1National Institute of Water and Atmospheric Research, P.O. Box 14-901, Wellington, New Zealand
2CSIRO Marine and Atmospheric Research, PMB 1, Aspendale Vic. 3195, Australia
3Australian Nuclear Science and Technology Organisation, PMB 1, Menai NSW 2234, Australia
4Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA
Abstract. 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 (12CH4, 13CH4 and 14CH4) 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, to assemble 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. Balancing both 12CH4 and 13CH4 budgets requires participation by a highly-fractionating atmospheric sink such as active chlorine (removing at least 10 Tg yr-1), which has been proposed independently. Examining a companion budget evolution for 14CH4 exposes uncertainties in inferring the fossil-methane source from atmospheric 14CH4 data. Specifically, methane evolution during the nuclear era is sensitive to the cycling dynamics of ''bomb 14C'' (originating from atmospheric weapons tests) through the biosphere. In addition, since ca 1970, direct production and release of 14CH4 from nuclear-power facilities is influential but poorly quantified. Atmospheric 14CH4 determinations in the nuclear era have the potential to better characterize biospheric carbon cycling and to better quantify the ill-determined nuclear-power source.