Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?

Little is known about how the methane source inventory and sinks have evolved over recent centuries. New and detailed records of methane mixing ratio and isotopic composition ((CH4)-C-12, (CH4)-C-13 and (CH4)-C-14) from analyses of air trapped in polar ice and firn can enhance this knowledge. We use existing bottom-up constructions of the source history, including EDGAR- based constructions, as inputs to a model of the evolving global budget for methane and for its carbon isotope composition through the 20th century. By matching such budgets to atmospheric data, we examine the constraints imposed by isotope information on those budget evolutions. Reconciling both (CH4)-C-12 and (CH4)-C-13 budgets with EDGAR-based source histories requires a combination of: a greater proportion of emissions from biomass burning and/or of fossil methane than EDGAR constructions suggest; a greater contribution from natural such emissions than is commonly supposed; and/or a significant role for active chlorine or other highly-fractionating tropospheric sink as has been independently proposed. Examining a companion budget evolution for (CH4)-C-14 exposes uncertainties in inferring the fossil-methane source from atmospheric (CH4)-C-14 data. Specifically, methane evolution during the nuclear era is sensitive to the cycling dynamics of bomb C-14 ( originating from atmospheric weapons tests) through the biosphere. In addition, since ca. 1970, direct production and release of (CH4)-C-14 from nuclear-power facilities is influential but poorly quantified. Atmospheric (CH4)-C-14 determinations in the nuclear era have the potential to better characterize both biospheric carbon cycling, from photosynthesis to methane synthesis, and the nuclear-power source.