Measurements of 13C in atmospheric methane made at Baring Head, New Zealand (41¿S), over the 4-year period, 1989--1993, display a persistent but highly variable seasonal cycle. Values for Δ13C peak in summer at about -46.9% and drop to around -47.5% in the late winter. Methane concentration shows a similar cycle, with winter peaks and summer minima. Similar features are observed at the New Zealand Antarctic station, Scott Base, at 78≡S. While the phase of the Δ13C cycle is consistent with a kinetic isotope effect that preferentially leaves methane enriched in 13C in the atmosphere after oxidation by OH, the amplitude of the cycle is much larger than expected from published laboratory measurements of the effect. We interpret the origin of this cycle and its interannual variability to be due to episodic southward transport of isotopically heavy methane from large-scale tropical biomass burning, possibly in conjunction with changes in the rate of interhemispheric transport in the troposphere. The Baring head 13C data show no significant secular trend from 1989 to mid-1991, followed by a rapid trend toward methane less enriched in 13C.

This indicates a major shift in the balance of the sources of atmospheric methane and precludes an increased sink strength. The trend in 13C since mid-1991 coincided with significant changes to the methane growth rate observed at Baring Head and at Scott Base: an elevated growth rate of about 15 parts per billion by volume (ppbv) during 1991 gave way to less than 3 ppbv yr-1 thereafter. A 2-box model of atmospheric methane (one box per hemispheric reservoir) sugest that (1) the recent decline in 13C in methane observed at Baring Head and Scott Base cannot have a solely northern hemispheric origin and (2) the most plausible origin is a recent reduction in methane released by biomass burning in the southern hemisphere, combined with a lower release rate of fossil methane in the northern hemisphere. ¿American Geophysical Union 1994