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Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions

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.

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Fig. 1: Reconstruction of atmospheric 14CH4 from firn air and ice core data.
Fig. 2: Growth in fossil CH4 emissions and fossil fuel consumption.

Data availability

The ice core and firn air 14CH4 data presented in Fig. 1 are provided in Supplementary Information Tables 2, 6. Additional measurements not provided in Supplementary Information Tables 18 are available via the NSF Arctic Data Center at https://doi.org/10.18739/A2599Z216.

Code availability

The code for the firn air inverse model and atmospheric box model (MATLAB) is available from the corresponding author upon request.

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Acknowledgements

This work was supported by US NSF awards OPP-1203779 (V.V.P.) OPP-1203686, OPP-0230452, ANT-0839031 (J.P.S.) ARC-1204084, ARC-1702920 (C.B.), a Packard Fellowship for Science and Engineering (V.V.P.), the National Institute of Water and Atmospheric Research through the Greenhouse Gases, Emissions and Carbon Cycle Science Programme (T.B.) and the Australian Government for the Centre for Accelerator Science at ANSTO through the National Collaborative Research Infrastructure Strategy (A.M.S.). We thank J. McConnell and P. Vallelonga for the interpretation of the ice core CFA data; P. Neff and E. Steig for sharing the ice-thinning model code; L. Davidge, J. Edwards, M. Pacicco and A. Adolph for assistance with firn air and ice core sampling; M. Jayred, L. Albershardt, T. Kuhl, D. Kirkpatrick and the US Ice Drilling programme for ice-drilling support; K. Gorham, J. Jenkins, D. Einerson, Polar Field Services and the 109th New York Air National Guard for logistical support; the Australian Antarctic Science Program for supporting the Law Dome drilling and firn air sampling and CSIRO GASLAB, in particular R. Langenfelds, for analysis of the firn air sample trace gas concentrations.

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Contributions

B.H. and V.V.P. designed the study and conducted field logistical and scientific preparations; B.H., V.V.P., M.N.D., C.B., P.F.P., R.B., J.S. and X.F. collected samples at Summit; B.H. measured [CO] and extracted CH4 and CO from firn air and ice core samples; C.B. developed the firn modelling code; B.H. and M.N.D. developed the box-model calculations; Q.H. and B.Y. graphitized the 14C samples; A.M.S. measured 14C; P.F.P. and I.V. measured δ13CO; S.E.M. measured δ13CH4; C.H. measured [CH4] and halogenated trace gases under the supervision of R.F.W.; E.D. supervised the firn air trace gas measurements; J.P.S. measured δXe/Kr, δKr/N2, δXe/N2 and δNe/N2 and collected Megadunes firn air samples; R.B. measured the δ15N of N2, the δ18O of O2, δO2/N2 and δAr/N2; D.E. collected and supervised the analyses of the Law Dome firn air samples; T.B. extracted CH4 from Megadunes and Law Dome samples; B.H. and V.V.P. analysed the data and B.H. drafted the manuscript with contribution from all authors.

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Correspondence to Benjamin Hmiel.

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Supplementary information

Supplementary Information

This file contains details regarding ice core and firn air sample collection and analyses, procedural and in situ cosmogenic corrections for 14CH4, forward and inverse modeling of firn air and ice core data, atmospheric box modeling of 14CH4 and 13CH4. It also contains 5 Supplementary Figures and 8 Supplementary Tables.

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Hmiel, B., Petrenko, V.V., Dyonisius, M.N. et al. Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions. Nature 578, 409–412 (2020). https://doi.org/10.1038/s41586-020-1991-8

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