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"Our results are not good news for those living in the western United States," write the study's authors, who used a combination of water and climate models to tease out what has driven significant changes in the region's hydrological cycle since 1950.

The researchers found human-caused emissions of heat-trapping greenhouse gases and aerosols to be major factors in the Western water cycle -- a result that surprised them, said lead author Tim Barnett, a climatologist at the Scripps Institution of Oceanography.

"When we started this project, I was leery we'd have success because the natural variability of the climate in the western U.S. is big," Barnett said. "We have droughts and wet periods that go way back through history."

But only when the researchers included GHG and aerosol emissions in their climate models did the simulations accurately reconstruct the river flow, snowpack and winter air temperature changes observed over the last half-century, from 1950-1999.

They estimate that human-caused climate change began having a noticeable impact on the region in the mid-1980s.

"It's pretty clear it's not natural variability," Barnett said, noting that the researchers tested and ruled out the influence of solar variation, volcanic emissions and precipitation patterns.

The study comes on the heels of the latest report of the Intergovernmental Panel on Climate Change, released last year, that predicted in the western United States there is a 90 percent chance that climate change will lead to "decreased snowpack, more winter flooding and reduced summer flows, exacerbating competition for over-allocated water resources" from agricultural power, residential and recreational sectors (Greenwire, April 17, 2007).

Given the earlier work, the new study is "not a terribly surprising result," said Chris Milly, a research hydrologist with the U.S. Geological Survey who said he agreed with Barnett's approach but would have liked to see more climate models used. "Warming there [in the West] and throughout the world is getting large enough that it's beginning to peak out above natural variability even on regional scales, not just global."

But what is notable is that Barnett's study is one of the first to try to figure out just how large an influence humans are having on the West's water cycle, said Milly, also the lead author of a policy essay that appears in tomorrow's print edition of Science.

Impact on water managers

The essay's authors -- all hydrologists -- argue that water managers must factor future climate change into plans for water infrastructure projects, which accounts for about $500 billion in global spending each year.

Past observations of streamflow, runoff, precipitation and snowpack are likely not a reliable base for planners looking to invest in new infrastructure, they conclude, recommending that planners discard the long-held concept of "stationarity," which the essay defines as "the idea that natural systems fluctuate within an unchanging envelope of variability."

In short, "if you're planning a project now that has to last 50 years, you have to look ahead at what the climate is going to be then," Milly told Land Letter. Even in regions where it may not be apparent now that warming is affecting water resources, climate is likely to become a factor in coming decades, he added.

But the mechanics of following the essay's recommendation to consider climate in planning may prove difficult.

"More communication between the scientific community and planners can be beneficial," Milly said. "The difficulty is in figuring out how does that communication occur? ... How do scientists give information that is meaningful to the planners?"