Hope for polar bears

Polar bears could be saved from extinction if greenhouse gas emissions are significantly reduced in the next decade or two, a study released Wednesday suggests.

The research, published in the Dec. 16 issue of Nature, found that if humans reduce greenhouse gas emissions considerably over the next 20 years, enough Arctic ice is likely to remain intact during late summer and early autumn for polar bears to survive.

It also concluded that substantial retention of the remaining ice through this century, as well as partial recovery of the ice that disappeared during the rapid ice loss, would be achieved.

Research from 2007 estimated that only about one-third of the world's 22,000 polar bears would remain by mid-century if the dramatic Arctic ice decline continued, and that eventually they could disappear completely. This led to the 2008 listing of polar bears as a threatened species.

The 2007 report coincided with the amount of summer sea-ice plunging to new record lows, prompting concern that there was a temperature threshold that could throw the ice into an irreversible decline.

This latest study, however, conducted by some of the same researchers as the 2007 reports, suggests that is not the case.

Tipping point

"If best possible wildlife management techniques were applied along with significant mitigation of emissions, polar bears might persist throughout their current range," Steven Amstrup, an emeritus researcher with the U.S. Geological Survey and the senior scientist with the Montana-based conservation organization Polar Bears International, said during a conference call.

"We did not see evidence of irreversible thresholds in our model outcomes. Sea-ice habitat features smoothly declined as temperatures rose. It does not appear that sea-ice tipping points would prevent mitigation from improving upon our 2007 projections. Therefore, conserving polar bears appears largely a matter of minimizing temperature rise."

Amstrup said the 2007 projections were based solely on the "business-as-usual greenhouse gas scenario" and did not consider the possibility of greenhouse gas mitigation.

The new research — funded primarily by the U.S. Geological Survey as well as the Forest Service, the U.S. Department of Energy and the National Science Foundation — used modelling by Cecilia Bitz, an associate professor of atmospheric sciences at University of Washington. The models found there is no tipping point that would result in the irreversible loss of summer sea ice when warming driven by greenhouse gases rose above a certain threshold.

The scientists were looking for a tipping point beyond which seasonal Arctic ice could not recover. They used a general circulation model with a sea-ice component particularly sensitive to rising temperatures, with major parts of it designed by Bitz.

Bitz said the researchers did not need to compare with other models, since they were using one that is extremely sensitive in the Arctic. This allowed them to "make a more conservative statement about the potential to slow the loss of sea ice."

Throughout the current study, the potential sea-ice outlook created by the general circulation model, as well as polar bear life history, were placed into a network model, to examine the relationship between polar bears and their environment. The results showed that retaining more of the sea-ice habitat by reducing greenhouse emissions would greatly increase the survival rate of the polar bears. 

Amstrup divided the Arctic into four separate ecoregions according to the nature of ice typically found there, and the 2007 study showed a very high likelihood that polar bears would become extinct in two of those regions given current trends in greenhouse gas emissions.

Polar bears depend on sea-ice to catch seals and other marine mammals. During seasons when they can't reach sea-ice, the bears mostly go without food, which is causing concern among researchers who have found that the periods when they don't have ice access have increased. These periods are expected to widen if the current levels of greenhouse gas emissions remain unchecked.

"The fact that there is no tipping point that we can see in the Arctic sea-ice, and there is no reason why mitigation efforts would not help polar bears to continue to exist, I think is one of the benefits that decision-makers may wish to consider as they think about possible strategies for mitigation," said Eric DeWeaver of the National Science Foundation.