Climate change doubles the risk of a major flood in California, study finds

As California grapples with the spread of wildfires and severe drought, the threat of flooding may seem like an unlikely cause for concern. The state could be hit by a megaflood in the not-too distant future, potentially putting millions across the state at risk, according to a new study.

The study, published Friday in Science Advances, found that climate change has doubled the chance of a catastrophic flood in California in the next four decades.

The study notes that California has had two historically severe droughts between 2012 and 2021, because the state is more accustomed to water scarcity than to overabundance in the modern era. It said historical and paleoclimate evidence suggests that the state is also subject to episodes of increased rainfall with climate change potentially accelerating the possibility of a major storm in the future.

The study notes that the state has seen extreme precipitation and severe subregional flooding events a number of times during the 20th century, including in 1969, 1986 and 1997. The study said that while those incidents hint at the latent potential of a future major flood, none have rivaled the 1861 'Great Flood of 1862.

It noted that the weekslong sequences of winter storms produced widespread catastrophic flooding across virtually all of California's lowlands, transforming the interior Sacramento and San Joaquin valleys into a vast inland sea nearly 300 miles in length. The effects of the Great Flood helped spur ARkStorm, a 2010 California statewide disaster scenario led by the U.S. Geological Survey.

It said that the event would likely cause widespread, catastrophic flooding, potentially causing the displacement of millions of people, the closure of key transportation corridors and nearly $1 trillion in overall economic losses. The researchers said that climate change is robustly increasing the frequency and magnitude of extremely severe storm sequences that can cause megaflood events in California, making such an event more likely.

Their analysis suggests that the present-day circa 2022 likelihood of historically rare to unprecedented 30 day precipitation accumulations has already increased substantially and that even modest additional increments of global warming will cause larger increases in likelihood, they said.

The study said that California could take measures to mitigate harm during a 21st century California megaflood, despite a grim outlook. It said that floodplain restoration and levee setbacks would help to lessen flood risk in urban areas. Emergency evacuation and contingency plans could be updated to reflect the possibility of inundation and transportation disruption extending far beyond that which has occurred in the past century, it said.