http://www.atmos-chem-phys-discuss.net/acp-2017-484/
Marine cloud brightening – as effective without clouds Lars Ahlm1,2,3, Andy Jones4, Camilla W. Stjern3,5, Helene Muri3, Ben Kravitz 6, and Jón Egill Kristjánsson31Department 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 Received: 19 May 2017 – Accepted for review: 28 May 2017 – Discussion started: 29 May 2017 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 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. Discuss., doi:10.5194/acp-2017-484, in review, 2017. -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/geoengineering. For more options, visit https://groups.google.com/d/optout.
