This past weekend, California’s capital lurched from one weather extreme to the other. On Sunday, Sacramento was drenched with 5.44 inches of rain in 24 hours, smashing a daily record that had stood since 1880. It came barely a week after the city went 212 days without measurable rain, again beating an 1880 record.
The weekend storms soaked much of Northern and Central California. Mount Tamalpais, just over the Golden Gate Bridge from San Francisco, received an astonishing 16.55 inches of rain in just 48 hours.
In the short term, that’s good news. An extreme drought that had steadily worsened since the start of 2020, while far from finished, has eased a little. And in Northern California, a brutal wildfire season is over — although the south of the state still faces the risk of flames fanned by the Santa Ana winds that can blow from the desert to the coast through the fall and into the winter.
But this year’s pattern of drought followed by deluge previews a dangerous trend that climate scientists have warned lies ahead for the Golden State. Droughts and floods are set to intensify in the coming decades as a warmer atmosphere parches the soil and then periodically dumps massive amounts of rain in one go, transported from the tropical Pacific in huge “atmospheric rivers” laden with water vapor. That has some scary consequences for a state with almost 40 million residents and an economy that would rank fifth in the world, measured by gross domestic product, if it were an independent nation.
“In a warming climate, some regions get wetter, some regions get drier, and some kind of do both,” Daniel Swain, a climate scientist at the University of California, Los Angeles, told BuzzFeed News. “There’s a rising risk of these extreme precipitation events and floods but also rising drought severity and worsening wildfires.”
Due to the growing levels of greenhouse gases in the atmosphere, the planet has warmed by an average of 1 degree Celsius since the 1880s. This has already had dire consequences for California, such as shortages of water for agriculture. In the decades to come, drought may force half a million acres of prime farmland out of production. At the same time, the state faces the prospect of a flood of biblical proportions that could surpass the impact of any weather-related disaster yet to befall the United States.
“We rarely seem to get the Goldilocks moment in California,” Jeff Mount, a senior fellow at the Public Policy Institute of California’s Water Policy Center, and founding director of the Center for Watershed Sciences at the University of California, Davis, told BuzzFeed News. “It’s either too dry or too wet.”
The storms that hit California over the weekend were caused by a large atmospheric river transporting a stream of damp air from the tropical Pacific — a phenomenon sometimes called the “Pineapple Express.” The system was also driven by a rapidly intensifying “bomb cyclone” spiraling off the coasts of Oregon and Washington state.
The latest data from the US Drought Monitor, released on Thursday, shows that the past weekend’s intense rainfall has so far made only a small dent in California’s ongoing drought.
The intense rain helped replenish California’s depleted reservoirs — but again to a limited extent. Parched soils act like a sponge, soaking up water rather than quickly discharging it into streams and rivers. So while water levels have bounced back in the largest reservoirs in Northern California over the past few days, storage is still massively below capacity.
Indeed, California’s system of capturing and storing water may become increasingly inadequate, given the impact of climate change. For decades, the state has relied heavily on the snowpack that built up in the Sierra Nevada mountains over the winter and then melted in a predictable fashion from April onward, filling reservoirs and steadily irrigating the state’s farmland.
“Historically, mountain snowpack has represented about 30% of the storage capacity,” Jeanine Jones, drought manager for the California Department of Water Resources, told BuzzFeed News.
Last weekend’s storms dropped nearly 3 feet of snow in the Sierra Nevada. But this may well have melted away once more by Thanksgiving, failing to contribute to the crucial spring snowpack. By the end of the century, given warming trends, the Sierra snowpack is expected to be a thing of the past. And with more and more of California’s precipitation falling as rain, water will become harder to capture and store.
“It doesn’t do you any good to build a new dam if you can’t fill it,” Mount said.
Addressing this problem will require huge investment. According to Mount, the solution involves moving captured rainfall around the state to recharge dwindling groundwater supplies.
Right now, California’s agriculture depends heavily on pumping water from beneath the surface in an unsustainable way. In recent decades, the amount removed from below ground in the agricultural heartland of the San Joaquin Valley has exceeded the amount replaced by some 1.8 million acre-feet per year — or about 11% of the region’s net water use.
In 2014, at the height of the previous multiyear drought, the California legislature passed the Sustainable Groundwater Management Act, which requires local agencies to develop plans to manage the state’s groundwater over the next 20 years. But how sustainability will be achieved, which will require groundwater reserves to be recharged using water captured across the state, remains a work in progress.
California dodged a bullet with the recent storms. As well as filling reservoirs and dousing wildfires, extreme rainfall events can be very destructive, causing floods, landslides, and mudflows — especially in areas burned by recent wildfires.
The storms did cause damage, including a rockslide that blocked Highway 70, which traverses the Sierra Nevada. But overall, the atmospheric river that hit the state did more good than harm.
Climate change is expected to shift the effects of atmospheric rivers coming in from the Pacific in a more destructive direction. According to a study from the Lawrence Berkeley National Laboratory, an increase in global average temperature by three degrees Celsius above preindustrial conditions would raise the proportion of atmospheric rivers that are “mostly or primarily hazardous” in the western US from 2% to 8%.
In February 2017, California was given a warning of the destructive power of incoming atmospheric rivers when the concrete spillway allowing overflow from the Oroville dam failed. More than 180,000 people living downstream were evacuated, as authorities contemplated the possibility of life-threatening floods.
Future storms could be much worse. In 2010, federal agencies and academic scientists led by the US Geological Survey ran a simulation called ARkStorm, to consider the effects of a procession of atmospheric rivers similar to those that hit California in the winter of 1861–62. This “Great Flood” is thought to have killed more than 4,000 people — or about 1% of the state’s population at the time.
The ARkStorm team noted that a similar event in the modern era would flood large parts of California’s Central Valley and put Sacramento under 10 to 20 feet of water. The study estimated that 1.5 million people could need to be evacuated and put the total cost of such a disaster at $725 billion — or more than $900 billion in current dollars. For comparison, Hurricane Katrina, which hit Louisiana and Mississippi in 2005 and heads the National Oceanic and Atmospheric Administration’s list of costly US disasters, is estimated to have caused just under $180 billion of damage in today’s dollars.
The Great Flood of 1861–62 was once thought to be an event likely to repeat less than once in 500 years. But in a rapidly warming world, bets about the frequency of such a disaster are off. Chillingly, the team behind the simulation of a modern equivalent concluded: “An ARkStorm is plausible, perhaps inevitable.” ●
Zahra Hirji contributed reporting for this story.