Dc. Lowe et al., THE 1991-1992 ATMOSPHERIC METHANE ANOMALY - SOUTHERN-HEMISPHERE C-13 DECREASE AND GROWTH-RATE FLUCTUATIONS, Geophysical research letters, 24(8), 1997, pp. 857-860
Measurements of atmospheric methane from 1989-1996 at Baring Head, New
Zealand, and at Scott Base, Antarctica show a seasonal cycle in the m
ixing ratio with a peak to peak amplitude of 28 ppb. This is superpose
d on a trend varying between 16 ppb yr(-1) and near zero. delta(13)C v
alues also show a seasonal cycle, with an amplitude of 0.1-0.3 parts p
er thousand, approximately 6 months out of phase with the mixing ratio
cycle. A pronounced negative anomaly in delta(13)C occurred in 1992 w
ith 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 d
elta(13)C together with the rapid drop and slow recovery in the latter
provide a stringent test of possible causes. Although a combination o
f causes cannot be ruled out, decreased emissions from an isotopically
heavy source such as biomass burning best meet the constraints of the
data.