The chance of a decade-long drought in the southwestern U.S. this century is at least 80 percent and the chance of a multi-decadal drought is as much as 50 percent, according to a research team that includes University of Arizona scientists.
The new report also suggests the current global climate models critically underestimate the risk of multi-decadal drought risk in the western U.S.
"It would be hard to overstate the risk of megadrought in the Southwest in this century," said lead author Toby R. Ault, who began the research at the UA as part of his doctoral work.
The research is the first to put numbers on estimates of the risk of future megadrought in the U.S., said Ault, now an assistant professor of earth and atmospheric sciences at Cornell University in Ithaca, New York.
The study is published in the American Meteorological Society's Journal of Climate.
Droughts of 20 to 60 years have occurred in the Southwest in the past, paleoclimate records such as tree rings show.
Ault and his colleagues used information from paleoclimate records, the current climate models and created innovative statistical tools to generate quantitative estimates of drought.
"Nature can spit out megadroughts at a higher rate than the current models show," he said. "Even without climate change, these prolonged periods of aridity are not uncommon."
Adding in climate change makes the risk of megadrought higher, he said.
UA co-author Julia Cole said, "Arizona has been in a long-term drought since 1999. Now imagine the current drought lasting twice as long - that's the kind of disaster that's happened in the past. Our work suggests such a drought is more likely in a warming world."
In computer models, while California, Arizona and New Mexico will likely face drought, the researchers show the chances for drought in some areas of Washington, Montana and Idaho may decrease.
"These results help us take the long view of future drought risk in the Southwest -- and the picture is not pretty. We hope this opens up new discussions about how to best use and conserve the precious water that we have," said Cole, a UA professor of geosciences and of atmospheric sciences.
Other co-authors of the study, "Assessing the Risk of Persistent Drought Using Climate Model Simulations and Paleoclimate Data," include Jonathan T. Overpeck and David M. Meko of the UA and Gregory T. Pederson of the U.S. Geological Survey in Bozeman, Montana.
The National Science Foundation, National Center for Atmospheric Research, the U.S. Geological Survey and the National Oceanic and Atmospheric Administration funded the research.