A megaflood may flood California by 8 feet

Wildfires and droughts are not the only natural disasters that California will continue to experience in the future. According to study released on Friday, a catastrophic megaflood may send so much water to some sections of the state that it might submerge whole stop signs on a residential street.

It is part of an examination into a “plausible worst-case scenario,” according to scientists. Their study, which was published in Science Advances, focused on two catastrophic flooding scenarios: one based on recent historical climate data and the other on the expected climate for the end of this century, between 2081-2100.

Using climate simulations and high-resolution weather models, scientists determined that California should be prepared for potential impacts in the decades to come.

The historical model, called as ArkHist in their paper and based on data from 1996 to 2005, suggests that a megaflood may send up to 85 inches of rain to the Sierra Nevada in California. Under this scenario, the state would also see increased precipitation intensities, with eight out of thirty days of “heavy precipitation” in coastal regions and fourteen out of thirty days in mountain regions. In general, extensive regions may anticipate more than 1.5 feet of precipitation, with widespread areas in the Sierra Nevada and certain points in the Coast, Transverse, and Cascade ranges receiving more than twice that amount.

Daniel Swain, a climate scientist at the University of California, Los Angeles, and co-author of the study, said in a UCLA news release that “the storm sequence is nearly always larger” in the future models.

“There is more rain overall, more severe hourly rainfall, and greater wind,” he stated.

New modeling research reveals that #ClimateChange may have already doubled the danger of catastrophic flooding in California, and that future warming will certainly lead to substantially bigger increases in flood risk. https://t.co/xzTqrZVys image.twitter.com/KUS96JhBhn

12 August 2022 — Science Advances (@ScienceAdvances)
Under the future model, which is based on a scenario of continued rapid growth of greenhouse gas emissions and global warming, precipitation would increase from more than two feet to more than two and a half feet – essentially doubling the amount of precipitation the state would receive under the historical model.

Some regions of the Sierra Nevada and southern Cascades might have a full month of intense precipitation. Also, the number of hours with heavy precipitation has increased by 220%.

This future modeling might be catastrophic for some regions.

Swain said in a UCLA news release that there are areas that get more than 100 liquid-equivalent inches of precipitation every month. “On summits that are still considerably below freezing despite warming, accumulations of twenty feet or more of snow may be found. However, until you reach the level of South Lake Tahoe and below, there is just rain. There would be an increase in runoff.”

Swain said in his own interpretation of the evidence that the greatest cause for worry is the increasing runoff into rivers and streams, which raises the danger of flooding.

According to their analysis, in a future high-emissions scenario, runoff would be 200 to 400 percent larger than historical levels, which will have enormous consequences for the Sacramento and San Joaquin River flood plains. According to Swain, these regions are home to both historic flood deposits and millions of Californians.

“Flood risk during an event similar to either of these scenarios will bring widespread and severe flood risk to nearly the entire state,” Swain said. “However, the extreme increases in projected surface runoff in the Sacramento and San Joaquin basins are of particular concern given the confluence of high pre-existing risk in these regions and a large population that has never historically experienced flooding of this magnitude.”

According to the researchers, all scenarios depict a bleak future, but climate change and humanity’ continued contribution to global warming via greenhouse gas emissions would only make matters worse.

Based on their historical simulation, they discovered that for every 1oC of global warming, the yearly chance of an incident increased dramatically. Comparing this year to 1920, climate change has already raised the risk of such an occurrence by almost 105%.

And if the world continues on its current course of high emissions over the next 40 years, the chance climbs by almost 374 percent, scientists warned.

The top graph depicts the occurrence of severe 30-day precipitation accumulations over the state of California as simulated by the CESM1-LENS ensemble. The data is derived from the historical CESM1-LENS simulations from 1920 to 2005 and the RCP8.5 scenario from 2006 to 2100. The lower graph depicts the yearly probability of extreme 30-day cumulative precipitation occurrences as a function of the predicted global mean surface temperature (GMST; K) anomaly for the 40-member ensemble.
“Current planned carbon reduction objectives will certainly result in an extra 1-1.5oC of warming above and beyond what we have already seen,” Swain said on his website. Therefore, it is quite probable that California will suffer future rises in megastorms capable of causing megaflood conditions.

Such an occurrence would be unusual and deadly, but not unprecedented in California. According to scholars, the Great Flood of 1861-1862 transformed the Sacramento and San Joaquin basins into a 300-mile-long “vast inland sea.” Experts estimate that floods of this magnitude occur five to seven times every thousand years.

This storm was the impetus for the U.S. Geological Survey to develop ARkStorm 1.0 in 2010, a system that constructed a hypothetical storm system of comparable intensity to estimate its influence in the current day. This study determined that a catastrophic occurrence comparable to the Great Flood would result in “widespread, life-threatening flooding” and an economic loss exceeding $750 billion in 2010 values, or $1 trillion in 2022 dollars. This would be “the most costly geophysical calamity in the history of the world to date.”

ARkStorm 2.0, “a new severe storm and flood scenario redesigned for the climate change age,” was utilized in the most recent research.

And although the world must seek to reduce global emissions and hence the likelihood of megastorm occurrences, adaptation must also be a priority since there will be at least some significant shift, experts say.

According to the experts, the danger has been “widely underrated.”

New research lead by @xingyhuang and I on the rising danger of a #megaflood in California due to #ClimateChange is published today in @ScienceAdvances! This document outlines the new #ARkStorm2 scenarios in depth and will serve as the foundation for future development. (Thread:1/n) https://t.co/z5n9h3ZUhk

— Daniel Swain (@Weather West) August 12, 2022
Swain tweeted, “All of this shows that California has to prepare for a growing risk of catastrophic floods, a danger that was grossly overestimated even in the absence of climate change, but which is now increasing.” On his website, he said that the state’s water and flood control policies and infrastructure must be “significantly redesigned for the climate of the twenty-first century.”

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