A study published Monday, however, turns those odds in a bad way, saying extreme weather swings from brutal dry spells to intense storms will become increasingly frequent, a phenomenon the authors dub “precipitation whiplash.”
Because of the warming atmosphere, the type of storms that produced the record flooding 156 years ago will probably be three to four times more frequent by the end of this ceyntury. That means San Francisco and Los Angeles are more likely than not to see an 1862-style deluge by 2060, according to the research published in the peer-reviewed journal Nature Climate Change.
Such a series of storms, involving about 40 days of punishing rain, would become more of a 50-year event — a 1-in-50 chance of happening in any given year, the authors figure.
“We know it can happen because it’s happened before,” said Daniel Swain, lead author of the new study and a climate researcher at UCLA. “But it’s something that’s outside anyone’s living memory and outside the planning envelope. There are questions about whether the infrastructure could stand up to this event.”
The rapid shift last year from a five-year drought to an extraordinarily wet winter is a taste of the whiplash that the authors expect more of. Northern California is projected to see 25 percent more of these dramatic transitions, and Southern California will see 100 percent more.
The study’s findings are a departure from most climate research, which has focused on long-term trends showing gradual warming over time with no major difference in total rainfall. The new paper suggests that the biggest problems may lie in the less-studied short term.
“If we only look at the average, we’re smoothing out everything we care about,” Swain explained. “There may be very little change in the averages (long term), but there are very large increases in the likelihood of extreme events like drought and flooding” in the short term.
The new study confirms that precipitation in California over the long haul is unlikely to change much, but also finds that it will come less often and in more intense bursts.
The main driver is water vapor in the atmosphere. As the air warms and evaporation increases with climate change, moisture levels rise, leaving more water for storms to wring out. There’s also research suggesting that changes in atmospheric circulation are tending to drive bigger storms.
The research also suggests that California’s wet season will shorten, with more rain arriving in winter and less in the fall and spring.
The increased likelihood of an 1862-type series of storms is a product of these projected precipitation trends.
“It’s still a rare event,” Swain said. “But it goes from being something at the outer end of what’s plausible to something that is a foreseeable thing in the not-too-distant future.”
When the mega-flood hit in January 1862, California had only been a state for a dozen years. Weather reports and damage assessments are sparse, but what records are available reveal disaster up and down the state.
The rain and snow, which probably arrived in a series of atmospheric rivers — bands of moisture carried by the jet stream over the Pacific — did not let up, bringing at least 40 days of precipitation. Meteorologists suspect that temperatures spiked toward the end of the storm series, causing much of the new snowfall to melt.
As a result, the Central Valley filled up with water for as much as 300 miles and as deep as 30 feet, reports suggest. Whole towns were submerged, and most roads and bridges became impassable. The state capital was temporarily shifted from Sacramento to San Francisco.
Flooding was rampant in the Bay Area, too.
While California has since developed one of the nation’s most extensive systems of dams and dikes to aid in flood control, many experts say the state’s waterworks will not entirely hold up to an 1862-level flood.
“We can only build to a certain level of flood protection,” said Dale Cox, a project manager who studies risk reduction for the U.S. Geological Survey. “Levees will be overtopped, they will be breached, and you will have problems as it relates to infrastructure.”
Cox, who is not affiliated with the new study, led a disaster preparedness exercise in 2011 that mimicked a flood similar to the one in 1862. The drill projected more than 1.5 million evacuations and over $725 billion in losses — greater than any earthquake would likely generate.
The authors of the new study arrived at their conclusions by simulating the climate impacts of increased greenhouse gases in the atmosphere, at the current rates of human emissions. If reductions are made, which is the goal of the 2015 Paris Agreement, the authors said the likelihood of more volatile weather, and a flood like in 1862, would drop significantly.
Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander
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