California, where earthquakes, droughts and wildfires have shaped life for generations, also faces the growing threat of another kind of calamity, one whose fury would be felt across the entire state.

This one will come from the sky.

According to new research, it will very likely take shape one winter in the Pacific, near Hawaii. No one knows exactly when, but from the vast expanse of tropical air around the Equator, atmospheric currents will pluck out a long tendril of water vapor and funnel it toward the West Coast. This vapor plume will be enormous, hundreds of miles wide and more than 1,200 miles long, and seething with ferocious winds. It will be carrying so much water that if you converted it all to liquid, its flow would be about 26 times what the Mississippi River discharges into the Gulf of Mexico at any given moment.

When this torpedo of moisture reaches California, it will crash into the mountains and be forced upward. This will cool its payload of vapor and kick off weeks and waves of rain and snow.

The superstorm that Californians have long feared will have begun. In centuries past, great rains deluged the Pacific coast, and strong storms in recent decades have caused havoc and ruin. But, because of climate change, this one would be worse than any in living memory.

Drenching rain will pummel cities and towns. At times, the hills around Los Angeles could get nearly 2 inches of rain an hour. Heavy rain and snow in the Sierra Nevada will test dams in the Central Valley, one of the world’s most productive farm belts.

While all this has been happening, another filament of moisture-laden air will have formed over the Pacific and begun hurtling toward California. Then another. And another.

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After a month, nearly 16 inches of precipitation, on average, will have fallen across the state. Large swaths of mountainous areas will have gotten much more. Communities might be ravaged beyond resettling. None of the state’s major industries, from tech and Hollywood to farming and oil, will be untouched.

The coming superstorm — really, a rapid procession of what scientists call atmospheric rivers — will be the ultimate test of the dams, levees and bypasses California has built to impound nature’s might.

But in a state where scarcity of water has long been the central fact of existence, global warming is not only worsening droughts and wildfires. Because warmer air can hold more moisture, atmospheric rivers can carry bigger cargoes of precipitation. The infrastructure design standards, hazard maps and disaster response plans that protected California from flooding in the past might soon be out of date.

As humans burn fossil fuels and heat up the planet, we have already increased the chances each year that California will experience a monthlong, statewide megastorm of this severity to roughly 1 in 50, according to a new study published Friday. In the coming decades, if global average temperatures climb by another 1.8 degrees Fahrenheit, or 1 degree Celsius — and current trends suggest they might — then the likelihood of such storms will go up further, to nearly 1 in 30.

At the same time, the risk of megastorms that are rarer but even stronger, with much fiercer downpours, will rise as well.

These are alarming possibilities. But geological evidence suggests the West has been struck by cataclysmic floods several times over the past millennium, and the new study provides the most advanced look yet at how this threat is evolving in the age of human-caused global warming.

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“We got kind of lucky to avoid it in the 20th century,” said Daniel Swain, a climate scientist at the University of California, Los Angeles, who prepared the new study with Xingying Huang of the National Center for Atmospheric Research in Boulder, Colorado. “I would be very surprised to avoid it occurring in the 21st.”

Wetter wet years

Donald G. Sullivan was lying in bed one night, early in his career as a scientist, when he realized his data might hold a startling secret.

For his master’s research at the University of California, Berkeley, he had sampled the sediment beneath a remote Sierra lake and was hoping to study the history of vegetation in the area. But a lot of the pollen in his sediment cores didn’t seem to be from nearby. How had it gotten there?

When he X-rayed the cores, he found layers where the sediment was denser. Maybe, he surmised, these layers were filled with sand and silt that had washed in during floods.

It was only late that night that he tried to estimate the ages of the layers. They lined up neatly with other records of West Coast megafloods.

“That’s when it clicked,” said Sullivan, who is now at the University of Denver.

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His findings, from 1982, showed that major floods hadn’t been exceptionally rare occurrences in the Sacramento Valley over the past eight centuries. They took place every 100 to 200 years. And in the decades since, advancements in modeling have helped scientists evaluate how quickly the risks are rising because of climate change.

For their new study, Huang and Swain replayed portions of the 20th and 21st centuries using 40 simulations of the global climate. Extreme weather events, by definition, don’t occur very often. So by using computer models to create realistic alternate histories of the past, present and future climate, scientists can study a longer record of events than the real world offers.

Swain and Huang looked at all the monthlong California storms that took place during two time segments in the simulations, one in the recent past and the other in a future with high global warming. They then used a weather model to produce detailed play-by-plays of where and when the storms dump their water.

Those details matter. There are “so many different factors” that make an atmospheric river deadly or benign, Huang said.

Climate change might be affecting atmospheric rivers in other ways, too, said F. Martin Ralph of the Scripps Institution of Oceanography at the University of California, San Diego. How strong their winds are, for instance, or how long they last.

Scientists are also working to improve atmospheric river forecasts, which is no easy task as the West experiences increasingly sharp shifts from very dry conditions to very wet and back again.

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“My scientific gut says there’s change happening,” Ralph said. “And we just haven’t quite pinned down how to detect it adequately.”

Katrina 2.0

How do you protect a place as vast as California from a colossal storm? Two ways, said David Peterson, a veteran engineer. Change where the water goes, or change where the people are. Ideally, both. But neither is easy.

Firebaugh is a quiet, mostly Hispanic city of 8,100 people, one of many small communities that power the Central Valley’s prodigious agricultural economy. Firebaugh also sits right on the San Joaquin River.

For a sleepless stretch of early 2017, Ben Gallegos, Firebaugh’s city manager, did little but watch the river rise and debate whether to evacuate half the town. Water was threatening homes, schools, churches and the wastewater treatment plant.

Luckily, the river stopped rising. Still, the experience led Gallegos to apply for tens of millions in funding for new and improved levees around Firebaugh.

Levees change where the water goes, giving rivers more room to swell before they inundate the land. Levee failures in New Orleans were what turned Katrina into an epochal catastrophe, and after that storm, California toughened levee standards in urbanized areas of the Sacramento and San Joaquin Valleys.

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But the plodding pace of work has set people on edge. At a recent public hearing in Stockton on flood risk, Elias stood up and highlighted some troubling math.

The Department of Water Resources says up to $30 billion in investment is needed over the next 30 years. Yet over the past 15 years, the state managed to spend only $3.5 billion.

“We have to find ways to get ahead of the curve,” Elias said. “We don’t want to have a Katrina 2.0 play out right here in the heart of Stockton.”