Many people are attracted to large parts of California for their reliably pleasant Mediterranean climate. It can be a welcome break for visitors weary of Nor’easters and scorching summers. But in coming decades, California and the rest of the West Coast could see increasingly wild swings in weather – a consequence of continued climate change.

That’s the conclusion of a new study published in Nature Climate Change by researchers at the University of California at Los Angeles, UC-Irvine, and The Nature Conservancy. The group ran computer simulations that suggest the state will experience more abrupt transitions between severe drought and winter deluges as the century advances.

“My colleagues and I set out to assess how the increasing emission of greenhouse gases will affect California precipitation,” lead author Daniel Swain, from UCLA, wrote in a post at his California Weather Blog. “Rather than considering average precipitation, as most previous studies have, we instead focused on wet and dry precipitation extremes specifically using a large ensemble of climate model simulations.”

Swings between drought and flood long have been part of California’s past, so the state is no stranger to that kind of weather volatility, Swain says. But in a warming world, those swings are expected to grow more frequent, the researchers found.

A ‘serious test’ for state’s stressed water resources

“We find that the occurrence of both extreme wet and extreme dry events in California – and of rapid transitions between the two – will likely increase with atmospheric greenhouse gas concentrations,” Swain wrote. “The rising risk of historically unprecedented precipitation extremes will seriously test California’s existing water storage, distribution, and flood protection infrastructure.”

Over the past two years, the state has experienced this kind of weather whiplash. California’s drought between 2012 and 2016 hammered its agriculture industry and led to water use restrictions statewide.

But by early 2017, winter storms were inundating the state – and the rainy season ended up ranking the second rainiest in 122 years, with a state average of more than 30 inches of rain between October 2016 and March 2017. A series of “atmospheric river” storms carried massive amounts of moisture from the tropics in the Pacific Ocean to the West Coast, bringing desperately needed snowpack to the Sierra but also drenching much of the state with torrential rain. California’s water infrastructure had problems coping: a spillway at the Oroville Dam at the north end of the state caved in, forcing the emergency evacuation of a quarter million people.

And just this past fall, scorching hot weather and powerful winds fueled deadly wildfires across California – most prominently in Santa Rosa, north of San Francisco, and in Montecito near Santa Barbara. These were followed by huge rainstorms, with Montecito in particular enduring deadly mudslides.

More intense weather variability ahead

The latest study quantifies how this kind of weather variability is expected to intensify:

• Very wet rainy seasons similar to the 2016-17 winter have historically occurred about four times a century. By 2100, this frequency is expected to increase by 100 percent to 200 percent – in other words, to eight to 12 times a century.
• By 2100, the number of extremely dry years similar to those seen in 1976-77 and 2013-14, now expected once every 100 years on average, could occur 50 percent to 150 percent more frequently than today – that is, occurring 1-1/2 to 2-1/2 times a century.
• Extremely wet years and extremely dry years are expected to become more common, and the abrupt switch between one to the other could also happen more frequently: these transitions could occur 50 percent to 100 percent more often as the century advances.

Abrupt transitions between extremely wet and extremely dry periods can lead to catastrophe, as with California’s cycle of wildfires and mudslides.

How bad can such extremes get? Well, the biblical winter of 1861-62 offers a hint. The Great Flood of 1862 was preceded by a month-and-a-half of atmospheric river storms that turned California’s Central Valley into an inland sea, drowning thousands of people and horses and 800,000 cattle under 30 feet of water in some places. Los Angeles was drenched with 66 inches of rain; a normal year currently yields about 15 inches. Sacramento, at the junction of the American and Sacramento rivers, was hit particularly hard. Snow buried the Sierra, only to be washed down mountain slopes and into streams and rivers after warm rainstorms pelted the mountain range.

Back in 2010, the U.S. Geological Survey projected what a similar series of storms in modern times might do to the state. USGS dubbed the project “ARkStorm.” As much as 10 feet of rain falls over a month in the nightmare scenario, flooding thousands of square miles. Widespread flooding and thousands of mudslides devastate California, costing some $720 billion (a conservative number eight years later). Flood depths in some areas reach 20 feet, and 1.5 million people are evacuated from their homes.

Swain’s analysis suggests the risk of a 40-day deluge like the one that caused the 1862 flood will grow by 300 percent to 400 percent by 2100. In his blog, he includes another chilling result from his study (the italics are in the original):

“One specific statistic that my colleagues and I found particularly eyebrow-raising: on our current emissions trajectory, at least one occurrence of an 1862-level precipitation event is more likely than not over the next 40 years (between 2018 and 2060), with multiple occurrences plausible between now and the end of the century. In practical terms, this means that what is today considered to be the 200-year flood – an event that would overwhelm the vast majority of California’s flood defenses and water infrastructure – will become the “40-50-year flood” in the coming decades.”

More to read:

“Climate change could leave Californians with ‘weather whiplash,’” CNN, April 23, 2018″

“Megastorms could drown massive portions of California,” Scientific American, January 2013.

“Great flood in California; great destruction of property; damage $10,000,000,” New York Times, Jan. 21, 1862

“An incredible 45-day storm turned California into a 300-mile-long sea … and it could happen again,” Business Insider, Feb. 17, 2016