Tropical Storm Hilary made landfall in Southern California this week, causing flooding, downed power lines, and mudslides in the region before becoming a post-tropical and moving across other parts of the Western U.S.
California is no stranger to flooding and landslides — and climate change makes the likelihood and severity of these disasters even worse. One mechanism behind this growing risk is the growing intensity and frequency of atmospheric rivers, which are long and narrow bands of concentrated moisture that can cause extreme precipitation.
Flood damages from atmospheric river storms in the Western U.S. could triple to $3 billion a year by the end of the century due to climate change, according to Scripps Institution of Oceanography at UC San Diego. A single major precipitation event can have significant impact. Earlier in 2023, a series of atmospheric river storms in California cost the U.S. up to $7 billion in economic losses.
But unlike atmospheric rivers, tropical storms rarely make landfall in California, and Hilary was the first such storm to hit the state since 1997. Relatively cool temperatures off the Pacific Coast typically weaken storms before they reach the state, and east-to-west winds also tend to blow storms towards the open ocean.
ClimateCheck Advisor Daniel Swain, a climate researcher at UCLA, writes on the Weather West blog that warming oceans under climate change could plausibly, “to a modest but probably not very large degree” increase the odds of storms similar to Hilary hitting California.
“An oft-asked question this week: will global warming increase the odds of landfalling tropical cyclones in California? My answer: It’s plausible, to a modest but probably not very large degree. However, there have not yet been any formal studies looking into this. So why do I say it’s plausible, but as yet undemonstrated? Well, warming oceans will probably somewhat weaken barrier #1. Oceans near California still won’t be warm enough to sustain tropical cyclogenesis even in a much warmer climate, but they may allow storms with a favorable trajectory to weaken less quickly (especially during strong El Nino years, like the present). The other two barriers, however, will likely remain largely intact–meaning that such events will most likely remain rare, but perhaps somewhat less so. (I’ll update this hypothesis in the future if relevant studies pop up!)”
Swain works with ClimateCheck on developing metrics, leveraging existing data, generating new data on climate risk, and communicating information about our products. He received his PhD in Earth System Science from Stanford University, and holds joint appointments at the Institute of the Environment and Sustainability at UCLA, the Capacity Center for Climate and Weather Extremes at NCAR, and at The Nature Conservancy. His research focuses on the dynamics and impacts of extreme climate events in a warming climate.
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