There are only three major parts of the globe that are substantially covered with ice and snow all year round. The Arctic, the Antarctic, and the least well known, the Greater Himalayas, or “the abode of the snows” in Sanskrit. This majestic arc of mountains, which rims the Tibetan Plateau, begins in Inner Asia with the Tian Shan Range, becomes the Hindu Kush in northern Afghanistan, joins with the Karakorum in northern Pakistan, and then merges into the Himalayas above Nepal, Bhutan, India, and southwest China. Because these mountains encompass the largest nonpolar ice mass in the world—some 46,298 glaciers covering 17 percent of the area’s land mass—the region is known as the “third pole.” Since time immemorial they have held vast quantities of fresh water in frozen reserve for the people of Asia.

Now it is under increasing stress. While the effects of global warming on the world’s ice systems elsewhere are well known, scientists are now documenting two special threats to these Himalayan glaciers that are less well understood. One is that global warming appears to be rapidly accelerating temperature elevations in certain high-altitude zones, particularly those nearest the equator, which includes the Greater Himalayas. The other is the effect of soot from the millions of wood and coal-burning stoves in India and China and the combustion of diesel fuel and coal, which is turning the surface of the Himalayan glaciers into heat-absorbing, rather than heat-reflecting, landforms, only further accelerating the warming process.

These threats could not come to a more vulnerable part of the globe. While the melting of the North and South Poles may raise ocean levels with disastrous effect, the melting of these Himalayan glaciers will have a more direct long-term effect on the hundreds of millions of people who live along rivers fed by their seasonal runoff. Scientists warn that by 2070 there could be a 43 percent decrease in land mass covered by these once seemingly eternal aspects of our planet’s architecture. In numerous and complex ways this loss will affect Asia’s 10 major rivers—the Yellow, Yangtze, Mekong, Salween, Irrawaddy, Brahmaputra, Ganges, Indus, Amu Darya, and Tarim. It is here, among the huge modern-day populations of Asia, that the melting of the Greater Himalayas’ glaciers will have the most significant impact during the coming decades and centuries.

Recent revelations that the Fourth Assessment Report of the U.N.’s Intergovernmental Panel on Climate Change erroneously claimed that there was a “likelihood” that Himalayan glaciers would disappear by 2035, “and perhaps sooner,” embarrassed the report’s authors; but this mistake has not altered the reality that many glaciers in the region are, in fact, rapidly receding. Nor does it scientifically invalidate the panel’s overall conclusion that because “more than one-sixth of the world’s population lives in glacier or snowmelt-fed river basins and will be affected by the seasonal shifts in stream flow,” a serious downstream problem is unfolding. A 2005 World Wildlife Fund report observes that in Nepal, India, and China, “glacial melt will affect freshwater flows with dramatic adverse effects on biodiversity, and people and livelihoods, with a possible long-term implication on regional food security.”

Glacial melt is critical to Asian rivers because of its perfect timing, during the hot, dry spring and fall “shoulder seasons” just before and after the monsoon, when rivers need added volume to keep constant flows. Any disruption of these flows—especially when a monsoon is late, weak, or fails, as it has this past year in drought-stricken southwest China (through which the Yangtze, Mekong, and Irrawaddy rivers all pass)—can significantly disturb life for hundreds of millions of people downstream. Indeed, on a recent trip to China’s Guizhou province, I observed the effect of such droughts on tens of millions of people, evident in the brown fields, bone-dry reservoirs, and rivers that have ceased to flow.

The glaciers that make up this alpine cryosphere are actually constantly moving “rivers of ice” that begin in their “accumulation zones” high on mountainsides, where snows fall and are compressed into “firn,” the blue ice that gives glaciers their air of frozen purity. Pushed by their own immense weight, glaciers make their slow, gravity-driven progress downward, carving out valleys and gathering up so much debris that they often look like conveyor belts for rocks. In its lower reaches, a glacier “calves,” or sloughs off, giant pieces of itself in a process that turns ice into the meltwaters that feed these river systems. Most of the world’s “reference glaciers”—those 200-plus that have been under observation over the past 60 years by the Switzerland-based U.N. World Glacier Monitoring Service—are now sloughing off ice faster than it accumulates, and have begun to record significant losses.

Over the past century, heat-trapping greenhouse gases have caused an average rise in global temperature of 0.74 degrees C. But many scientists contend that, unless current emission rates are radically curbed, it is possible that future average temperatures could rise by as much as 4.3 degrees C. Ice ecosystems are particularly delicate, and could be thrown out of balance by shifts of even a degree or two. What is more, scientists have recently been recording unusually large temperature rises in certain high-altitude regions. On the Tibetan Plateau, for instance, warming has increased at upwards of three times the global average. Two of the world’s leading glaciologists, Lonnie Thompson of Ohio State University’s Byrd Polar Research Center, and Yao Tandong, director of the Chinese Academy of Sciences’ Institute of Tibetan Plateau Research, have concluded that temperatures in the Himalayas are likely to rise by as much as 5 to 6 degrees C over the next century.

“The current warming at high elevations in the mid to low latitudes is unprecedented for at least the last 2 millennia,” writes Thompson in the Proceedings of the National Academy of Sciences. “The continuing retreat of most mid to low-latitude glaciers, many having persisted for thousands of years, signals a recent and abrupt change in the Earth’s climate system.” This change, he writes, “may signal that the climate system has exceeded a critical threshold and that most low-latitude, high-altitude glaciers are likely to disappear in the near future.” As Zheng Guoguang, head of the China Meteorological Administration, recently added, “If the warming continues, millions of people in western China will face floods in the short term and drought in the long run.”

As important as rising temperatures are in understanding what is happening to Tibetan Plateau glaciers, another major factor is also causing damage. Atmospheric brown clouds, or ABCs, are aerosol suspensions of very small particles created by the inefficient combustion of coal, kerosene, and diesel oil, as well as biofuels from home cooking fires. They now regularly hang like a pall over densely populated areas of the Indian subcontinent and China and have the capacity to both absorb and reflect solar heat.

Dust and black-carbon soot particles absorb heat, and thus increase global warming. But ABCs also contain sulfate and nitrate particles, which act in reverse like little mirrors reflecting even more incoming heat back out of the atmosphere. UC San Diego climate scientist Veerabhadran Ramanathan and his colleague Y. Feng believe that this “dimming” or “surface cooling effect” has actually “masked a significant fraction” of the overall warming effect of already emitted greenhouse gases. But their masking effect is only temporary, because heat-reflecting particles fall out of brown clouds soon after they are emitted, while the greenhouse gases last in the atmosphere for decades. Moreover, if mankind ever does find a way to clean up these toxic clouds, the two scientists say, it would be “like removing the mask.” Very quickly thereafter, they warn, the planet could experience a rapid leap of approximately 1.6 degrees C in additional warming.

Furthermore, because Himalayan surface ice is now darkening as carbon migrates up onto the Tibetan Plateau, another melting dynamic has been set in play. James Hansen, director of NASA’s Goddard Institute for Space Studies, and Yao Tandong, perhaps China’s best-known glaciologist, have been doing research and fieldwork on the glaciers, and they report that more and more black carbon is being deposited on Himalayan glaciers via the warm, moisture-laden, southerly monsoon winds that sweep it up onto the Tibetan Plateau, where it falls as snow. As temperatures rise at high altitudes, more of the soot-laden snow melts before any new snow can bury it and compress it into glacial ice. So with successive meltings, greater concentrations of black soot build up, turning the surface of glaciers into giant collectors of solar heat.

Stopping this process will require finding ways to reduce, and not merely slow, the rate of increase of greenhouse-gas emissions and carbon soot levels in the atmosphere. Such reductions would require new regional organizations and strategies. India and China have begun to discuss the threat to glaciers but have yet to take any meaningful joint action. In fact, the Indian minister of environment and forests, Jairam Ramesh, recently released a report questioning whether glaciers in the region are melting precipitously at all.

Perhaps leaders should just go see the glaciers themselves. Between 1950 and 1980 about half the glaciers in the region were receding. Now that figure is 95 percent, and on a trip I recently made to the Tibetan Plateau in Yunnan province to observe the Yongming and Baishui glaciers, the two southernmost and lowest-latitude glaciers in the Greater Himalayan Range, what was striking was just how gritty they now look and how far they have receded. Admittedly, these are two of the fastest-melting in the Greater Himalayas, but their reduced size and grimy appearance were a jarring reminder of the threat their wasting away ultimately poses not only to the region, but to all the people living downstream. (Newsweek: http://www.newsweek.com/2010/05/28/the-most-endangered-glaciers.html)

Schell is the Arthur Ross director of the Asia Society’s Center on U.S.-China Relations. This piece is adapted from “The Message From The Glaciers” in the New York Review of Books.