The 1991-1992 atmospheric methane anomaly: Southern hemisphere 13C decrease and growth rate fluctuations

Measurements of atmospheric methane from 1989–1996 at Baring Head, New Zealand, and at Scott Base, Antarctica show a seasonal cycle in the mixing ratio with a peak to peak amplitude of 28 ppb. This is superposed on a trend varying between 16 ppb yr−1 and near zero. δ13C values also show a seasonal cycle, with an amplitude of 0.1–0.3‰, approximately 6 months out of phase with the mixing ratio cycle. A pronounced negative anomaly in δ13C occurred in 1992 with annual average values dropping from −47.08‰ to −47.28‰. From 1992 to 1996, average δ13C values recovered slowly at an average rate of about 0.04‰ yr−1. The simultaneous changes in the mixing ratio growth rate and δ13C together with the rapid drop and slow recovery in the latter provide a stringent test of possible causes. Although a combination of causes cannot be ruled out, decreased emissions from an isotopically heavy source such as biomass burning best meet the constraints of the data.