For example, the estimate of 6 percent reduction did not include some of the fourth-generation climate model runs that tend to predict a dryer West. The high-end estimate of 45 percent relied on an analysis that overestimated how much increased temperatures would cut flows from the Colorado’s high-elevation headwaters. 

UW co-author Dennis Lettenmaier said, "The Colorado River is kind of ground zero for drying in the southwestern U.S."

In addition, the report calls on scientists to focus on what is known, rather than on uncertainties, when helping managers understand the implications of multiple studies.

"We hope this paper sheds some light on how to interpret results from the new generation of climate models, and why there's an expectation that there will be a range of values, even when analyzing output from the same models," said Lettenmaier, a UW professor of civil and environmental engineering.

The authors include leaders in Western water issues, including specialists in atmospheric sciences, hydrology and paleoclimate.

The other co-authors are Bradley Udall of the University of Colorado in Boulder; Daniel Cayan, Tapash Das and Hugo Hidalgo of Scripps Institution of Oceanography, UC San Diego; the UA’s Holly Hartmann and Kiyomi Morino; Levi Brekke of the Bureau of Reclamation; Gregory McCabe of the U.S. Geological Survey in Denver; Robert Webb and Martin Hoerling of the National Oceanographic and Atmospheric Administration in Boulder; and Kevin Werner of the National Weather Service in Salt Lake City.

The team identified four main reasons for the variation in flow estimates among the studies reviewed.

The most important source of difference was that different studies used different climate models and levels of future greenhouse gas emissions to predict future flows.

Next in importance was how well the models captured the effect of topography on the distribution of snow in the Colorado River's mountainous headwaters.

Third was which rainfall-runoff model was used to show how future changes in precipitation and temperature would affect the land's ability to absorb, evaporate or transport water.

The final source of differences in flow rates was which method was used to apply information from the large-scale global climate models to smaller-scale regional hydrology models.

The authors intend scientists, policymakers and stakeholders to use the new study to judge future estimates.

The research was funded by the National Oceanic and Atmospheric Administration through its Regional Integrated Sciences and Assessments Program and its National Integrated Drought Information System.

Overpeck said, "Temperature affects a bunch of things, but especially the snow in Colorado."

"Drought and climate warming are a one-two punch for our water supply,” he said. “We can lessen one of those two blows by reducing global warming by reducing our use of fossil fuels."


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