It is not an earthquake. This is not a huge drought. It is actually quite the opposite.
enormous.
Daniel Swain, a UCLA climate scientist and co-author of the study, describes the megaflood as a “high-risk flood event across a wide area that has the potential to cause catastrophic community impacts in affected areas.” He said the mega-flood is similar to the 1,000-year flash flood events seen this summer in the St. Louis and Kentucky area, but across a much wider area, such as the entire state of California.
These massive floods, which experts say will turn California’s lowlands into a “broad inland sea,” may have previously occurred once in the state’s lifetime. But experts say climate change is increasing the likelihood of these catastrophic disasters, causing them to occur more frequently every 25 to 50 years.
California is naturally prone to these atmospheric river floods, and major floods have occurred from them before — but climate change is raising expectations, and millions of people could be affected.
The study said that atmospheric rivers can become cascading for weeks on end, as seen in this animation. This episode was set up by Xingying Huang, one of the study’s authors, which shows water vapor transport and potential precipitation accumulation in specific time slices over a 30-day scenario.
A flood of the size that fills this valley is possibly the most expensive geophysical disaster to date, costing more than a trillion dollars in losses and devastating lowland areas of the state, including Los Angeles and Orange counties, according to the study.
That would be five times more than the cost of Hurricane Katrina, the current costliest disaster in US history.
The study showed that “such a flood in modern California would likely exceed the damage caused by a large-scale earthquake by a significant margin.”
This study is the first in a three-part series examining the effects of a future mega event in California. The next two phases are expected to be launched within two to three years.
“Ultimately, one of our goals is not only to scientifically understand these events, but also to help California prepare for them,” Swain said. “It’s a question of when (the megaphase) happens and not whether it happens.”
It’s happened before. It will happen again, but worse, scientists warn
More than 150 years ago, a powerful series of atmospheric rivers inundated the Golden State, causing one of the most extraordinary floods in history after a drought that left the West dry for decades.
Communities were demolished in minutes.
It was the winter of 1861-1862, and a historic massive river transformed the San Joaquin and Sacramento valleys into a “vast but temporary inland sea,” according to the study. Some areas had up to 30 feet of water for weeks, obliterating infrastructure, farmland, and towns.
Sacramento, the new state’s capital at the time, was under ten feet of debris-strewn water for several months.
The disaster began in December 1861, when nearly 15 feet of snow fell in the Sierra Nevada. Frequent rivers in the atmosphere dropped warm rain for 43 days afterward, dumping water down mountain slopes and into valleys.
Four thousand people lost their lives, a third of the state’s property was destroyed, a quarter of California’s cattle were drowned or starved, and one in eight homes suffered a complete loss due to flood waters.
In addition, a quarter of California’s economy was obliterated, resulting in statewide bankruptcy.
Swain warns that such a huge stream will happen again, but it is worse and more frequent.
The study warns that “climate change has already increased the risk of a California mega-flood scenario (1862), but future climate warming is likely to increase the risk even further.”
Swain added that many of today’s major cities with millions of residents are built directly on top of ancient flood deposits, putting even more people at risk.
About 500,000 people lived in California in 1862. Now, the state has a population of over 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 precipitation the atmosphere can hold and causes more water to fall into the air as rain, which can lead to immediate flooding. Both are and will continue to occur in California.
The new study shows a rapid increase in the likelihood that strong to extreme weather will occur for a week during the cold season. An atmospheric river is a long, narrow region of intense moisture in the atmosphere that can transmit moisture for thousands of miles, like a fire hose in the sky. It usually brings beneficial rains to drought-prone areas such as California but can quickly become dangerous as the climate warms.
Historically, these atmospheric winter rivers fell with feet of snow in the Sierra Nevada, but as the climate warms, more snow will fall in the form of rain. Instead of slowly dissolving over time, everything fades, accumulates, and overflows instantly.
With a neighbor like the Pacific Ocean, California has “an endless reservoir of water vapor outside,” Swain added.
California’s mountainous terrain and wildfire hazards make it particularly vulnerable to flooding. Long burn scars from wildfires can create a steep, slick surface for water flow and debris. As wildfires become more and more widespread thanks to climate change, more areas are exposed to these debris flows.
Although models show that such a huge influx is inevitable, experts say there are ways to mitigate the excessive loss.
“I think the scale of the losses (floods) can be significantly reduced by doing certain kinds of things to revamp our flood management, our water management systems, and our disaster preparedness,” Swain said.
Everyone can make a small effort in the fight against climate change, said Huang, a project scientist at the National Center for Atmospheric Research and a co-author on the study.
“If we work together to reduce emissions in the future, we can also reduce the risks of extreme events,” Huang said.