The world’s supply of fresh water is in trouble as mountain ice vanishes

High in the Himalaya, near the base of the Gangotri glacier, water burbles along a narrow river. Pebbles, carried in the small river’s flow, pling as they carom downstream.

This water will flow thousands of miles, eventually feeding people, farms, and the natural world on the vast, dry Indus plain. Many of the more than 200 million people in the downstream basin rely on water that comes from this stream and others like it.

But climate change is hitting those high mountain regions more brutally than the world on average. That change is putting the “water towers” like this one, and the billions of people that depend on them, in ever more precarious positions.

The high mountains cradle more ice and snow in their peaks than exists anywhere else on the planet besides the poles. Over 200,000 glaciers, piles of snow, high-elevation lakes and wetlands: All in all, the high mountains contain about half of all the fresh water humans use.

The high-mountain “water towers” of the planet act like giant storage tanks with valves on them. The system more or less works like this: Snow falls, filling the tank, and then it melts out slowly over days, weeks, months, or years—a natural valve that smooths out the boom-bust pattern in outflow that would otherwise occur.

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