Arid Areas of the U.S. Face Prospect of ‘Megadroughts’

By Tim Radford, Climate News NetworkThis Creative Commons-licensed piece first appeared at Climate News Network.

LONDON — The Central Plains and Southwest region of the US face “unprecedented” droughts later this century, according to new research.

While Midwest states have experienced ever more flooding over the last 50 years, the regions already suffering from extremes of aridity are being warned to expect megadroughts worse than any conditions in the last 1,000 years.

Climate scientist Benjamin Cook, of NASA’s Goddard Institute for Space Studies, New York, and colleagues report in a new journal, Science Advances, that they looked at historical evidence, climate projections and ways of calculating soil moisture.

They found that the drought conditions of the future American west will be more severe than the hottest, most arid extended droughts of the 12th and 13th centuries, which are thought to have played a role in ending the once-flourishing Pueblo culture of the American Southwest.

Computer predictions

The growth rings of trees provided the evidence for reconstructions of what climatologists call the warm Medieval period, and the researchers matched the picture from the past with 17 different computer model predictions of the climate later in the 21st century.

The conclusions were ominous: nearly all the models predicted that the Plains and the Southwest would become drier than at any time in the last 1,000 years.

Even though winter rain and snowfall could increase in parts of California — currently in the grip of calamitous drought — in the decades to come, overall there will be lower cold season precipitation and, because of higher temperatures, ever more evaporation and ever more water demand for the surviving vegetation.

The authors conclude: “Ultimately, the consistency of our results suggests an exceptionally high risk of a multidecadal megadrought occurring over the Central Plains and Southwest regions during the late 21st century, a level of aridity exceeding even the persistent megadroughts that characterised the Medieval era.”

I was honestly surprised at just how dry the future is likely to be

Co-author Toby Ault, head of the Emergent Climate Risk Lab at Cornell University, warned of future megadroughts only last year. He says: “I was honestly surprised at just how dry the future is likely to be.”

But to the north, in the American Midwest, conditions have begun to change in a different way. Iman Mallakour and Gabriele Villarini, of the Department of Civil and Environmental Engineering at the University of Iowa, collected evidence from 774 stream gauges in 14 states from 1962 to 2011.

Flood events

They report in Nature Climate Change that a third of them had recorded a greater number of flood events, and only one in 10 recorded a decrease.

The pattern of increase extended from North Dakota south to Iowa and Missouri, and east to Illinois, Indiana and Ohio.

The region was hit by economically-disastrous, billion-dollar floods in 1993, 2008, 2011, 2013 and 2014. The researchers wanted to see whether flooding was really on the increase, or whether perception of greater flooding was what they called “an artefact of our relatively short collective memory”.

The result is a confirmation of perceived increase. It was not an artefact.

“It’s not that big floods are getting bigger, but that we have been experiencing a larger number of big floods,” Dr Villarini says.