Giant rivers of moist air that curl off from the tropics are responsible for most of the flooding in the Western United States, and scientists say these atmospheric rivers will become more intense as the planet warms.
A new study is now providing some insight into what that could look like in the future.
The research, published Wednesday, shows how much damage the most intense atmospheric rivers today are already doing, finding that just 10 atmospheric river events caused nearly half the flood damage in the U.S. West over the past four decades, adding up to billions of dollars.
These powerful rivers of water vapor can carry twice as much moisture as the Amazon River and extend for hundreds of miles. Once they cross land, their accompanying storms can trigger heavy rainfall and mudslides, and snow in the higher elevations.
In the new study, researchers with the Scripps Institution of Oceanography at the University of California San Diego and the U.S. Army Corps of Engineers analyzed 40 years of data from the National Flood Insurance Program. They found that storms associated with atmospheric rivers caused more than 99 percent of all flood damage in parts of coastal Oregon and California. The 10 most extreme atmospheric rivers caused nearly half of all the flood damage in the West between 1978 and 2017, about $23 billion, the study concluded.
Global warming is already intensifying rainfall from atmospheric rivers, the authors said.
"In a warmer climate, extreme ARs (atmospheric rivers) will become more intense as they become wetter, longer and wider; there is some indication that this is already happening in association with observed Pacific Ocean warming," they wrote in the scientific journal Science Advances.
The findings show it's urgent for communities in the path of atmospheric rivers to adapt and "prepare for worst-case scenarios," said Scripps scientist Tom Corringham, the lead author of the study.
That includes the risk of megafloods dubbed "ARk storms," like the extreme atmospheric river in 1861 that flooded California's Central Valley with so much water that the newly elected governor, Leland Stanford, traveled to his inauguration in a rowboat.
The risk of ARk storms and other related climate extremes intensified by global warming should be on the radar, Corringham said on Tuesday, shortly after attending what he called a make-or-break session at the UN climate talks in Spain on implementing the Paris climate agreement.
Corringham said trends of increasing impacts from extreme flooding events is clear, including from increasingly intense hurricanes.
The atmospheric river story bolsters this," he said. "Extreme events will have significant economic impacts. It will get worse if we don't act now to stabilize the global climate system. If we care about economic growth, we need to take action."
What Are Atmospheric Rivers?
Atmospheric rivers are powerful streams that carry huge amounts of water vapor from the tropics to Earth's continents and polar regions. Sometimes they're called tropical plumes, and along the West Coast of the U.S. some people call them "pineapple express" storms because they carry moisture from sources near the Hawaiian Islands.
They can average 250 to 500 miles wide and about 1.8 miles deep. When they flow up over land, especially mountains, the water vapor condenses and falls as rain or snow.
Not all atmospheric rivers are extreme. Some bring moderate rainfall that is critical for ecosystems, as well as for maintaining stream flows and recharging groundwater aquifers. But the storms from the stronger atmospheric rivers can cause destructive mudslides and flooding.
Coastal areas are most affected, but depending on exactly where they make landfall and what direction they are moving, atmospheric rivers can affect states as far east as Colorado and Wyoming. Last winter's extreme snow and avalanche cycle in the Colorado Rockies was fueled by an atmospheric river, Corringham said.
As they intensify in a warming atmosphere, they could also exacerbate the growing problem of rain-on-snow floods as far inland as the Idaho panhandle and Montana, he added.
Bigger, Badder Storms as Temperatures Rise
A 2018 study identified a 70-year trend of atmospheric rivers sending more moisture surging ashore.
Variations year to year and decade to decade can be caused by cycles like El Niño, but the study picked out a long-term trend driven by global warming, said author Duane Waliser, chief scientist with the NASA Jet Propulsion Lab Earth Science and Technology Directorate.
"The upshot was, you will, if you're standing somewhere on the California coast (between 2070 and 2100), experience atmospheric river conditions 40 percent more than now. The number of events decreases, but they get about 20 percent longer and 25 percent wider than today. Even if you have a smaller number, you are more likely to be affected," he said.
Waliser's research showed the number of atmospheric rivers will decrease by about 10 percent by the 2070 to 2100 time period, but that they will be carrying more moisture. Globally, that adds up to "pronounced increases" in the amount of moisture carried by the rivers if global warming continues at its current pace.
The reason is simple, Waliser said. Warming oceans release more moisture to the air, and a warmer atmosphere can hold more moisture, carried along by atmospheric rivers until it hits land and releases the water vapor as rain or snow.
Additionally, global warming is also changing the undulations of the normal west-to-east wind patterns in the mid latitudes of both hemispheres in a way that contributes to the intensification of atmospheric rivers, he added.
Chile, South Africa and parts of Australia and New Zealand are also vulnerable to flood risks from increased atmospheric river intensity. Related research shows they could become twice as frequent in parts of Western Europe, where some areas are particularly vulnerable to wind storms associated with atmospheric rivers, Corringham said.
Building Resilience to Storms
The new study includes suggestions on how communities can become more resilient in the face of the increasing flood threats from atmospheric rivers.
Among other things, it says communities should discourage new development in areas at risk from such flooding, restore natural floodplains and develop greener infrastructure, along with hardening existing flood walls, dams and spillways.
At best, such improvements could be an opportunity to boost water supplies, Corringham said. Preliminary results from a pilot reservoir operations project he works on with the Center for Western Weather and Water Extremes at Lake Sonoma show that existing atmospheric river forecasts could increase water supplies by up to 50 percent. But many reservoir plans have not been significantly updated since their dams were built.
There has been talk about making real changes for quite a while, he said, but in recent years, "there's more of a willingness to experiment."