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⁻², 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 Earth9s 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.