Methane and the Ch4–related Greenhouse Effect over the past 400 Million Years

Natural variations in the tropospheric CH4 concentration, excluding short bursts from geospheric reservoirs, have been estimated for the past 400 Ma by scaling a wetland CH4 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 CH4 fluxes were used as inputs into the Cambridge 2-D chemistry-transport model to determine the equilibrium atmospheric response. The approach suggests tropospheric CH4 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 , after accounting for the indirect effects of increased stratospheric H2O and tropospheric ozone. Assuming co-variance of N2O with CO2 and CH4, 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 CH4 history probably played a secondary role to atmospheric CO2 and the evolution of the Sun in driving climate change, the combined effects CH4 and N2O appear to be sufficiently large to warrant incorporation into global modeling studies of past warm climates.