California's wild extremes of drought and floods to worsen as climate warms

Over the past couple of years, California lurched from its worst drought ever to disastrous, record flooding.

Now, a new study suggests the frequency of these rapid, year-to-year swings from extreme dry to wet conditions — which the study authors dub "precipitation whiplash events" — may become more common in California’s future as a consequence of man-made global warming.

The study authors say the frequency of whiplash events — in which the region transitions suddenly from very dry to very wet conditions — "will double in southern California by the end of the century." 

That would mean such whiplash events would occur around eight times per century in a warmer climate versus four times per century in a more stable climate, said UCLA climate scientist Daniel Swain, lead author of the study.

The extremes would also increase in northern California, but not at the dramatic rate of southern California.

Swain and his team of researchers used computer model simulations to predict the future climate patterns in California. 

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Of particular concern would be a repeat of the infamous 1862 flood that likely killed thousands of people and left much of the state's central valley underwater: A repeat "would probably lead to considerable loss of life and economic damages approaching a trillion dollars," the study said. 

That's about a third of the state's annual gross domestic product, which is the most of any U.S. state and makes California the world's sixth-biggest economy. 

Here's the science behind how it works: The amount of water vapor in the atmosphere increases rapidly as the atmosphere warms. This big increase in water vapor can lift the "ceiling" on extreme rainstorms and snowstorms, leading to a greatly increased potential for very wet events.

Global warming could also make the dry times even drier. In addition to the water vapor increase, climate change is likely to change prevailing wind and storm track patterns over the Pacific in a way that favors both wetter California winters in some years and drier winters in other years, Swain said.

Overall, this large shift in precipitation patterns "may pose large challenges for regional water management in the 21st century." Extensive upgrades to flood control and water infrastructure would have to be made across the state to deal with the additional precipitation in wet winters.

Daniel Cayan, a climate researcher at Scripps Institution of Oceanography who was not involved in the study, told the Los Angeles Times that the study "pushed the envelope forward in elucidating how the extremes on the wet and dry side have increased."

"The thing that I think is really nice about this paper is that they identify … a period in which there could be a really profound wet spell. That's very important," Cayan said.

The study only focused on California: "This region is a bit different than other places, since we have such a narrow and well-defined rainy season with naturally large swings from wet to dry from year to year," Swain said.

"But there is abundant evidence from other studies that extreme wet events are increasing in most areas, yet that warming will (somewhat counterintuitively) also bring increased aridity to many of these same places."

The study appeared in the peer-reviewed British journal Nature Climate Change.