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Researchers from half a dozen universities investigated what the future might hold in terms of rainfall and soil moisture, two measurements of drought. The team, led by UC Santa Barbara's Samantha Stevenson, found that many regions of the world will enter permanent dry or wet conditions in the coming decades, under modern definitions. The findings, published in the Proceedings of the National Academy of Sciences, reveal the importance of rethinking how we classify these events as well as how we respond to them.
"Essentially, we need to stop thinking about returning to normal as a thing that is possible," said Stevenson, an assistant professor in the Bren School of Environmental Science & Management. This idea affects both how we define drought and pluvial (abnormally wet) events and how we adapt to a changing environment.
A drought is when conditions are drier than expected. But this concept becomes vague when the baseline itself is in flux. Stevenson suggests that, for some applications, it's more productive to frame drought relative to this changing background state, rather than a region's historical range of water availability.
To predict future precipitation and soil moisture levels, Stevenson and her colleagues turned to a new collection of climate models from different research institutions. Researchers had run each model many times with slightly different initial conditions, in what scientists call an "ensemble." Since the climate is an inherently chaotic system, researchers use ensembles to account for some of this unpredictability.
The results show a world where certain regions are in permanent drought while others experience perennial pluvial for the rest of the 21st century. The team calculated the year in which average soil moisture will exceed the threshold that defines either a megadrought or a megapluvial. "In other words, at what point do average conditions exceed what we would consider a megadrought if it happened now, [and never return to 'normal']" Stevenson said.
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