The atmospheric CH₄ increase since the Last Glacial Maximum: (2) Interactions with oxidants

Two studies of the effect of changing CH₄ fluxes on global tropospheric oxidant levels, O₃, OH, H₂O₂, have been performed with a multi-box photochemical model. (1) A sensitivity study is made by scaling back CH₄, CO and NO emissions relative to present-day budgets. When the CH₄ ice core record is compared to calculated CH₄ abundances, corresponding CH₄ fluxes for the pre-industrial Holocene (PIH) and Last Glacial Maximum (LGM) are fairly well-constrained: 175–225 Tg CH₄/yr for PIH and 100–130 Tg CH₄/yr for LGM. Except for OH at fluxes of 100–200 Tg CH₄/yr, preindustrial oxidant concentrations levels are not narrowly defined by the CH₄ record. The small range of CH₄ flux and OH abundance at the LGM is due to strong CH₄–OH feedbacks. (2) Specific scenarios for CH₄/CO/NO are selected to represent sources for the PIH and LGM. The CH₄ budget is taken from an evaluation of wetlands and other natural sources. For CO and NO, apparent O₃ levels and ice-core-derived H₂O₂ for the PIH are used to constrain PIH CO and NO fluxes. These fluxes are further scaled back to simulate the LGM, and perturbed temperature, precipitation and stratosphere-troposphere exchange are prescribed according to the GISS GCM. For the PIH changes for global abundance relative to present day are: 44% less O₃; 20% more OH and 56% less H₂O₂. For the LGM, with 120 Tg CH₄/yr and surface temperature 4–5 K lower than today, global changes are 56% less O₃, 32% more OH and 59% less H₂O₂. Calculated preindustrial oxidant changes are in reasonable agreement with other studies based on one-, two- and three-dimensional models, although differences among model physics preclude a definitive comparison. There is consensus that OH has decreased since the Last Glacial Maximum, in contrast to projections for future OH, on which models are in disagreement. Model validation of oxidant concentrations requires more ice-core data. Preindustrial OH would be inferred best by using a model with ice-core measurements of species that are uncoupled from the CH₄–CO—OH—O₃ cycle. Candidates might be marine gases of moderately long lifetime, e.g., CH3Cl or OCS.