Megastorm expected to eventually pound California
Statewide massive rain will challenge state
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 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 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 hurtled toward California. Then another. And another.
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 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 study provides the most advanced look yet at how this threat is evolving in the age of human-caused global warming.
The researchers specifically considered hypothetical storms that are extreme but realistic, and which would probably strain California’s flood preparations. According to their findings, powerful storms that once would not have been expected to occur in an average human lifetime are fast becoming ones with significant risks of happening during the span of a home mortgage.
“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 study with Xingying Huang of the National Center for Atmospheric Research in Boulder, Colo. “I would be very surprised to avoid it occurring in the 21st.”
On Feb. 12, 2017, almost 190,000 people living north of Sacramento received an urgent order: Get out. Now. Part of the tallest dam in America was verging on collapse.
That day, Ronald Stork was in another part of the state, where he was worrying about precisely this kind of disaster — at a different dam.
Standing with binoculars near California’s New Exchequer Dam, he dreaded what might happen if large amounts of water were ever sent through the dam’s spillways. Stork, a policy expert with the conservation group Friends of the River, had seen on a previous visit to Exchequer the nearby earth was fractured and could be easily eroded. If enough water rushed through, it might cause major erosion and destabilize the spillways.
He only learned later that his fears were playing out in real time, 150 miles north. At the Oroville Dam, a 770-foot-tall facility built in the 1960s, water from atmospheric rivers was washing away soil and rock beneath the dam’s emergency spillway, which is essentially a hillside next to the main chute that acts like an overflow drain in a bathtub. The top of the emergency spillway looked like it might buckle, which would send a wall of water cascading toward the cities below.
Stork had no idea this was happening until he got home to Sacramento and found his neighbor in a panic. The neighbor’s mother lived downriver from Oroville. She didn’t drive anymore. How was he going to get her out?
Stork had filed motions and written letters to officials, starting in 2001, about vulnerabilities at Oroville. People were now in danger because nobody had listened. “It was nearly soul crushing,” he said.
“With flood hazard, it’s never the fastball that hits you,” said Nicholas Pinter, an earth scientist at the University of California, Davis. “It’s the curveball that comes from a direction you don’t anticipate. And Oroville was one of those.”
Such perils had lurked at Oroville for so long because California’s Department of Water Resources had been “overconfident and complacent” about its infrastructure, tending to react to problems rather than preempt them, independent investigators later wrote in a report. It is not clear this culture is changing, even as the 21st century climate threatens to test the state’s aging dams in new ways. One recent study estimated that climate change had boosted precipitation from the 2017 storms at Oroville by up to 15 percent.
A year and a half after the crisis, crews were busy rebuilding Oroville’s emergency spillway when the federal hydropower regulator wrote to the state with some unsettling news: The reconstructed emergency spillway will not be big enough to safely handle the “probable maximum flood,” or the largest amount of water that might ever fall there.
This is the standard most major hydroelectric projects in the United States have to meet. The idea is that spillways should basically never fail because of excessive rain.
Today, scientists say they believe climate change might be increasing “probable maximum” precipitation levels at many dams. When the Oroville evacuation was ordered, nowhere near that much water had been flowing through the dam’s emergency spillway.
Yet California officials have downplayed these concerns about the capacity of Oroville’s emergency spillway, which were raised by the Federal Energy Regulatory Commission. Such extreme flows are a “remote” possibility, they argued in a letter last year. Therefore, further upgrades at Oroville aren’t urgently needed. “People could die,” Stork said. “And it bothers the hell out of me.”
Donald 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. 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.
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 study, published in the journal Science Advances, Huang and Swain replayed portions of the 20th and 21st centuries using 40 simulations of the global climate. 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, and chose one of the most intense events from each period. 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.
In the high Sierras, for example, atmospheric rivers today largely bring snow. But higher temperatures are shifting the balance toward rain. Some of this rain can fall on snowpack that accumulated earlier, melting it and sending even more water toward towns and cities below.
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: Some storms stall, barraging an area for days on end, while others blow through quickly.
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. In October, strong storms broke records in Sacramento and other places. Yet this January through March was the driest in the Sierra Nevada in more than a century.