California warned it cannot ignore trillion dollar flood threat

A team of researchers have warned that California faces a real danger of future catastrophic floods.

The team from UCLA and the National Centre for Atmospheric Research were prompted into the research by a flood that occurred in 1862, which was called the “ArkStorm scenario,” reflecting the potential for an event of biblical proportions.

To account for the additional flood-worsening effects of climate change, scientists have completed the first part of ArkStorm 2.0. The results were stark.

“In the future scenario, the storm sequence is bigger in almost every respect,” said Daniel Swain, UCLA climate scientist and co-author of the paper. “There’s more rain overall, more intense rainfall on an hourly basis and stronger wind.”

In total, the research predicts that end-of-the-century storms will generate 200% to 400% more runoff in the Sierra Nevada Mountains due to increased precipitation and more precipitation falling as rain, not snow.

The researchers used a combination of new high-resolution weather modelling and existing climate models to compare two extreme scenarios: one that would occur about once per century in the recent historical climate and another in the projected climate of 2081-2100. Both would involve a long series of storms fuelled by atmospheric rivers over the course of a month.

The paper also simulated how the storms would affect parts of California at a local level.

“There are localised spots that get over 100 liquid-equivalent inches of water in the month,” Swain said, referring to the future scenario. “On 10,000-foot peaks, which are still somewhat below freezing even with warming, you get 20-foot-plus snow accumulations. But once you get down to South Lake Tahoe level and lower in elevation, it’s all rain. There would be much more runoff.”

The paper, which was co-authored by climate scientist Xingying Huang, found that historical climate change has already doubled the likelihood of such an extreme storm scenario, building on previous UCLA research showing increases in extreme precipitation events and more common major floods in California. The study also found that further large increases in “megastorm” risk are likely with each additional degree of global warming this century.

“Modelling extreme weather behaviour is crucial to helping all communities understand flood risk even during periods of drought like the one we’re experiencing right now,” said Karla Nemeth, director of the California Department of Water Resources, which provided funding for the study. “The department will use this report to identify the risks, seek resources, support the Central Valley Flood Protection Plan, and help educate all Californians so we can understand the risk of flooding in our communities and be prepared.”

With drought and wildfire getting so much attention, Californians may have lost sight of extreme flooding, Swain said. “There is potential for bad wildfires every year in California, but a lot of years go by when there’s no major flood news. People forget about it.”

Stockton, Fresno and Los Angeles would be under water even with today’s extensive collection of reservoirs, levees and bypasses. It is estimated that it would be a $1 trillion disaster, larger than any in world history.

Though no flood so large has happened since the 19th century, climate modelling and the paleoclimate record — including river sediment deposits dating back thousands of years — shows that it typically happened every 100 to 200 years in the pre-climate change era.

The ArkStorm flood is also known as “the Other Big One” after the nickname of an expected major earthquake on the San Andreas Fault. But, unlike an earthquake, the ArkStorm would lead to catastrophe across a much larger area.

“Every major population centre in California would get hit at once — probably parts of Nevada and other adjacent states, too,” Swain said.

The effects on infrastructure would complicate relief efforts, with major interstate freeways such as the I-5 and I-80 likely shut down for weeks or months, Swain said. Economic and supply chain effects would be felt globally.

The first ArkStorm exercise concluded that it would not be possible to evacuate the 5 to 10 million people who would be displaced by flood waters, even with weeks of notice from meteorologists and climatologists. While it helped inform flood planning in some regions, the exercise was limited due to lack of organized resources and funding, Swain said.

California has already seen increases in climate-driven drought and record-breaking wildfires, Swain said. With climate change-amplified flooding, ArkStorm 2.0 aims to get ahead of the curve.


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