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Published December 1, 2020 | Published
Journal Article Open

Solar geoengineering may not prevent strong warming from direct effects of CO₂ on stratocumulus cloud cover

Abstract

Discussions of countering global warming with solar geoengineering assume that warming owing to rising greenhouse-gas concentrations can be compensated by artificially reducing the amount of sunlight Earth absorbs. However, solar geoengineering may not be fail-safe to prevent global warming because CO₂ can directly affect cloud cover: It reduces cloud cover by modulating the longwave radiative cooling within the atmosphere. This effect is not mitigated by solar geoengineering. Here, we use idealized high-resolution simulations of clouds to show that, even under a sustained solar geoengineering scenario with initially only modest warming, subtropical stratocumulus clouds gradually thin and may eventually break up into scattered cumulus clouds, at concentrations exceeding 1,700 parts per million (ppm). Because stratocumulus clouds cover large swaths of subtropical oceans and cool Earth by reflecting incident sunlight, their loss would trigger strong (about 5 K) global warming. Thus, the results highlight that, at least in this extreme and idealized scenario, solar geoengineering may not suffice to counter greenhouse-gas-driven global warming.

Additional Information

© 2020 National Academy of Sciences. Published under the PNAS license. Edited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved October 7, 2020 (received for review February 27, 2020). PNAS first published November 16, 2020. We thank Clare Singer for assistance with data processing. This research was made possible by the generosity of Eric and Wendy Schmidt by recommendation of the Schmidt Futures program, by Earthrise Alliance, Mountain Philanthropies, the Paul G. Allen Family Foundation, Charles Trimble, and NSF Grant AGS-1835860. The computations were performed on the California Institute of Technology's (Caltech's) High Performance Cluster, which is partially supported by a grant from the Gordon and Betty Moore Foundation. Part of this research was carried out at the Jet Propulsion Laboratory, Caltech, under a contract with NASA. C.M.K. and K.G.P. were at Caltech while carrying out this research. Data Availability: All data needed to evaluate the conclusions in the paper are present in the paper. The source code for the simulations is available at climate-dynamics.org/software/#pycles. Author contributions: T.S. and C.M.K. designed research; T.S., C.M.K., and K.G.P. performed research; T.S., C.M.K., and K.G.P. analyzed data; and T.S. wrote the paper. The authors declare no competing interest. This article is a PNAS Direct Submission.

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August 22, 2023
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October 20, 2023