There's been a lot of bad news for polar bears recently. In 2007, for example, scientists reported that if global warming continues unabated, the population could drop two-thirds by 2050. But it's not a lost cause, according to researchers who have quantified for the first time how much polar bears would benefit from reductions of greenhouse gas emissions.
A recently released study concludes that significant cuts in greenhouse gas emissions over the next two decades could give polar bears a chance of surviving over the long term by preserving the Arctic sea ice on which they depend.
Polar bears need sea ice to survive. They spend the summer on the ice hunting for seals and other prey. As the summertime extent of sea ice has shrunk in the Arctic over the past several decades, some populations have declined.
Polar bears depend on sea ice to gain access to their primary food source, ringed and bearded seals. During seasons when they cannot reach the ice the bears go hungry and can lose two pounds of body weight a day. As the ice-free periods have increased, the animals have had to last longer without food.
In 2007, a massive reduction in sea ice raised concerns that the Arctic might have reached a tipping point, and sea ice would start to melt even more rapidly. Dwindling ice, which reflects the sun's heat, could mean that the dark waters would absorb so much heat that they would melt the remaining ice at an ever faster pace.
Some media reports jumped to the conclusion that polar bears are doomed, says wildlife biologist Steven Amstrup of Polar Bears International, a small research and education nonprofit in Bozeman, Montana. Luckily, the amount of summer sea ice bounced back the following year.
Amstrup and several climate scientists decided to investigate two questions. First, does a tipping point even exist for Arctic sea ice? Second, what would be the impact of decreasing greenhouse gas emissions on the sea ice? "Although it seems reasonable to expect that reducing emissions would benefit polar bears and their habitat, no studies had been done to test whether this was actually true," Amstrup said at a telephone press conference.
The team picked five scenarios for greenhouse gas emissions and plugged them into a climate model developed by the National Center for Atmospheric Research in Boulder, Colorado. The scenarios ranged from "business as usual" (in which emissions continue to increase and the concentration of carbon dioxide reaches 689 parts per million by the end of the century) to aggressive cuts that reduce CO2 levels to the 2000 concentration of 368 ppm.
The researchers found no evidence of a tipping point that would lead to sudden loss of sea ice. Instead, all of the model scenarios showed that sea ice would decline at a steady rate as global mean annual temperature rose. (The finding is consistent with other studies, which used different methods, that have come out since Amstrup's group began its study.)
"This is the first time that the issue of tipping points has been explicitly considered within the context of polar bears," says wildlife biologist Andrew Derocher of the University of Alberta in Canada, who was not involved in the research. "Clearly, the prognosis for the species is vastly better without a tipping point," he says, because a rapid loss of sea ice would be catastrophic for the species.
To figure out what reductions in greenhouse gas emissions would mean for the fate of the polar bears, the researchers compared the rates of sea ice loss under the various scenarios. As they report online in Nature, the business-as-usual scenario led to a 50% loss of optimal polar bear habitat (sea ice must be not too thin and not too thick) by mid century. Under the mitigation scenario, only 20% of the ice was lost. By combining this result with other data, such as measurements of how long the best sea ice lasts each year, Amstrup's team was able to estimate how much benefit would accrue to polar bears in four large regions.
Under the business-as-usual scenario, the chance of polar bears vanishing from these regions ranges from about 50% to roughly 80% by mid century, depending on the region. In contrast, if emissions are reduced, the chance of extinction is negligible for polar bears in two regions, and 25% to 50% for the other two.
Amstrup suggests that if policymakers can cut greenhouse gas emissions enough to stabilize atmospheric concentrations of carbon dioxide at or below 450 parts per million, enough Arctic ice is likely to remain during the late summer and early autumn to allow polar bears to survive. Current carbon dioxide concentrations are now at roughly 390 parts per million.
"Conserving polar bears appears to be largely a matter of minimizing temperature rise," Amstrup says. And if management of the species is also optimized—by minimizing hunting and disruptions from oil and gas extraction—polar bear populations are likely to increase in northern Canada and Greenland.
Amstrup said he wanted the public to know that the 2007 Geological Survey projections were based on a "business as usual" scenario that did not incorporate steep emissions cuts.
"If people and leaders think there's nothing to do, they will do nothing," said Amstrup, who now serves as a senior scientist with the Montana-based conservation group Polar Bears International. "We have now shown there is something that can be done to save polar bears. This problem is not irreversible. "
That's a much brighter picture for polar bears, if emissions can be brought under control. "One of the reasons why I believe this paper is so important is that it corrects the notion that caught hold in the media and some parts of the public that there was nothing that could be done to save polar bears from extinction," says Deroche.
The findings come just a week before the Obama administration faces a court-imposed deadline on whether to upgrade polar bears' status under the Endangered Species Act from "threatened" to "endangered." The Interior Department formally added polar bears to the endangered species list in May 2008 on the grounds that warming temperatures were eroding the sea ice the bears need to survive, but department officials under both President Obama and George W. Bush have refrained from using the listing as justification for limiting greenhouse gases linked to climate change.
But when the Interior Department listed the polar bear as threatened it also adopted what is known as the "4D rule," which allows it to disregard how activities outside a species' immediate range could affect its survival. The federal government is not able to invoke this rule for endangered species, which is why a coalition of environmental groups are seeking to upgrade the bears' status to endangered in federal court.
"Global warming is not just a future threat for the polar bear or for the rest of us. It's here now," said Kassie Siegel, who directs the Climate Law Institute at the Center for Biological Diversity and whose group is the lead plaintiff in the suit over the bears' status. "The Obama government needs to acknowledge the reality that global warming has arrived and grant the polar bear the 'endangered' status it desperately needs."
Science Now, "How to Save Polar Bears", by Erik Stoksted, accessed December 18, 2010
Washington Post, "Curbing carbon emissions can save polar bears, new study says", accessed December 18, 2010
Nature, "Greenhouse Gas Mitigation Can reduce Sea Ice Loss and increase polar bear persistence", accessed December 18, 2010
The Daily Mail, "Polar bears CAN survive global warming 'but only if temperature rise stays below 1.25C'", accessed December 18, 2010
Christian Science Monitor, "Polar bear 'doomed'? Only if greenhouse-gas emissions aren't cut", accessed December 18, 2010
Christian Science Monitor, "Where polar bears might go if climate change doesn't slow", accessed December 18, 2010