Subtropical Marine Cloud Brightening Suppresses the El Niño–Southern Oscillation

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2025EF006522

Authors

C. Xing, S. Stevenson, J. Fasullo, C. Harrison, C. Chen, J. Wan, J. Coupe, C. Pfleger

First published: 04 August 2025

https://doi.org/10.1029/2025EF006522

Abstract

Stratospheric aerosol injection (SAI) and marine cloud brightening (MCB) are two proposed methods of compensating for greenhouse gas-induced warming by reflecting incoming solar radiation. However, their effects on the El Niño–Southern Oscillation (ENSO), a critical mode of climate variability, are poorly understood. Here we use ensembles of climate model simulations to show that deploying MCB in the subtropical eastern Pacific dramatically reduces ENSO amplitude by approximately 61%, while SAI has a negligible impact. MCB increases cloud albedo, which cools the subtropical eastern Pacific and triggers a loss of moist static energy. This cooling promotes atmospheric subsidence, dries the tropical Pacific, and intensifies the trade winds. The ultimate effect is a dramatic reduction in all air-sea feedback processes operating during ENSO, which we demonstrate using a mixed-layer heat budget. This contrast between the MCB and SAI impacts on ENSO shows that the choice of climate intervention strategy used to mitigate global warming has drastic regional implications.

Key Points

Marine cloud brightening in the eastern Pacific cuts El Niño-Southern Oscillation (ENSO) amplitude through limiting key air-sea interactions

Stratospheric aerosol injection shows little effect on ENSO variability

The ENSO response to cloud brightening shows the need to assess geoengineering's impacts on variability in addition to the mean state

Plain Language Summary

Global warming has significantly impacted human societies and ecosystems over the past several decades. Solar geoengineering offers a potential way to cool the planet by reflecting sunlight, including two popular approaches: stratospheric aerosol injection (SAI), which cools the atmosphere by mimicking volcanic eruptions, and marine cloud brightening (MCB), which increases the reflectivity of low ocean clouds by adding sea salt aerosols. Because the El Niño-Southern Oscillation (ENSO)—a key climate pattern in the tropical Pacific—strongly affects global temperature and precipitation, it is crucial to understand how these methods might alter ENSO. Our climate model simulations show that subtropical eastern Pacific MCB reduces ENSO variability by about 61% by cooling the subtropical eastern Pacific, strengthening the trade winds, and suppressing essential ocean-atmosphere interactions, while SAI has little to no effect on ENSO variability. These findings underscore the need to thoroughly assess the broader impacts of solar geoengineering to avoid unintended consequences.

Source: AGU