The study also found that more changes are on tap for Western snowpacks.
Those findings may come as a surprise this year, when the West is getting so much snow that skiers and snowmobilers are dying in avalanches in places that normally don't get that much snow.
But that doesn't mean the future won't see significantly less snowfall.
"We'll still have wet years and dry years," Barnett said. "People have a problem distinguishing weather events and things that happen on a long scale. ... It's important to think of climate on time scales of a decade or more."
"Global warming is an abstraction to most people," he said. "Well, the people who live in the West, if they haven't already, are going to very shortly find out what global warming really means to them."
Barnett predicts a crisis in water management that will require not only government action but individual sacrifices.
Governments may be able to help by building more dams to store early season runoff for the increasingly dry summers.
"I don't know how much room they have to put in dams," he said, "but I think we're in a situation where the environmentalists are going to have to stand down a little bit."
Lining aqueducts and irrigation canals to prevent seepage losses will also help, he said.
But individual conservation is also needed. "When you brush your teeth, do you leave the water running—maybe wasting a gallon of water?" he asked. "Multiply a gallon of water [a day] by 30 million people ..."
In addition, he suggests that desert-region growth may need to be restricted.
"Right now we're at, or very close to, the limit of what the Southwest can sustain. And yet they're building huge subdivisions in the desert," he said.
"Where is the water going to come from?"
Crisis or Opportunity?
Not all scientists are as pessimistic.
Molly Brown of NASA's Goddard Space Flight Center in Greenbelt, Maryland, is a co-author of another article in the current issue of Science, which looks at how farmers in Africa can adapt to their own changing climate by switching to better suited crops or improved seed varieties.
In Africa the good news is that there are lots of things that can be done, Brown said.
"By applying some technology, we can overcome the area's changing climate."
The American West is far more technologically advanced, but that doesn't mean there isn't room for conservation-related improvement, she said.
"We can apply the same thing to the Colorado River," she added. "The bottom line is that we're pretending that water is an endless resource."
What's needed, she said, is recognizing the need for a new attitude and using the changes as an opportunity to improve resource use.
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