Polar bears may be threatened, but they aren't yet doomed.
While
Arctic sea ice will continue to retreat under the glare of rising global temperatures, the ice is unlikely to collapse in spectacular fashion, causing hope that, with aggressive greenhouse gas emissions cuts and wildlife management, polar bears may retain viable habitat into the next century, a team of scientists reports in a paper to be published tomorrow in Nature.
Several years ago, government scientists projected that two-thirds of the world's polar bears would go extinct by midcentury under current emissions scenarios, a finding that ultimately prompted the
George W. Bush administration to list the bear as threatened. Those estimates, though uncertain in their specifics, remain unchanged by the current work, said Steven Amstrup, senior scientist at Polar Bears International and former biologist for the U.S. Geological Survey, who co-authored both studies.
However, what seems increasingly unlikely is that the retreat of summer sea ice -- the base for bears' pursuit of seals, their highway system and their mating grounds -- could cascade out of control. Rather, its decline is entirely contingent on controlling human emissions of greenhouse gases, Amstrup said.
"Conserving polar bears largely seems to be a matter of containing temperature rise," he said.
The notion that no "tipping point" exists for Arctic ice decline has spread in
climate science for several years, supported by deeper examination of the North's physics. Initially, the media exaggerated fears that the loss of ice, which naturally reflects light, would expose more heat-absorbing water to the sun, causing runaway decline. However, scientists now widely believe this feedback is balanced by a host of other phenomena, like increased flows of hot air from the tropics, improved ice formation efficiency under thinning conditions and the region's general cloudiness.
The sea ice episode should be a cautionary tale, wrote Dirk Notz, a climatologist at the Max Planck Institute for Meteorology, last year. Melting thresholds likely exist for land-bound glaciers in Greenland and Antarctica, he said, and scientists should not risk their credibility by conflating those claims with Arctic sea ice. Most prominently, NASA climatologist James Hansen has included Arctic ice in his list of possible tipping points, along with melting permafrost, glacier melt and ecosystem collapse.
Despite the growing scientific awareness that ice loss has an inch-by-inch relationship to rising temperatures, though, the public has largely been left with the message that prospects were grim for polar bears, no matter what steps were taken to limit
global warming, Amstrup said. That message was hardly a call to action and, more importantly from a scientific view, lacked validity.
"If people and leaders feel there's nothing they can do, they will do nothing," he said.
'Messy literature'
The projections published by Amstrup should be taken with a grain of salt, independent scientists said. Most models incorporating sea ice fall short of predicting the actual loss seen in the Arctic over the past several decades, and systems like cloud cover are not well understood. Indeed, over the past few years a sometimes acrimonious debate has arisen as to whether existing ice models were unintentionally introducing errors to compensate for errors.
However, despite these gross differences, there's one thing nearly every model agrees on: that there is a gradual relationship between rising temperatures and ice loss, said Michael Winton, an Arctic modeler at the Geophysical Fluid Dynamics Lab at Princeton University.
Arctic sea ice is a "messy literature," Winton said. Substantial natural variability is needed to match even the most sensitive models, like the one used in the Nature study, to observed changes, he said. "The outstanding question is, 'Are the models sufficiently sensitive? Are we missing something?'"
Given these uncertainties, it has been perplexing to scientists that the public seized on "tipping point" scenarios for sea ice, the one area where there is large agreement. Those fears, which peaked in 2007, were likely exacerbated by the stark retreat in sea ice that year. The Arctic lost more than 1.6 million square kilometers of ice, an area larger than Alaska; by September, sea ice covered half the area it had during the early 1950s.
However, since that shocking decline, the ice has modestly expanded during the summer, perhaps the best evidence that Arctic ice won't drop off a cliff, said Eric DeWeaver, a co-author on the Nature paper and physical climatologist at the National Science Foundation.
The 2007 loss was "spectacular," he said, but "one would not expect to see it very often."
Scientists do expect that ice fluctuations will become increasingly steep and difficult to predict, largely thanks to the floes' declining girth. Simply put, the thinner ice is more susceptible to the weather.
Sweltering summers will cause large retreats in sea ice, while chilly years will cause equally large increases. (The mid-1990s saw a one-year ice advance almost as large as the 2007 loss.) The era of Arctic ice impassively gliding through these variations is over.
Read More
http://www.nytimes.com/gwire/2010/12/15/15greenwire-no-tipping-point-for-sea-ice-in-polar-bears-fu-29018.html
