Rising atmospheric methane: 2007-2014 growth and isotopic shift

Nisbet, E. G. and Dlugokencky, E. J. and Manning, M. R. and Lowry, D. and Fisher, R. E. and France, J. L. and Michel, S. E. and Miller, J. B. and White, J. W. C. and Vaughn, B. and Bousquet, P. and Pyle, J. A. and Warwick, N. J. and Cain, M. and Brownlow, R. and Zazzeri, G. and Lanoisellé, M. and Manning, A. C. and Gloor, E. and Worthy, D. E. J. and Brunke, E.-G. and Labuschagne, C. and Wolff, E. W. and Ganesan, A. L. (2016) Rising atmospheric methane: 2007-2014 growth and isotopic shift. Global Biogeochemical Cycles, 30 (9). pp. 1356-1370. ISSN 0886-6236 DOI https://doi.org/10.1002/2016gb005406

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Abstract

From 2007 to 2013, the globally averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr−1. Simultaneously, δ13CCH4 (a measure of the 13C/12C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests that the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics, for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short-term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13C-depleted values and its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate that fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.

Item Type:	Article
Uncontrolled Keywords:	2016AREP; IA71
Subjects:	01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions:	01 - Climate Change and Earth-Ocean Atmosphere Systems 07 - Gold Open Access
Journal or Publication Title:	Global Biogeochemical Cycles
Volume:	30
Page Range:	pp. 1356-1370
Identification Number:	https://doi.org/10.1002/2016gb005406
Depositing User:	Sarah Humbert
Date Deposited:	11 Apr 2017 19:39
Last Modified:	11 Apr 2017 19:41
URI:	http://eprints.esc.cam.ac.uk/id/eprint/3907

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