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Polar bear 'doomed'? Only if greenhouse-gas emissions aren't cut.

Greenhouse-gas emissions worldwide must be cut to avoid a disappearance of summer sea ice in the Arctic, which is crucial to the polar bear.

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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.

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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.

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