Around one-sixth of the world’s population is dependant on glaciers and seasonal snow packs for their water supply. But climate change is disrupting the annual flow of water downstream from snowy mountainous regions, says this paper. The hydrological changes taking place will impact on future water availability, and these impacts are likely to be severe.As global temperatures increase, less winter precipitation falls as snow and the melting of winter snow occurs earlier in spring. Even without any changes in precipitation intensity, both of these effects lead to a shift in peak river runoff to winter and early spring, away from summer and autumn when demand is highest. Where reservoir storage capacities are not sufficient, much of the winter runoff will be lost.The negative impacts of melting glaciers will probably be felt the hardest in the Himalaya–Hindu Kush region (China, India, and parts of Asia), home to more than half the worlds population. Here, glacial water provides nearly three-quarters of the summer flow in the Ganges and 50–60% of the flow in other major rivers. In China, 23% of the population lives in the western regions, where glacial melt provides the principal dry season water source. Another hard-hit region will be the South-American Andes, where glacial shrinking will reduce the river flow used for potable water and hydropower, requiring huge adjustments on the part of local peoples and economies.For countries in sensitive areas, time may be running out. The authors say it is still uncertain how much can be done to mitigate against the impacts, as warming will continue for several decades even if greenhouse gas concentrations are maintained at their current level. However, they anticipate a strategic planning response to the prospects of reduced water supplies.