlier than usual, the report says.
Once on land, they are interacting with people more often, sometimes giving the false impression that their populations are increasing.
In fact, the report says, continued warming in the Arctic will cause the bears to become "increasingly food-stressed," which will most likely cause a significant decline in numbers.
Researchers from NASA and the Canadian Wildlife Service studied satellite data to measure the extent of sea ice in several regions of the Arctic since 1978.
Looking at the Western Hudson Bay region, for example, scientists found that the warmer temperatures were causing the sea ice to break up seven days to eight days earlier per decade over the last 30 years.
That means polar bears in that region have had their prime hunting period cut by as much as 24 days.
"There is a lot of variability, but in general the trend is for that date to come earlier and earlier in the spring," said Claire Parkinson, a climatologist at NASA's Goddard Space Flight Center and co-author of the study.
The earlier breakup means significantly less time for polar bears to use the ice to hunt seals and build needed energy reserves, according to co-author Ian Stirling, a polar bear expert with the Canadian Wildlife Service.
"Seventy to 80 percent of the energy they will use for the entire year is taken in between April and the ice breakup in mid-July," Stirling said.
"If they feed for a shorter period of time, they're going to accumulate less fat. At the same time, they're going to be on land and fasting for longer periods of time. So there's a double whammy."
Scientists say the irony is that among some native Inuit hunters in the region, the increased number of bears seen on land has been "interpreted as evidence that the populations were growing," the report says.
In several cases, this even led to polar bear hunting quotas being raised, allowing more of the bears to be killed.
Researchers say that the population is being inaccurately -- and unintentionally -- inflated as more bears come on shore, where they are spotted by people, and that it is very unlikely that populations are increasing.
Population data collected in the Western Hudson Bay, for example, shows that polar bear numbers have actually declined from about 1,200 bears in 1989 to about 950 in 2004 -- a 22-percent drop.
As the sea ice disappears, experts say there are few food options on land.
Bears are left to scavenge for dead whales or seals that wash up on shore, for example.
"There are a few small sources of nutrition, but they're not enough to sustain the population as a whole," Stirling said.
The lack of food is apparently forcing some polar bears to resort to cannibalism.
In June, Canadian and U.S. scientists reported three separate incidents in which polar bears had attacked and eaten other polar bears.
"It is very clear that the length of the sea-ice season is getting shorter," Parkinson said.
The primary reason, according to the report, is global warming.
The earlier breakup of the ice is "significantly correlated with, and most likely caused by, climate warming," the report says.
Earth has warmed by approximately 1 degree Fahrenheit in the last 100 years, which virtually all climate scientists agree is a result of humans burning fossil fuels, releasing carbon dioxide and other gasses that exacerbate the heat-trapping greenhouse effect.
One degree is only a global average, scientists point out, and places like the Arctic have warmed much more than that.
In the Davis Strait, for example, Parkinson and NASA researcher Josefino Comiso found that temperatures had increased an average of more than 4.5 degrees Fahrenheit from 1981 to 2003 -- one of the largest increases ever recorded in the Arctic.
Given the current and future impacts of global warming, the overall trend is not encouraging for polar bears -- though with approximately 20,000 polar bears to 25,000 polar bears in the wild, Stirling says, they are not endangered yet.
"It's hard to be optimistic when you look at the big picture," Stirling said. "Polar bears are clearly not endangered right now. But if the climate continues to warm -- as it's predicted to do -- then the outlook isn't that great. We're going to continue to lose more ice, and that is not going to be good for polar bears."