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But a crisis is brewing on the roof of the world, and it rests on a curious paradox: For all its seeming might and immutability, this geologic expanse is more vulnerable to climate change than almost anywhere else on Earth. The Tibetan Plateau as a whole is heating up twice as fast as the global average of 1.3°F over the past century—and in some places even faster. These warming rates, unprecedented for at least two millennia, are merciless on the glaciers, whose rare confluence of high altitudes and low latitudes make them especially sensitive to shifts in climate.

For thousands of years the glaciers have formed what Lonnie Thompson, a glaciologist at Ohio State University, calls "Asia's freshwater bank account"—an immense storehouse whose buildup of new ice and snow (deposits) has historically offset its annual runoff (withdrawals). Glacial melt plays its most vital role before and after the rainy season, when it supplies a greater portion of the flow in every river from the Yangtze (which irrigates more than half of China's rice) to the Ganges and the Indus (key to the agricultural heartlands of India and Pakistan).

But over the past half century, the balance has been lost, perhaps irrevocably. Of the 680 glaciers Chinese scientists monitor closely on the Tibetan Plateau, 95 percent are shedding more ice than they're adding, with the heaviest losses on its southern and eastern edges. "These glaciers are not simply retreating," Thompson says. "They're losing mass from the surface down." The ice cover in this portion of the plateau has shrunk more than 6 percent since the 1970s—and the damage is still greater in Tajikistan and northern India, with 35 percent and 20 percent declines respectively over the past five decades.

The rate of melting is not uniform, and a number of glaciers in the Karakoram Range on the western edge of the plateau are actually advancing. This anomaly may result from increases in snowfall in the higher latitude—and therefore colder—Karakorams, where snow and ice are less vulnerable to small temperature increases. The gaps in scientific knowledge are still great, and in the Tibetan Plateau they are deepened by the region's remoteness and political sensitivity—as well as by the inherent complexities of climate science.

Though scientists argue about the rate and cause of glacial retreat, most don't deny that it's happening. And they believe the worst may be yet to come. The more dark areas that are exposed by melting, the more sunlight is absorbed than reflected, causing temperatures to rise faster. (Some climatologists believe this warming feedback loop could intensify the Asian monsoon, triggering more violent storms and flooding in places such as Bangladesh and Myanmar.) If current trends hold, Chinese scientists believe that 40 percent of the plateau's glaciers could disappear by 2050. "Full-scale glacier shrinkage is inevitable," says Yao Tandong, a glaciologist at China's Institute of Tibetan Plateau Research. "And it will lead to ecological catastrophe."

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