Global warming will worsen drought and reduce flows on the Colorado River, a key water source for Southern California and six other Western states, according to a report released Wednesday.
The study, prepared by a National Research Council committee, paints a sobering picture of the future as the water needs of a rapidly expanding population test the limits of a river system further strained by the effects of climate change.
"The basin is going to face increasingly costly, controversial and unavoidable trade-off choices," said Ernest Smerdon, who chaired the panel of academicians and scientists who wrote the report. "Increasing demands are impeding the region's ability to cope with droughts and water shortages."
The authors concluded that there was no easy solution. Such measures as conservation, desalination and water recycling will all help, they said, but won't offer a panacea.
The report, which examined climate modeling and tree-ring data, reaffirms a more pessimistic assessment of river hydrology that has emerged in recent years.
Scientists have concluded that historically the Colorado River system, which supplies water to 25 million people and several million acres of crop and ranch land, has been drier and more prone to severe drought than was the case in the early 20th century, when the river's flows were divvied up among the seven states in the basin.
That period, it turns out, was unusually wet, prompting an overly generous estimate of how much water would be available to farms and cities. Ancient tree rings, which provide graphic evidence of past precipitation patterns, indicate it had been three centuries since the basin was last awash in that much water.
Tree ring data also show that in the past the basin experienced more prolonged and more serious droughts than anything in modern times -- including the most recent one, which began in 2000 and has left the river's biggest reservoirs, Lake Mead and Lake Powell, about half empty.
"Now, that is not a happy picture," Smerdon said. "What we want people to do is to be aware of these scientific facts and take them into account as they plan for development, for drought contingency plans."
Global warming will only make matters worse, said Connie Woodhouse, a University of Arizona associate professor of geography who helped write the report. "It's going to enhance the droughts" that are part of the natural climate cycle.
Although the river's flows returned to normal in 2005 after five years of dramatic lows, they fell again last year. Federal scientists say that if they remain below average or even average, Lake Mead may never completely refill.
The study, sponsored by the National Academies, of which the research council is a part; the U.S. Bureau of Reclamation; and several water agencies, including the Metropolitan Water District of Southern California, says temperatures have risen in the western U.S. in the last century and are expected to keep climbing.
"All of the models are predicting very significant warming in the future," said Smerdon, a former engineering college dean at the University of Arizona.
The report acknowledges some uncertainty over whether climate change will alter the amount of precipitation in the basin. Even if it doesn't, Smerdon added, "the preponderance of scientific evidence suggests that warmer temperatures will reduce Colorado River stream flow and water supplies."
That is because the snowpack that feeds the Colorado will shrink as the climate grows hotter. More precipitation will fall as rain instead of snow, and the snowpack will melt earlier, diminishing late spring runoff.
As the mercury rises, water demands will also increase. There will be more evaporation from croplands and reservoirs, and wild land vegetation will suck more moisture from the soil, reducing runoff.
"I think a reasonably savvy person would have to say you're going to see less water in that river," said Brad Udall, director of Western Water Assessment at the University of Colorado, who was not a member of the report committee. "The real question is how much less. Is it 5% less or is it 40% less, and by when?"
The report recommends that officials undertake a comprehensive study of the region's urban water use and demand, while it praises recent efforts by basin states to forge agreements on how to cope with future water shortages.
The U.S. Department of the Interior, which oversees the Colorado's management, is reviewing a drought plan the states put together last year.
Because California holds some of the most senior rights to the river, Arizona, Nevada and Mexico would all see cuts in deliveries before California, said Roger Patterson, the MWD's assistant general manager.
He said that under the states' drought proposal, there is only a 1% chance that shortages will fall to a level that will affect California.
The river is a major source for the MWD, Southern California's biggest urban water supplier. Last year about 30% of the agency's water deliveries came from the Colorado, which provides about one-tenth of California's overall urban and agricultural supplies.
Most of the state's river allotment -- the largest in the system -- goes to the Imperial Irrigation District, which distributes water to the sprawling croplands of the Imperial Valley, one of the nation's biggest lettuce producers.
Noting that farms remain the dominant water user in the Colorado River basin, the report called agricultural water "the most important and perhaps final large reservoir of available water for urban use in the arid U.S. West."
But that supply is also finite, the authors emphasized, and shifting water from farm to city can have negative impacts. Transfers can hurt rural economies, lower food production and rob wildlife of leaking irrigation water that nourishes important habitat.