Global warming is already shrinking the Colorado River, the most important waterway in the American southwest, and it could reduce the flow by more than a third by the end of the century, two scientists say.
The river's volume has dropped more than 19 per cent during a drought gripping the region since 2000, and a shortage of rain and snow can account for only about two-thirds of that decline, according to hydrology researchers Brad Udall of Colorado State University and Jonathan Overpeck of the University of Arizona.
In a study published last week in the journal Water Resources Research, they concluded that the rest of the decline is due to a warming atmosphere induced by climate change, which is drawing more moisture out of the Colorado River Basin's waterways, snowbanks, plants and soil by evaporation and other means.
Their projections could signal big problems for cities and farms which span across seven states and Mexico. The river supplies water to about 40 million people and 16,317sq/km of farmland.
"Fifteen years into the 21st century, the emerging reality is that climate change is already depleting the Colorado River water supplies at the upper end of the range suggested by previously published projections," the researchers wrote.
Using existing climate models, the researchers said that much of the decline in precipitation should have produced a reduction of about 11.4 per cent in the river flow, not the 19.3 per cent that occurred.
They concluded that the rest was due to higher temperatures, which increased evaporation from water and soil, sucked more moisture from snow and sent more water from plant leaves into the atmosphere.
