ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-6961-2011 Large methane releases lead to strong aerosol forcing and reduced cloudiness Kurtén T. 1 2 Zhou L. 1 Makkonen R. 1 Merikanto J. 1 Räisänen P. 3 Boy M. 1 Richards N. 4 Rap A. 4 Smolander S. 1 Sogachev A. 5 Guenther A. 6 Mann G. W. 4 Carslaw K. 4 Kulmala M. 1 Department of Physics, University of Helsinki, P.O. Box 64, 00014, Helsingin Yliopisto, Finland Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100, København Ø, Denmark Finnish Meteorological Institute, P.O. Box 503, 00101, Helsinki, Finland Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK Wind Energy Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Building 118, P.O. Box 49, 4000, Roskilde, Denmark Atmospheric Chemistry Division, NESL/NCAR, Boulder CO, USA 18 07 2011 11 14 6961 6969 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/6961/2011/acp-11-6961-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/6961/2011/acp-11-6961-2011.pdf

The release of vast quantities of methane into the atmosphere as a result of clathrate destabilization is a potential mechanism for rapid amplification of global warming. Previous studies have calculated the enhanced warming based mainly on the radiative effect of the methane itself, with smaller contributions from the associated carbon dioxide or ozone increases. Here, we study the effect of strongly elevated methane (CH<sub>4</sub>) levels on oxidant and aerosol particle concentrations using a combination of chemistry-transport and general circulation models. A 10-fold increase in methane concentrations is predicted to significantly decrease hydroxyl radical (OH) concentrations, while moderately increasing ozone (O<sub>3</sub>). These changes lead to a 70 % increase in the atmospheric lifetime of methane, and an 18 % decrease in global mean cloud droplet number concentrations (CDNC). The CDNC change causes a radiative forcing that is comparable in magnitude to the longwave radiative forcing ("enhanced greenhouse effect") of the added methane. Together, the indirect CH<sub>4</sub>-O<sub>3</sub> and CH<sub>4</sub>-OH-aerosol forcings could more than double the warming effect of large methane increases. Our findings may help explain the anomalously large temperature changes associated with historic methane releases.

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