Marine Stratocumulus Clouds With More Coarse Sea Spray Aerosols Are Brighter

The idea of cooling the Earth by marine cloud brightening is well established. All prior studies considered enhancing cloud albedo only with fine aerosols (FA). Adding coarse sea spray aerosols (CSA, radius>1 μm) has been thought to have the opposite effect. Using nearly a decade of satellite obs...

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Published in:Geophysical research letters Vol. 52; no. 7
Main Authors: Liu, Fan, Mao, Feiyue, Rosenfeld, Daniel, Pan, Zengxin, Zang, Lin, Zhu, Yannian, Gong, Wei
Format: Journal Article
Language:English
Published: Washington John Wiley & Sons, Inc 16.04.2025
Wiley
Subjects:
Aerosols
aerosol‐cloud interaction
Albedo
Brightening
Cloud albedo
Clouds
Cooling
Cooling effects
Earth
Feasibility
Geoengineering
Global aerosols
Marine aerosols
marine cloud brightening
Oceans
radiation
Radiative cooling
Satellite observation
Satellites
Sea spray
Sea spray aerosols
Spray
Stratocumulus clouds
Winds
ISSN:0094-8276, 1944-8007
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Summary:The idea of cooling the Earth by marine cloud brightening is well established. All prior studies considered enhancing cloud albedo only with fine aerosols (FA). Adding coarse sea spray aerosols (CSA, radius>1 μm) has been thought to have the opposite effect. Using nearly a decade of satellite observations and global aerosol reanalysis, we found that the maximum radiative cooling effect from marine stratocumulus occurs when FA is around 3 μg m−3 and CSA is around 30 μg m−3. Under low winds and high stability conditions, optimal FA and CSA can enhance cooling by −95 W m−2, nearly 60% more than adding FA alone. This CRE response to FA and CSA was consistently observed across various cloud‐controlling factors, thus minimizing the probability of being caused by meteorological co‐variability. These findings improve our understanding of how different aerosols affect Earth's climate, improve the evaluation of cooling achieved through marine cloud brightening, and support its feasibility. Plain Language Summary Geoengineering aims to cool the Earth by artificially injecting aerosols into marine stratocumulus clouds over the oceans by flight or ship, called marine cloud brightening (MCB). Previous studies mainly focused on the cooling effects of fine aerosols (FA), while coarse sea salt aerosols (CSA, radius >1 μm) were thought to cause warming. Due to technical limitations, CSA is unintentionally introduced during sea spray injection in MCB project. This raises concerns about the feasibility of MCB using only fine sea spray, as it's difficult to avoid the tail of large spray drops. However, by analyzing nearly 10 years of satellite observations and global aerosol reanalysis, we found that the largest cooling effect of marine stratocumulus clouds is associated with FA of about 3 μg m−3 and CSA of around 30 μg m−3. This optimal combination yields a 60% greater radiative cooling enhancement than optimal FA can achieve alone. The scheme of injecting CSA at MCB project appears as an advantage rather than a limitation and makes MCB closer to practical. Key Points Maximum cooling of marine stratocumulus occurs at about 3 μg m−3 of fine aerosols and 30 μg m−3 of coarse sea salt Optimal fine and coarse sea salt aerosol is associated with up to −95 W m−2 more cloud cooling than in the clean condition Observational evidence eliminates concerns about the previously assumed warming effect of coarse sea salt in marine cloud brightening
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ISSN:0094-8276
1944-8007
DOI:10.1029/2024GL113718