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."