Many Californians fear being “big,” but not as much as you might think.
This is not an earthquake. And this is not a mega drought. It’s actually quite the opposite.
A mega flood.
A new study in Science Advances Climate change has already doubled California’s chances of catastrophic flooding in four decades. Experts say it will be unlike anything anyone alive today has ever experienced.
Daniel Swain, a climate scientist at UCLA and a researcher involved in the study, described it as “a very severe flood event over a wide region that could have catastrophic impacts on communities in affected areas.” He said megafloods are similar to the 1,000-year flash flood events seen this summer in the St. Louis area and Kentucky, but over a much wider area, like the entire state of California.
The massive flooding, which experts say will turn California’s lowlands into a “vast inland sea,” has previously been a once-in-a-lifetime occurrence in the state. But experts say climate change is increasing the likelihood of these catastrophic disasters, making them more likely to occur every 25 to 50 years.
California is naturally prone to these floods from atmospheric rivers, and major floods from them have occurred before — but climate change is a warning, and millions of people could be affected.
The study suggests that atmospheric rivers can change continuously over weeks, as seen in this animation. Xingying Huang, one of the study’s authors, created the loop, which illustrates water vapor transport and potential precipitation accumulation in selected time slices over a 30-day scenario.
The study’s authors sampled California’s Central Valley, an area with the most destruction, including Sacramento, Fresno and Bakersfield. The Central Valley, roughly the size of Vermont and Massachusetts, produces a quarter of the nation’s food supply. US Geological Survey.
A flood the size of this valley fill would be the costliest geophysical disaster to date, costing more than $1 trillion in damages and devastating low-lying areas of the state, including Los Angeles and Orange counties, according to the study.
That’s more than 5 times the cost of Hurricane Katrina, the current costliest disaster in US history.
“Such a flood event in modern California would exceed the damage caused by a large-scale earthquake by a significant margin,” the study shows.
This study is the first in a three-part series studying the consequences of a future megaflood event in California. The next two phases are expected to be rolled out in two to three years.
“Ultimately, one of our goals is not only to scientifically understand these events, but to help California prepare for them,” Swain said. “It’s more a question of when (the mega-flood) will happen.”
More than 150 years ago, a powerful series of atmospheric rivers drenched the Golden State in one of the most exceptional floods in history, following decades of western dry weather.
Communities were demolished within minutes.
It was the winter of 1861-1862 and a historic mega-flood turned the San Joaquin and Sacramento valleys into a “temporary but vast inland sea,” according to the study. In some areas, the water was up to 30 feet for weeks, destroying infrastructure, farmland and towns.
Sacramento, the new state capital at the time, had been under ten feet of debris-laden water for months.
The disaster began in December 1861 when nearly 15 feet of snow fell in the Sierra Nevada. Repeated atmospheric rivers poured down warm rains for 43 days, pouring water down mountain slopes and valleys.
Four thousand people lost their lives, a third of the state’s property was destroyed, a quarter of California’s livestock population drowned or starved to death, and one in eight homes was completely lost to the floodwaters.
In addition, one-fourth of California’s economy was destroyed, resulting in the bankruptcy of the entire state.
Swain warns that a flood like this will happen again, but worse and more often.
“We find that climate change has already increased the risk of an (1862) megaflood scenario in California, but future climate warming will increase the risk even more sharply,” the study warns.
Many of today’s major cities with millions of residents are built directly on top of ancient floodplains, Swain added, putting more people in harm’s way.
About 500,000 people lived in California in 1862. Now, the population of the state is more than 39 million.
“When this (flood) happens again, the consequences will be very different than they were in the 1860s,” Swain said.
Climate change increases the amount of rain that the atmosphere can withstand and more water in the air falls as rain, leading to flash floods. Both occur regularly in California.
The new study shows a rapid increase in the likelihood of a week-long, strong-to-severe storm surge during the cold season. An atmospheric river is a long, narrow region of high moisture in the atmosphere that can transport moisture thousands of miles across the sky like a fire hose. They usually bring beneficial rain to drought-prone areas like California, but can quickly become dangerous with a warming climate.
Historically these winter atmospheric rivers have dumped snow in the Sierra Nevada, but as the climate warms, more snow falls as rain. Instead of melting slowly over time, it all runs off, accumulates, and floods instantly.
Along with neighbors like the Pacific Ocean, Swain added, California has an “infinite reservoir of sea vapor.”
California’s mountainous terrain and wildfire risk make it particularly vulnerable to flooding. Burn scars from wildfires can create a steep, thin surface for water and debris to run off. As wildfires grow larger and burn more areas due to climate change, more areas are exposed to these debris flows.
Although models show this megaflood is inevitable, experts say there are ways to mitigate the excess loss.
“I think we can significantly reduce the amount of (megaflood) losses by doing some kind of things to streamline our flood management and our water management systems and our disaster preparedness,” Swain said.
Huang, a project scientist at the National Center for Atmospheric Research and a researcher involved in the study, said everyone can do a little to help fight climate change.
“If we work together to reduce future emissions, we can also reduce the risk of extreme events,” Huang said.