(CNN) – Many Californians fear the “Big One,” but it may not be what you think.
It’s not an earthquake. And it’s not the mega drought. It’s actually the complete opposite.
A mega flood.
A new study from Science Advances shows that climate change has already doubled the likelihood of catastrophic flooding in California over the next four decades. And experts say it would be unlike anything anyone alive today has ever experienced.
Daniel Swain, a climate scientist at UCLA and a researcher involved in the study, describes a megaflood as “a very severe flood event across a wide region that has the potential to have catastrophic effects on communities in affected areas.” He said a megaflood is similar to the 1,000-year flash flood events seen in the St. Louis and Kentucky area this summer, but over a much larger area, such as the entire state of California.
These massive floods, which experts say would turn the California lowlands into a “huge inland sea,” could have happened in the state once before. However, experts say that climate change is increasing the likelihood of these catastrophic disasters, making them more likely to occur every 25 to 50 years.
Climate change is increasing heavy rain events, making flash floods more frequent, as has been noted several times this summer Eastern Kentucky, st louisand even inside California’s Death Valley National Park.
California is inherently vulnerable to these floods from atmospheric flows, and major floods from them have occurred before — but climate change is raising the stakes and millions of people could be affected.
The study states that atmospheric fluxes can become consecutive for weeks, as seen in this animation. Xingying Huang, one of the authors of the study, created this loop that illustrates water vapor transport and potential precipitation accumulation at selected time intervals during the 30-day scenario.
The area of greatest destruction would be the Central Valley of California, including Sacramento, Fresno and Bakersfield, the study authors’ project. The Central Valley, which is about the size of Vermont and Massachusetts combined, produces a quarter of the country’s food supply, according to the US Geological Survey.
A flood large enough to fill this valley has the potential to become the costliest geophysical disaster to date, causing over $1 trillion in losses and affecting the state’s lowland areas, including Los Angeles and Orange, according to the study County, devastated.
That would be more than five times the cost of Hurricane Katrina, currently the costliest disaster in US history.
“Such a flooding event in modern California would likely exceed the damage of a large earthquake by many times,” the study showed.
This study is the first phase of a three-part series examining the impact of a future megaflood event 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’s more of a question of when than if (the mega-flood) will happen.”
It’s happened before. It will happen again, but worse, scientists warn
Over 150 years ago, a powerful series of atmospheric flows inundated the Golden State, causing one of the most extraordinary floods in history after a dry spell that had parched the West for decades.
Communities were demolished within minutes.
It was the winter of 1861-1862, and a historic megaflood transformed the San Joaquin and Sacramento Valleys into a “temporary but vast inland sea,” according to the study. Some areas had water as high as 30 feet for weeks, destroying infrastructure, farmland, and cities.
Sacramento, then the new state capital, lay under ten feet of debris-filled water for months.
Disaster began in December 1861, when almost 15 feet of snow fell in the Sierra Nevada. Recurrent atmospheric flows then dropped warm rain for 43 days, pouring water down the mountainsides and into the valleys.
Four thousand lives were lost, a third of state property was destroyed, a quarter of California’s cattle population drowned or starved, and one in eight homes was completely destroyed by flooding.
Additionally, a quarter of California’s economy was wiped out, leading to statewide bankruptcy.
Swain warns that a mega-flood like this will happen again, but worse and more frequently.
“We find that climate change has already increased the risk of a megaflood scenario (1862) in California, but that future global warming is likely to result in even greater increases in risk,” the study warns.
Many of today’s major cities of millions were built directly on top of ancient flood deposits, Swain added, putting far more people at risk.
In 1862 about 500,000 people lived in California. Today the state has over 39 million inhabitants.
“If this (flood) occurs again, the aftermath would be vastly different from what it was in the 1860s,” Swain said.
Climate change is increasing the amount of rain the atmosphere can hold, causing more airborne water to fall than rain, which can cause instant flooding. Both are and will continue to occur in California.
The new study shows a rapid increase in the likelihood of week-long, recurring strong to extreme atmospheric flows during the cool season. An atmospheric flow is a long, narrow region of high moisture in the atmosphere that can carry moisture thousands of miles like a fire hose in the sky. They typically bring beneficial rainfall to drought-prone regions like California, but could quickly become dangerous with a warming climate.
Historically, these winter-atmosphere rivers shed snow in the Sierra Nevada, but as the climate warms, more snow will fall than rain. Rather than slowly melting away over time, everything drains, piles up, and immediately floods.
With a neighbor like the Pacific Ocean, California has “an infinite reservoir of water vapor offshore,” Swain added.
California’s mountainous terrain and wildfire risk make it particularly vulnerable to flooding. Permanent burn scars from wildfires can create a steep, smooth surface for water and debris to run off. As wildfires grow larger and burn more land thanks to climate change, more areas are vulnerable to these debris flows.
Although models show that this mega flood is inevitable, experts say there are ways to mitigate excessive losses.
“I think the magnitude of (megaflood) losses can be significantly reduced by doing certain things to overhaul 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 make a small contribution to combating climate change.
“If we work together to reduce future emissions, we can also reduce the risk of extreme events,” Huang said.
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