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. New research published Monday in Nature identifies the most important and vulnerable water towers in the world. The research creates a template for world leaders—many of whom gathered at a major annual climate summit last week—to follow as they consider how to prioritize climate adaptation efforts in the face of unprecedented, rapid change.

“We all need water. We’re 90 percent water, we require fresh water,” says Michele Koppes, a climate and glacier scientist at the University of British Columbia and an author of the report. “We have big demands on the water from these water towers, and we have to understand better how they’re changing.”