Adams then researched manmade reservoirs in the target regions and found that the vast majority do not have the capacity to store the extra, early runoff.
"We were surprised how many places [lacked extra capacity]", Barnett said. "[Reservoirs] were built on the assumption that water availability throughout the year wouldn't change."
In addition to the western United States and Canada, hard-hit regions include parts of Europe, South America west of the Andes, and much of central Asia from northern India across to China and Russia.
About one-sixth of the world's populationmore than a billion peopleinhabit these areas. The regions at risk also account for about a quarter of the world's economic output.
"What are those people going to do?" Barnett asked. "Just sit there and be thirsty and watch their crops die?"
Christopher Milly, a hydrologist with the U.S. Geological Survey, said that Barnett and Adam's model draws on two decades of "really robust" research.
But climate scientists are less confident in predicting how, rather than where, global warming will affect total annual precipitation and river flow.
In a companion article in Nature, Milly used historical data from a dozen climate models to make a composite that accurately predicted changes in 20th century river flow across the globe.
Extending the model into the future, he estimated that by 2050 the western U.S., southern Africa, and areas surrounding the Mediterranean Sea will receive 10 to 30 percent less runoff than they currently do.
"Our contribution has been to show that the models can match observations of trends in water availability," Milly said of his study.
Add this shortage to a shift in peak runoff from summer to spring, and the potential for crisis looms, warns Barnett.
As an example, he points to the Klamath River in northern California and Oregon. In 2001 the river ran about 25 percent lower than normal, leading to a tussle between conservationists and farmers over the water that remained. A court ruling led to a release of water to save two endangered species of sucker fish.
But in 2002, when the river ran low again, the farmers won and salmon lost. As water was diverted to agriculture, thousands of salmon died, perhaps as much as half of the river's spawning population, according to the Oregon Natural Resources Council.
More difficult water choices loom, Barnett says.
"The Klamath situation is a harbinger of things to come," he said. "You can see the whole future sort of happening right there."
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