At the end of this century, scientists believe that megadroughts – extended drought events that can last two decades or more – will be more severe and longer than they are today in western United States. Coupled with worsening climate change, those droughts will likely be more dangerous and extreme.

“In the future, not only will a given decade probably be drier on average,” said co-author Flavio Lehner, assistant professor of Earth and atmospheric sciences in the College of Agriculture and Life Sciences. The department is shared with the College of Engineering. “But that decade will also feature more extremes within that decade, than for example, a decade today.”

The research, “Twenty-First Century Hydroclimate: A Continually Changing Baseline, With More Frequent Extremes,” published March 14 in the Proceedings of the National Academy of Sciences. The paper was led by Samantha Stevenson, assistant professor, University of California, Santa Barbara. Joining Stevenson and Lehner on the paper is John Fasullo ’90, a project scientist from the National Center for Atmospheric Research.

“If you think of a decade in the future, let’s say from 2090 to 2100, it’s going to be a warmer world and – in certain drought-prone areas – also consistently drier,” Lehner said. “Here, we were able to better quantify the timing and spatial pattern of this transition using a so-called climate model large ensemble, which offers an unprecedented amount of data.”

The large ensemble simulations represent many possible futures and are used by climate scientists to derive robust statistics of decades ahead, Lehner said. “People now ask if this is a ‘new normal,’ but – with worsening climate change – it’s rather a constantly changing world,” he said.

Lehner explained that droughts or megadroughts are triggered naturally, but can be made worse by climate change. “We’re transitioning into a different state that has little to do with the last 50 or 100 years of statistics,” he said.

“There will still be variability, though, like higher heat and lower heat,” Lehner said. “Long-term change is attributable to greenhouse gases and anthropogenically induced warming. The small wiggles in smaller trends are just weather variations that happen year to year. In any given year, the weather can be a little warmer, or drier or wetter or a little colder.”

Another paper focusing on drought, “Precipitation Trends Determine Future Occurrences of Compound Hot–Dry Events,” published March 14 in Nature Climate Change, used the same climate model large ensembles to show that future precipitation changes will be key to modulating future compound hot–dry events.

Lehner served as a co-author on this paper; Emanuele Bevacqua, of Helmholtz Centre for Environmental Research, was the lead.

The paper shows that compound hot-dry events – rare combinations of hot summers that are also very dry – become frequent and intense. “But there is uncertainty with regards to the frequency and intensity,” he said, “thanks to our relatively poor understanding of how rainfall is changing in different places around the world.”

“We know the world is warming up everywhere,” Lehner said. “But we don’t know with certainty whether it’s going to rain more or rain less or going to stay the same.”

With this work, scientists aim to reduce such uncertainty. “On the one hand, we are making progress in quantifying changes in hot-dry compound events thanks to the new large ensembles,” Lehner said. “On the other hand, we need to better understand the role of rainfall in the future.”