BOSTON, MASS - From melting glaciers, to earlier spring seasons, to the collapsing fringes of the Antarctic ice sheet: climatic change is underway at the Earth's poles and high latitudes, according to research presented at the American Association for the Advancement of Science (AAAS) Annual Meeting today.
While it's not clear yet that these changes are the result of human-induced greenhouse warming, scientists say, the rate of change has accelerated markedly in recent decades.
Researchers discussed recent developments at high latitudes and their possible implications for sea level-rise at a session entitled "Deciphering the Complex Changes in Snow and Ice," at the AAAS Annual Meeting.
The world's glaciers and ice caps have been shrinking faster since the 1980s than they did during the last several millennia, according to estimates by Mark Meier of the University of Colorado at Boulder. Meier analyzed information about glacier volumes worldwide, from several thousand years ago to the present, and studied the last 40 years in more detail. He estimates that sea level is likely to rise perhaps twice as much as the International Panel on Climatic Change (IPCC) recently predicted.
"The IPCC's projections are too low for several reasons," he said.
According to Meier, the projections don't consider that glaciers seem to be growing increasingly sensitive as air temperatures get warmer. In addition, smaller glaciers melt faster than large ones because a greater percentage of their surface area is at lower altitudes. Finally, as temperatures increase, large glaciers that are currently not contributing to sea-level rise will begin to do so.
Meier also analyzed measurements of glacier runoff and precipitation, and found that the increase in melting seems to have intensified the hydrological cycle at high altitudes. That is, the amount of water evaporating from the Earth's surface and returning as precipitation has also amplified.
"It's sort of enigmatic, because it means temperatures are warming, but snowfall is increasing," Meier said.
Also important for understanding the consequences of climate change, at least in Antarctica, is the behavior of ice shelves, the steep ice cliffs at the edge of an ice sheet.
Research by Ted Scambos of the University of Colorado at Boulder, suggests that ice shelves in the Antarctic may be more vulnerable to warming-induced break up than previously thought.
He and his colleagues developed a model for how cracks push their way through an ice shelf, and applied it to the so-called "Larson B Ice Shelf" in the Antarctic. In the model, a relatively small amount of melted water on the surface seeps into fractures in the ice, breaking up the ice shelf when it freezes. The shelf then collapses surprisingly quickly, without first having to warm all the way through, as scientists had generally assumed.
"We found that ice shelves thought to be stable are probably susceptible to breakup," Scambos said.
Since the late 1970s, ice shelves in some of the northernmost areas of the Antarctic ice sheet have exhibited the dramatic breakup style that Scambos and his colleagues attribute to this "inside-out" disintegration process. Before that, the shelves shed their ice in a more gradual fashion, he said.
The researchers have observed melt ponds forming in these particular regions, and believe this melting may be triggering the ice-shelf collapse. It would take several more decades of warming, however, before the actively shedding Ross Ice Shelf would be likely to undergo such a change, Scambos noted.
"This area is clearly experiencing a strong regional warming," Scambos said. It's too early, however, to make a connection between this trend and greenhouse warming caused by humans, Scambos cautioned.
Melting ice from somewhere else in the Antarctic ice sheet is contributing to sea-level rise, but the source of the meltwater has been a mystery. Scientists are now closing in, according to Robert Bindschadler of NASA Goddard Space Flight Center,
He and his colleagues initially thought the fast-moving ice streams within the West Antarctic Ice Sheet might be the culprit.
"We studied these ice streams for a long time, but they're not providing the signal for a large increase in sea-level rise. One has stopped, and another major one is decelerating," Bindschadler said.
On the other hand, a certain region making up approximately 20 percent of the ice sheet does appear to be retreating and thinning quite rapidly, Bindschadler and his colleagues have found. This region includes the basin that feeds the Pine Island and Thwaites Glaciers.
"That's where the action is," Bindschadler said.
He noted that these glaciers may behave quite differently from the well-studied ice streams, so more research is needed before scientists can say exactly what the West Antarctic ice sheet is doing now and what it will do in the future.
Evidence of recent warming isn't limited to the Antarctic. The northern high latitudes are showing key signs of change, according to Mark Serreze of the University of Colorado at Boulder.
He cited the thinning and breakup of sea ice, the warming of water masses deep in the oceans, the diminishing of snow cover, and the thawing of permafrost in Alaska and Russia. In addition, new research by Serreze suggests that Siberian rivers have begun discharging more freshwater into the sea.
"Together, these data create a coherent picture of high latitude change," Serreze said.
Researchers have yet to determine whether this change is the result of human activity, or whether it simply corresponds to natural atmospheric variations that occur on the order of decades, according to Serreze.
"It isn't resolved yet. To be a responsible scientist you have to be a fence sitter on this issue," he said.
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