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The longest drought, redefined

Climate scientists reconsider the meaning and implications of drought

Maps of the American West have featured ever darker shades of red over the past two decades. The colors illustrate the unprecedented drought blighting the region. In some areas, conditions have blown past severe and extreme drought into exceptional drought. But rather than add more superlatives to the descriptions, one group of scientists believes it's time to reconsider the very definition of drought.

Researchers from several universities investigated what the future might hold in terms of rainfall and soil moisture, two measurements of drought. The team, led by University of California, 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, supported by the U.S. National Science Foundation and published in 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. This idea, she believes, affects both how to define drought and pluvial, or abnormally wet, events and how people 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.

"As climate changes, we have to change the way we think about climate, as what used to be extreme becomes the new normal," said Eric DeWeaver, a program director in NSF's Division of Atmospheric and Geospace Sciences. "The idea of redefining droughts and pluvials relative to a moving baseline could be very helpful for efforts to manage water resources."

To predict future precipitation and soil moisture levels, Stevenson and her colleagues turned to a new collection of climate models. The researchers ran each model multiple 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 conditions for the rest of the 21st century. The team is calculating 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?" Stevenson asked.