Trick-or-treating in Fairbanks, Alaska, deep in the state's interior, tends to be a parka-and-mukluks affair.
Some 13 years ago, Larry Hinzman recalls, he and his young daughters endured Halloween temperatures that plummeted to 30 degrees below zero F. as they shuffled through the neighborhood.
This year, however, the Halloween forecast calls for a relatively balmy 8 below F., capping a month where temperatures often have hovered near freezing. For October in Fairbanks, "that's not unusual. That's mutant," says Dr. Hinzman, a University of Alaska hydrologist.
The Arctic has long been viewed as the canary in the coal mine for global warming. Now, say many researchers, the canary not only is teetering on its perch; it may have reached the tumbling point.
To monitor such changes more closely, scientists globally have launched a scientific field assault in the region. Earlier this month, the International Council for Science met in Suzhou, China, and formally launched the International Polar Year slated to run from 2007 to 2008.
In addition, representatives from national and international scientific organizations are scheduled to meet in Copenhagen, Denmark, next month to lay out an ambitious 10- to 15-year polar science agenda, which will extend the IPY's yearlong effort.
The intense examination is likely to be fueled by a series of recent studies, including one Dr. Hinzman and colleagues have published in the current issue of the journal Climatic Change: signs of change are showing up just about everywhere researchers look.
The signs are seen in shrinking summer sea ice, the northward march of tree and shrubs, and longer periods from spring to fall when the ground is snow-free. But perhaps more telling, scientists say they see evidence that the changes are beginning to feed on themselves, building momentum for which researchers say they currently see no effective brakes.
Scientists say this acceleration comes from "positive feedbacks" - self-reinforcing trends, which are the climate's version of irritating squeals from mishandled sound gear at a rock concert. If left unchecked by countervailing forces, positive feedbacks have the potential to destablize the Earth's climate, tipping it into a new regime.
Not so long ago, these feedbacks appeared largely in computer simulations of human-induced climate change. Now, research groups are openly asserting in peer-reviewed journals that these feedbacks are taking hold in the world of snow and ice gauges, stream flow measurements, satellite images, and core samples.
"What used to be whispered in a speculative way - hey, do you think the feedbacks are kicking in - is more like: It's starting to look like the feedbacks may be kicking in," says Donald Perovich, an Arctic sea-ice specialist at the US Army Corps of Engineers' Cold Regions Research and Engineering Laboratory in Hanover, N.H.
Others are less tentative. "Recent changes in the length of the snow-free season have triggered a set of interlinked feedbacks that will amplify future rates of summer warming," according to a study of Arctic changes in a recent issue of the journal Science.
The Arctic's sensitivity to change lies in its two seasons - frozen and thawed, researchers say. Everything from whole ecosystems to foundations for buildings and pipelines are tailored to those two states. With so much hanging on the freezing point of water, a three-degree change in average temperature in Barrow, Alaska or Fairbanks can have a far more profound effect than a three-degree change in Houston. And those changes are likely to reverberate far beyond the top of the world, Hinzman adds. "This isn't just an Arctic issue. The changes up here are going to cascade to the temperate regions around the world."
Researchers cite two broad categories of change as examples - summer ice cover in the Arctic Ocean and snow cover on land. Both are responsible for reflecting sunlight back into space, cooling the region.
A research group which includes Jonathan Overpeck, with the University of Arizona at Tucson, recently published an overview of sea-ice research indicating that the Arctic Ocean is likely to be ice-free in the summer within a century - a situation that the region hasn't experienced for at least a million years, the team says. "Probably the biggest surprise ...was that no one could envision any interaction ... that would act naturally to stop the trajectory," Dr. Overpeck notes.
Less summer ice allows the darker oceans to absorb more sunlight and thus more heat. More heat, in turn, means still less summer ice and thinner ice when it starts to re-form in the fall.
Many scientists expect that the additional fresh water from the summer melt could work its way into the North Atlantic, possibly slowing a major ocean conveyor belt that drives currents - and hence regional climates - worldwide. And these reductions in sea ice are occurring long after natural fluctuations in climate have shifted to states that would favor less summertime loss.
As with ice, snow cover performs the same reflective function. But a longer snow-free season allows the ground to warm earlier and stay warm longer, further lengthening the snow-free period. With warmer ground comes a range of effects to soil, plants, and permafrost that can further boost the region's ability to absorb sunlight and warm up.
Researchers also are concerned about Greenland's slowly melting ice cap. If current trends continue over several centuries, the cap could add as much as seven meters to the average sea level.
"There has to be some environmental controls to slow these effects," Hinzman says. The Arctic has undergone warming trends in the past, he notes, and recovered. "So something must regulate these processes."
Finding them, then determining whether they can blunt the changes under way, he says, will be critical to providing the forecasts that will help the region adapt.