Posted on 7 February 2023 by Guest Author

This is a re-post from Yale Climate Connections by Jeff Masters. While predicated on accepted scientific findings, this article includes conclusions of the author and is presented to our readers as an informed perspective.

Asequence of nine atmospheric rivers hammered California during a three-week period in January 2023, bringing over 700 landslides, power outages affecting more than 500,000 people, and heavy rains that triggered flooding and levee breaches. On a statewide basis, about 11 inches of rain fell; 20 deaths were blamed on the weather, with damages estimated at over $1 billion.

But the storm damages were a pale shadow of the havoc a true California megaflood would wreak.

The Golden State has a long history of cataclysmic floods, which have occurred about every 200 to 400 years — most recently in the Great Flood of 1861-62. And a future warmer climate will likely significantly increase the risk of even more extreme floods. In particular, a 2022 study found that, relative to a century ago, climate change has already doubled the risk of a present-day megastorm, and more than tripled the risk of a trillion-dollar megaflood of the type that could swamp the Central Valley. 

Given the increased risk, it is more likely than not that many of you reading this will see a California megaflood costing tens of billions in your lifetime.

This is the third part of a three-part series on California’s vulnerability to a megaflood. Part One examined the results of a 2011 study introducing the potential impacts of a scenario, known as “ARkStorm,” which would be a repeat of California’s Great Flood of 1861-62 — though the study did not take climate change into account. Part Two looked at how California is preparing its dams for future great floods. Here, in Part Three, we’ll look at the increasing future threat of a California megaflood in a warming climate.

The ARkStorm 2.0 scenario

A 2011 government study introduced the “ARkStorm” scenario, finding that a megaflood in California could swamp the state’s Central Valley and cause more than $1 trillion in damage.

A 2022 study by Xinging Huang and Daniel Swain updates that work in a scenario called “ARkStorm 2.0,” using data and computer modeling advances not available in 2011.

The new study used climate modeling to develop a plausible megastorm in the present-day climate (1995-2005), which they called ARkHist. They also developed a more extreme case in a much warmer world, called ARkFuture.

Both scenarios featured a weeks-long parade of atmospheric river storms during the winter months. A high-resolution weather model was then run, using the climate model as input, in order to produce detailed “synthetic weather forecasts” for California. (For readers familiar with weather models, it was WRF with grid boxes 3 km on a side.)

The modeled storms not only brought massive precipitation accumulations – they also produced very high precipitation intensities (that is, very heavy precipitation during a single hour or day). This would greatly increase flash flood and landslide/debris flow risk – especially since climate change is bringing California a major increase in large and intense wildfires, making denuded slopes more vulnerable to flooding.

The ARkHist scenario involved storms that produced slightly less precipitation than the Great Flood of 1861-62 but slightly more than the wettest winters of the past 100 years. This scenario was thought to have a recurrence interval of once every 90-100 years. That means it has a 1-1.1% chance of occurring in a given year, or 26-28% chance in 30 years. A storm of this magnitude would be capable of causing tens of billions of dollars in damage; a 2022 study estimated that a flood with a 1-in-100-year return period affecting only the Los Angeles area would likely inundate property worth $56 billion to a depth of a foot or more.

The ARkFuture scenario, which would be a catastrophic event capable of causing more than $1 trillion in damage, had a recurrence interval of every 400 years in the current climate. That works out to a 0.25% chance in a given year, or a 6% chance in 30 years.

The study concluded that every additional degree Celsius of global warming slightly more than doubles the risk of a megaflood. ARkHist-level events have a 1% chance per year of occurring (1-in-100-year recurrence) with Earth’s current global warming level of 1.2 degree Celsius above pre-industrial temperatures. If global warming hits 2.2 degrees, an ARkHist-level megastorm would have a 2.2% chance of occurring per year (a 1-in-45-year recurrence interval) — a dramatic increase in risk, especially given the catastrophic nature of a megaflood.

A warmer climate is already leading to stronger atmospheric rivers hitting California

One of the best-understood impacts of global warming on weather is that it increases the odds of heavy precipitation events. A warmer atmosphere can hold more water vapor, which leads to an increase in the heaviest downpours — including those in California’s atmospheric rivers: “The frequency and severity of landfalling ‘atmospheric rivers’ on the U.S. West Coast … will increase as a result of increasing evaporation and resulting higher atmospheric water vapor that occurs with increasing temperature,” according to a medium-confidence conclusion of the 2017 Fourth National Climate Assessment, a sweeping government report that outlines how climate change is affecting the U.S.

Wetter atmospheric rivers are already being observed. A 2022 case study found that human-caused climate change increased the amount of rainfall from two February 2017 atmospheric rivers by about 11% and 15%, respectively. As discussed in Part Two of this series, the Oroville Dam spillway nearly suffered a catastrophic failure because of these heavy rains, prompting the evacuation of over 180,000 people.

If the same events were to take place in an even warmer world with 541 parts per million of carbon dioxide in the atmosphere — projected to occur in the second half of the 21st century — the researchers found rainfall quantities in the two atmospheric rivers would have been another 9% and 26% higher, respectively.

Extreme rain does not necessarily mean extreme flooding

Although extreme precipitation is increasing because of climate change and will continue to increase as the planet warms, that doesn’t necessarily mean that extreme flooding will also increase. Floods are influenced by a variety of factors, including, very importantly, how wet the soils are. California is suffering from increasing drought, and when heavy rains fall on dry soils, it usually takes a greater amount of rain to induce flooding — though very dry, drought-baked soils can be impervious to water, increasing runoff.

A 2015 study in the journal Climactic Change found that very heavy precipitation — in the 99th percentile — in the contiguous U.S. resulted in 99th-percentile flooding only 36% of the time. The odds of 99th-percentile flooding increased to 62% when the soils were already moist, though. With California increasingly suffering extreme drought conditions prior to experiencing intense stormy periods, this will raise the bar on the amount of rain required to generate a megaflood during some years. In addition, when reservoirs are low because of pervasive drought, the risk of flooding and dam failures is reduced, since reservoirs can store a lot of floodwater.

More rain, less snow in store for California

As the climate warms, more wintertime precipitation in California’s mountains falls as rain instead of snow. This increases flood risk, since rain immediately creates runoff, while melting snow provides a more gradual release of water. The ARkStorm 2.0 model runs also found multiple potential “rain-on-snow” events at higher elevations, which could further add to runoff (though this is a very complex issue, and the uncertainty in how this phenomenon will change in the future is high).

In the Sacramento and San Joaquin River watersheds, the peak runoff in the future warmer climate scenario (ARkFuture) was as much as 200-400% higher than ARkHist runoff, despite precipitation totals that were only about 50% higher. At lower elevations (except in the southeastern deserts of California), peak runoff also increased by a considerably wider margin than precipitation (runoff increases of 60-100%, compared to precipitation increases of 30-60%).

In a blog post at weatherwest.com, report co-author Daniel Swain said, “Flood risk during an event like either of these scenarios will bring widespread and severe flood risk to nearly the entire state, but 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 experienced flooding of this magnitude historically.”

El Niño brings higher megaflood odds

The ARkstorm 2.0 study found that the top eight simulated 30-day “megastorm” events occurred during El Niño conditions, and seven of them occurred during moderate to strong El Niño events. These results suggest that reservoir operators should be more aggressive managing for floods during El Niño events – something that forecasts can give advance warning of several months in advance. But it’s not a guarantee: The floods of January 2023 and during the Great Flood of 1861-62 both occurred when El Niño was not present.

California must prepare for increasingly extreme floods and droughts

California’s weather over the past two months has abruptly switched from extreme drought to extreme flooding. It’s a particularly striking example of the exacerbation of precipitation extremes that a warming climate is likely to continue producing in an area naturally prone to weather whiplash, as documented by Daniel Swain and coauthors in a 2018 paper in Nature Climate Change, “Increasing precipitation volatility in twenty-first century California.” California’s water management system was designed for the climate of the 20th century, and a rapid and costly upgrade to the climate of the 21st century is urgently needed to prepare for a future of increasingly extreme droughts and floods.

For example, it is critical that flood planners give more room for rivers to flood by moving levees back to widen river channels and thus allow rivers to reclaim their ancestral floodplains.

Read: Could leaving ‘room for the river’ help protect communities from floods?

But giving more room for rivers requires purchase of riverside land, a difficult proposition in a state where land values are high and public finances are tight. Converting that land to flood relief and wildlife habitat also means losing the property taxes the government collects.

One success story, though, is in the city of West Sacramento, where a stretch of the Sacramento River has more room to flow thanks to a new “setback” levee — a second levee built in 2011 farther from the main levee lining the river. When the river is high, floodwater has room to flow through the tree-filled space between the two levees, instead of flooding the dozen or so homes that used to lie there, which the city bought out. An additional setback levee is being constructed just southeast of the Sacramento International Airport.

Improved forecast techniques could also inform reservoir operators on how to reduce flood risk. A pilot project for this has begun for two reservoirs in California, aided by data taken by both the NOAA and Air Force Hurricane Hunters. This project also studied how floodwaters might be used to recharge underground aquifers. Locating more underground features known as paleovalleys may aid in this effort.

In December 2022, the Central Valley Flood Protection Board approved a plan to spend more than $3 billion in the next five years and $30 billion over the next 30 years for infrastructure upgrades, emergency preparation and floodplain restoration in California’s Central Valley. In addition, one of the authors of the ARkStorm 2.0 study, Dr. Swain, has been asked to testify in front of the state legislature Feb. 1 on the risk of California megafloods during a public hearing.

Given California’s megaflood history and the potential increasing flood risk from climate change, it is essential for the state to continue to upgrade its flood infrastructure, policies, and flood-awareness efforts. It’s good to see the state has taken positive steps in that direction.

The other two parts of this three-part series:
Part One: The other ‘big one’: How a megaflood could swamp California’s Central Valley
Part Two: If a megaflood strikes California, these dams might be at risk