A cholera victim is rushed to a clinic in Port-au-Prince. Image by William Wheeler. Haiti, 2012.

In the Nepalese Himalayas in 2009, I trekked into the Langtang Valley, just short of the Tibetan border, and to a village of empty plywood cabins. The arrival of the summer monsoon season had chased the trekkers away.

Just uphill was a Buddhist temple and, behind it, a wrinkled sea of gray ice reached up the steep mountain walls into the clouds – the Langtang Lirung glacier, one of thousands that make up the largest body of ice outside the poles. In the winter, these glaciers capture precipitation that melts off in warmer months to feed the Ganges, Indus, and Brahmaputra rivers – and 1.5 billion people in eight countries who depend on them.

At night I could hear the thunderous crackling of distant avalanches on peaks above. By day, I saw shepherds, whose yaks couldn't withstand the summer heat, chanting a prayer for safe passage to higher, cooler climes.

The monsoon, seasonal rain that sweeps across the Indian subcontinent before crashing into the world's tallest mountains, was late, causing the worst drought in 30 years in Mumbai (Bombay), a thousand miles south.

Villagers talked of the arrival of mosquitoes – heralds of warmer summers and milder winters. The accelerated glacial melt is expected to increase floods in countries downstream over coming decades; earlier melts can reduce water when it's needed most.

In the long run, says Madhav Karki, director of the International Centre for Integrated Mountain Development, the rivers themselves may become seasonal, with potentially profound effects on the countries below.

Water is power

My travels in South Asia were a reminder of an ancient truth, often lost at the magical turn of a tap: A society's fate turns on its water supply. Water is power.

Covering global water issues, I've seen up close how the gap between the water rich and the water poor is often the line between life and death. In Haiti, I met people who took their water from rivers or nearby wells. Since the outbreak of cholera, those very water sources threaten their lives.

In Bangladesh, I saw how too much water creates problems: More intense rain deluges, that one scientist with the United Nations' Intergovernmental Panel on Climate Change told me might already reflect climate change, had increased flooding and river erosion. I met people whose homes, crops, and water supplies were repeatedly wiped out. In a country with a population roughly half that of the United States packed into an area about the size of Idaho, they had few options but to move to low-lying, vulnerable coastal lands. Or they joined the nearly 1 billion slum dwellers worldwide trying to ascend the economic ladder and increasing demand for water in all its forms.

Water: source of life and conflict

In Pakistan, I saw how water crises are not self-contained. Several analysts and historians I talked to that summer believe the initial spark of the region's most enduring conflict – the Kashmir dispute between India and Pakistan over the Muslim region – was perhaps less about religious differences and more about control of the region's vital water resources.

Kashmir is home to the headwaters of the Indus River, Pakistan's primary water lifeline. India also harnesses some of the river's flow for hydropower. But the fragile status quo that governs sharing of the river is under threat from booming population demands and the impacts of climate change. Both nations are racing to complete hydroelectric dams along the Kashmir rivers, elevating tensions. India's projects are of such size and scope to worry Pakistan about water shortages at critical times and massive deluges at other times.

Water stress has triggered unrest in both countries. In India, competition for water has set communities against each other. When I visited Pakistan in 2009, water stress had recently triggered food riots, bringing the military out to guard grain elevators; and it stoked protest and sectarian grievances in one region that some feared was on the verge of revolt. Water stolen from public pipes and then resold from tankers is a lucrative industry in Karachi, which depends on the Indus. And upstream elites divert large quantities of water before it reaches the city. But most Pakistanis blame their water problems on Indian dams – part of an alleged strategy to "make Pakistan into a desert."

The fear is an example of water's psychological impact. "If there is a war here in the future, it will be over water," the former chairman of Pakistan's Securities and Exchange Commission, Tariq Hassan, told me. International water disputes worried him, but so did domestic water conflict. "That could be tomorrow."

More recently, militant groups have used water to mobilize anti-India sentiment: Hafiz Saeed, the founder of the Pakistani militant group allegedly behind the 2008 terror attack in Mumbai, accused India of "water terrorism."

At the time I was there in 2009, officials were occupied with fighting the Taliban in the Swat Valley and dealing with more than a million displaced people. In Mardan, where some had fled to, I watched thousands of tents rapidly filling in a green field. The fortunate among them were close to blue water tanks spray-painted "UNICEF." The next year, after they'd returned home to the Swat Valley, they were hit by a devastating flood from the Indus – a phenomenon that will be intensified by accelerated glacial melting and more erratic monsoons.

The challenge of understanding the water crisis is recognizing the myriad ways water shapes lives and how the narrow margins on which many survive may change.

The consequences will fall the hardest where the margins are thinnest. In Islamabad, I stood on a dusty hill above a small, polluted stream where hundreds of tents were set up to receive refugees from the Swat Valley conflict. A doctor made rounds, treating waterborne illnesses. It was a scene that would be repeated after the floods the next year, and it didn't inspire optimism about global preparedness for the complex challenges ahead. Pointing at the dry, sunbaked slope beneath his feet, one refugee told me: "There is no life here."

Project

The majority of India's water sources are polluted. A lack of access to safe water contributes to a fifth of its communicable diseases. Each day in the booming, nuclear-armed nation, diarrhea alone kills more than 1,600 people.

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