Methane and the CH 4-related greenhouse effect over the past 400 million years

Abstract

Natural variations in the tropospheric CH 4 concentration, excluding short bursts from geospheric reservoirs, have been estimated for the past 400 Ma by scaling a wetland CH 4 emission estimate for the middle Pliocene (ca. 3.6-2.6 Ma) by the relative rate of coal basin deposition at any given time in the past. Wetland CH 4 fluxes were used as inputs into the Cambridge 2-D chemistry-transport model to determine the equilibrium atmospheric response. The approach suggests tropospheric CH 4 concentrations reached exceptionally high values of ∼12,000 ppb during the Permo-Carboniferous, when tropical swamplands were widespread, fell to minimum levels (∼100 ppb) during the Triassic 'coal gap', averaged around 2000 to 4000 ppb during the Mesozoic and < 1000 ppb in the Cenozoic. Peak Permo-Carboniferous CH4 levels could have contributed additional radiative forcing of ∼3 to 4 W m -2, after accounting for the indirect effects of increased stratospheric H 2O and tropospheric ozone. Assuming co-variance of N 2O with CO 2 and CH 4, we predict a combined additional forcing by these two trace greenhouse gases of up to 4 W m -2 during the warm Mesozoic. Although variations in Earth's Phanerozoic CH 4 history probably played a secondary role to atmospheric CO 2 and the evolution of the Sun in driving climate change, the combined effects CH 4 and N 2O appear to be sufficiently large to warrant incorporation into global modeling studies of past warm climates.