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Methane and the CH₄ related greenhouse effect over the past 400 million years

David Beerling^*,, Robert A. Berner^**, Fred T. Mackenzie^***, Michael B. Harfoot and John A. Pyle^,

^* Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom
^** Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511
^*** Department of Oceanography, School of Ocean Science and Technology, University of Hawaii, Honolulu, Hawaii 96822
Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge CB1 1EW, United Kingdom
^, National Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge CB1 1EW, United Kingdom

Corresponding author: d.j.beerling{at}sheffield.ac.uk

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 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.