Science doi:10.1126/science.1152538 (2008)

Credit: CANDACE HARTLEY

In the western US, where water is perhaps the most precious natural resource, anthropogenic global warming is responsible for more than half of the well-documented changes to the hydrological cycle from 1950 to 1999, researchers report. Over the last half of the twentieth century, the region's mountains received less winter snow and more rain, with snow melting earlier, causing rivers to flow more strongly in the spring and more weakly in the summer.

Tim Barnett of the Scripps Institution of Oceanography, US, and colleagues analyzed hydroclimatic data from the region for the 1950–1999 period and compared observed changes to results from sophisticated climate models. They found that up to 60 percent of the observed hydrological changes resulted from man-made greenhouse gases and aerosols, with other factors, such as variations in total precipitation, solar activity and volcanic eruptions, unable to account for the changes.

Climate models project more of the same, the scientists say. The region, whose population is steadily growing, faces water shortages because of insufficient capacity to store ever-increasing early mountain runoffs, the anticipated transfer of water from agriculture to cities, and other constraints. The researchers urge immediate action to forestall “a coming crisis in water supply”.