COP15: Climate-Change Conference

The road to Khardung La begins in the Indian town of Leh on the northwestern fringe of the Himalayas. Exhaust-spewing army trucks rattle up the side of dry rock, past Buddhist monasteries clinging to the craggy mountainside and alongside small farms barely scraping fertility from the earth. Khardung La, the highest motorable mountain pass in the world, is more than 18,000 ft. above sea level, the air so thin that just standing there a few minutes leaves you feeling as if your head might lift off like a balloon. But if 65-year-old Syed Iqbal Hasnain is bothered by the altitude, he isn't showing it. The Indian glaciologist hops lightly from a car and walks to the edge of the pass, beneath fluttering Buddhist prayer flags. The rock is dusted with early winter snow, and there might not be much more this season or next, he says.

Reports from Leh indicate that precipitation has dropped during the past quarter-century as temperatures have risen, a possible consequence of climate change. But the real threat is to the heart of the greater Himalayas and the vast Tibetan Plateau, where more than 40,000 sq. mi. of glaciers hold water in the largest collection of land ice outside the polar regions. "These glaciers are central to the region," says Hasnain, looking over Khardung La. "If we don't have snow and ice here, people will die." (See pictures of Himalayan glaciers under threat.)

Scientists call it the third pole — but when it comes to clear and present threats from climate change, it may rank first. The high-altitude glaciers of the Himalayas and the Tibetan Plateau — which cover parts of India, Pakistan, Nepal, Bhutan and China — are the water tower of Asia. When the ice thaws and the snow melts every spring, the glaciers birth the great rivers of the region, the mightiest river system in the world: the Ganges, the Indus, the Brahmaputra, the Mekong, the Yellow, the Yangtze. Together, these rivers give material and spiritual sustenance to 3 billion people, nearly half of the world's population — and all are nursed by Himalayan ice. Monsoons come and go, filling the rivers at times and then leaving them lethargic, but the ice melt has always been regular and dependable in a region where water — or the lack of it — defines civilization. "This isn't like the polar ice caps," says Shubash Lohani, an officer with the Nepal program of the World Wildlife Fund (WWF). "You have a huge population downstream from the Himalayas who are dependent on it." (Watch TIME's video "Creating New Land for Climate Refugees in Bangladesh.")

It's a population that is stressed for water, even if the ice doesn't disappear. According to the International Water Management Institute (IWMI), most of South Asia is already in a state of water scarcity, as is much of China. At the same time, the population in this part of the world is set to expand, even as economic growth increases competition for water used in agriculture and industry. (See pictures of a glacier melting in Peru.)

Regardless of the impact of climate change, there is a widening gap between water supplies and needs. In fact, a new report from the international consulting group McKinsey & Co. estimates that by 2030, India alone will have only 50% of the water that it needs under a business-as-usual scenario. Nor is Asia the only region that will grapple with water scarcity in a warmer world: the McKinsey report estimates that the globe will have 40% less water than it needs by 2030 if nothing is done to change current consumption patterns. "The countries where water is already scarce are going to be the ones really vulnerable to climate change," says Colin Chartres, director general of the IWMI. (See how global warming is threatening penguins.)

That makes the security of the Himalayan glaciers all the more important for the region and their potential loss all the more threatening. While it's difficult to get a comprehensive assessment of the tens of thousands of glaciers in the Himalayas — all above 10,000 ft. — independent scientific studies indicate that the third pole is melting fast, probably because of warming temperatures brought on by climate change. Since 1960, almost a fifth of the Indian Himalayas' ice coverage has disappeared, and the 2007 global-warming assessment by the Intergovernmental Panel on Climate Change judged that glaciers in the Himalayas were "receding faster than at any other place in the world." If global warming goes unchecked, the Himalayan melt will certainly get worse. This year Chinese researchers projected a 43% decrease in glaciated area by 2070. If that happens, the impact could be catastrophic. Losing Himalayan meltwater would only stress the remaining resources further. High-mountain states like Nepal and Bhutan could suffer flash floods as glacial lakes gave way under the rush of accelerated melting. And since the rivers of the Himalayas are shared by nuclear powers that have engaged in violent conflict over the past half-century — India, Pakistan and China — the threat of a war over water can't be denied. "The warming of the past 20 years is getting more and more intense," says Yao Tandong, head of China's Institute of Tibetan Plateau Research. "If warming continues, [the impact] will be even more serious."

Watch TIME's video "The Icy Clues to Global Warming."

Watch "Looking at Ice to See Our Past."