Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 3 2006 10.5194/acpd-6-4995-2006 http://www.atmos-chem-phys-discuss.net/6/4995/2006/ http://www.atmos-chem-phys-discuss.net/6/4995/2006/acpd-6-4995-2006.html http://www.atmos-chem-phys-discuss.net/6/4995/2006/acpd-6-4995-2006.pdf 4995 5038 2006-06-21 Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us? K. R. Lassey D. M. Etheridge D. C. Lowe A. M. Smith D. F. Ferretti National Institute of Water and Atmospheric Research, P.O. Box 14-901, Wellington, New Zealand CSIRO Marine and Atmospheric Research, PMB 1, Aspendale Vic. 3195, Australia Australian Nuclear Science and Technology Organisation, PMB 1, Menai NSW 2234, Australia Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, USA 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.