Atmospheric methane in the southern hemisphere: The recent decline in source strengths inferred from concentration and isotope data
- NIWA Climate, Lower Hutt (New Zealand)
- Univ. of London Egham, Surrey (United Kingdom)
The authors present and review the evidence for the declining growth rate in atmospheric methane (CH{sub 4}) since ca 1992. The NOAA/CMDL records of methane concentration from a network of 28 sites implicate source reductions in the northern hemisphere, though an increase in the methane consumption rate cannot be ruled out. The New Zealand (NZ) NIWA laboratory has maintained since 1989 complementary concentration records at the NZ clean air site at Baring Head (41 S) and at Scott Base, Antarctica (78 S). These records show nearly identical features, and confirm a reduction in growth rate since early 1992, falling to less than 2 ppbv/yr in early 1994. In addition to the methane concentration record, the NIWA laboratory maintains a companion high precision time series of methane`s {sup 13}C/{sup 12}C isotope ratio. This broadens considerably the scope for identifying the origins of the reduction in methane growth rate. The main features of the isotope data are as follows. The isotope ratio displays no discernible secular trend until about 1992 after which it falls markedly (i.e., the methane becomes more depleted in {sup 13}C) over about 2 years. It is too early to be sure how this secular change will develop, as it is masked by a surprisingly large and irregular seasonal cycle. The trend in {sup 13}C depletion suggests that the declining growth rate cannot be due to methane being consumed more quickly, as this process preferentially consumes {sup 12}CH{sub 4}. Rather, it would instead be due to a reduction in a significant methane source which is relatively enriched in {sup 13}C. Such sources are either fossil methane from gas fields and/or coal mines, or methane generated pyrogenically in the large scale burning of biomass. These possibilities are canvassed, and a simple quantitative model is used to explore the possibilities. 53 refs., 4 figs., 2 tabs.
- OSTI ID:
- 182795
- Report Number(s):
- CONF-940426-; ISBN 0-923204-11-3; TRN: IM9608%%160
- Resource Relation:
- Conference: International conference on global climate change: science, policy and mitigation strategies, Phoenix, AZ (United States), 5-8 Apr 1994; Other Information: PBD: 1994; Related Information: Is Part Of Global climate change: Science, policy, and mitigation strategies. Proceedings of the Air and Waste Management Association international specialty conference; Mathai, C.V. [ed.] [Arizona Public Service Co., Phoenix, AZ (United States)]; Stensland, G. [ed.] [Illinois State Water Survey, Champaign, IL (United States)]; PB: 1117 p.
- Country of Publication:
- United States
- Language:
- English
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