As global carbon emissions continue to rise and the window for conventional climate mitigation narrows, interest in geoengineering,deliberately manipulating the Earth’s climate,has grown. Two of the most prominent proposals are marine cloud brightening (MCB) and stratospheric aerosol injection (SAI). Both aim to reflect more sunlight back into space, cooling the planet. But a new study from the University of California, Santa Barbara, published in Earth’s Future, reveals that the two methods would have dramatically different effects on one of Earth’s most important climate systems, and that the consequences of deploying one of them could be far-reaching and fast.
“Two interventions can get to the same warming target globally and have extremely different regional climate impacts,” said co-author Samantha Stevenson, associate professor at UCSB’s Bren School of Environmental Science and Management. “The most important question is: are we thinking of all of the potential consequences?”
The two approaches
Marine cloud brightening involves spraying fine sea-salt particles into low-lying clouds over the ocean, typically less than 2 kilometers above the surface. The salt particles make cloud droplets smaller and more numerous, increasing the clouds’ reflectivity. Stratospheric aerosol injection, by contrast, releases sulfate particles into the stratosphere, where they spread globally and produce a more uniform cooling effect.
The UCSB team modeled what would happen if MCB was deployed over the subtropical eastern Pacific,a region often proposed as an optimal location because the clouds there are especially susceptible to brightening. The answer, according to the study: the El Niño-Southern Oscillation (ENSO) would weaken by approximately 61 percent within a decade.
“We thought the proposals could have impacts, but we didn’t expect two-thirds of ENSO’s variance to disappear,” said lead author Chen Xing, a doctoral student at the Bren School.
Why MCB disrupts ENSO
ENSO is the periodic cycle of warming (El Niño) and cooling (La Niña) in the tropical Pacific that drives weather patterns across the globe, influencing rainfall in the Americas, Africa, Australia, and Asia, with cascading effects on agriculture, fisheries, and public health.
The mechanism behind MCB’s disruption is a cascade of regional feedbacks. Spraying salt particles into clouds over the subtropical eastern Pacific cools the ocean surface below and reduces rainfall (smaller cloud droplets are less likely to coalesce into raindrops). The cooler, drier air spreads into the central Pacific, reducing evaporation, weakening atmospheric circulation, and strengthening equatorial trade winds. Those stronger winds drive increased upwelling of cold water from the deep ocean, further cooling the surface. The cycle suppresses the temperature gradient that normally drives ENSO variability.
“It just does not naturally drop 60 percent in 10 years, even under climate change,” Stevenson said. “It’s hard to get ENSO to change by that much that quickly.”
The timing is critical. Naturally occurring changes in ENSO amplitude, whether from internal variability or greenhouse-gas forcing, unfold over decades to centuries. A 61 percent collapse in under a decade is unprecedented in the observational record and in most climate model simulations.
SAI leaves ENSO largely unchanged
In contrast, the team found that stratospheric aerosol injection had no meaningful effect on ENSO. Because SAI particles are injected high in the stratosphere and spread globally, they produce a uniform cooling effect that does not trigger the regional ocean-atmosphere feedbacks that destabilize ENSO.
This does not mean SAI is without risks,the study only examined ENSO impacts, and SAI has been linked to other concerns, including stratospheric ozone depletion, altered precipitation patterns, and the risk of “termination shock” if deployment is suddenly stopped. But for the specific question of ENSO disruption, SAI appears far safer than MCB.
Caveats and context
The researchers are careful to note that the study does not reject marine cloud brightening as a concept. The disruption is location-specific,deploying MCB in the eastern Pacific is the problem. MCB deployed elsewhere might still provide global cooling, but would require substantially larger effort.
“The study is not saying that all MCB is going to kill ENSO,” Stevenson said. “We’re just saying that this happens if you do it in this specific region.”
The researchers also emphasize that not intervening carries its own risks. Unchecked climate change will alter ENSO in ways that remain uncertain,some models suggest more extreme El Niño events, others suggest a weakening. Either way, the decision is not between geoengineering and a stable climate, but between different sets of risks.
The team plans to next study how different geoengineering strategies affect marine ecosystems, including ocean productivity and fisheries,effects that would compound the ENSO disruption concerns.
Source: Xing C, Stevenson S, Fasullo J, Harrison C, Chen C, Wan J, Coupe J, Pfleger C. Subtropical Marine Cloud Brightening Suppresses the El Niño-Southern Oscillation. Earth’s Future (2025). DOI: 10.1029/2025EF006522