A paper in Nature by Tim Barnett at Scripps and Jenny Adam and Dennis Lettenmaier at the University of Washington suggests climate change problems for folks who depend on snowpack for their water supplies:

All currently available climate models predict a near-surface warming trend under the influence of rising levels of greenhouse gases in the atmosphere…. In a warmer world, 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 storage capacities are not sufficient, much of the winter runoff will immediately be lost to the oceans. With more than one-sixth of the Earth’s population relying on glaciers and seasonal snow packs for their water supply, the consequences of these hydrological changes for future water availability—predicted with high confidence and already diagnosed in some regions—are likely to be severe.

For those of us in the western U.S.:

There is not enough reservoir storage capacity over most of the West to handle this shift in maximum runoff and so most of the ‘early water’ will be passed on to the oceans. These hydrological changes have considerable impacts on water availability.

The one exception, they note, is the Colorado River basin, where the massive multi-year storage behind the big dams is a sufficient buffer against the early snowmelt timing problem. But the Washington team, in Climatic Change last year, published results of modelling studies suggesting that, independent of snomelt timing, overall Colorado Basin runoff could be significantly less in a warmed world.