Marine cloud brightening in eastern Pacific could slash El Niño, study finds

Study finds regional MCB could collapse most of ENSO

A new modeling study from the University of California, Santa Barbara suggests that one prominent proposed method of solar geoengineering — marine cloud brightening (MCB) — could severely weaken the El Niño Southern Oscillation (ENSO) if deployed in the subtropical eastern Pacific, while a competing method, stratospheric aerosol injection (SAI), appears to leave the cycle largely intact. The research, published in the journal Earth’s Future and summarized by ScienceDaily, is being framed by its authors as a warning against rushing regional climate interventions into deployment before their side effects are understood.

“We need to be careful about implementing geoengineering proposals before we fully understand what’s going to happen,” first author Chen Xing, a doctoral student at UCSB’s Bren School of Environmental Science & Management, told ScienceDaily. The study was co-authored with fellow graduate student Cali Pfleger and their advisor, Associate Professor Samantha Stevenson.

Why ENSO matters to weather and food systems

The researchers began by studying how geoengineering might affect marine ecosystems but quickly shifted focus to ENSO, a tropical Pacific climate cycle that repeats every two to seven years and redistributes warm ocean water across the basin. During El Niño phases, warmer waters push toward the west coasts of the Americas, often producing wetter winters in California. During La Niña, warmer water remains farther west and monsoon rains tend to strengthen across parts of South and Southeast Asia.

ENSO’s swings influence rainfall, agriculture and storm patterns well beyond the Pacific Rim. Any technology that materially weakens the cycle would therefore have global consequences for water management, crop yields and disaster planning, the authors argue.

How the two cooling strategies differ

Both MCB and SAI are designed to cool the planet by reflecting more sunlight back into space, and both work by spraying particles into the atmosphere — but the similarities largely end there. MCB involves spraying sea salt particles less than two kilometers above the ocean surface, where they seed marine clouds with smaller and more numerous droplets, making the clouds brighter and more reflective. Because the effect is local to where the particles are released, MCB has often been proposed for deployment over the eastern sides of ocean basins, where its cooling potential would be strongest.

SAI, by contrast, injects sulfate particles into the stratosphere, where they spread around the globe and produce a more uniform dimming effect. According to ScienceDaily’s summary of the study, that difference in altitude and distribution turned out to be decisive for ENSO.

A 61% drop in ENSO amplitude

According to the paper, simulations showed that deploying MCB specifically over the subtropical eastern Pacific “dramatically reduces ENSO amplitude by approximately 61%.” “It’s hard to get ENSO to change by that much that quickly,” Stevenson said.

The mechanism the authors describe runs through local weather. Brighter clouds cool the ocean surface beneath them while reducing rainfall, because the smaller droplets are less likely to coalesce into raindrops. Cooler, drier air then spreads into the central Pacific, evaporation drops, atmospheric circulation weakens, and equatorial winds strengthen. Those stronger winds increase the upwelling of colder deep water, which further cools the surface and dampens the warm-cold swings that define ENSO.

“We thought the proposals could have impacts, but we didn’t expect two-thirds of ENSO’s variance to disappear,” Xing said. His blunt takeaway: “Don’t do MCB over the eastern Pacific Ocean because it might cause super strong chain reactions from ENSO’s disappearance.”

Why stratospheric aerosols left ENSO largely untouched

In the same model experiments, stratospheric aerosol injection produced almost no measurable change to ENSO. The researchers attribute the difference to the geography of cooling. MCB concentrates its particle plume near the surface in a single region, sharply altering the local balance of heat, moisture and wind. SAI’s sulfate aerosols spread widely before settling, creating a more even cooling that does not disrupt the tropical Pacific’s atmospheric circulation in the same way.

The result, the authors stress, is not an endorsement of SAI. Stevenson emphasized that the paper should not be read as a blanket rejection of marine cloud brightening: “We’re not saying that all MCB is going to kill ENSO. We’re just saying that this happens if you do it in this specific region.”

Broader context: record heat underscores the pressure to act

The study lands against a backdrop of intensifying extreme weather that is making the political case for climate intervention louder. According to a series of reports compiled by The Guardian’s extreme weather vertical, France recorded about 2,025 excess deaths during its hottest week in a record June heatwave, with public health authorities warning the toll could rise. Provisional readings in the UK reached 37.7°C, while Germany hit an all-time high of 41.7°C. CNN reported that the Great American State Fair in Washington, DC, was forced to postpone activities as temperatures reached triple digits (100°F/38°C or higher), part of a multiday heat wave straining the eastern US power grid ahead of the Fourth of July holiday.

Separately, The Guardian reported at least 59 deaths in flooding in Côte d’Ivoire and flash floods in Kentucky that killed four people, alongside continued research, cited by the Guardian, suggesting Australian-style dam-management modeling could have reduced peak flood depths during the 2022 Lismore disaster in New South Wales. None of these events is directly linked to the UCSB modeling study, but researchers have long argued that the worsening climate impacts raise the political pressure to consider geoengineering despite its risks.

Limits of the finding and what to watch

Several caveats accompany the result. The 61% figure comes from a single climate model’s response to a specific MCB deployment scenario over the subtropical eastern Pacific, and the paper has not yet been subject to the broader model intercomparisons that the field typically requires before policy conclusions are drawn. Independent replication and comparison with other major models — including those used in the Geoengineering Model Intercomparison Project — will be needed to test whether the ENSO collapse is a robust feature of the climate system or an artifact of one model’s sensitivity.

Readers should also note that the paper does not model physical side effects, governance challenges or termination risks associated with either technique, and the study has not been independently reported by outlets beyond ScienceDaily at the time of writing. For now, the authors’ position is narrow but pointed: not a moratorium on solar geoengineering, but a clear warning that regional MCB proposals targeting the eastern Pacific could trigger unintended consequences on a climate cycle that billions of people depend on.