Limited impact of El Niño–Southern Oscillation on variability and growth rate of atmospheric methane

• Main Authors: H. Schaefer, D. Smale, S. E. Nichol, T. M. Bromley, G. W. Brailsford, R. J. Martin, R. Moss, S. Englund Michel, J. W. C. White
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-10-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/15/6371/2018/bg-15-6371-2018.pdf
• ISSN: 1726-4170; 1726-4189

<p>The El Niño–Southern Oscillation (ENSO) has been suggested as a strong forcing in the methane cycle and as a driver of recent trends in global atmospheric methane mole fractions [CH₄]. Such a sensitivity of the global CH₄ budget to climate events would have important repercussions for climate change mitigation strategies and the accuracy of projections for future greenhouse forcing. Here, we test the impact of ENSO on atmospheric CH₄ in a correlation analysis. We use local and global records of [CH₄], as well as stable carbon isotopic records of atmospheric CH₄ (<i>δ</i>¹³CH₄), which are particularly sensitive to the combined ENSO effects on CH₄ production from wetlands and biomass burning. We use a variety of nominal, smoothed, and detrended time series including growth rate records. We find that at most 36&thinsp;% of the variability in [CH₄] and <i>δ</i>¹³CH₄ is attributable to ENSO, but only for detrended records in the southern tropics. Trend-bearing records from the southern tropics, as well as all studied hemispheric and global records, show a minor impact of ENSO, i.e. &lt;&thinsp;24&thinsp;% of variability explained. Additional analyses using hydrogen cyanide (HCN) records show a detectable ENSO influence on biomass burning (up to 51&thinsp;%–55&thinsp;%), suggesting that it is wetland CH₄ production that responds less to ENSO than previously suggested. Dynamics of the removal by hydroxyl likely counteract the variation in emissions, but the expected isotope signal is not evident. It is possible that other processes obscure the ENSO signal, which itself indicates a minor influence of the latter on global CH₄ emissions. Trends like the recent rise in atmospheric [CH₄] can therefore not be attributed to ENSO. This leaves anthropogenic methane sources as the likely driver, which must be mitigated to reduce anthropogenic climate change.</p>