More than 40 million people who depend on Colorado river could be facing a severe water shortage as the weather continues to grow warm and the population increases.
Climate modellers at the Columbia university's Lamont-Doherty Earth Observatory predict a 10 percent drop in the Colorado river's flow over the next few decades.
The drop, they say, is enough to disrupt long time water-sharing agreements between farms and cities in the American Southwest - from Denver to Los Angeles to Tucson, and through California's Imperial Valley.
"It may not sound like a phenomenally large amount except the water and the river is already over-allocated," said Richard Seager, climate scientist at the Columbia Earth Observatory, who led the study, the journal Nature Climate Change reports.
The study expands on findings published in 2007 in the journal Science that the American Southwest is becoming more arid as temperatures rise and rainfall patterns shift from human-caused climate change, according to a Columbia statement.
It also comes on the heels of a major study of the Colorado river basin by the US department of interior that projected longer and more severe droughts by 2060, and a nine percent decline in the Colorado's flows.
"The projections are spot on," said Bradley Udall, expert on hydrology and policy of the American West, at the University of Colorado, Boulder.
"Everyone wondered what the next generation of models would say. Now we have a study that suggests we better take seriously the drying projections ahead."