Droughts in the U.S. Southwest and Central Plains during the second half of the XXI century, could be drier and longer than drought conditions seen in those regions in the last millennium, says a team of scientists writing in the journal Science Advances.
North American drought projections: soil moisture 30 cm below ground projected through 2090-2100 for high emissions scenario RCP 8.5. Image credit: NASA’s Goddard Space Flight Center.
The team says the drying would surpass in severity any of the decades-long megadroughts that occurred much earlier during the past millennium – one of which has been tied by some scientists to the decline of the Anasazi or Ancient Pueblo Peoples in the 13th century.
Many studies have already predicted that the Southwest could dry due to global warming, but this one, based on projections from several climate models, is the first to say that such drying could exceed the worst conditions of the distant past.
“Natural droughts like the 1930s Dust Bowl and the current drought in the Southwest have historically lasted maybe a decade or a little less. What these results are saying is we’re going to get a drought similar to those events, but it is probably going to last at least 30 to 35 years,” said study lead author Dr Benjamin Cook of the NASA Goddard Institute for Space Studies and the Lamont-Doherty Earth Observatory.
“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. 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,” said co-author Dr Jason Smerdon of the Lamont-Doherty Earth Observatory.
“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. It all showed this really, really significant drying,” Dr Cook added.
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.
The current drought directly affects more than 64 million people in the Southwest and Southern Plains 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.
“Changes in precipitation, temperature and drought, and the consequences it has for our society are likely to be the most immediate climate impacts we experience as a result of greenhouse gas emissions. The findings require us to think rather immediately about how we could and would adapt,” said Dr Kevin Anchukaitis of the Woods Hole Oceanographic Institution, who was not involved in the study.
Much of our knowledge about past droughts comes from extensive study of tree rings conducted by Lamont-Doherty scientist Dr Edward Cook (Benjamin’s father) and others, who in 2009 created the North American Drought Atlas.
In the new study, Dr Cook, Dr Smerdon, and their colleague Dr Toby Ault of Cornell University, 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 team found it matched well with other measures, and that it provides a bridge between the climate models and drought in observations.
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 Prof David Stahle of the University of Arkansas, who was not involved in the study, though he worked on the North American Drought Atlas.
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Benjamin I. Cook et al. 2015. Unprecedented 21st century drought risk in the American Southwest and Central Plains. Science Advances, vol. 1, no. 1; doi: 10.1126/sciadv.1400082
