California’s droughts and deluges are a sign of the weather “whiplash” to come

When California was in the midst of its wettest winter in 100 years in February of last year, 20 inches of rain fell in the Sierra Nevada over just three days, sending a record flow of water into the lake held back by the tallest dam in the United States.

Completed in 1968, the aging Oroville Dam was suddenly holding back a reservoir at 151 percent of its capacity, forcing operators to relieve the pressure via a spillway. But they soon noticed unusual flow patterns, and when they stopped the outflow, they saw that the concrete had buckled and a gaping, widening crater had formed in the middle of the spillway.

With a looming threat of collapse — and the possibility of trillions of gallons of water suddenly escaping — authorities ordered 188,000 people downstream to evacuate.

The water level eventually fell without breaching the dam, and engineers shut down the spillway to investigate the damage. But the whole ordeal was jarring for California, which had just been parched by a record drought that left lakes dry, lawns yellow, and restaurants not allowed to serve water unless it was specifically requested.

New research published Monday in the journal Nature Climate Change says “precipitation whiplash” like this is going to happen more often in California as the climate changes. Both the rainstorms and the droughts are going to get more severe. And the state’s roads, bridges, and dams aren’t ready for it.

Daniel Swain, a climate scientist at the University of California Los Angeles and an author of the new paper, studies how global climate change plays out in regional extreme weather. Many of the discussions on the impacts of climate change revolve around averages and extremes, he says, while the more practical and overlooked concern is actually volatility.

“We also call it ‘whiplash,’” Swain said. “Essentially [it’s] referring to rapid transitions between extremes of opposing character, so between extreme wet and extreme dry. Or in this case, from extreme dry to extreme wet.”

California makes a great case study for whiplash. Its summers are very dry, but it can get massive precipitation in the winter. And season to season, it can shift from no rain to downpours from atmospheric rivers.

In the Nature Climate Change study, Swain and his collaborators used historical records and model simulations to project how wet and dry spells would change in California over the coming century.

The team found that the number of very high-precipitation seasons will increase between 100 and 200 percent across California relative to the 2016-’17 season under a business-as-usual greenhouse gas emissions scenario. The number of extremely dry years will also increase, especially in the southern part of the state.

And seasonal whiplash events in California where the weather goes from extremely wet to extremely dry and vice versa will increase by 50 percent by the end of the century.

The researchers also studied the probability of having another flood event like the Great Flood of 1862, the largest flood on record for California, Nevada, and Oregon, which killed thousands and created inland seas. They found that the likelihood of such a deluge will increase threefold over the coming century compared to preindustrial times.

Another key finding is that worst flooding doesn’t necessarily coincide with the wettest winters. An extremely wet season isn’t as harmful if the precipitation is spread out over months, giving rivers, waterways, and sewage systems time to absorb the water. If more rain falls than the environment or infrastructure can handle at a given time, it starts gushing through gullies and inundating streets.

As a result, flooding can increase even when the amount of precipitation averaged over the season holds steady. This sharpening of precipitation peaks is projected to increase as well, so more rain will fall over shorter time periods.

That’s because as average temperatures rise globally, more moisture enters the atmosphere, increasing the amount of available precipitation. But this moisture also perturbs air currents, so in some years, Pacific moisture is all channeled toward California via atmospheric rivers while in other years, this concentrated plume discharges elsewhere, leaving the Golden State to dry out.

The upshot is that even though precipitation will only see a modest increase on average throughout the state in the coming years, the consequences will be more severe because some of the individual storms are expected to be more intense.

These findings highlight how talking about climate change solely in terms of averages obscures its true impacts.

“The problem with that is it potentially can miss a lot of the things we actually care about, especially in California, which is a place that has a climate that is intrinsically variable,” Swain said. “The mean doesn’t really describe the typical climate state here.”

And the threat of a whiplash poses a huge challenge for California’s infrastructure, which is already struggling to anticipate and build for the more modest impacts of climate change since few planners know how to incorporate climate forecasts into their work.

“No one was trained for this when I was going to school, and that’s true for a lot of state managers,” said Adrienne Alvord, the western states director for the Union of Concerned Scientists. “When you are talking about a change in the climate that means the outliers are now the norm, how do you plan for that?”

Previously, planners would look at 100-year and 500-year averages of likelihoods for different types of weather events, essentially building infrastructure to handle averages with a few nominal provisions for extremes. But that often leads to inadequate planning for the inevitable storm surges and flash floods that could lead to catastrophes.

One example of this is the San Francisco-Oakland Bay Bridge, a $6.5 billion, 10-lane elevated roadway carrying 270,000 vehicles a day. It’s built to withstand a one-in-1,500-year earthquake, but a 2015 report from the Metropolitan Transportation Commission found that parts of it would be underwater during a 50-year storm surge even at today’s sea levels:

It was found that this baseline scenario results in inundation across the westbound lanes of the I-80 approach, the westbound portion of the toll plaza, the Emeryville Crescent tidal wetland, Radio Beach, three radio towers and associated facilities, and several partially paved access roads.

The report adds that “sensitive assets may suffer irreversible damage if exposed to any amount of water, even temporarily.” The harm from a storm surge is therefore far more extensive than from just the slowly rising seas.

“The problem, as with earthquakes, is not the mean but the more extreme events,” Alvord said.

And with a projected increase in whiplash seasons, planners will have to simultaneously balance competing problems, like implementing flood control measures while also trying to store water ahead of shortages. Meanwhile, a flood akin to the one in 1862 hasn’t yet tested California’s modern infrastructure, and researchers expect that such an event would wreak upward of $1 trillion in damages to the state.

“This is emerging science, and it hasn’t made its way into the planning process, and that’s what we have to address,” Alvord said.