SCIENCE -- December 16, 2010 at 10:17 AM EDT
Polar Bears Have a Chance -- If Drastic Steps are Taken
A study by a team of high-profile agencies says that polar bears have a chance -- if (and that's a big "if") we sharply cut our greenhouse gas emissions.
Polar bears depend on sea ice platforms to hunt for seals and other prey. When sea ice melts early in the season, food-deprived bears are forced to come ashore sooner and live longer off the fat stored in their body. That fat is like the amount of gas in a car; it only lasts so long, says Steven Amstrup, senior scientist at Polar Bears International and lead author of the study, which appeared in the journal, Nature on Thursday. And this year, scientists have observed shorter, skinnier polar bears and starving bear cubs in Canada's Hudson Bay.
The report references a 2007 study by a United States Geological Survey team, which found that two-thirds of the world's polar bears would disappear by mid-century if greenhouse gas emissions continue at present levels.
But there's another possible storyline, according to this report, whose authors include scientists from USGS, U.S. Forest Service, the National Science Foundation and the National Center for Atmospheric Research. "There is, in fact, something we can do to preserve a future for polar bears," Amstrup says. "Our 2007 reports suggested that polar bears could be extinct by the end of the century, and many people glommed onto that finding."
This study relied on climate modeling which showed that sea ice melting is not necessarily irreversible, as some feared. Gloomier predictions came in part from a theory that there was a so-called "tipping point", meaning once a temperature threshold was crossed, ice would abruptly collapse and disappear.
But climate models show a consistent rate of ice melt, indicating there is no such tipping point, says Eric DeWeaver, a climate scientist and co-author of the study. As you increase the temperature by a degree, a degree and a half, the whole time you're seeing a loss of sea ice proportional to the temperature increase."
The modeling was done at the National Center for Atmospheric Research, in Boulder, Color. Sea ice is not easy to model. You have to take several moving pieces into account, DeWeaver said. Freezing, thawing, cloud cover and ocean properties. "It's difficult to model how sea ice moves around, what happens when it collides and produces ridges."
An estimated 20,000 to 25,000 polar bears are alive today. They are divided into 19 different sub-populations, and some are faring worse than others. Bears in the Western Hudson Bay, for example, have seen a 22% decline in the last 15 years. Usually bears are off the sea ice for 120 days, but that period stretched as long as 163 days for some bears this year, said Doug Inkley of the National Wildlife Federation.
"That's 163 days without feeding," he said. "We have already been witnessing the decline of the polar bears in their southern most regions, because of climate change. And that's only a glimmer of the future, the tip of the iceberg of what's in store for polar bears."
The scenario in this week's study harbors hope for a collective effort by humanity. To achieve it would require a 70 percent reduction from 2000 levels by the end of the century, and decarbonizing world electricity production by mid-century. But is that realistic?
"We're currently in the midst of a debate on whether to take action to deal with climate change, and it's a fairly strident and contentious debate," DeWeaver said. "But in order for decision makers to actually do something about climate change, they have to be convinced that there's something they can do about it."
While Inkley agrees with the report's findings, he says we still have far to go. "Climate models make it clear that if we continue to pump CO2 into the atmosphere unabated, we have serious problems, not just for polar bears, but for millions of other species," he said. "There is hope, if we take actions. But right now, we don't have that plan of action."