The trend in atmospheric methane delta C-13 implications for isotopic constraints on the global methane budget

A recent paper by Tans [1997] has drawn attention to the isotopic disequili brium that inevitably prevails when atmospheric methane is not in steady st ate with its sources, noting in particular the very slow adjustment of the isotopic signature delta(13)C toward its steady state. Our aim in this pape r is to clarify the nature of disequilibrium effects on delta(13)C(CH4) and to assess their likely magnitudes in the global atmosphere over recent dec ades. We use a simple model simulation incorporating a plausible scenario o f the global methane source history over 1700-2010, which includes an uncha nged source since 1990. The simulation of both mixing ratio and delta(13)C compare favorably with the secular features of a 10-year data set (1988-199 8) from Baring Head, New Zealand, and of a 17-year data set (1978-1995) in air archived from Cape Grim, Australia. This corroborates a recent analysis of those data sets and their compatibility with stabilized sources. We sho w that the slow adjustment of delta(13)C toward steady state arises from th e effect of isotope fractionation on the cancellation of contributing terms to delta(13)C. We explore the implications of disequilibrium for the usual practice of relating delta(13)C values in the atmosphere to those in the a ggregate source through a shift induced by fractionation and quantify the f laws in this practice. Finally, we examine the sensitivity of the atmospher ic secular response, in both mixing ratio and delta(13)C, to sustained chan ges in source and sink and show that delta(13)C is a potentially powerful d iagnostic of such changes.