By Will Dunham
WASHINGTON (Reuters) - A water supply crisis is looming in the western United States thanks to human-caused climate change that already has altered the region's river flows, snow pack and air temperatures, scientists said.
Trends over the past half century foreshadow a worsening decline in water, perhaps the region's most valuable natural resource, even as population and demand expands in western states, researchers led by a scientist at the Scripps Institution of Oceanography wrote in the journal Science on Thursday.
Up to 60 percent of changes in three key factors affecting the West's water cycle -- river flow, winter air temperatures and snow pack -- are due to human-caused climate change, they determined using multiple computer models and data analysis.
"Our results are not good news for those living in the western United States," wrote the team led by Tim Barnett, a climate expert at Scripps Institution, part of the University of California at San Diego.
"It foretells of water shortages, lack of storage capability to meet seasonally changing river flow, transfers of water from agriculture to urban uses and other critical impacts."
Barnett said computer models point to a looming crisis in water supply in the coming two decades.
It has been clear for some time that the climate has been changing in the western United States, and the question was whether it was due to natural variability or driven by climate change related to human-produced greenhouse gases and aerosols, the scientists said.
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While the western United States has experienced natural wet and dry cycles in the past, current water flow trends differ in length and strength from past natural variations, the scientists found. The changes match those expected from the impacts of human activity on climate.
The researchers tracked water flows in three major western river systems -- Columbia, Colorado and Sacramento/San Joaquin rivers.
Changes over the past half century have meant less snow pack and more rain in the mountains, rivers with greatly reduced flows by summer and overall drier summers in the region, they noted.
"At this point in time, there's not much we can do to change that," said Barnett, who worked with experts at the U.S. government's Lawrence Livermore National Laboratory, the University of Washington in Seattle and the National Institute for Environmental Studies in Japan.
"We're going to have to adapt our infrastructure and some of our societal needs to fit the way the world is changing," Barnett said in a telephone interview.
"Water shortages throughout the west, hydroelectric power reductions, heat waves -- the whole litany of things that go with global warming."
Another group of researchers, writing in the same journal, said leaders who set water policies worldwide must take climate change into account when planning for the future.
Until now, water policies have relied on the premise that historical water patterns could be counted on to continue. But human-induced changes to Earth's climate have begun to shift the averages and the extremes for rainfall, snowfall, evaporation and stream flows, Christopher Milly of the U.S. Geological Survey and colleagues said.
"Our best current estimates are that water availability will increase substantially in northern Eurasia, Alaska, Canada and some tropical regions, and decrease substantially in southern Europe, the Middle East, southern Africa and southwestern North America," Milly said in a statement.
(Editing by Maggie Fox)