{"id":809,"date":"2026-02-17T14:25:00","date_gmt":"2026-02-17T14:25:00","guid":{"rendered":"https:\/\/flatplanplus.io\/bbc-sciencefocus\/?p=809"},"modified":"2026-02-17T14:25:00","modified_gmt":"2026-02-17T14:25:00","slug":"a-biblical-megaflood-could-hit-the-us-at-any-time-and-were-not-prepared","status":"publish","type":"post","link":"https:\/\/flatplanplus.io\/bbc-sciencefocus\/2026\/02\/17\/a-biblical-megaflood-could-hit-the-us-at-any-time-and-were-not-prepared\/","title":{"rendered":"A biblical megaflood could hit the US at any time. And we’re not prepared"},"content":{"rendered":"\n
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Environment<\/p>\n\n\n\n

A biblical megaflood could hit the US at any time. And we’re not prepared<\/h1>\n\n\n\n

The damage could eclipse anything in modern American history<\/p>\n\n\n\n

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By Tom Howarth<\/p>\n\n\n\n

–<\/p>\n<\/div>\n<\/div>\n\n\n\n

\"Flooded
Photo credit: Getty<\/figcaption><\/figure>\n<\/div>\n<\/div>\n\n\n\n

California. The Golden State. Home to almost 40 million people and the world\u2019s fourth-largest economy, it is a global capital of technological innovation and the epicentre of the entertainment industry. But beneath the glitz and glamour, prosperity and prestige, the state is a natural disaster hotspot. <\/p>\n\n\n\n

From earthquakes to wildfires, Californians are accustomed to battling the forces of nature. In 2025 alone, wildfires ravaged more than 500,000 acres<\/a> of land statewide, killing 31 people and causing hundreds of billions of dollars<\/a> worth of damage. <\/p>\n\n\n\n

Wildfires \u2013 and how to prevent, mitigate and fight them \u2013 have dominated California\u2019s disaster planning in recent years. And with good reason: the state has been in a constant state of drought for 25 years, only becoming drought-free for the first time this January. The last five years in particular have seen some of the worst fire seasons in living memory, with millions of acres scorched.<\/p>\n\n\n\n

But California is built on extremes at both ends. Alongside vast wealth sits deep poverty; alongside years of drought and fire come episodes of torrential rain and devastating floods.<\/p>\n\n\n\n

Only in 2023, hundreds of thousands were left without power as storms and flooding battered the state. Highways collapsed, trains derailed, and thousands were evacuated from their homes. <\/p>\n\n\n\n

That sounds bad enough, but according to the latest research, episodes like this pale in comparison to what\u2019s to come. <\/p>\n\n\n\n

Scientists and officials are now preparing for not one threatening storm, but a 30-day maelstrom of megastorms unlike anything seen in the state for almost 200 years. <\/p>\n\n\n\n

Such a scenario was always possible, but rising global temperatures are making it more likely \u2013 and far more destructive.<\/p>\n\n\n\n

\u201cIt was always a when, not if,\u201d says Dr Daniel Swain<\/a>, a climate scientist at UCLA, who co-authored the study<\/a> warning of the coming storm. \u201cBefore global warming, that \u2018when\u2019 might have been centuries away. Now it\u2019s quite likely to be within my own lifetime.\u201d<\/p>\n\n\n\n

This storm system, dubbed \u2018ARkStorm 2.0\u2019, could strike this year or in 60 years \u2013 no one knows for sure. Whenever it does, it is likely to be one of the most costly disasters in global history. The only question is whether California can prepare in time.<\/p>\n\n\n\n

\"A
Flooding has always been a risk in California, but an ARkStorm-type event would be on a different scale than anything seen for hundreds of years. Photo credit: Getty<\/figcaption><\/figure>\n\n\n\n

What is an ARkStorm?<\/h2>\n\n\n\n

Short for \u2018Atmospheric River 1,000-year Storm\u2019, the ARkStorms are hypothetical \u2013 yet very plausible \u2013 megastorm scenarios. Those familiar with weather on the US West Coast may already know atmospheric rivers (ARs) well. <\/p>\n\n\n\n

These vast corridors of water vapour flow through the sky, transporting moisture from the tropics towards the poles. When they collide with land and mountain ranges, that vapour condenses into prolonged, heavy rainfall or snowfall.<\/p>\n\n\n\n

One well-known example is the so-called Pineapple Express, a strong AR that funnels warm, moisture-laden air from near Hawaii towards the west coast of North America. <\/p>\n\n\n\n

Though often strong rainfall events, ARs are important to water supplies in the region, contributing up to 50 per cent<\/a> of California\u2019s annual precipitation. <\/p>\n\n\n\n

However, when these systems stall, or multiple hit in quick succession, they can dump extraordinary amounts of rain and snow in a short time, dramatically raising the risk of landslides, river flooding and reservoir overflow.<\/p>\n\n\n\n

\"Satellite
The largest atmospheric rivers can transport 15 times the amount of water as the flow of the Mississippi River at its mouth. Photo credit: NASA MODIS\/TERRA\/NASA Worldview<\/figcaption><\/figure>\n\n\n\n

In 2010, a team of interdisciplinary scientists led by the US Geological Survey developed the first ARkStorm scenario: a sequence of super-strong atmospheric rivers striking California just days apart. <\/p>\n\n\n\n

Far from being a speculative worst-case, the scenario was designed to mirror real historical events, including devastating storms in 1861-62 that lasted 45 days and turned hundreds of miles of the Central Valley into an inland sea.<\/p>\n\n\n\n

Today, if an ARkStorm hit, it would be the most costly geophysical disaster of all time. <\/p>\n\n\n\n

In such an event, damages are expected to exceed $1tn, with flood depths in some areas up to 6m (20ft). Millions of people would need to be evacuated, as water invaded populated areas from San Francisco to Los Angeles. <\/p>\n\n\n\n

In 2022, researchers revisited the ARkStorm scenario to factor in one \u2013 pretty important \u2013 factor that was left out of the original analysis: our old friend climate change. Suffice to say that if the first scenario was troubling, ARkStorm 2.0 was truly mind-melting. <\/p>\n\n\n\n

The mother of all ARkStorms<\/h2>\n\n\n\n

Climate change impacts rainfall events, and in particular extreme rainfall events, in two main ways. <\/p>\n\n\n\n

First, warmer air can hold more water vapour. For every degree Celsius of warming, the atmosphere\u2019s moisture-holding capacity increases by around 7 per cent<\/a>, giving storms more fuel to work with.<\/p>\n\n\n\n

Swain and his colleagues call this the \u201cexpanding atmospheric sponge effect\u201d. <\/p>\n\n\n\n

He says: \u201cYou can essentially just imagine an ordinary kitchen sponge that grows progressively larger with warming. It has the capacity to absorb much more water, so you can soak up much more water.\u201d <\/p>\n\n\n\n

This is why, under a warming climate, average precipitation will largely stay the same, and droughts and wildfires will still be commonplace because more water is locked up in the atmosphere, rather than on the ground. <\/p>\n\n\n\n

\u201cThe air is demanding more water more rapidly from the landscape and drying it out faster,\u201d Swain continues. \u201cBut of course, if you have a very large sponge that\u2019s completely saturated and you wring it out, it yields more water. And that\u2019s analogous to the more intense precipitation.\u201d <\/p>\n\n\n\n

\"This
Strong atmospheric rivers brought widespread flooding to California in 2023, such as here in Planada, where 83 per cent of households experienced economic losses. Photo credit: Getty<\/figcaption><\/figure>\n\n\n\n

In ARkStorm 2.0<\/a> simulations, atmospheric rivers transport around a quarter more moisture in future climates than they do today, driving much heavier cumulative rainfall.<\/p>\n\n\n\n

Second, warming raises the snowline. In the past, much of the Sierra Nevada mountain range\u2019s winter precipitation fell as snow, temporarily locking water away in mountain snowpack and releasing it gradually during spring melt. <\/p>\n\n\n\n

In a warmer climate, more precipitation falls as rain, and rain that lands on existing snow can rapidly melt it, sending large volumes of water downhill all at once.<\/p>\n\n\n\n

A bigger sponge and higher snowline is what Swain aptly calls the \u201cdouble whammy effect.\u201d <\/p>\n\n\n\n

\u201cYou get increased precipitation intensity, but also a large shift from snow to rain. What once would have fallen as snow and not contributed immediately to flood risk is now falling as rain and turning straight into runoff.\u201d<\/p>\n\n\n\n

Together, these effects drive dramatic increases in peak flows in rivers and waterways. In parts of the southern Sierra Nevada, ARkStorm 2.0 modelling suggests runoff could be 200-400 per cent higher than in historical megastorm equivalents.<\/p>\n\n\n\n

From rare disaster to rising risk<\/h2>\n\n\n\n

Historically, floods on the scale of California\u2019s Great Flood of 1861-62 were thought to occur perhaps once every century or two. That event turned the Central Valley into an inland sea nearly 300 miles long, drowning farms, towns and transport routes, and forcing the state capital to temporarily relocate from Sacramento to San Francisco.<\/p>\n\n\n\n

But ARkStorm 2.0 modelling suggests that climate change has already shifted those odds. Compared with conditions in the early 20th century, the likelihood of a megaflood-capable storm sequence has more than doubled. <\/p>\n\n\n\n

If emissions are not curbed, by 2060, catastrophic events that previously occurred once every two centuries are set to occur three times per century<\/em>, the study suggested.<\/p>\n\n\n\n

\u201cFlooding has been an underestimated risk historically relative to more visible natural disasters like wildfires and earthquakes,\u201d Swain says. <\/p>\n\n\n\n

\u201cBut it\u2019s now a risk which is increasing over time, which is going to compound the consequences of that historical underestimation if we don\u2019t do things differently.\u201d <\/p>\n\n\n\n

\"View
This illustration from 29 March 1862 shows Sacramento during the Great Flood. The city will once again be in the firing line when an ARkStorm hits. Photo credit: Getty<\/figcaption><\/figure>\n\n\n\n

The Central Valley is the danger zone<\/h2>\n\n\n\n

If ARkStorm 2.0 were to strike, flooding would not be evenly distributed across the state. The greatest risks converge on the Central Valley, where runoff from the Sierra Nevada funnels into the Sacramento and San Joaquin river systems before spreading across some of the most heavily engineered floodplains in the world.<\/p>\n\n\n\n

This vast low-lying region is both California\u2019s agricultural heartland and a critical artery of its water supply system. It is criss-crossed by hundreds of miles of levees, canals, pumping stations and reservoirs designed to protect cities, farmland and transport corridors from seasonal floods.<\/p>\n\n\n\n

But those defences were largely built for a climate in which winter precipitation arrived in shorter bursts and snowmelt dominated flows in spring.<\/p>\n\n\n\n

\u201cThe San Joaquin Valley in particular really stands out to me as being of great concern,\u201d Swain says. <\/p>\n\n\n\n

\u201cIt\u2019s the breadbasket region, but it\u2019s also got cities home to several million people, and it\u2019s where much of the state\u2019s water infrastructure that supplies Southern California is located\u2026 But it\u2019s also where we expect the largest increases in flood severity and magnitude in a warming climate.\u201d <\/p>\n\n\n\n

In ARkStorm 2.0 simulations, runoff in key Sierra Nevada drainage basins often exceeds anything seen in historical megastorms, with little overlap between past and future extremes. <\/p>\n\n\n\n

In practical terms, that means flood protection infrastructure may face water levels well beyond those it was designed to withstand \u2013 and for far longer than engineers ever anticipated.<\/p>\n\n\n\n

\"Summary
Summary map showing ARkStorm 1.0 predicted maximum daily precipitation, plausible flooded areas, maximum winds and landslide susceptibility. Photo credit: Plumlee et al. (2015) via USGS<\/figcaption><\/figure>\n\n\n\n

Mapping a disaster that has never happened<\/h2>\n\n\n\n

To turn storm simulations into maps of real-world flooding, researchers need detailed inundation modelling \u2013 complex analyses that simulate how water would move across landscapes, interact with levees and spill into towns and infrastructure.<\/p>\n\n\n\n

Those studies are now underway as part of the ARkStorm 2.0 programme, using national-scale flood models developed by Oak Ridge National Laboratory. <\/p>\n\n\n\n

The aim is to generate high-resolution flood maps that state and local agencies can use for planning and emergency response, and according to a spokesperson for the lab, the first results from this work may be available in the autumn.<\/p>\n\n\n\n

\u201cThat\u2019s one of the positive outcomes for the state,\u201d says Michael Anderson, California\u2019s state climatologist at the Department of Water Resources. <\/p>\n\n\n\n

\u201cWe\u2019ll have a tool that lets us anticipate where flooding concerns may be, and start figuring out what to do about it.\u201d <\/p>\n\n\n\n

Until those maps are complete, however, emergency planners lack a full picture of exactly which neighbourhoods, roads and facilities would be underwater in a true megaflood scenario.<\/p>\n\n\n\n

\"San
Emergency responders rescue a dog during atmospheric river-driven flooding in 2023, Merced, California. Photo credit: Getty<\/figcaption><\/figure>\n\n\n\n

In the meantime, officials in some areas have begun to prepare through large-scale emergency exercises \u2013 similar to war games but in this case, the enemy is water. <\/p>\n\n\n\n

One such exercise took place back in 2013 and 2014 in response to the ARkStorm 1.0 simulations. Researchers at the Desert Research Institute (DRI) downscaled the scenario to the Lake Tahoe region, bringing together more than 300 people to take part in discussions and tabletop drills, testing how they would respond in emergency situations to threats such as landslides, road closures and prolonged power outages.<\/p>\n\n\n\n

\u201cJust getting people in the same room, talking to each other was important,\u201d says Dr Christine Albano, an associate research professor at the DRI. <\/p>\n\n\n\n

\u201cThere are an infinite number of ways an ARkStorm could play out, but these exercises help people know who to call and give people a tangible sense of what could happen.\u201d <\/p>\n\n\n\n

According to Albano, that first exercise has already yielded benefits when flooding occurred in 2017. She says it improved preparedness and responses, including helping agencies pre-position equipment, staff, fuel and supplies to ensure they could continue operating during power outages. <\/p>\n\n\n\n

The DRI is now using the ARkStorm 2.0 scenario to run planning exercises with emergency services and utilities, and has developed an online tool<\/a> that allows local officials to explore how prolonged flooding could affect their communities. <\/p>\n\n\n\n

But Albano says they currently don\u2019t have funding to run more tabletop exercises for ARkStorm 2.0, despite the scale of the risk.<\/p>\n\n\n\n

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Read More:<\/h3>\n\n\n\n