Modeling the response to changes in tropospheric methane concentration; Application to the Permian-Triassic boundary

We discuss model experiments valid for the Permian-Triassic boundary in which we explore the impact of changes in tropospheric methane concentration. For scenarios relevant to methane clathrate release, we consider surface methane concentration with values up to 5000 times its preindustrial concentration. We employ a comprehensive three-dimensional tropospheric-stratospheric model with chemistry that allows for the feedbacks between chemistry and climate. We show that stratospheric ozone starts collapsing for methane surface concentrations on the order of 1000 times their preindustrial concentration. At 5000 times, more than half of the total ozone column has disappeared. As a result a large rise (up to a factor of 7) in surface UV-B radiation is found. Other chemical consequences include a rise in CO and ozone surface concentrations; although becoming very large (up to 17 ppmv for CO), neither seems to reach lethal values according to present-day life forms. Finally, we show that tropospheric OH does not collapse for any of the scenarios; a corollary of this is a finite methane lifetime (45 years at the most). As a result, if methane were to increase significantly enough over a short period, the associated UV-B increase and/or deterioration of surface conditions could provide an explanation for the landmass extinction at the Permian-Triassic boundary.