Many regions of the world will enter nearly permanent drought or pluvial (wet) conditions in the coming decades, according to researchers from half a dozen institutions, including the University of Hawaiʻi at Mānoa, who investigated what the future might hold in terms of rainfall and soil moisture. 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.
In some areas of the western United States, for instance, 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.
“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 (SOEST). “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.”
What is drought in a changing climate?
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 a pluvial. 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 United States may have already crossed this benchmark, and there are other places headed that way, including Australia, southern Africa and western Europe.
“If regions are projected to be in permanent drought conditions, then what really is a drought?” asked Sloan Coats, assistant professor in the SOEST Department of Earth Sciences and an author of the study. “The terms drought and pluvial, and the thresholds that are used to define them, can become vague in a changing climate.”
Since climate is an inherently chaotic system, ensembles can be used to robustly account for changes to drought and pluvial thresholds with climate change. 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.
This work is an example of UH Mānoa’s goal of Excellence in Research: Advancing the Research and Creative Work Enterprise (PDF), one of four goals identified in the 2015–25 Strategic Plan (PDF), updated in December 2020.
