1Department of Physics, University of Helsinki, P.O. Box 64, 00014 Helsingin Yliopisto, Finland
2Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 København Ø, Denmark
3Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
4Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK
5Wind Energy Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, Building 118, P.O. Box 49, 4000 Roskilde, Denmark
6Atmospheric Chemistry Division, NESL/NCAR, Boulder CO, USA
Abstract. 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 (CH4) 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 (O3). 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 CH4-O3 and CH4-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.