Polar bears – the white-furred poster children for global warming's impact on the Arctic – could step back from the brink of extinction over the course of this century if concerted efforts are made to reduce greenhouse-gas emissions and if rigorous wildlife management efforts are undertaken to keep bears and people safely apart.
That's the simple message behind a modeling study unveiled Wednesday by a team of federal and university scientists specializing in Arctic climate and polar-bear ecology.
In some respects, the conclusion isn't surprising, say some researchers who were not involved in the study. But, they add, buried in the study's details is a change in expectations for how the Arctic Ocean's sea ice is likely to behave as the climate warms. This change, if born out in the real world, could give Ursus maritimus more ice rafts to clamber over than might otherwise be the case.
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The study is being released even as environmentalists and scientists are pressing the federal government to list the bears as "endangered." In November, a US district court ruled that the Interior Department had to take a second look at its 2008 decision to list the bears as "threatened," rather than the more-dire "endangered." The department must turn its latest evaluation over to the court by Dec. 23.
Perched at the top of the Arctic food chain, polar bears are seen as key indicators of the overall health of the marine environment above the Arctic Circle.
Three years ago, summer sea ice that the bears rely on as platforms for hunting seals, as well as for breeding, receded the furthest since satellites first began monitoring sea-ice coverage in the Arctic Ocean in 1979.
Many specialists attributed the decline to the effects of global warming, as well as to changes in seasonal wind patters that drove broken sea ice out of the Arctic basin and into the North Atlantic, researchers noted.
The dramatic retreat of Arctic sea ice in the summer of 2007 prompted some researchers to warn that the system may have reached a tipping point that would lead to the disappearance of summer sea ice within the next several decades, regardless of actions humans took to reduce greenhouse-gas emissions.
That concern, in turn, helped elevate the polar bear to climate-icon status and reportedly fed into then-President George W. Bush's decision in 2008 to list the bear as "threatened" under the Endangered Species Act.
No 'tipping point'
The new study, however, finds no "tipping point" now or in this century in Arctic sea-ice decline, but rather a relatively steady fall-off in ice extent as average temperatures increase.
The study's authors say that this gives cause for hope that once greenhouse-gas concentrations stabilize, the average temperatures in the Arctic would respond in ways that would keep the sea ice from declining further, although natural shifts in seasonal weather patterns would continue to play its role in varying sea-ice extent from one year to the next.
"Polar bears do not appear to be doomed by what we have contributed already" to atmospheric concentrations of greenhouse gases, says Cecilia Bitz, an atmospheric scientist at the University of Washington in Seattle and a member of the team reporting its results in Thursday's issue of the journal Nature.
But, she cautions, "they [the bears] will be" if greenhouse-gas emissions follow – or exceed, as they have this decade – "business as usual" growth trajectories.
Several members of the team, led by US Geological Survey scientist Steven Amstrup and former University of Wisconsin climate scientist Eric DeWeaver, took part in an influential set of reports on Arctic climate change and its potential effect on polar bears in 2008.
If greenhouse-gas emissions from human industrial activities continued to follow the trajectory that existed at the time, the scientists reported then, the loss of summer sea ice could lead polar bears to vanish from two-thirds of the habitats they occupy by mid-century. By the end of the century the bears could be all but extinct worldwide, with some of them breeding with other types of bears to form hybrids – a phenomenon biologists are starting to see today.
At the time, however, the study didn't consider the possible effects from efforts to curb greenhouse gases, says Dr. Amstrup.
While "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," he says.
The current study aims to help fill that gap.
The team selected a climate model run out of the National Center for Atmospheric Research in Boulder, Colo., as their test bed. The model simulates an atmosphere more sensitive to changes in greenhouse-gas levels than many other models do, so it would tend to register the largest amount of warming for a given increase in these heat-trapping gases. And, the team says, the model does the best job of capturing changes in Arctic summer sea-ice extent as temperatures change, in ways that mimic well the changes scientists have observed in the real world.
Essentially, if a tipping point shows up in any model, it would be this one, the team posits. But none appeared under a range of emissions scenarios.
Under business-as-usual emissions, the decline in sea ice instead followed a more or less linear plunge as temperatures rise with rising greenhouse-gas concentrations. The model projects a minimum summer ice extent by around 2050 of about 100,000 square miles – bad news for the bears. This past year, by contrast, the sea-ice minimum covered roughly 2 million square miles.
Model predicts rebound
If those greenhouse-gas concentrations follow business-as-usual growth through 2020, then stabilize, sea-ice minimums would continue to shrink over the next decade or two, but rebound to an average extent of about 1 million square miles starting around 2050, the model indicated.
The team then linked their results to a model of polar bear habitats and found that in the absence of a tipping point, it would be possible to stabilize polar-bear populations in many – though not all – of their known ranges if the increase in global average temperatures increased by no more than 1.25 degrees Celsius (2.25 degrees Fahrenheit) above the 1980 to 1999 average.
The team cautions that in the real world, tipping points in fact may exist, if not from a climate standpoint, then from a purely ecological standpoint. Climate as well as hunting pressures could combine in ways that leave a polar-bear population with little in the way of genetic diversity to sustain itself. And their study, the team acknowledges, does not sa
The National Snow and Ice Data Center's Dr. Meier adds that some models do, in fact, show tipping points for Arctic sea-ice – a difference that might be traced to the way different models handle complex interactions between the sea, ice, and atmospheres.
But given all the uncertainties associated with the study, including how humans respond, the work nevertheless indicates that "in general, it's not too late to act," Meier says.