'Megadroughts’ predicted to ravage the Southwest

Houseboats sit idle in the low water of Lake Oroville with Bidwell Bar Bridge in the back ground near the end of last year when Calif ornia's second-larg est reservoir was close to the 1977 historic low at 26 percent of capacity. Scientists say the state’s current drought is unrelated to their dire predictions of the “mega droughts” that will plague the end of this century. less Houseboats sit idle in the low water of Lake Oroville with Bidwell Bar Bridge in the back ground near the end of last year when Calif ornia's second-larg est reservoir was close to the 1977 historic low at 26 ... more Photo: Leah Millis / The Chronicle Photo: Leah Millis / The Chronicle Image 1 of / 77 Caption Close 'Megadroughts’ predicted to ravage the Southwest 1 / 77 Back to Gallery

The Southwest, including California, along with the Great Plains states, will endure long-lasting “megadroughts” in the second half of this century, worse by far than anything seen in the past 1,000 years, a team of climate experts said Thursday.

The driving force behind the devastating droughts? Human-induced global warming, the team reported.

The new forecast is based on models of continued climate change that consider the slow pace of many nations to curb their output of greenhouse gases. The scientists contend there is at least a 20 percent chance that coming droughts will last 35 years or more, and a 50 percent chance that they will last 10 years or more.

“When you stack these model projections against the reconstruction of past climates, the results are so sobering that they have me ready to go out for a drink,” said Ken Caldeira, a climate scientist at the Carnegie Institution for Science and Stanford University, in an e-mail.

Caldeira, who was not connected with the study, said the scientists’ forecasts are based on “the most reliable model results available in the world today.”

Current drought unrelated

The report comes as California remains in a severe drought, but a leading scientist on the project said the current drought is not directly connected to the new forecast. “I do, however, want to be clear that our results do not say anything about the current and ongoing drought in California,” said climate scientist Benjamin I. Cook of the NASA Goddard Institute for Space Studies.

Cook worked with Toby R. Ault of Cornell University and Jason E. Smerdon of Columbia University’s Lamont-Doherty Earth Observatory to arrive at the forecast of an “unprecedented 21st century drought risk.”

Their report appears Friday in the new peer-reviewed journal Science Advances, published by the American Association for the Advancement of Science in Washington, which is meeting this week in San Jose.

The prediction of megadroughts, the report’s authors say, “contrasts sharply with the recent emphasis on uncertainty” in drought forecasts by the Intergovernmental Panel on Climate Change, which is considered the authoritative international agency on climate change.

“Future droughts will occur owing to significantly higher temperatures than ever recorded” in the Southwest and Great Plains regions, the scientists said, adding that these extremes are likely to cause “increased stress on natural ecosystems and agriculture.”

“In the not-too-distant future, the impending droughts will put a lot more pressure on all our resources,” said Kevin Anchukaitis, a climate researcher at the Woods Hole Oceanographic Institution on Cape Cod, Mass., who was not involved in the study. “We can head off some of the impact by reducing greenhouse gas emissions, but we’ll face difficult choices about reducing our hydropower capacity.”

Tree-ring records

To gather evidence of past megadroughts, the scientists used thousands of tree-ring records collected over many years by other researchers, as well as the histories of ancient droughts that affected the long-puzzling history of the Anasazi people in the American Southwest.

Those people and their widespread cultures disappeared around 1300 A.D. and the cause of their disappearance has long been disputed by archaeologists. But the tree-ring records and the radiocarbon dates of their plant remains show that they did undergo centuries of alternating heat-induced droughts — climaxed by what has been called the “great drought” of 1276 to 1299 A.D.

In his discussion of the megadrought report, Caldeira recalled visiting Anasazi ruins like Mesa Verde in Colorado and said “it looks like the droughts in store for us later this century will make the droughts that did in the Mesa Verde civilization look like child’s play.”

David Perlman is The San Francisco Chronicle’s science editor. E-mail: dperlman@sfchronicle.com