Future estimates of tropospheric ozone radiative forcing and methane turnover - the impact of climate change

We present a range of estimates for future radiative forcings due to changes in tropospheric ozone (O3T ). Ozone distributions were generated by the UKMO 3-D chemistry-transport model for 1990, 2030, 2060, and 2100, using four sets of boundary conditions. Anthropogenic emissions evolved following either the IPCC SRES “high” (A2) or “central” (B2) case. Each scenario was run with both a fixed (1990) climate, and with a changing climate, as generated by a coupled ocean-atmosphere GCM, forced with IS92a emissions. Calculated global mean O3T radiative forcings for the A2 (B2) cases for 1990-2100 were +0.43 (+0.22) W m when climate change was ignored; these fell to +0.27 (+0.09) Wm when climate change was included. Without climate change, CH4 lifetimes (τCH4) lengthened by 7-12 % between 1990 and 2100; however, when climate change was included, τCH4 fell by 0-5 %. Hence climate warming exerts a negative feedback on itself by enhancing O3T and CH4 destruction.