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 change s in tropospheric ozone (O-3T) Ozone distributions were generated by the UK MO 3-D chemistry-transport model for 1990, 2030, 2060, and 2100, using four sets of boundary conditions. Anthropogenic emissions evolved following eit her 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 generate d by a coupled ocean-atmosphere GCM, forced with IS92a emissions. Calculate d global mean O-3T radiative forcings for the A2 (B2) cases for 1990-2100 w ere +0.43 (+0.22) W m(-2) when climate change was ignored; these fell to +0 .27 (+0.09) W m(-2) when climate change was included. Without climate chang e, CH4 lifetimes (tau(CH4)) lengthened by 7-12 % between 1990 and 2100; how ever, when climate change was included, tau(CH4), fell by 0-5 %. Hence clim ate warming exerts a negative feedback on itself by enhancing O-3T and CH4 destruction.