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If not by drought then by megaflood: Climate change may have doubled risk of disastrous flooding in California

Already crippled by years of megadrought, California may have another climate change-induced worry on the horizon: economy-busting megafloods.

(CN) — In drought-ridden California's history, megafloods have occurred about every century. Not often, but from a geological standpoint they're not rare. The most notable megaflood in recent history, the Great Flood of 1862, came after a 20-year drought. The flood caused $100 million in damage, equal to $3.11 billion today, making it one of the costliest natural disasters to hit the disaster-prone state.

The 1862 megaflood occurred not as the result of a single storm, but after weeks of continuous rain and heavy snow in the Sierra. It culminated in a warm intense monster of a storm that melted all the snow that had fallen in the prior weeks. The resulting flood turned the Central Valley into an inland sea up to 30 feet deep and 4,000 people — 1% of the state's population at the time — lost their lives.

And now, new research and weather modeling indicates climate change is changing the timeline for so-called 100-year events. Using the state's geological and weather history, a previous weather model from 2011 and new climate data, researchers crafted a new model that accounts for climate change and discovered the risk of disastrous megafloods — the culmination of multiweek megastorm event — may have already doubled.

Daniel Swain, study author and climate scientist at the University of California Los Angeles Institute of the Environment and Sustainability, noted that the 2011 ARkStorm model didn't factor in climate change's effects on weather.

"One of the main goals in doing this was to exclusively account for climate change. The other was to leverage all these newer, more advanced tools that have come into existence and the broadening scientific knowledge surrounding these types of events and have come before in recent years," Swain said in a Zoom interview.

Amid campaigns and infrastructure surrounding water conservation and drought remedies, the state isn't ready to cope with intense rainfall and runoff. The megaflood events looming in the near future would cause trillions of dollars in damage — the costliest natural disaster on Earth to date — and a massive economic shutdown. Making up 14.6% of the nation's economy in 2021 and with a $3.3 trillion GDP, California holds its own on the global stage, beating the United Kingdom, France and India, all of which clock GDPs around $2.65 trillion.

In other words, California's megaflood will have global economic effects.

"It would wipe out agriculture, tourism, the movie industry, the tech industry — at least temporarily — for a multi-month, two-year period, all at once. Imagine even just economically what that would look like. That's what the first ARkStorm scenario showed, and that's not to mention the direct damages that would occur in people's houses, public structures, infrastructure and the lives that would be lost," Swain said in the interview.

This revolutionary model is phase one of three, looking at meteorology and what weather sequences lead to megaflooding events. The next phase will include assessing the damage in more detail and working closely with state departments to inform policy and prevent worst-case outcomes.

"That is the whole purpose of doing this — to improve societal resilience to extreme flood events, improve our preparedness, potentially retool our water and flood infrastructure in the long run in ways that accommodate not just increased risk of drought but also this increased risk of flood. And in some ways, those things can be complementary," Swain said. "There are lots of practical reasons why it's important, and this is something we're going to do as we move beyond the first phase."

Swain believes that one of the study's most interesting and alarming findings is a nearly linear increase between global temperature rise and megaflood likelihood.

"It goes from something that probably wouldn't happen for multiple generations — our children and our children's children could go without ever seeing one to being something that more likely than not we are going to experience in our lifetimes and potentially more than once. That's a huge shift," Swain said.

He added: "What can we do? These events are always going to cause problems. I don't think there's any way to fully adapt our way out of it. But there are going to be things that we're going to be able to implement that could tamp down some of the harms. Those are what we're hoping to do in the next couple of project phases."

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