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The science

What is an ARkStorm?

ARkStorm is short for Atmospheric River 1,000. It’s the name the U.S. Geological Survey gave a storm scenario it published in 2011: a run of severe winter storms like those of 1861–62, hitting the California we live in now.

The basics

First, what’s an atmospheric river?

An atmospheric river is a long, narrow band of moisture high in the sky. It carries water vapor across the Pacific toward the coast, like a river with no banks.

When one runs into California’s mountains, the water falls out as rain and snow. Most winters that’s good news, because these storms fill the reservoirs. The trouble starts when they arrive one after another and the ground, the rivers and the levees can’t keep up.

Clouds over the ocean, seen from orbit
ARkStorm 1.0 · USGS · 2011

The first study: what would 1862 do to modern California?

A team of 117 scientists, engineers, and policy and insurance experts designed a realistic storm, then followed the damage through: floods, landslides, power, water, roads and the economy.

Cars standing in floodwater outside a row of shops
500 to 1,000 years
Rainfall in many places beyond what’s expected once in that long.
300 by 20 miles
Flooding in the Central Valley: 300 miles long and 20 or more miles wide.
125 mph
Wind in some places. Across wider areas of the state, 60 mph.
1.5 million
Residents who could have to evacuate the inland region and the Delta counties.
Hundreds
of landslides damaging roads, highways and homes.
Weeks or months
to restore power, water, sewer and other lifelines.

What it would cost: about $725 billion

$400B
Property damage
$325B
Lost business
  • That’s nearly three times the loss in USGS’s own scenario for a severe Southern California earthquake.
  • Only $20 to $30 billion of the property loss would be recoverable through insurance.
  • California’s flood defenses are typically built for 100- to 200-year runoffs. This storm overwhelms them in many places.
All figures on this section: USGS Open-File Report 2010-1312, Overview of the ARkStorm Scenario (2011).
ARkStorm 2.0 · Science Advances · 2022

The update: the same storm in a warmer climate.

In August 2022, climate scientists Xingying Huang and Daniel Swain published the first part of ARkStorm 2.0. They built two month-long storm scenarios: one drawn from the recent climate, and one from the warmer climate expected late this century.

2×
Climate change has already doubled the likelihood of a storm sequence that could cause catastrophic flooding.
200–400%
increase in severity over the 2011 scenario, according to the update.
$1 trillion+
in damage in California, the update’s new estimate.
“In the future scenario, the storm sequence is bigger in almost every respect.”
Daniel Swain, UCLA climate scientist. UCLA Newsroom, August 2022.

Still California only

Like the first study, the update looks only at California. It doesn’t follow the damage past the state line.

What comes next

What the 2011 report said should happen.

  1. 01

    Test the plans.

    The report raised serious questions about whether existing federal, state and local disaster plans could handle a disaster this size.

  2. 02

    Pay now or pay later.

    A core question it posed: spend now to reduce the damage, or pay a lot more later for recovery.

  3. 03

    Tell the public.

    It said common messages to educate the public about the risk could be developed and consistently communicated. That’s why this site exists.

Sources