The 1991-1992 atmospheric methane anomaly: Southern Hemisphere C-13 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. delta(13)C values also show a seasonal cycle, with an amplitude of 0.1-0.3 parts per thousand, approximately 6 months out of phase with the mixing ratio cycle. A pronounced negative anomaly in delta(13)C occurred in 1992 with annual average values dropping from -47.08 parts per thousand to -47.28 parts per thousand. From 1992 to 1996, average delta(13)C values recovered slowly at an average rate of about 0.04 parts per thousand yr(-1). The simultaneous changes in the mixing ratio growth rate and delta(13)C 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.