(CNN) — Many Californians fear the “Big One,” but it may not be the natural disaster you imagine.
This is not an earthquake. And it’s not Megadraught. It’s actually the exact opposite: a huge flood.
A new study in Science Advances shows that climate change will double the likelihood of devastating floods in California over the next four decades. And experts say it will be unlike anything any living person has experienced.
Daniel Swain, a UCLA climate scientist and researcher who participated in the study, described a megaflood as “a very severe flood over a wide area that has the potential to cause devastating effects on society in the affected areas.” The scientist says a mega-flood is similar to the 1,000-year flash flood seen in the St. Louis and Kentucky area this summer, but over a much wider area, potentially engulfing the entire state of California.
These massive floods, which experts say will turn California’s lowlands into “huge inland seas,” could happen once in a lifetime in the state. But experts say climate change is increasing the likelihood of these devastating disasters, more like every 25 to 50 years.
Climate change supercharges intense rainfall events, making flash floods more regular, as seen several times this summer in eastern Kentucky, St. Louis and even California’s Death Valley National Park Is.
California is naturally prone to these atmospheric river floods, and major floods from them have already occurred, but climate change is heading east, and millions of people could be affected.
According to the study, atmospheric rivers can be continuous for weeks, as seen in this animation. One of the study’s authors, Xingying Huang, created this loop, which shows the transport of water vapor and the potential accumulation of precipitation over certain time segments during the 30-day scenario.
The region with the most destruction would be California’s Central Valley, which includes Sacramento, Fresno and Bakersfield, study authors project. The Central Valley, which is about the size of Vermont and Massachusetts combined, produces a quarter of the nation’s food supply, according to the US Geological Survey.
According to the study, this canyon-sized flood could be the costliest geophysical catastrophe ever, with an estimated more than $1 trillion in damages and devastating low-lying areas in the state, including Los Angeles and Orange counties. ,
It would be five times the cost of Hurricane Katrina, the costliest devastation in US history.
“Such flooding in modern-day California could be much greater than the damage of a major earthquake,” the study said.
This study is the first of a three-part series studying the impacts of future megafloods in California. The next two phases are expected to be released in two to three years.
“Ultimately, one of our goals is not only to understand these events scientifically, but also to help California prepare for them,” Swain said. “It is a question of when it (mega-flood) happens and when it doesn’t.”
It has happened before. It will happen again, but worse, scientists warn
More than 150 years ago, a mighty series of atmospheric rivers flooded the Golden State, causing the most extraordinary floods in history after a time of drought that left the West dry for decades.
Within minutes the communities were demolished.
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. Some areas had up to 9 meters of water for weeks, destroying infrastructure, farmland and towns.
Sacramento, the state’s new capital at the time, was under ten feet of debris-laden water for months.
The devastation began in December 1861, when about 4.5 meters of snow fell in the Sierra Nevada. After that, for 43 days repeatedly atmospheric rivers dropped hot rain, watering the mountains 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, and one in eight homes suffered a total loss of floodwaters.
Additionally, a quarter of California’s economy was wiped out, bankrupting the entire state.
Swain warns that a major flood like this will happen again, but that it will be worse and more frequent.
“We found that climate change has already increased the risk of a mega-flood in California, but future climate warming will increase the risk even further,” warns the study.
Many of today’s large cities, with millions of people, are built directly on old flood deposits, Swain said, putting many more people at risk.
In 1862, about 500,000 people lived in California. Now, the population of the state is more than 39 million.
“When this (flood) happens again, the results will be very different than in the 1860s,” Swain said.
Climate change increases the amount of precipitation that the atmosphere can hold and causes more water to fall into the air in the form of rain, which can lead to immediate flooding. Both things happen and will continue to happen in California.
The new study shows a rapid increase in the likelihood of strong or extreme week-long atmospheric rivers in cold weather. An atmospheric river is a long, narrow area of high humidity in the atmosphere that can carry moisture thousands of miles across the sky like a fire hose. They often bring beneficial rain to drought-prone areas such as California, but can become dangerous as the climate warms.
Historically, these winter atmospheric rivers dropped meters of snow on the Sierra Nevada, but as the climate warmed, more snow would fall in the form of rain. Instead of melting slowly over time, it would all flow away, accumulate and flood immediately.
With a neighbor like the Pacific Ocean, California has “an infinite reservoir of water vapor in the ocean,” Swain said.
California’s mountainous terrain and risk of wildfires make it particularly vulnerable to flooding. Burn marks from wildfires can create a steep, slippery surface for water and debris to drain out. As wildfires increase and climate change burns more land, more areas are susceptible to these debris flows.
Although models show that these major floods are inevitable, experts say there are ways to reduce the extreme damage.
“I think the extent of the damage (from a mega-flood) can be significantly reduced by doing certain kinds of things to improve 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 who participated in the study, said there is little everyone can do to combat climate change.
“If we work together to reduce future emissions, we can also reduce the risk of extreme events,” Huang said.