Effects of strongly enhanced atmospheric methane concentrations in a fully coupled chemistry-climate model

Kurzfassung

Methane (CH4) is the second most important anthropogenic greenhouse gas and its atmospheric abundance is rising rapidly at the moment (Nisbet et al. 2019). In this Master’s thesis the effects of doubled and fivefold present-day CH4 lower boundary mixing ratios are examined on the basis of sensitivity simulations performed with the chemistry-climate model EMAC. The model set-up includes a mixed layer ocean (MLO) model, which allows for tropospheric temperatures to adjust to the forcing, in contrast to a very similar previous study with prescribed oceanic parameters (Winterstein et al. 2019). The objective of this thesis is to identify chemical and physical interactions in the MLO set-up and to compare these to the response in the set-up with prescribed oceanic conditions to determine the implications of tropospheric warming and associated feedbacks. The enhanced CH4 lower boundary mixing ratios lead to increases of stratospheric water vapour (SWV) mixing ratios in the order of 50 % for doubled CH4 and 250 % for fivefold CH4. Ozone (O3 ) mixing ratios in the lowermost stratosphere decrease up to 5 % (10 %) in the experiment with doubled (fivefold) CH4. Furthermore, the O3 mixing ratio increases in the middle stratosphere up to about 2 hPa and decreases above. Changes of SWV and O3 feed back on the abundance of the hydroxyl radical (OH). The OH mixing ratio in the stratosphere increases, which in turn leads to enhanced CH4 oxidation and results in the shortening of the stratospheric CH4 lifetime. The chemical interactions that act in the stratosphere in the MLO set-up compare well with the results of Winterstein et al. (2019). For the first time, an estimate of the near surface temperature respond following the doubling or fivefolding of present-day CH4 mixing ratios using a fully coupled chemistry-climate model could be made. The near surface global and annual mean temperature changes are calculated to be 0.45 ± 0.17 K for the CH4 doubling and 1.31 ± 0.07 K for the CH4 fivefolding. The tropospheric warming further leads to the strengthening of the Brewer-Dobson circulation in the MLO set-up. O3 decrease and CH4 increase in the lowermost tropical stratosphere indicate a more distinct strengthening of tropical upwelling in the MLO set-up compared to the set-up with prescribed oceanic parameters. The strongly enhanced CH4 mixing ratios lead to decreasing OH mixing ratios in the troposphere, which in turn results in a prolongation of the CH4 tropospheric lifetime. The tropospheric CH4 lifetime is found to increase linearly with the CH4 lower boundary condition for the three simulated CH4 levels. The lifetime increase is about 3 % (13 %) points weaker for CH4 doubling (fivefolding) in the MLO set-up than in the set-up analysed by Winterstein et al. (2019) in line with a weaker decrease of OH. The O3 mixing ratio increases throughout the troposphere in the MLO set-up as well as in the set-up with prescribed oceanic parameters. CH4 oxidation influences the NOx cycle in the troposphere so that the formation of O3 is favoured and the depletion is reduced. When removing the chemically induced response from the O3 response pattern in the MLO set-up, the O3 response to slow climate feedbacks remains. This pattern is consistent with the O3 response to slow climate feedbacks induced by increases of CO2.