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The potential impacts extend far beyond the glaciers. On the Tibetan Plateau, especially its dry northern flank, people are already affected by a warmer climate. The grasslands and wetlands are deteriorating, and the permafrost that feeds them with spring and summer melt is retreating to higher elevations. Thousands of lakes have dried up. Desert now covers about one-sixth of the plateau, and in places sand dunes lap across the highlands like waves in a yellow sea. The herders who once thrived here are running out of options.

Along the plateau's southern edge, by contrast, many communities are coping with too much water. In alpine villages like Mingyong, the glacial melt has swelled rivers, with welcome side effects: expanded croplands and longer growing seasons. But such benefits often hide deeper costs. In Mingyong, surging meltwater has carried away topsoil; elsewhere, excess runoff has been blamed for more frequent flooding and landslides. In the mountains from Pakistan to Bhutan, thousands of glacial lakes have formed, many potentially unstable. Among the more dangerous is Imja Tsho, at 16,400 feet on the trail to Nepal's Island Peak. Fifty years ago the lake didn't exist; today, swollen by melt, it is a mile long and 300 feet deep. If it ever burst through its loose wall of moraine, it would drown the Sherpa villages in the valley below.

This situation—too much water, too little water—captures, in miniature, the trajectory of the overall crisis. Even if melting glaciers provide an abundance of water in the short run, they portend a frightening endgame: the eventual depletion of Asia's greatest rivers. Nobody can predict exactly when the glacier retreat will translate into a sharp drop in runoff. Whether it happens in 10, 30, or 50 years depends on local conditions, but the collateral damage across the region could be devastating. Along with acute water and electricity shortages, experts predict a plunge in food production, widespread migration in the face of ecological changes, even conflicts between Asian powers.

The nomads' tent is a pinprick of white against a canvas of green and brown. There is no other sign of human existence on the 14,000-foot-high prairie that seems to extend to the end of the world. As a vehicle rattles toward the tent, two young men emerge, their long black hair horizontal in the wind. Ba O and his brother Tsering are part of an unbroken line of Tibetan nomads who for at least a thousand years have led their herds to summer grazing grounds near the headwaters of the Yangtze and Yellow Rivers.

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