19:00, 31 January 2008, NewScientist.com news service, Jim Giles

The water shortages gripping the western US are the result of global warming, not natural variations in climate, according to a bleak study by hydrologists. The results suggest that water disputes will plague the region in the future and damage economic growth unless action is taken now, warn researchers.

About 60% of the changes seen in river flow in the western US are due to warming caused by humans, their study suggests.

Key indicators have hinted at looming water problems for many years. More rain and less snow has been falling in mountain ranges such as the Rockies, for example. River levels, which depend on melting snow from the mountains during the spring and summer, have fallen as a result.

Spring temperatures also increased by around 0.35 °C per decade during the second half of the last century, further cutting snow levels.

Fluctuating levels

These trends spell trouble for managing water supplies. For example, Lake Mead, a 640 square kilometre reservoir that straddles the Arizona-Nevada border, fell to its lowest level for 40 years in 2007. It is just one of many reservoirs topped up by the Colorado River, which supplies water for around 30 million people.

Water levels fluctuate naturally and Lake Mead and other reservoirs have experienced drought before. But Tim Barnett of Scripps Institution of Oceanography in La Jolla, California, US, and his colleagues now say that recent changes cannot be explained by random changes in rainfall, or any other aspects of the weather.

They used two climate models to predict how warming caused by rising greenhouse gas levels would affect local hydrology. The results suggest that around 60% of the changes seen in river flow are due to man-made warming. Natural variability accounts for the remaining 40%, but cannot account for all the changes on its own.

Dashed hopes

Hopes that the drought is temporary have been dashed by the analysis, prompting leading researchers to intensify their calls for policy change.

Peter Gleick, a water policy expert at the Pacific Institute, an independent think tank based on Oakland, California, says that water use in his state could be reduced by a fifth by 2030, even if the population and economy continue to grow.

Many sectors need to change to achieve that goal. Agriculture would have to shift towards drip irrigation, in which small amounts of water are focused on individual plants, rather than whole areas being sprayed. Home owners would also have to adopt toilets and washing machines that use less water, he says.

Gleick is cautiously optimistic about the chances about the willingness of politicians to make the necessary changes. New standards for water-saving toilets were adopted in 2007, for example. "We can do the things we want for less water," he says. "But we need more federal and state legislation."

Journal reference: Science (DOI: 10.1126/science/1152538)