A SIMPLE-MODEL FOR ESTIMATING METHANE CONCENTRATION AND LIFETIME VARIATIONS

A simple methane model is presented in which lifetime changes are expr essed as a function of CH4 concentration and emissions of NOx, CO and NMHCs. The model parameters define the relative sensitivities of lifet ime to these determining factors. The parameterized model is fitted to results from five more complex atmospheric chemistry models and to 19 90 IPCC concentration projections. The IPCC data and four of the five models are well fitted, implying that the models have similar relative sensitivities. However, overall sensitivities of lifetime to changes in atmospheric composition vary widely from model to model. The parame terized model is used to estimate the history of past methane emission s, lifetime changes and OH variations, with estimates of uncertainties . The pre-industrial lifetime is estimated to be 15-34% lower than tod ay. This implies that: 23-55% of past concentration changes are due to lifetime changes. Pre-industrial emissions are found to be much highe r (220-330 TgCH(4)/y) than the best estimate of present natural emissi ons (155 TgCH(4)/y). The change in emissions since pre-industrial time s is estimated to lie in the range 160-260 TgCH(4)/y, compared with th e current best guess for anthropogenic emissions of 360 TgCH(4)/y. The se results imply either that current estimates of anthropogenic emissi ons are too high and/or that there have been large changes in natural emissions. 1992 IPCC emissions scenarios are used to give projections of future concentration and lifetime changes, together with their unce rtainties. For any given emissions scenario, these uncertainties are l arge. In terms of future radiative forcing and global-mean temperature changes over 1990-2100 they correspond to uncertainties of at least /- 0.2 Wm(-2) and +/- 0.1 degrees C, respectively.