How Big a Climate Threat Are Atmospheric Rivers?

In 1862, a flood of biblical proportions hit California. In the state’s Central Valley region, the “Great Flood” spawned lakes that grew to be 20 miles wide and 250 miles long. It killed thousands, devastating entire towns and huge swaths of farmland. In Sacramento, the state capital, flood waters forced Leland Stanford, the newly elected governor, to travel in a rowboat to his inauguration. 

Some scientists suspect that atmospheric rivers, plumes of water vapor that transport water from the poles and dump heavy rain or snow when they hit land, caused the extreme flooding in 1862. These rivers in the sky have become increasingly powerful in the decades since, pummeling California and the western United States the past few winters. They will likely grow even more severe in a warming world. Western states need to prepare accordingly.

How is climate change affecting atmospheric rivers?

The American West is no stranger to atmospheric rivers. These normal weather events hit every winter and can benefit communities by replenishing water supplies and putting out wildfires. Atmospheric rivers provide up to half of California’s annual precipitation, which helps to ease drought. According to the U.S. Geological Survey, atmospheric rivers broke 33 to 74 percent of droughts on the West Coast between 1950 and 2010.

However, the rivers can bring extensive flooding and landslides as well, costing lives and causing massive damage. In 1862, California’s population was about half a million. Now, it is close to forty million. If a flood like the Great Flood hit today, it could displace five to ten million people and inflict one trillion dollars in damages, according to a University of California, Los Angeles study. Atmospheric rivers can also spur increased vegetation growth, resulting in larger wildfires.

Climate change will likely make the flooding caused by atmospheric rivers more difficult to manage. A recent study funded by the Department of Energy showed that climate change and global heating could cause total precipitation in future atmospheric rivers to increase by as much as 40 percent. California is already enduring wetter storms: scientists found that climate change increased the amount of rainfall in two atmospheric rivers in 2017 by between 11 and 15 percent. 

Beyond bringing heavier precipitation, climate change could also make atmospheric rivers more intense. The rivers could become 25 percent wider and longer as well as last longer, according to a NASA study. Atmospheric rivers could also come more quickly one after another, compounding their effects. Simultaneously, milder atmospheric rivers will become less frequent, according to a recent study by the National Oceanic and Atmospheric Administration.

Warmer temperatures also mean these storms more often fall as rain instead of snow. This poses challenges for the western states that rely on atmospheric rivers for their water supply year-round. For example, atmospheric rivers are currently responsible for around one-fourth of snowpack in the Sierra Nevada, the large mountain range located primarily in California. If more storms fall as rain than as snow, the Sierra Nevada could have less snowpack in the future.

Normally, the Sierra Nevada provides nearly one-third of California’s water supply, as the snowpack slowly melts and releases water that fills California reservoirs through the spring and summer. If this snowpack shrinks, California loses out on a crucial water source it relies on during drier times. Rainstorms falling on existing snowpacks also cause water runoff—as the rain on snow can cause snowmelt—resulting in even greater flooding than the initial rain or typical snowmelt alone.

What kind of damage can atmospheric rivers inflict?

When strong atmospheric rivers hit, they threaten human wellbeing. The storms can kill, with people drowned as floodwaters rise. In early February this year, Los Angeles received a foot of rain over the course of only a few days, resulting in floods that left nine dead. The National Weather Service described the storm as “one of the most dramatic weather days in recent memory.” Last year, atmospheric rivers killed twenty people in California.

Flooding can also spread disease when sewage overflows or when families crowd into evacuation centers. After the most recent storms in California, the state saw the spread of coccidioidomycosis, or Valley fever, a deadly fungal disease. These fungi thrive in rain-drought cycles. The California Department of Public Health has recorded over nine-thousand new cases of Valley fever with onset dates in 2023, the highest ever total reported by the department. 

Beyond health challenges, atmospheric rivers also pose serious risks to infrastructure. This year, in addition to the Los Angeles storms, a month’s worth of rain fell on San Diego in just one day. These storms in southern California left three hundred thousand people without power. The economic cost from the destruction is now estimated to reach $11 billion in damages, as droves of homes, roadways, and other infrastructure were destroyed. Meanwhile, atmospheric rivers in late 2022 and early 2023 caused between $5 and $7 billion in losses, Moody’s RMS estimated. Those numbers are even larger than recent historical models. A 2019 study by researchers at the University of California, San Diego, found that atmospheric rivers pose a $1-billion-a-year flood risk to the western United States. 

Those damages are so severe in part because flood protections, including dams, levees, and spillways, were built for the climatic conditions of the past. As human emissions drive worsening storms, outdated infrastructure fails under the new precipitation extremes. The Oroville Dam in California is a case in point. The fifty-year-old dam was no match for two atmospheric rivers that hit in 2017. Its emergency spillway struggled to control the deluge of water. With the dam threatening to overflow, 180,000 people evacuated their homes.

What can states do?

States should act now to build resilience to bigger, wetter atmospheric rivers, by:

Improving flood resistance infrastructure. States should improve building codes and land-use practices to better address larger influxes of water. For instance, state and local governments could impose building elevation requirements, increase drainage capacity, and improve water storage by adding more green spaces. Los Angeles’s decision to invest in “sponge” infrastructure, replacing impermeable concrete surfaces with permeable natural ones, helped the city avoid the worst impacts of flooding this winter. Instead of having the water run into the ocean, the new infrastructure collected 8.6 billion gallons of water for households’ future use. States should also discourage further development in floodplains to avoid future damage. 

Increasing public messaging and transparency. States should strengthen requirements for sellers to disclose the extent and repair costs for prior flooding to potential buyers. Emergency and evacuation plans should account for flooding that far exceeds what has occurred in the recent past. Owners and operators of critical infrastructure should plan for more extreme conditions. 

Improving meteorological systems. More accurate and advanced meteorological modeling could help reservoir managers better anticipate increased precipitation. These adjustments could prevent water from overflowing and destroying nearby infrastructure and allow for closer tracking of atmospheric rivers’ movements to better evaluate potentially affected areas.

Reducing greenhouse gas emissions. While emissions climb, global temperatures will continue to increase, making atmospheric rivers more dangerous. According to the latest calculations, global efforts are significantly off track to meet the Paris Agreement goals. In fact, carbon emissions in 2023 were at the highest levels ever recorded. Lowering greenhouse gas emissions is the best long-term line of defense. 

Will Merrow created the graphics for this article.