Articles | Volume 18, issue 2
Atmos. Chem. Phys., 18, 621–634, 2018

Special issue: The Geoengineering Model Intercomparison Project (GeoMIP):...

Atmos. Chem. Phys., 18, 621–634, 2018

Research article 19 Jan 2018

Research article | 19 Jan 2018

Response to marine cloud brightening in a multi-model ensemble

Camilla W. Stjern1,2, Helene Muri2, Lars Ahlm2,3,4, Olivier Boucher5, Jason N. S. Cole6, Duoying Ji7, Andy Jones8, Jim Haywood8, Ben Kravitz9, Andrew Lenton10, John C. Moore7,11,12, Ulrike Niemeier13, Steven J. Phipps14, Hauke Schmidt13, Shingo Watanabe15, and Jón Egill Kristjánsson2,† Camilla W. Stjern et al.
  • 1CICERO Center for International Climate and Environmental Research Oslo, Oslo, Norway
  • 2Department of Geosciences, University of Oslo, Oslo, Norway
  • 3Department of Meteorology, Stockholm University, Stockholm, Sweden
  • 4Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
  • 5Laboratoire de météorologie dynamique, Université Pierre et Marie Curie, Paris, France
  • 6Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, Victoria, Canada
  • 7College of Global Change and Earth System Science, Beijing Normal University, Beijing, China
  • 8Met Office Hadley Centre, Exeter, UK
  • 9Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, USA
  • 10CSIRO Oceans and Atmosphere, Hobart, Australia
  • 11Joint Center for Global Change Studies, Beijing, 100875, China
  • 12Arctic Centre, University of Lapland, P.O. Box 122, 96101 Rovaniemi, Finland
  • 13Max Planck Institute for Meteorology, Hamburg, Germany
  • 14Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia
  • 15Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
  • deceased

Abstract. Here we show results from Earth system model simulations from the marine cloud brightening experiment G4cdnc of the Geoengineering Model Intercomparison Project (GeoMIP). The nine contributing models prescribe a 50 % increase in the cloud droplet number concentration (CDNC) of low clouds over the global oceans in an experiment dubbed G4cdnc, with the purpose of counteracting the radiative forcing due to anthropogenic greenhouse gases under the RCP4.5 scenario. The model ensemble median effective radiative forcing (ERF) amounts to −1.9 W m−2, with a substantial inter-model spread of −0.6 to −2.5 W m−2. The large spread is partly related to the considerable differences in clouds and their representation between the models, with an underestimation of low clouds in several of the models. All models predict a statistically significant temperature decrease with a median of (for years 2020–2069) −0.96 [−0.17 to −1.21] K relative to the RCP4.5 scenario, with particularly strong cooling over low-latitude continents. Globally averaged there is a weak but significant precipitation decrease of −2.35 [−0.57 to −2.96] % due to a colder climate, but at low latitudes there is a 1.19 % increase over land. This increase is part of a circulation change where a strong negative top-of-atmosphere (TOA) shortwave forcing over subtropical oceans, caused by increased albedo associated with the increasing CDNC, is compensated for by rising motion and positive TOA longwave signals over adjacent land regions.

Short summary
Marine cloud brightening (MCB) has been proposed to help limit global warming. We present here the first multi-model assessment of idealized MCB simulations from the Geoengineering Model Intercomparison Project. While all models predict a global cooling as intended, there is considerable spread between the models both in terms of radiative forcing and the climate response, largely linked to the substantial differences in the models' representation of clouds.
Final-revised paper