It could be time to reconsider the very definition of drought, according to new research from half a dozen institutions, including the University of Hawai‘i at Mānoa, which investigated what the future might have in store for rainfall and soil moisture.
Many regions of the world will enter nearly permanent drought or pluvial (wet) conditions in the coming decades, according to a press release from UH-Mānoa. The findings, published in Proceedings of the National Academy of Sciences, reveal the importance of rethinking how these events are classified as well as how communities adapt to a changing environment.
“When we talk about being in a drought, the presumption is that eventually the drought will end and conditions will return to normal,” said Samantha Stevenson, lead author of the study, assistant professor at the University of California, Santa Barbara and former postdoctoral fellow at UH-Mānoa’s School of Ocean and Earth Science and Technology, in the press release. “But if we’re never returning to normal, then we need to adapt all of the ways that we manage water with the expectation that normal will continually be drier and drier every year.”
In some areas of the western United States, for example, conditions have blown past severe and extreme drought into exceptional drought. But rather than add more superlatives to the descriptions, it could be time to reconsider the very definition of drought.
“If regions are projected to be in permanent drought conditions, then what really is a drought?” asked Sloan Coats, assistant professor in the School of Ocean and Earth Science and Technology Department of Earth Sciences and an author of the study, in the press release. “The terms drought and pluvial, and the thresholds that are used to define them, can become vague in a changing climate.”
According to the release, a drought or pluvial is when conditions are drier or wetter than some threshold, usually defined by the historical range of water availability. To project future rainfall and soil moisture, the researchers turned to a new collection of climate models, each run many times with slightly different initial conditions, in what scientists call an “ensemble.”
The team used the ensembles to calculate the year in which soil moisture will exceed the threshold that historically defined either a drought or pluvial. The press release said the results show that many regions are projected to be in permanent drought or pluvial conditions by the end of the 21st century.
The western U.S. might have already crossed this benchmark, and there are other places headed that way, including Australia, southern Africa and western Europe.
The researchers found that regions in permanent drought or pluvial conditions will continue to experience soil moisture variations similar to today, but relative to those much drier or wetter thresholds. Precipitation, on the other hand, will become much more extreme, with implications for water management. For instance, in addition to adapting infrastructure to drier soil moisture in the American West, that infrastructure will also need to handle more intense rainfall.
“Essentially, we need to stop thinking about returning to normal as a thing that is possible,” said Stevenson.