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.
