Approximately 75 percent of the world's freshwater is stored in glacial ice, much of it in mountain areas. Mountain glaciers help regulate water supply: they collect and store water as ice during wetter, colder periods and release it into river networks during drier, warmer ones. In some of the driest parts of the world, mountain glaciers supply as much as 95 percent of an area's water. This water sustains agriculture, livestock, and the needs of all people living downstream.

Since 1900, most glaciers around the world have been retreating, or losing more water volume than they've gained. This retreat is linked to global warming, which is accelerating melting at the glaciers' surface. Mountain glaciers are further at risk because projected warming at high elevations may be twice as great as at sea level—an added six degrees Celsius versus three degrees Celsius by 2100. As heat is trapped in the atmosphere, it holds more water vapor. When water vapor rises, it condenses, releasing this additional heat into the upper atmosphere.

These findings have frightening implications for the billion or more Asian people who depend on the Himalayan glaciers as the source of their water. The Himalayas contain the largest store of freshwater outside the polar ice caps. More than 45,000 glaciers build up each winter and melt in the spring, feeding seven major rivers, including the Yangtze, Yellow, Brahmaputra, and Mekong. Yet today, Himalayan glaciers are dwindling far faster than expected. In fact, if melting continues at current levels, two-thirds of these glaciers may no longer exist in 2050. Before drought conditions would sweep over the region, the rapid glacial melting would produce widespread destructive flooding of downstream lands and communities.

Glacial melting is not limited to the Himalayas. Glaciers everywhere are losing volume. For example, since 1970, glaciers in South America's Andes Mountains have lost 20 percent of their volume, threatening the water supply of 30 million people. Thus, global warming's effect on ice could have serious consequences for human societies all over the world.

While an immediate global commitment to reduce greenhouse gas emissions and put an end to destructive forest burning and cutting practices might help slow global warming and, in turn, glacial melting, other measures must be considered. Dams built to contain the melted water can help in certain cases. But dam construction requires lots of money and resources and often faces opposition from local area and downstream residents. Instead, cities might seek out alternative water supply sources, develop new ways to collect and store rainfall, and practice conservation. Agricultural communities might plant alternative crops that require less water, and install advanced irrigation systems.

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