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The trend and interannual variability of methane sources are derived frommulti-annual simulations of tropospheric photochemistry using a 3-D globalchemistry-transport model. Our semi-inverse analysis uses the fifteen years(1979--1993) re-analysis of ECMWF meteorological data and annually varyingemissions including photo-chemistry, in conjunction with observed CH4concentration distributions and trends derived from the NOAA-CMDL surface stations. Dividing the world in four zonal regions(45--90 N, 0--45 N, 0--45 S, 45--90 S) we find good agreement in each region between (top-down) calculatedemission trends from model simulations and (bottom-up) estimated anthropogenic emission trends based on the EDGAR global anthropogenicemission database, which amounts for the period 1979--1993 2.7 Tg CH4yr-1. Also the top-down determined total global methane emission compares well with the total of the bottom-up estimates. We use thedifference between the bottom-up and top-down determined emission trends tocalculate residual emissions. These residual emissions represent the inter-annual variability of the methane emissions. Simulations have beenperformed in which the year-to-year meteorology, the emissions of ozone precursor gases, and the stratospheric ozone column distribution are eithervaried, or kept constant. In studies of methane trends it is most importantto include the trends and variability of the oxidant fields. The analyses reveals that the variability of the emissions is of the order of8Tg CH4 yr-1, and likely related to wetland emissions and/orbiomass burning

Trends and inter-annual variability of methane emissions derived from 1979-1993 global CTM simulations