In August, Governor Gavin Newsom and officials from the Department of Water Resources released a new Water Supply Strategy, saying that because of California’s “hotter, drier climate,” the state needed to find at least 10 percent more water to supply its farms, cities, and industry by 2040.
“We are experiencing extreme, sustained drought conditions in California and across the American West caused by hotter, drier weather,” states the plan. “Our warming climate means that a greater share of the rain and snowfall we receive will be absorbed by dry soils, consumed by thirsty plants, and evaporated into the air.”
The plan says that steadily rising temperatures will overcome even a year or two of better-than-average or average rainfall in Southern California — as in 2018 and 2019 — and will not close what state officials call an “evaporative gap” that threatens California’s water supply.
This new state plan follows the climate science on “aridification.” That’s the scientific term for the “drying trend” that young climate scientist Samantha Stevenson of UCSB’s Bren School of Environmental Science and Engineering identified this year in an extensive global study of the 21st-century hydroclimate.
Stevenson said that she wanted to provoke new thinking about what we call drought.
“Drought is already normal in much of the western United States and other parts of the world, such as western Europe,” Stevenson said. “Part of the reason I wrote the paper was to try to say that we need to think about what we mean when we say ‘drought,’ because we’ve been using these definitions based on expectations from 40 years ago. What happens if you know the drought is never going to end?”
Stevenson’s work finds that “the soil moisture changes are so large that conditions that would be considered a megadrought” in western Europe and North America will become average. Stevenson said that the team’s modeling shows that the drying trend has in fact already emerged from the data in our region. What scientists call “megadrought” has become our norm.
Peter Gleick, a prominent researcher in water and climate in California at the Pacific Institute since the 1980s, seconded Stevenson’s finding that the concept of a “normal” climate has become profoundly misleading in the West.
“In general, the science about increasing drought severity and “aridification” is strong and worrisome, and builds on concerns about climate and water that scientists have been raising for literally decades,” Gleick said. “The climate is changing. What used to be normal is no longer normal, and we’re not approaching a new, stable normal — a ‘new normal.’ Rather we’re entering a period of rapid, unstable changes, and we’re not adequately prepared.”
An example of the breakdown of climate normality leading to rapid, unstable changes in the West could be found right next door to Stevenson in the work of Danielle Touma, her post-doctoral researcher at the Bren School last year, who this year published with Stevenson and others a paper on the deadly debris flows and floods that can occur when a deluge follows a firestorm.
This happened in January 2018 in Montecito, a scene that shocked Stevenson, Touma’s mentor at the Bren School, to the core.
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“I moved to Santa Barbara in October of 2017, two months before the Thomas Fire. I’m from the East, but I’ve lived in the West a long time, and I thought I knew about fires from places like Colorado,” Stevenson said. “But then I got to California and the Thomas Fire happened. It was apocalyptic — very scary. And then the rainstorm happened and the debris flows and it was kind of a wake-up call for me. I wondered how climate change would affect these sort of ‘compound events.’ And then Danielle arrived and was actually interested in quantifying that.”
Touma said that she was unusual as a climate researcher in that she had experience with both extreme fire weather and extreme rainfall events. Despite her familiarity with the data, she herself was surprised by the findings from the climate models, which show a 100 percent increase in such “compound events” in California by the end of the century, and a 700 percent increase in the Pacific Northwest. The study finds that extreme fire weather events will be followed within five years by three extreme rainfall events in the same locations in the West 90 percent of the time.
That means that this century, extreme wildfires in the West will usually be followed in short order by extreme rain, with the potential for massive damage. Daniel Swain, a colleague of Touma’s at the National Center for Atmospheric Research, described the significance of her study.
“The risk of post-fire hydrologic hazards is not just increasing because we’re seeing more intense wildfire burning conditions, although we are and that’s part of the story,” Swain said. “The other half of the story is that the very most extreme precipitation events are likely to increase rapidly as well. And so you put these two and two together, and we’re getting more extreme wildfires on the one hand, and more extreme precipitation on the other hand.”
“These kinds of signals are emerging a lot earlier than you would think,” Touma said, speaking gently of potential disasters. Both Touma and Stevenson said that they did not intend to become climate scientists earlier in their careers, but were drawn to the field for idealistic reasons.
“I didn’t start out as a climate scientist; I was an astronomer when I first went to grad school,” Stevenson said. “I wanted to tried to understand those big questions in space, but I found that given the magnitude of the climate crisis that was already unfolding in 2006, that those questions aren’t the ones we need to be focusing on when things were literally starting to heat up. I wanted to do science that will actually help humanity in some way.”
Touma began her career as a civil engineer interested in designing water systems, but when she began to look into the design work, she discovered that engineers had not adequately integrated the climate models to understand the impact on streams and rivers since the 1970s.
“So when I went to do my PhD, I decided to focus on climate change and its impacts,” she said, adding that Stevenson was the first woman mentor she had had in her career.
“It was really important for me to have that mentorship,” she said “Not only because Sam’s a woman, but because she’s an amazing scientist.”
As for Stevenson, she said that she is the designated climate modeler at the Bren School, and she is excited to work with graduate researchers and students in the cross-disciplinary environment of the graduate school. She especially likes working with students interested in climate.
“In California, when I tell people what I do, the reaction I often hear is: ‘Save us!’” she said. “It’s hard to argue that climate change doesn’t exist when we’re seeing the hottest temperatures we’ve ever seen and the largest fires that have ever happened year after year.”