Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions

Benjamin Hmiel (), V. V. Petrenko, M. N. Dyonisius, C. Buizert, A. M. Smith, P. F. Place, C. Harth, R. Beaudette, Q. Hua, B. Yang, I. Vimont, S. E. Michel, J. P. Severinghaus, D. Etheridge, T. Bromley, J. Schmitt, X. Faïn, R. F. Weiss and E. Dlugokencky
Additional contact information
Benjamin Hmiel: University of Rochester (UR)
V. V. Petrenko: University of Rochester (UR)
M. N. Dyonisius: University of Rochester (UR)
C. Buizert: Oregon State University (OSU)
A. M. Smith: Australian Nuclear Science and Technology Organisation (ANSTO)
P. F. Place: University of Rochester (UR)
C. Harth: University of California San Diego
R. Beaudette: University of California San Diego
Q. Hua: Australian Nuclear Science and Technology Organisation (ANSTO)
B. Yang: Australian Nuclear Science and Technology Organisation (ANSTO)
I. Vimont: University of Colorado and National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Division (GMD)
S. E. Michel: University of Colorado
J. P. Severinghaus: University of California San Diego
D. Etheridge: Climate Science Centre, Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere
T. Bromley: National Institute of Water and Atmospheric Research (NIWA)
J. Schmitt: University of Bern
X. Faïn: University of Grenoble Alpes, CNRS, IRD, Grenoble INP, Institut des Géosciences de l’Environnement (IGE)
R. F. Weiss: University of California San Diego
E. Dlugokencky: NOAA, Earth System Research Laboratory (ESRL)

Nature, 2020, vol. 578, issue 7795, 409-412

Abstract: Abstract Atmospheric methane (CH4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era1. Fossil fuel extraction and use are among the largest anthropogenic sources of CH4 emissions, but the precise magnitude of these contributions is a subject of debate2,3. Carbon-14 in CH4 (14CH4) can be used to distinguish between fossil (14C-free) CH4 emissions and contemporaneous biogenic sources; however, poorly constrained direct 14CH4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century4,5. Moreover, the partitioning of total fossil CH4 emissions (presently 172 to 195 teragrams CH4 per year)2,3 between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH4 per year6,7. Geological emissions were less than 15.4 teragrams CH4 per year at the end of the Pleistocene, about 11,600 years ago8, but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core 14CH4 measurements to show that natural geological CH4 emissions to the atmosphere were about 1.6 teragrams CH4 per year, with a maximum of 5.4 teragrams CH4 per year (95 per cent confidence limit)—an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH4 emissions are underestimated by about 38 to 58 teragrams CH4 per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH4 budget, and will help to inform strategies for targeted emission reductions9,10.

Date: 2020
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DOI: 10.1038/s41586-020-1991-8

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