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Firefighters battling the deadly wildfires that swept through Los Angeles in January 2025 are hampered by limited freshwater supplies. So, when the winds are calm enough, experienced pilots flying planes aptly called Super Scoopers scoop up 1500 gallons of seawater at a time and drop it on the flames with pinpoint accuracy. Using seawater to fight fires may seem like a simple solution—the Pacific Ocean has a seemingly endless supply of water. In emergencies like those facing Southern California, it’s often the only quick fix, though the operation can be risky in choppy ocean conditions.
But seawater also has its drawbacks.
Salt water corrodes firefighting equipment and can damage ecosystems, especially those like the scrublands around Los Angeles that are not normally exposed to seawater. Gardeners know that a small amount of salt (added, say, as fertilizer) doesn't harm plants, but too much salt can stress and kill plants. While the effects of adding seawater to ecosystems are not yet fully understood, we can get an idea of what to expect by considering the effects of rising sea levels.
Seawater experiment in a coastal forest
As an ecosystem ecologist at the Smithsonian Center for Environmental Research, I lead a new experiment called TEMPEST, which was designed to understand how and why historically salt-free coastal forests are responding to the first impact of saltwater. Global sea level rise has averaged about 8 inches over the past century, and that water has pushed saltwater into forests, farms, and neighborhoods across the United States that previously had only freshwater.
As the rate of sea level rise accelerates, storms push seawater further inland, eventually killing trees and creating ghost forests, a consequence of climate change that is widespread in the U.S. and around the world. At our TEMPEST test sites, we pump saltwater from the nearby Chesapeake Bay into tanks and then spray it onto the forest soil surface quickly enough to saturate the soil for about 10 hours at a time. This simulates the saltwater surge during a severe storm.
Our riparian forest was little affected by the first 10 hours of saltwater exposure in June 2022, and grew normally for the rest of the year. We increased the exposure to 20 hours in June 2023, and the forest still seemed mostly unaffected, although the tulip poplars were slower to extract water from the soil, which may be an early warning signal.
The situation changed after a 30-hour exposure in June 2024. The leaves of the tulip poplar in the forests began to turn brown in mid-August, a few weeks earlier than usual. By mid-September, the forest canopy was bare, as if winter had begun. These changes did not occur in a neighboring plot, which we treated in the same way, but with fresh water rather than seawater. The initial resilience of our forest can be explained in part by the relatively low salt content of the water in this estuary, where water from freshwater rivers and the salty ocean mix. Rainfall after the experiments in 2022 and 2023 washed the salts out of the soil.
But after the 2024 experiment, there was a severe drought, so the salts were trapped in the soil. The trees' longer exposure to salty soil after our 2024 experiment may have exceeded their ability to tolerate those conditions. The seawater that is dumped on the Southern California fires is pure salty ocean water. The conditions there were very dry, especially compared to our forest site on the East Coast.
Changes are noticeable in the ground
Our research team is still trying to understand all the factors that limit a forest's tolerance to salt water, and how our findings apply to other ecosystems, such as those in the Los Angeles area. The trees' leaves turning from green to brown long before fall was a surprise, but there were other surprises hidden in the soil beneath our feet.
Rainwater percolating through soil is usually clear, but about a month after the first and only 10-hour exposure to salt water in 2022, the groundwater turned brown and remained that way for two years. The brown color comes from carbon-based compounds leaching from dead plant material. It’s a process similar to brewing tea. Our lab experiments suggest that the salt caused clay and other particles in the soil to disperse and move. Such changes in soil chemistry and structure can persist for years.
Sea level rise increases coastal exposure
While ocean water can help fight fires, there are reasons why firefighters prefer freshwater sources—if freshwater is available. Meanwhile, the U.S. coast is facing more intense and frequent saltwater impacts as rising global temperatures accelerate sea levels, flooding forests, fields, and farms, with unknown risks to coastal landscapes.
