THE IMPACT OF THE FUTURE SCENARIOS FOR METHANE AND OTHER CHEMICALLY ACTIVE GASES ON THE GWP OF METHANE

Usually for the calculation of GWP (greenhouse warming potential) a pulse in emission Of CO2 and the gas considered is overlayed on constant background concentration levels of CH4, CO and NO(x). The pulse is expressed as a decaying perturbation in concentration with constant e-folding time. Because the decay of a concentration perturbation in CH4 is dependent on OH concentrations which are in turn dependent on CH4 and other gases, the concept of a constant e-folding time is not suitable to estimate GWPs into the future properly. It is shown by applying our time-dependent, interactive chemical radiative convective model for two hemispheres with oceans, that, especially because of the CH4-OH feedback, the GWP can be very sensitive to the assumptions on future emissions. In this model also the indirect radiative forcing by altitude dependent production of tropospheric ozone (and, in the limitations of 1 D-models, stratospheric water vapor) is treated self-consistently. Several scenarios are studied. For example, using the IPCC-B-scenario for CH4, CO and NO(x) emissions as background with a methane emission pulse in 1989 on top yields for methane a GWP of 22 for an integration time of 20 years, while assuming a constant background and a constant lifetime for the CH4 perturbation yields only 19. A share of about 4 in both cases can be attributed to the production of tropospheric ozone. For an integration time of 50 years the relative difference in GWP between the two scenarios exceeds 30%.