February 12, 2015 The Earth Institute at Columbia University
During the second half of the 21st century, the U.S. Southwest and Great Plains will face persistent drought worse than anything seen in times ancient or modern, with the drying conditions “driven primarily” by human-induced global warming, a new study predicts.
The research says the drying would surpass in severity any of the decades-long “megadroughts” that occurred much earlier during the past 1,000 years — one of which has been tied by some researchers to the decline of the Anasazi or Ancient Pueblo Peoples in the Colorado Plateau in the late 13th century. Many studies have already predicted that the Southwest could dry due to global warming, but this is the first to say that such drying could exceed the worst conditions of the distant past. The impacts today would be devastating, given the region’s much larger population and use of resources.
“We are the first to do this kind of quantitative comparison between the projections and the distant past, and the story is a bit bleak,” said Jason E. Smerdon, a co-author and climate scientist at the Lamont-Doherty Earth Observatory, part of the Earth Institute at Columbia University. “Even when selecting for the worst megadrought-dominated period, the 21st century projections make the megadroughts seem like quaint walks through the Garden of Eden.”
“The surprising thing to us was really how consistent the response was over these regions, nearly regardless of what model we used or what soil moisture metric we looked at,” said lead author Benjamin I. Cook of the NASA Goddard Institute for Space Studies and the Lamont-Doherty Earth Observatory. “It all showed this really, really significant drying.”
The new study, “Unprecedented 21st-Century Drought Risk in the American Southwest and Central Plains,” will be featured in the inaugural edition of the new online journal Science Advances, produced by the American Association for the Advancement of Science, which also publishes the leading journal Science.
Today, 11 of the past 14 years have been drought years in much of the American West, including California, Nevada, New Mexico and Arizona and across the Southern Plains to Texas and Oklahoma, according to the U.S. Drought Monitor, a collaboration of U.S. government agencies.
The current drought directly affects more than 64 million people in the Southwest and Southern Plains, according to NASA, and many more are indirectly affected because of the impacts on agricultural regions.
Shrinking water supplies have forced western states to impose water use restrictions; aquifers are being drawn down to unsustainable levels, and major surface reservoirs such as Lake Mead and Lake Powell are at historically low levels. This winter’s snowpack in the Sierras, a major water source for Los Angeles and other cities, is less than a quarter of what authorities call a “normal” level, according to a February report from the Los Angeles Department of Water and Power. California water officials last year cut off the flow of water from the northern part of the state to the south, forcing farmers in the Central Valley to leave hundreds of thousands of acres unplanted.
“Changes in precipitation, temperature and drought, and the consequences it has for our society — which is critically dependent on our freshwater resources for food, electricity and industry — are likely to be the most immediate climate impacts we experience as a result of greenhouse gas emissions,” said Kevin Anchukaitis, a climate researcher at the Woods Hole Oceanographic Institution. Anchukaitis said the findings “require us to think rather immediately about how we could and would adapt.”
Much of our knowledge about past droughts comes from extensive study of tree rings conducted by Lamont-Doherty scientist Edward Cook (Benjamin’s father) and others, who in 2009 created the North American Drought Atlas. The atlas recreates the history of drought over the previous 2,005 years, based on hundreds of tree-ring chronologies, gleaned in turn from tens of thousands of tree samples across the United States, Mexico and parts of Canada.
For the current study, researchers used data from the atlas to represent past climate, and applied three different measures for drought — two soil moisture measurements at varying depths, and a version of the Palmer Drought Severity Index, which gauges precipitation and evaporation and transpiration — the net input of water into the land. While some have questioned how accurately the Palmer drought index truly reflects soil moisture, the researchers found it matched well with other measures, and that it “provides a bridge between the [climate] models and drought in observations,” Cook said.
The researchers applied 17 different climate models to analyze the future impact of rising average temperatures on the regions. And, they compared two different global warming scenarios — one with “business as usual,” projecting a continued rise in emissions of the greenhouse gases that contribute to global warming; and a second scenario in which emissions are moderated.
By most of those measures, they came to the same conclusions.
“The results … are extremely unfavorable for the continuation of agricultural and water resource management as they are currently practiced in the Great Plains and southwestern United States,” said David Stahle, professor in the Department of Geosciences at the University of Arkansas and director of the Tree-Ring Laboratory there. Stahle was not involved in the study, though he worked on the North American Drought Atlas.
Smerdon said he and his colleagues are confident in their results. The effects of CO2 on higher average temperature and the subsequent connection to drying in the Southwest and Great Plains emerge as a “strong signal” across the majority of the models, regardless of the drought metrics that are used, he said. And, he added, they are consistent with many previous studies.
Anchukaitis said the paper “provides an elegant and convincing connection” between reconstructions of past climate and the models pointing to the risk of future drought.
Benjamin I. Cook, Toby R. Ault, Jason E. Smerdon. Unprecedented 21st century drought risk in the American Southwest and Central Plains. Science Advances, 12 February 2015 DOI: 10.1126/sciadv.1400082
By David Perlman SF Chron February 12, 2015 Updated: February 12, 2015 9:18pm
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.”
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 Arizona 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.”