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. �2016. The Authors. This article has been contributed to by U.S. Government employees and their work is in the public domain in the U.S.A.
Department of Earth Sciences, Royal Holloway, University of London, Egham, United Kingdom; US National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Boulder, CO, United States; Climate Change Research Institute, School of Geography Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand; Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom; Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, CO, United States; Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, United States; Laboratoire des Sciences du Climat et de l'Environnement, Gif-sur-Yvette, France; Department of Chemistry, University of Cambridge, Cambridge, United Kingdom; National Centre for Atmospheric Science, Cambridge, United Kingdom; School of Geography, University of Leeds, Leeds, United Kingdom; Environment Canada, Downsview, ON, Canada; South African Weather Service, Stellenbosch, South Africa; School of Physical and Chemical Sciences, North-West University, Potchefstroom, South Africa; Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom; School of Geographical Sciences, University of Bristol, Bristol, United Kingdom
Recommended Citation:
Nisbet E,G,, Dlugokencky E,et al. Rising atmospheric methane: 2007–2014 growth and isotopic shift[J]. Global Biogeochemical Cycles,2016-01-01,30(9)