The role of methane for chemistry-climate interactions

Abstract

Methane (CH4), the second most important greenhouse gas directly emitted by human activity, is removed from the atmosphere via chemical decomposition. The chemical sink of CH4 depends on the temperature and on the abundance of its reaction partners, of which the hydroxyl radical (OH) is the most important. Thus, the atmospheric lifetime of CH4 is not constant and changes of the latter feed back on the atmospheric CH4 abundance, which has implications for its potential as a greenhouse gas. Motivated by this, the present thesis investigates the response of the atmospheric CH4 abundance, as a consequence to changes of its chemical sink, in a warming climate on the basis of chemistry-climate model simulations. The essential innovation of the simulation set-up is that CH4 emission fluxes are used instead of prescribed CH4 mixing ratios at the surface. This means that changes of the chemical sink can feed back on the CH4 mixing ratios, and that also secondary feedbacks can evolve without constraints. Using this model configuration, sensitivity simulations with, either increased atmospheric mixing ratios of carbon dioxide (CO2), or increased surface emissions of CH4 are performed. While the CO2 perturbation affects the chemical composition of the atmosphere only indirectly by induced temperature changes, chemical interactions play an important role for the direct response following the CH4 perturbation. The increased CH4 emissions reduce the abundance of OH, and thereby extend the atmospheric lifetime of CH4. As a result of this process, the CH4 mixing ratios increase by a larger factor than the emissions. In addition, the chemical decomposition of CH4 affects the abundance of ozone (O3) and stratospheric water vapour. The radiative effects of the corresponding composition changes are important contributions to the total radiative forcing of the CH4 perturbation. The composition changes caused by the isolated effect of tropospheric warming induced by, either the CO2, or the CH4 perturbation, are qualitatively the same. Warming and moistening of the troposphere lead to a shortening of the CH4 lifetime, and correspondingly to a reduction of CH4 mixing ratios. The fact, that the CH4 mixing ratios explicitly respond to changes of the chemical sink, enables secondary feedbacks of, e.g. O3 and OH. The climate response of tropospheric O3 is influenced by a variety of processes, the quantitative importance of which is estimated by an attribution method. The climate responses of CH4 and O3 induce negative radiative effects, which means that they are expected to dampen the resulting change of the global surface air temperature. Finally, the results suggest that the CH4 perturbation induces the same response of the global surface air temperature per specified effective radiative forcing as the CO2 perturbation, which confirms the usefulness of the effective radiative forcing framework for CH4 perturbations.