Rising atmospheric methane and its isotopic shifta

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A paper published in Global Biogeochemical Cycles has clarified the importance of different sources contributing to the atmospheric methane burden during 2007-2014. The research used measurements taken by the USA National Oceanic and Atmospheric Administration (NOAA) Cooperative Global Air Sampling Network, and investigated changes in mole fractions and isotopic composition.

The results showed that the globally averaged mole fraction of CH4 increased by 5.7 ± 1.2 ppb yr-1 up to 2013, then by 12.5 ± 0.4 ppb in 2014. Accompanying this change was a shift in δ13CCH4 to more negative values. At the remote monitoring station at Ascension Island, this shift was −0.24 ± 0.02‰, but the same trend was observed globally.  These global changes will potentially be driven by varying processes in different regions, which the authors attempt to untangle.

In 2007 there was an increase in CH4 concentrations at high northern latitudes, which was caused by an increase in late summer emissions from wetlands, stimulated by higher temperatures. However, during 2008-2013 this Arctic increase fell below global means, indicating that no new CH4 emissions developed during this time.  In 2014, strong increases commenced again, but this time in line with global means. For the entire period, isotopic data shows a trend to more depleted δ13CCH4, indicative of wetland sources.

For Ascension Island, methane increases have been sustained for the duration of monitoring. Assuming a linear trend, isotopic change is in a negative direction and of approximately −0.03‰ yr−1. Measurements from Cape Point (South Africa) and the South Pole were similar to the Ascension ones. The authors point out that the southern Amazon is a distant source of air for Ascension Island.  They therefore posit that increased rainfall and warmth may have been responsible for increased CH4 emissions from tropical wetlands, as recent years have seen flood levels above long-term averages. The paper also emphasise the importance of emissions from ruminants in the tropics, but point to a lack of data regarding the δ13CCH4 values of such emissions.

Lead author, Professor Euan Nisbet at Royal Holloway University of London said: “Our results go against conventional thinking that the recent increase in atmospheric methane must be caused by increased emissions from natural gas, oil, and coal production. Our analysis of methane’s isotopic composition clearly points to increased emissions from microbial sources, such as wetlands or agriculture.”

To conclude, the authors stress the magnitude of the global increase in methane concentrations; 60 ppb in nine years. They write: “if methane growth continues, and is indeed driven by biogenic emissions, the present increase is already becoming exceptional, beyond the largest events in the last millennium.”

Reference. Nisbet, E.G., Dlugokeencky, E.J., Manning, M.R., et al. Rising atmospheric methane: 2007-2014 growth and isotopic shift. 2016. Global Biogeochemical Cycles,DOI: 10.1002/2016GB005406.

Image is from the above paper, under a Creative Commons Attribution 4.0 International License.

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