Though the IPCC report stated that the risk of the region’s glaciers “disappearing by the year 2035 and perhaps sooner is very high,” the new study finds that ice cover is stable in the Karakoram mountains, a northern range that holds about half of the Himalaya’s store of frozen water.
That’s not to imply that water reservoirs on what’s often called the roof of the world aren’t under stress. Throughout most Himalayan ranges, roughly 65 percent of the studied glaciers were shrinking, Dirk Scherler of the University of Potsdam, Germany, and his colleagues report in the January 23 Nature Geoscience. But in Karakoram, 58 percent of studied glaciers were stable or slowly expanding up to 12 meters per year.
Scherler’s team pored over satellite images of 286 glaciers throughout the Himalayas. Collected between 2000 and 2008, they showed a consistent trend everywhere except the Karakoram: a reduction in the area of glacial cover. Many glaciers in those regions also were stagnant — not flowing — which, Scherler says, is an indicator of poor health.
The new findings are consistent with what Kenneth Hewitt of Wilfrid Laurier University in Waterloo, Ontario, has observed, and point to the fact “that the picture of climate change effects in high Asia is much more complicated than most people realize.”
Indeed, for much of the past century Karakoram’s glaciers were in retreat. A 2005 paper by Hewitt described a turnaround that commenced only in the late 1990s.
In the new study, Scherler’s team looked for factors that might affect the responsiveness of Himalayan glaciers to regional warming. A rocky blanket quickly emerged as a major one.
In general, the warmer the air above a glacier becomes the faster exposed ice will melt. A thin veneer of dust or grit will darken glaciers, increasing the amount of heat they absorb and exaggerating their warming, much as a dark roof becomes hotter in sunlight than a light gray one. But once the depth of any rock cover exceeds several centimeters, it will insulate ice from the sun’s warming rays. In some lower reaches of Himalayan glaciers, especially in the Karakoram, rock debris can include house-size boulders, Scherler observes.
In this range, it seems, rocky rubble eroded from uphill peaks serves to decouple the effects of regional warming from glacial retreats. The new analysis found retreat rates varied in the Himalaya “from high for debris-free glaciers to zero for glaciers with debris cover greater than 20 percent.”
What the satellite data, which surveys the extent of a glacier’s coverage, can’t establish is how much area glaciers might be thinning. Such information requires ground measurements, Hewitt says, which are particularly rare in this part of the world. But they could also prove quite crucial. He notes that some data emerging from India and China suggest that the diffuse fallout of soot from local industries, traffic and cook stoves might be subtly darkening debris-free portions of Himalayan glaciers — and constitute “a more significant factor than even temperature change in their melting.”
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