Flooding rains and record snow in California last week marked another extreme swing of the state’s climate pendulum. The widespread downpours triggered mudslides that damaged homes and roads near some of the huge fire scars from last summer, and also brought some of the water the state will need to end a months-long hot and dry streak and douse a record-setting wildfire season that extended into January. 

The storm included a long stream of moisture from the subtropical Pacific Ocean, called an atmospheric river, that inundated the state from north to south for three days. Atmospheric rivers are concentrated streams of moist air, generally more than 1,200 miles long, up to 620 miles wide and about 1.8 miles deep.

Last week’s torrent hit Central California hardest. Several spots reported 24-hour rainfall records and the deluge washed out a 150-foot section of Highway 1 in the vulnerable Big Sur area, where the road clings to unstable mountains that drop steeply to the sea. Just two years ago, another nearby section of the highway was also destroyed by an intense storm.

Climate scientists say it’s important to understand how atmospheric rivers will change as global warming drives more extremes. They’ve recently created a scale to rate their strength. Between 1978 and 2017, the 10 most intense atmospheric rivers caused nearly half of all flood damage in the Western United States.

It could get worse. Stronger atmospheric rivers are part of California’s “whiplash” climate future, said Daniel Swain, a climate scientist with the National Center for Atmospheric Research. “One of the things that’s really clear, the intense ones are going to become much more intense,” he said. 

A 2018 study he co-authored showed that the odds for a scenario like the storm that caused the Great Flood of 1862, when Sacramento was partially destroyed, are “perhaps as high as 50% between 2018 and 2060.” A flood of that size would devastate towns and swamp much of California’s Central Valley, a breadbasket for the entire country.

At the other extreme are climate studies showing that California’s future droughts will be hotter and more intense. Even some years with near normal seasonal rain and snow can end up dry as increasingly hot summers simply steam the water out of the ground.

“The events that were once thought to have been vanishingly rare, are not so rare anymore,” Swain said. “What we’re not used to are these really extreme events, above the top end of the scale. Those are the ones that test infrastructure.”

Even absent an extreme event, changes to the timing, intensity and location of the moist flows will affect water supplies for towns, agriculture and natural ecosystems, he added. Water managers need to be prepared for climate-driven feast or famine cycles of rain, snow and runoff, for instance operating dams more flexibly to capture as much water as possible during big downpours, while still holding back enough to reduce river flood risks.

Building the necessary resilience won’t be easy, but since a flood on the scale of the 1862 event would cause billions in damage, figuring out ways to prepare for such risks are well worth it, Swain said. In the meantime, the threats from surging atmospheric rivers are rising fast. 

“This is a problem in a society where we build our infrastructure to last many decades, on the assumption that our climate does not change over 50 years,” he said. Many risks from climate extremes will become “greatly elevated with regard to how we’ve designed our infrastructure.”

Important Global Climate Pumps

Hazards similar to those in California are driving research into how global warming will change atmospheric rivers around the world, said University of Lisbon climate scientist Alexandre Ramos. In Europe, those intensifying moisture streams will inundate the British Isles and Scandinavia more frequently, but a northward shift detected by climate scientists could leave other areas thirsty.

Globally, atmospheric rivers are the largest transporters of freshwater and also distribute heat energy from equatorial regions toward the poles. Those flows are shifting because global warming is expanding bulging domes of stable, warm and dry air in the subtropics, which shunts atmospheric rivers poleward, Ramos said.

A wet Atlantic storm that pummeled parts of the already soggy United Kingdom last week carried swaths of moist air from around Florida all the way across the Atlantic in a weather pattern similar to an atmospheric river, climate and weather researcher Simon Lee wrote on Twitter.

The atmosphere can hold about 7 percent more moisture for every 1 degree Celsius of warming, so every atmospheric river can potentially hold more moisture. One recent study found that there has been an increase in the number of atmospheric rivers since the 1980s, Ramos said. 

But they are still relatively rare—about four per year—making it hard for researchers to separate a global warming signal from natural factors that could affect their frequency. But projections for the future are robust, showing that, with continued warming, up to eight atmospheric rivers could affect Europe each year, with some climate models even showing their frequency tripling, he said.

“What we are seeing is, the rainy seasons are getting shorter, and the precipitation will become more extreme, and the extremes will become more frequent,” Ramos said. “Increased temperature means more water in the atmosphere, lowering the threshold for extremes.”

In Portugal, like in California, atmospheric rivers are often beneficial drought-busters that fill reservoirs, so it’s also a problem if they don’t materialize, Ramos said. His research, and other studies, suggest that global warming may already be shifting atmospheric rivers poleward. He said the research describes how recent droughts in South Africa and Australia could be linked with the poleward shift of atmospheric rivers.

Why Atmospheric Rivers Are Becoming More Dangerous

Swain’s most recent study zoomed in on California and scrutinized the moisture, temperature and wind speeds of the strongest atmospheric rivers from ground level to their upper boundary, about two miles high, to assess the risks of extreme rain and snow.

Since they are infrequent, it’s hard to get a statistically valid idea about how they have changed up to now, even when grouping the data from multiple atmospheric rivers together. But figuring out the mechanics of individual events at a detailed level will help predict how they’ll behave in a warmer climate in the future, he said.

Swain’s analysis delivered detailed projections of increased precipitation from atmospheric rivers: 25 percent to 50 percent more across the San Joaquin Valley, a 25 percent to 30 percent increase in the northern Sacramento valley, and 80 percent more in the Owens Valley, east of the Sierra Nevada.

“We were looking at the really rare ones,” he said of the atmospheric rivers he studied. “That’s part of why we did this study, looking at events that happen a couple of times per century.”  

By comparing the results of their analysis with observations from historic extreme storms, “It’s almost as if we had a 400 year long record,” he said. “It’s the best we can do.” 

In the strongest atmospheric rivers, the study found a 10 percent to 40 percent increase in total accumulated precipitation. It found an even greater increase in hourly maximum precipitation rates that are above the level that could be caused solely by the increased water vapor in a warmer atmosphere, which means global warming is intensifying atmospheric rivers beyond just adding water, Swain said. 

The large increase in maximum hourly precipitation intensity is critical because short blasts of intense precipitation pose a much greater risk of flash flooding and other hazards, such as debris flows and mudslides, than do equally large accumulations occurring over a longer period of time, the study found.

They also found that rain and snowfall from atmospheric rivers is increasing the most, proportionately, in areas that were previously sheltered from the phenomenon, like in the rain shadows on the downwind, lee sides of mountain ranges. That could increase unexpected flood and water management challenges in areas where they are not expected.

“These findings imply substantial challenges for water and flood management in California, given future increases in intense atmospheric river-induced precipitation extremes,” the researchers wrote in their study.

Swain said it’s also important to remember that future atmospheric rivers in a warmer climate will change snow to rain, which increases the risks of flooding even more, based on current expectations of the snow-rain ratio. Snow, of course, piles up and melts slowly, while rain runs off into streams and rivers right away.

With continued global warming, the study suggests that streamflows could increase beyond what would be expected just from the precipitation increase alone, because “a substantially greater fraction of precipitation is likely to fall as rain rather than snow,” the study concluded. 

For Swain, the study elicited deep concerns about the prospects for an incredibly damaging megaflood.

“By 2060, there is a 50-50 chance of seeing a megastorm on the scale that swamped the Central Valley,” he said. “Given that would be a catastrophic flood event, I don’t think we’re fully prepared for that. For me, that almost means that, by late century, it’s almost an inevitability. That’s pretty amazing for an event that happened 5 times per millennium up to now.”

“I’m not going to call these findings good news,” he said. “But they could help start discussions about how to mitigate the risks.”