Credit: © ISTOCKPHOTO / SARA_WINTER

Glacial meltwater can be a crucial water source for human populations living in river basins, raising concerns about the affect of shrinking glaciers. But a study now shows the most populated regions derive their water primarily from other sources, such as precipitation, suggesting they may be less affected by dwindling glaciers.

Ben Marzeion and colleagues at the University of Innsbruck, Austria, estimated the relative contribution of glacier melt water to total water availability for 18 of the world's large river basins, from the Ganges to the Danube1. Taking account of the total water supplied by the glaciers and the number of people living at a given altitude, they calculated a population impact index for each river basin. They found that water supply from glaciers becomes less important downriver — where populations are concentrated.

The team found if an increase in air temperature caused melt-water production to double, this would have only a small impact on overall water availability. The researchers note that such changes would still have detrimental effects for numerous high mountain communities.