Southwest USA faces long term 'megadroughts' this century - The Ecologist

Tim Radford & Oliver Tickell

17th February 2015

The mean state of drought in the late 21st century over the Central Plains and Southwest will likely exceed even the most severe megadrought periods of the Medieval era.

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 - an unusually warm period climatologists call the Medieval Climatic Anomaly - which brought an end to the once-flourishing Ancient Pueblo culture of the American Southwest, forcing the people to migrate to other areas.

They report: "We have demonstrated that the mean state of drought in the late 21st century over the Central Plains and Southwest will likely exceed even the most severe megadrought periods of the Medieval era in both high and moderate future emissions scenarios, representing an unprecedented fundamental climate shift with respect to the last millennium.

"Notably, the drying in our assessment is robust across models and moisture balance metrics. Our analysis thus contrasts sharply with the recent emphasis on uncertainty about drought projections for these regions, including the most recent Intergovernmental Panel on Climate Change assessment report."

A remarkably drier future far outside the contemporary experience

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.

"Our results point to a remarkably drier future that falls far outside the contemporary experience of natural and human systems in Western North America, conditions that may present a substantial challenge to adaptation.

"Human populations in this region, and their associated water resources demands, have been increasing rapidly in recent decades, and these trends are expected to continue for years to come.

"Future droughts will occur in a significantly warmer world with higher temperatures than recent historical events, conditions that are likely to be a major added stress on both natural ecosystems and agriculture."

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."

And to the north, more frequent severe floods

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.

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."

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.

"While observational records from the central United States present limited evidence of significant changes in the magnitude of floodpeaks, strong evidence points to an increasing frequency of flooding", the paper explains. "These changes in flood hydrology result from changes in both seasonal rainfall and temperature across this region."

The result is a confirmation of the perceived increase, says Dr Villarini: "It's not that big floods are getting bigger, but that we have been experiencing a larger number of big floods."

The papers:

Tim Radford writes for Climate News Network.

Oliver Tickell edits The Ecologist.