Role of climate feedback on methane and ozone studied with a coupled ocean-atmosphere-chemistry model.

We present results from two experiments carried out with a coupled ocean-atmosphere-tropospheric chemistry model run continously over the period 1990 to 2100. In the control experiment, climate is unforced, but emissions of trace gases to the chemical model increase in line with an illustrative scenario for future trace gas emissions with medium high growth. In the climate change experiment trace gas emissions are identical to the control, but climate is also forced using greenhouse gas concentrations and SO2 emissions from the same scenario. Global average methane in the climate change experiment increased from 1670 ppbv in 1990 to 3230 ppbv Ly 2100, compared to 3650 ppbv by 2100 in the control. The methane increase in the control experiment is therefore 27 % more than in the control. This difference is due to both temperature and OH changes which increase the rate of methane oxidation and act in the opposite direction to the negative feedback of methane on itself through OH. Mid-latitude northern hemisphere ozone concentrations in July for the mid-troposphere rose from 39 ppbv in 1990s to 64 ppbv in the 2090s in the control experiment and to 49 ppbv in the climate change experiment. The direct role of climate change is therefore predicted to be a negative feedback on the radiative forcing from the change to tropospheric ozone and methane concentrations.