A new study is offering a dire prediction for the U.S. state of California, where scientists say catastrophic flooding could become twice as likely in the future due to the effects of climate change.

Researchers from UCLA and the National Center for Atmospheric Research completed and published the results of the first part of their "ArkStorm" 2.0 study, looking into possible climate-induced flooding of a "biblical" proportion, also known as "the Other Big One" in reference to an expected major earthquake on the San Andreas Fault.

The study, published on Aug. 12 in the peer-reviewed journal Science Advances, found that historical climate change has already doubled the likelihood of an extreme storm scenario, with the chance of a "megastorm" expected to increase with each additional degree of global warming this century.

The study projects that end-of-the-century storms will create between 200 and 400 per cent more runoff in the Sierra Nevada Mountains due to increased precipitation, more of which will fall as rain instead of snow.

"In the future scenario, the storm sequence is bigger in almost every respect," Daniel Swain, UCLA climate scientist and co-author of the paper, said in a press release.

"There's more rain overall, more intense rainfall on an hourly basis and stronger wind."

While droughts and wildfires tend to get a great deal of attention, Swain said Californians might be losing sight of extreme flooding

"There is potential for bad wildfires every year in California, but a lot of years go by when there's no major flood news," he said. "People forget about it."

The state has experienced major flooding in the past, but the researchers say nothing has reached the scale of the Great Flood of 1862, when floodwaters stretched up to 300 miles long and 60 miles wide across California's Central Valley, at a time when no flood management infrastructure existed.

The population of California has increased significantly since then from about 500,000 to nearly 40 million today, the researchers say.

Were a similar Great Flood to occur now, the scientists say parts of Sacramento, Stockton, Fresno and Los Angeles would be underwater – a disaster to the tune of US$1 trillion in economic losses and larger than any in world history.

Flooding is already taking a toll on west coast economies, with major flooding in British Columbia causing $450 million in insured damage, according to the Insurance Bureau of Canada.

The study comes nearly a year after record flooding in November 2021 displaced at least 15,000 people in British Columbia.

Prior to that, the province experienced a record-breaking heat wave over the summer.

Using new high-resolution weather modelling and existing climate models, the study's researchers compared two extreme scenarios: one occurring about once a century under recent historical climates and the other under a projected climate between the years 2081 and 2100.

The scientists say both scenarios would see a long series of storms fuelled by atmospheric rivers – long narrow regions in the atmosphere that carry water vapour – over the course of a month. The B.C. flooding last November has been tied to the presence of atmospheric rivers.

Climate change increases the amount of rain the atmosphere can hold, a CNN report on the study says, leading to more water falling as rain, which can lead to immediate flooding.

Referring to the future scenario, Swain said there are localized spots that could get the equivalent of more than 100 inches of water in a month.

Increased runoff could lead to devastating landslides and debris, especially in hilly areas burned by wildfires, the researchers say.

Major interstate freeways such as the I-5 and I-80 would likely shut down for weeks or months, affecting the economy and supply chains globally, the researchers add.

But even if meteorologists and climatologists gave weeks of notice about the flooding, the scientists say it would still not be possible for the five to 10 million people displaced by floodwaters to evacuate their homes.

The researchers highlight that the study was limited due to a lack of organized resources and funding.

Advanced flood simulations done with the support of federal and state agencies are in the works, the scientists say, adding they hope to map where flooding could be worst.

With files from CTV News, The Canadian Press and CNN