Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us? K. R. Lassey1, D. M. Etheridge2, D. C. Lowe1, A. M. Smith3, and D. F. Ferretti1,4 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, Campus Box 450, 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, 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 12CH4 and 13CH4 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 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
both biospheric carbon cycling, from photosynthesis to methane synthesis, and
the nuclear-power source.
Citation: Lassey, K. R., Etheridge, D. M., Lowe, D. C., Smith, A. M., and Ferretti, D. F.: Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us?, Atmos. Chem. Phys., 7, 2119-2139, doi:10.5194/acp-7-2119-2007, 2007.