The climatic effects of hygroscopic growth of sulfate aerosols in the stratosphere
Open accessAbstract
Abstract Solar geoengineering by deliberate injection of sulfate aerosols in the stratosphere is one of the proposed options to counter anthropogenic climate warming. In this study, we focus on the effect of a specific microphysical property of sulfate aerosols in the stratosphere: hygroscopic growth—the tendency of particles to grow by accumulating water. We show that stratospheric sulfate aerosols, for a given mass of sulfates, cause more cooling when prescribed at the lower levels of the stratosphere because of hygroscopic growth. The larger relative humidity in the lower stratosphere causes an increase in the aerosol size through hygroscopic growth that leads to a larger scattering efficiency. In our study, hygroscopic growth provides an additional cooling of 23% (0.7 K) when 20 Mt‐SO4 of sulfate aerosols, an amount that approximately offsets the warming due to a doubling of CO2, are prescribed at 100 hPa. The hygroscopic effect becomes weaker at higher levels as relative humidity decreases with height. Hygroscopic growth also leads to secondary effects such as an increase in near‐infrared shortwave absorption by the aerosols that causes a decrease in high clouds and an increase in stratospheric water vapor. The altitude dependence of the effects of hygroscopic growth is opposite to that of sedimentation effects or the fast adjustment effects due to aerosol‐induced warming identified in a recent study.
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Altitude also impacts relative humidity conditions and the degree of hygroscopic growth, which could affect aqueous aerosol sizes, compositions and resulting optical properties, though this is highly dependent on the size of aerosol injected.
“Altitude also impacts relative humidity conditions and the degree of hygroscopic growth, which could affect aqueous aerosol sizes, compositions and resulting optical properties23, though this is highly dependent on the size of aerosol injected24.”
From Engineering and logistical concerns add practical limitations to stratospheric aerosol injection strategies (2025) · cites this paper as [23]
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Experimental — accuracy not yet measuredThe extent of cooling depends on the amount of aerosol released and the height of injection; lower stratosphere injection produces more cooling.
“The extent of cooling for example not only depends on how much aerosol is released, but the height of injection in atmosphere (lower stratosphere injection produces more cooling) (Bala, 2009; Krishnamohan et al., 2020).”
From A Fate Worse Than Warming? Stratospheric Aerosol Injection and Global Catastrophic Risk (2021) · cites this paper as Krishnamohan et al., 2020
The Introduction says cooling efficiency depends on aerosol amount and injection altitude, and the Abstract reports more cooling for a fixed mass of sulfate aerosols prescribed in the lower stratosphere. But the Conclusions say this was an idealized prescribed-aerosol study, not an injection study, and that its lower-altitude result is opposite to altitude effects found for other processes. The claim overgeneralizes that conditional result to lower-stratosphere injection. (LLM, conf=0.88)
Experimental — accuracy not yet measured
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- Authors
- Krishnamohan, K. S., Bala, G., Cao, L., Duan, L., & Caldeira, K.
- Affiliations
- Indian Institute of Science Bangalore (IN)
- Zhejiang University (CN)
- Carnegie Institution for Science (US)
- Corresponding author
- Krishnamohan, K.S.
- Venue
- Earth s Future
- Publisher
- American Geophysical Union (AGU)
- Published
- 2020-02
- Volume
- 8
- Issue
- 2
- ISSN
- 2328-4277 (ISSN-L)
- OpenAlex
- W2999702683
- Cited by
- 21
- References
- 48
- Citations by year
- 2026: 5 · 2025: 3 · 2024: 3 · 2023: 3 · 2022: 3 · 2021: 3 · 2020: 1
- FWCI
- 0.93
- Type
- journal article
- Language
- en
- Open access
- gold
- doi.org
- Licence
- Locations
- Earth s Future (publishedVersion, cc-by-nc-nd)
- NOT FOUND REPOSITORY (Indian Institute of Science Bangalore) (acceptedVersion)
- DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
- Funders
- Department of Science and Technology, Ministry of Science and Technology, India
- Topics
- Physical Sciences › Environmental Science › Global and Planetary Change › Climate Change and Geoengineering
- Physical Sciences › Environmental Science › Global and Planetary Change › Atmospheric aerosols and clouds
- Physical Sciences › Earth and Planetary Sciences › Atmospheric Science › Atmospheric chemistry and aerosols
- Keywords
- solar geoengineering, stratospheric sulfate aerosols, hygroscopic growth, aerosol microphysics, relative humidity, altitude dependence, shortwave radiation absorption, stratospheric water vapor, global cooling
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