Global warming will have a devastating effect on water availability in the western United States, a new climate forecast predicts.
The report involved more than two dozen scientists and engineers from around the country who undertook the study as a test of a national climate forecasting effort.
What they found doesn't bode well for the West.
Even the report's best-case scenario predicted water supplies would fall far short of future demands by cities, farms and wildlife, generating critical water-rights' issues that have already surfaced during the West's current drought.
"You'd like there to be some good news in there somewhere, but unfortunately there is not," said Scripps Institution of Oceanography research marine physicist Tim Barnett.
The study predicts overall precipitation levels are likely to remain constant, but warmer temperatures mean what would have fallen as snow will instead come down as rain.
Currently, the snowpack acts a natural reservoir, storing water through the winter so it will melt and be released during the spring and summer when demand spikes. If that precipitation falls as winter rain, however, it will fill rivers and streams at a time of year when demand is low.
To create the forecasts, scientists began two years ago with current observations of the state of the world's oceans - those vast reservoirs of heat that drive climate - and worked to translate that into real effects on precipitation and temperature on the West's three most important river systems: the Columbia, Sacramento and Colorado river basins.
According to some research, global warming is due to an increase in atmospheric greenhouse gases, principally carbon dioxide from the burning of oil, gas and coal. Global temperatures are thought to have risen by about 1.1 degrees over the last century, with the top few thousand feet of ocean waters increasing by about one-tenth of a degree.
Among the new study's forecasts for the next 25 to 50 years:
Reservoir levels along the Colorado River will drop by more than a third, and releases by 17 percent. The lower levels and flows will cut hydropower generation by as much as 40 percent.
The Sacramento River will see reduced reliability in the volumes of water available for irrigation, cities and hydropower. With less fresh water, the Sacramento Delta will increase in salinity, disrupting the ecosystem.
Along the Columbia River system, there will be either water in the summer and fall to generate electricity, or in the spring and summer for salmon runs - but not both.
"The problem is you basically can't resolve that trade-off," said Dennis Lettenmaier, a professor of civil and environmental engineering at the University of Washington.
The continued growth in the population of the West will exacerbate the problem. Indeed, that alone makes for a crisis, said Bill Patzert, a National Aeronautics and Space Administration research oceanographer who was not connected with the new research.
"The problem in the West is not climate change, it's too many ... people using too much water," Patzert said. "If nothing happens, we're in trouble. If something happens, it's worse."
The study included researchers from institutions including Scripps, the University of Washington, the Energy Department and the U.S. Geological Survey. The results are expected to appear in a future issue of the journal Climatic Change.