Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 17, 13071-13087, 2017
https://doi.org/10.5194/acp-17-13071-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
06 Nov 2017
Marine cloud brightening – as effective without clouds
Lars Ahlm1,2,3, Andy Jones4, Camilla W. Stjern3,5, Helene Muri3, Ben Kravitz6, and Jón Egill Kristjánsson3,† 1Department of Meteorology, Stockholm University, Stockholm, Sweden
2Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
3Department of Geosciences, University of Oslo, Oslo, Norway
4Met Office Hadley Centre, Exeter, UK
5Center for International Climate and Environmental Research – Oslo (CICERO), Oslo, Norway
6Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA
deceased
Abstract. Marine cloud brightening through sea spray injection has been proposed as a climate engineering method for avoiding the most severe consequences of global warming. A limitation of most of the previous modelling studies on marine cloud brightening is that they have either considered individual models or only investigated the effects of a specific increase in the number of cloud droplets. Here we present results from coordinated simulations with three Earth system models (ESMs) participating in the Geoengineering Model Intercomparison Project (GeoMIP) G4sea-salt experiment. Injection rates of accumulation-mode sea spray aerosol particles over ocean between 30° N and 30° S are set in each model to generate a global-mean effective radiative forcing (ERF) of −2.0 W m−2 at the top of the atmosphere. We find that the injection increases the cloud droplet number concentration in lower layers, reduces the cloud-top effective droplet radius, and increases the cloud optical depth over the injection area. We also find, however, that the global-mean clear-sky ERF by the injected particles is as large as the corresponding total ERF in all three ESMs, indicating a large potential of the aerosol direct effect in regions of low cloudiness. The largest enhancement in ERF due to the presence of clouds occur as expected in the subtropical stratocumulus regions off the west coasts of the American and African continents. However, outside these regions, the ERF is in general equally large in cloudy and clear-sky conditions. These findings suggest a more important role of the aerosol direct effect in sea spray climate engineering than previously thought.

Citation: Ahlm, L., Jones, A., Stjern, C. W., Muri, H., Kravitz, B., and Kristjánsson, J. E.: Marine cloud brightening – as effective without clouds, Atmos. Chem. Phys., 17, 13071-13087, https://doi.org/10.5194/acp-17-13071-2017, 2017.
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Short summary
We present results from coordinated simulations with three Earth system models focusing on the response of Earth’s radiation balance to the injection of sea salt particles. We find that in most regions the effective radiative forcing by the injected particles is equally large in cloudy and clear-sky conditions, suggesting a more important role of the aerosol direct effect in sea spray climate engineering than previously thought.
We present results from coordinated simulations with three Earth system models focusing on the...
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