Scientists already know how climate change is impacting the Western United States – higher temperatures have translated to earlier spring snow melts, precipitation is falling more as rain instead of snow at higher elevations and there’s reduced runoff and streamflow. “When it comes to global warming, a lot of what we know with really high confidence has to do with temperature,” said Daniel Swain, a climate scientist at the University of California Los Angeles Center for Climate Science.

But when it comes to how climate change is impacting precipitation, the story is more complicated. “Precipitation regimes on Earth vary greatly,” said Swain. “What that means is that it’s actually possible in many cases for the response of precipitation to climate change to be quite different from place to place.”

A new study, published Monday in the journal Nature Climate Change and coauthored by Swain, provides new insight into how precipitation is changing in California and why that’s going to be important to water managers and others. The paper found that the region’s “already variable year-to-year climate is likely to become even more volatile.”

Previous climate models have shown that there didn’t appear to be much change in annual average precipitation in California or changes were unknown, even under aggressive warming scenarios. But Swain and his colleagues wanted to know if that was because there was really little change in precipitation or if there were big changes at either end of the spectrum that balanced each other out when looking at yearly averages.

It turns out that the big story about precipitation in California has to do with extremes. Such precipitation whiplash is already occurring to some extent and will be increasing in California this century if nothing is done to curb greenhouse gas emissions.

The first kind of precipitation whiplash is from year to year, meaning that we’ll see more extraordinarily dry years followed by incredibly wet years – in the way California’s five-year drought ended in 2017 in one of the wettest winters on record.

The second kind of whiplash happens within California’s November-to-March wet season, where there are increasing likely to be very dry months followed by very wet months. That happened this year when the winter was nearly bone dry until an onslaught of March storms.

The researchers also found that the wet season seems to be shrinking. Instead of wet weather arriving throughout the regular five months or so of winter, there’s likely to be less precipitation during the autumn and spring months and more in the mid-winter months – although that wasn’t the case this year.

Surprisingly for researchers, many of these changes will occur across the entire state, although some aspects will hit Southern California harder.

“One of things that was robust everywhere was a really big increase in the likelihood of really big events, which is uniformly distributed from” Northern California to Southern California, Swain said.

Overall, the scientists found that precipitation extremes of different kinds will occur, with only small changes in the mean precipitation across the rainy season. But by the end of the century “the frequencies of both extreme dry seasons and whiplash events increase by over 50 percent over much of the state, and the frequency of extreme wet events increases by well over 100 percent nearly everywhere,” the report states. “This remarkable divergence between simulated future mean and extreme climate is especially pronounced across Southern California.”

The findings are important from a scientific perspective, Swain says, because little change versus huge swings in extremes suggests something really different about the way the atmosphere is responding to warming. But more important is what this means from a practical perspective. “If you try to adapt for precipitation that is the same as it used to be versus precipitation that is wildly more variable than it used to be – there are very different actions you might take,” said Swain.

Geeta Persad, a climate scientist with the Union of Concerned Scientists, says that the paper “demonstrates that we really have to be a lot smarter about how we integrate climate change into water management decisions.” And the kind of precipitation whiplash that was discussed in the findings is what can compound risks like the deadly Montecito mudslides this winter in Southern California, where heavy rains hit hillsides ravaged by a recent wildlife.

The paper also looked at the likelihood of major catastrophic events, like the megaflood that hit California during the winter of 1861–62, when several years of drought were followed by more than a month of high rainfall that devastated huge swathes of the state. The floods ended up being the worst in recorded history across much of the West.

The researchers found that a similar event “is more likely than not to occur at least once between 2018 and 2060, and that multiple occurrences are plausible by 2100 on a business-as-usual emissions trajectory.”

Said Swain: “That’s eyebrow-raising to me, given what the implications would be for infrastructure, for the environment, for everything else in California, as well.”

This underscores the need for water managers to plan, not for the averages, but for extremes, says Adrienne Alvord, Western States director at the Union of Concerned Scientists. “Imagine if our building codes were all designed for an average earthquake – that wouldn’t work very well when we get a really big one,” she said. “In terms of water infrastructure, what we need is for our water managers to be planning for the extreme.”

Part of the problem is that water managers don’t always have the training and background to understand what climate models mean, says Alvord. But California is working to change that with the passage of Assembly Bill 2800 in 2016 that makes climate change a key part of infrastructure planning.

“We created a process for the state to bring together scientists with the people who design and engineer infrastructure to discuss how we need to change our planning processes and that is going on right now,” she said. “There will be a series of recommendations that will be coming out this summer to try and help water managers better understand how to do this.”