Spraying seawater into the sky over the southeastern Pacific could weaken a rampaging El Niño before it wrecks harvests and scrambles weather worldwide, a new study finds.
The technique, marine cloud brightening, would need roughly 2,400 ships fitted with saltwater sprayers — about 2 percent of the world’s merchant fleet — misting sea salt into low clouds so they reflect more sunlight and cool the ocean below.
Published July 8 in Science Advances, the study comes from a team led by Jessica Wan of the Scripps Institution of Oceanography, working with researchers at the National Center for Atmospheric Research. They wanted to know whether cloud brightening could interrupt a single El Niño while it was still forming, not just slow decades of warming.
Researchers needed proof the idea could work outside a computer model. They found it, unexpectedly, in smoke.
The 2019-2020 “Black Summer” bushfires in Australia pumped so much smoke into the southeast Pacific that it brightened the cloud deck there. The ocean below cooled enough to help trigger the La Niña that followed from 2020 to 2023.
The team recreated that smoke-driven cooling in a climate model and got a nearly identical result. The match gave them confidence to test deliberate cloud brightening against real El Niño events. Co-author John Fasullo of NCAR had mapped the original bushfire effect in 2023, laying the groundwork.
Timing Determines Everything
Researchers ran the model against the two strongest El Niños of the past 30 years: 1997-98 and 2015-16. Spraying clouds from June through February, starting while the event was still building, nearly erased it, cooling the central Pacific by close to 1.9 degrees Celsius.
Waiting until December, after the El Niño had already peaked, barely helped. That late intervention cooled the ocean by less than a third as much.
The mechanism disrupts the Bjerknes feedback, the loop of shifting trade winds and rising sea temperatures that normally makes an El Niño grow stronger.
“MCB is effective at reducing the warming and wetting impacts of El Niño,” said Jessica Wan, the study’s lead author.
Cooling an El Niño early doesn’t come free. In the simulations, the same interventions that weakened El Niño often triggered a stronger, earlier La Niña the following year — its cold counterpart, which carries disruptions of its own.
The model also showed unexpected warming over Europe and Asia in some scenarios. Nudging one climate pattern, it turns out, can shove others in directions scientists don’t yet fully understand.
Not Ready for the Real Ocean
The researchers built their case entirely inside one climate model, CESM2, and caution the results could differ in other simulations. Nobody has tested cloud brightening at this scale in the real ocean, and the fleet of specialized spray ships doesn’t exist yet.
No one is proposing to test this on the El Niño building right now. But the authors argue the case for taking the idea seriously is growing as scientists look beyond cutting emissions.
Targeting short-term swings like El Niño, rather than only the planet’s long-term warming trend, could offer comparable protection from extreme weather through a shorter, less risky intervention, the researchers wrote.
The Black Summer smoke that inspired this study killed dozens of people and choked a continent for months. That the same disaster might point toward a tool for defusing future El Niños is, even to the researchers, an uneasy foundation for a climate strategy.
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