# 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, et al.

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Benjamin Hmiel, V V Petrenko, M N Dyonisius, C Buizert, A M Smith, et al. Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions. Nature, 2020, 578, pp.409 - 412. 10.1038/s41586-020-1991-8. hal-04945279

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# Preindustrial $^{14}$CH$_4$ indicates greater anthropogenic fossil CH$_4$ emissions

https://doi.org/10.1038/s41586-020-1991-8

Received: 27 May 2019

Accepted: 27 November 2019

Published online: 19 February 2020

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Benjamin Hmiel<fnref n="1" />,², V. V. Petrenko<fnref n="1" />, M. N. Dyonisius<fnref n="1" />, C. Buizert², A. M. Smith³, P. F. Place<fnref n="1" />, C. Harth⁴, R. Beaudette⁴, Q. Hua³, B. Yang³, I. Vimont⁵, S. E. Michel⁶, J. P. Severinghaus⁴, D. Etheridge⁷, T. Bromley⁸, J. Schmitt⁹, X. Faïn¹⁰, R. F. Weiss⁴ & E. Dlugokencky¹¹

Atmospheric methane (CH$_4$) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era<fnref n="1" />. Fossil fuel extraction and use are among the largest anthropogenic sources of CH$_4$ emissions, but the precise magnitude of these contributions is a subject of debate²,³. Carbon-14 in CH$_4$ ($^{14}$CH$_4$) can be used to distinguish between fossil ($^{14}$C-free) CH$_4$ emissions and contemporaneous biogenic sources; however, poorly constrained direct $^{14}$CH$_4$ emissions from nuclear reactors have complicated this approach since the middle of the 20th century⁴,⁵. Moreover, the partitioning of total fossil CH$_4$ emissions (presently 172 to 195 teragrams CH$_4$ 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 CH$_4$ per year⁶,⁷. Geological emissions were less than 15.4 teragrams CH$_4$ 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 $^{14}$CH$_4$ measurements to show that natural geological CH$_4$ emissions to the atmosphere were about 1.6 teragrams CH$_4$ per year, with a maximum of 5.4 teragrams CH$_4$ per year (95 per cent confidence limit)—an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH$_4$ emissions are underestimated by about 38 to 58 teragrams CH$_4$ 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 CH$_4$ budget, and will help to inform strategies for targeted emission reductions⁹,¹⁰.

Atmospheric measurements of carbon-13 in methane (δ¹³CH$_4$) have been used to estimate the fossil fraction of the contemporaneous CH$_4$ budget³. This approach relies on having accurate estimates of the δ¹³C signatures of the major CH$_4$ source categories (fossil, microbial and biomass burning) and the strength of the biomass burning source. Large uncertainties in these parameters in the past preclude accurate δ¹³CH$_4$-based estimates of preindustrial-era fossil CH$_4$ emissions⁸,¹¹,¹². Radiocarbon ($^{14}$C) is an ideal tracer for quantifying the fossil component of the atmospheric CH$_4$ budget because all $^{14}$C in fossil CH$_4$ has decayed. By contrast, biogenic CH$_4$ sources (wetlands, biomass burning) have a $^{14}$C activity similar to that of contemporaneous atmospheric CO$_2$ (ref. ⁴,⁸). Interpretation of atmospheric $^{14}$CH$_4$ measurements from 1987–2000 suggests that the fossil fraction of the contemporary CH$_4$ budget is

30 ± 2.3% (ref. ¹³; ¹o). However, the interpretation of atmospheric $^{14}$CH$_4$ in recent decades has been complicated by (1) rapidly changing atmospheric $^{14}$CO$_2$ (from above-ground nuclear testing and fossil fuel emissions) that propagates into biospheric CH$_4$ emissions¹³, and (2) direct $^{14}$CH$_4$ emissions from nuclear power plants⁴,⁵. By contrast, palaeoatmospheric $^{14}$CH$_4$ measurements from ice cores offer a direct constraint on natural geological CH$_4$ emissions without these complications. Whereas geological CH$_4$ emissions have the potential to change on tectonic- and glacial-cycle timescales¹⁴, they have very probably been constant over the past few centuries. The preindustrial-era emission estimates can therefore be applied to the modern CH$_4$ budget with confidence.

Ice core $^{14}$CH$_4$ analysis is challenging owing to both the very large sample requirement (-1,000 kg of ice) and interference from in situ

<footnote n="1">¹Department of Earth and Environmental Sciences, University of Rochester (UR), Rochester, NY, USA. ²College of Earth, Ocean and Atmospheric Sciences, Oregon State University (OSU), Corvallis, OR, USA. ³Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, New South Wales, Australia. ⁴Scripps Institution of Oceanography (SIO), University of California San Diego, La Jolla, CA, USA. ⁵Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado and National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Division (GMD), Boulder, CO, USA. ⁶Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO, USA. ⁷Climate Science Centre, Commonwealth Scientific and Industrial Research Organisation (CSIRO) Oceans and Atmosphere, Aspendale, Victoria, Australia. ⁸National Institute of Water and Atmospheric Research (NIWA), Wellington, New Zealand. ⁹Climate and Environmental Physics, Physics Institute and Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland. ¹⁰University of Grenoble Alpes, CNRS, IRD, Grenoble INP, Institut des Géosciences de l'Environnement (IGE), Grenoble, France. ¹²NOAA, Earth System Research Laboratory (ESRL), Boulder, CO, USA.
²e-mail: bhmiel@ur.rochester.edu</footnote>
cosmogenic ¹⁴C production within the ice crystal lattice¹⁵. We address the former by using a large-diameter ice drill and a large-volume ice-melting apparatus (Supplementary Information section 1) to obtain sufficient CH₄ (-20 µg C) for ¹⁴C analysis by accelerator mass spectrometry. To address the latter, we follow the established⁸,¹⁶ approach of analysing ¹⁴C of carbon monoxide (CO) in parallel with ¹⁴CH₄. ¹⁴CO is very sensitive to in situ cosmogenic ¹⁴C production¹⁵ and can be used to precisely establish the effective cosmic ray exposure history of each sample. We then correct the ¹⁴CH₄ data using the known in situ cosmogenic ¹⁴CH₄/¹⁴CO production ratio in ice¹⁵ (Supplementary Information sections 5, 6). The in situ cosmogenic ¹⁴CH₄ component in the samples used in this study is much lower (<2% of total ¹⁴CH₄) than in ablation-zone ice used in previous palaeoatmospheric ¹⁴CH₄ studies (~30% of total ¹⁴CH₄)⁸,¹⁶. We present new ¹⁴CH₄ data from large-volume ice core samples and firn air sampling from Summit, Greenland, which we combine with prior firn air ¹⁴CH₄ measurements from Law Dome DSSW20K⁴ and Megadunes¹⁷, Antarctica. Our combined record spans from about 1750 to 2013 and captures the evolution of atmospheric ¹⁴CH₄ since the preindustrial era (Fig. 1). The movement of gases within the firn and closure into bubbles is characterized using a firn air transport model¹⁸, and the time series of atmospheric ¹⁴CH₄ is reconstructed using a matrix inversion technique¹⁹,²⁰ (Supplementary Information section 9).

Our atmospheric ¹⁴CH₄ reconstruction (Fig. 1) is indistinguishable from the ¹⁴CO₂-derived contemporaneous biogenic ¹⁴CH₄ signature (blue curve, Supplementary Information section 10) before 1880, suggesting very low natural geological CH₄ emissions. Atmospheric ¹⁴CH₄ began to decrease around 1880, coincident with substantial increases in the use of coal, oil and natural gas (Fig. 2)²¹. The precise timing of the ¹⁴CH₄ minimum (in the 1940s in our reconstruction) is difficult to establish owing to the broad age distributions of individual firn air and ice core samples, as well as the smoothing applied by the matrix inversion technique to address the non-uniqueness of the solution¹⁹. Beyond this fossil ¹⁴C minimum, our samples are affected by the propagation of ¹⁴C from atmospheric nuclear testing into the carbon cycle²² and by emissions from nuclear power plants (starting in the 1970s), which drove a sustained ¹⁴CH₄ increase despite decreasing ¹⁴CO₂⁴,⁵. We calculate the fossil CH₄ fraction and develop a time series of fossil CH₄ emissions (Fig. 2) using a one-box atmospheric model (Supplementary Information section 10). The broad age distributions of our air samples (Supplementary Fig. 3) result in a smoothed representation of the atmospheric ¹⁴CH₄ history that cannot capture the abrupt increase of bomb ¹⁴CO₂ (and subsequently ¹⁴CH₄) starting in 1955. Therefore, we interpret the fossil CH₄ fraction only before the 1940s. We find an increase in the total (geological plus anthropogenic) fossil emissions from negligible CH₄ emissions in the mid-19th century to 64.8 teragrams CH₄ per year (Tg CH₄ yr⁻¹) in 1940.

Assuming that the oldest ice core ¹⁴CH₄ sample in our reconstruction (mean age 1756 AD; Fig. 1) is devoid of anthropogenic fossil CH₄ contributions, we use the contemporaneous biogenic ¹⁴CH₄ source signature to calculate the natural geological CH₄ emissions during the preindustrial era: 1.6 Tg CH₄ yr⁻¹ with a 95% confidence interval (CI) maximum of 5.4 Tg CH₄ yr⁻¹ (Supplementary Information section 10. Supplementary Fig. 5). Our 95% confidence limit of 5.4 Tg CH₄ yr⁻¹ agrees well with, and provides a tighter constraint than, the only other published ¹⁴CH₄-based estimate of natural geological CH₄ emissions from ice cores, which sampled air from the most recent deglaciation (0 to 15.4 Tg CH₄ yr⁻¹, 95% CI range)⁸.

Our result is much lower than estimates from recent source inventory ('bottom-up') studies typically used in global CH₄ budgets², which suggest natural geological emissions of ~40–60 Tg CH₄ yr⁻¹ (ref. ⁶). A recent study⁷ aimed at developing gridded maps of geological CH₄ emissions revised this estimate downwards to 37 Tg CH₄ yr⁻¹ on the basis of data and modelling specifically targeted for gridding; however, the CH₄ emissions increased to 43–50 Tg CH₄ yr⁻¹ when extrapolated to account for temporal variability in mud volcano eruptions and onshore

<figure><img src="image_3.png" /><figcaption>**Fig. 1 | Reconstruction of atmospheric ¹⁴CH₄ from firn air and ice core data.**
a, Global CH₄ mole fraction, [CH₄], reconstructed from ice core, firn air and atmospheric measurements¹. ppb, parts per billion. b, Reconstructed history of atmospheric Δ¹⁴CH₄ from firn air and ice core samples (this study). Dotted lines represent the 95% confidence range based on all calculated ¹⁴CH₄ histories using three different inversion methods (Supplementary Information section 9). Ice core and firn air Δ¹⁴CH₄ measurements are shown at the mean age of the modelled air age distribution. Vertical error bars on the Δ¹⁴CH₄ data from each site represent the 2σ uncertainty for each sample after corrections (Supplementary Information Tables 2, 6), and horizontal error bars represent ±2σ, where Δ is the spectral width of the sample-air age distribution²⁰. We also plot the ¹⁴CH₄ signature of the contemporaneous biogenic source (blue; Supplementary Information section 10). Our time series begins in 1850 because the age distributions of the collected ice core samples have poor coverage of air from -1780 to 1850 (Supplementary Information section 10, Supplementary Fig. 3B).</figcaption></figure>

or submarine geological seeps that lack location-specific measurements. Natural fossil CH₄ emissions of about 40 Tg CH₄ yr⁻¹ (out of total preindustrial-era CH₄ emissions of 215 Tg CH₄ yr⁻¹; Supplementary Fig. 5) would result in a preindustrial-era Δ¹⁴CH₄ of around -185‰, which is in clear disagreement with our data (1.5‰ ± 21.2‰, 2σ; Fig. 1). Bringing our ¹⁴C results into agreement with the bottom-up estimates of natural fossil CH₄ emissions would require an order-of-magnitude larger correction for in situ cosmogenic ¹⁴CH₄. This would in turn require either an order-of-magnitude higher ¹⁴CO content in the sampled ice or an order-of-magnitude higher in situ ¹⁴CH₄/¹⁴CO production ratio; both of these possibilities are well outside the respective uncertainties. The added uncertainties arising from the in situ and procedural corrections to the measured ¹⁴CH₄ are also too small to explain the disagreement
<figure><img src="image_4.png" /><figcaption>**Fig. 2 |** Growth in fossil CH₄ emissions and fossil fuel consumption.

a, Historical fossil fuel energy consumption²¹. b, Calculated total fossil CH₄ emission history (solid line) from the one-box model (Supplementary Information section 10). The dashed lines show the 95% confidence interval.</figcaption></figure>

(Supplementary Information section 10, Supplementary Information Table 8).

Diffuse microseepage (24 Tg CH₄ yr⁻¹), macro-seeps and mud volcanoes (8.1 Tg CH₄ yr⁻¹), submarine seepage (>7 Tg CH₄ yr⁻¹) and geothermal manifestations (5.7 Tg CH₄ yr⁻¹) represent the main categories of natural geological CH₄ emissions in the latest comprehensive bottom-up analysis⁷. Each of these four categories is nearly equivalent to, or exceeds, our upper bound (at 95% confidence) on the total preindustrial-era geological CH₄ emissions (5.4 Tg CH₄ yr⁻¹). Emission estimates for diffuse microseepage are based on limited flux-chamber measurements in regions of known gas seepage (for example, ref. ²³), which are scaled up to a global flux estimate based on the total dryland area situated above hydrocarbon reservoirs (~10% of Earth's total land surface area), the percentage of measurements that show a positive flux, and emission rates chosen on the basis of several geological factors⁷. It is possible that the uncertainties associated with such global upscaling are much larger than reported, resulting in an overestimation by an order of magnitude or more. Similarly, emission estimates from macro-seeps, mud volcanoes and geothermal manifestations are derived from limited observations, which are scaled up to a global total⁷. To provide a sense of scale for the extrapolation in the case of mud volcanoes, ~0.0026 Tg CH₄ yr⁻¹ of measured CH₄ emissions (table S2 in the supplement of ref. ⁷) are scaled up to 6.1 Tg CH₄ yr⁻¹ (table 2 in ref. ⁷).

With regard to submarine seepage, recent studies suggest that CH₄ emissions to the atmosphere are probably very low owing to rapid dissolution of rising bubbles²⁴ and rapid oxidation of dissolved CH₄ (ref. ²⁵). Furthermore, ¹⁴CH₄ measurements in surface waters indicate minimal quantities of fossil CH₄ even in shallow waters over areas of active seeps or methane hydrate degradation²⁶. Our atmospheric ¹⁴CH₄ measurements for the preindustrial era indicate that either (1) the uncertainties associated with global upscaling of geological emissions from discrete measurements result in overestimation by an order of magnitude, or (2) geological CH₄ emissions quantified by these measurements were not present in the preindustrial era and may have been triggered by fossil fuel extraction from hydrocarbon reservoirs or other anthropogenic activity such as groundwater aquifer depletion. If the latter is true, such emissions cannot be considered natural.

A recent study³ used ice core δ¹³CH₄ measurements to arrive at a natural geological CH₄ emission estimate that is on par with what is indicated by bottom-up methods (~50 Tg CH₄ yr⁻¹). However, ref. ⁸ showed that ice core δ¹³CH₄ data do not provide a strong constraint on preindustrial-era geological emissions and are also compatible with a minimal geological source. Measurements of ethane²⁷ in ice cores have also been used to suggest considerable emissions of fossil CH₄ during the preindustrial era. However, this is also an ambiguous constraint because ice core measurements of ethane mole fraction cannot discriminate between contributions from biomass burning (a major source) and natural geological emissions¹¹. Our preindustrial-era ¹⁴CH₄ measurements, by contrast, place an unambiguous constraint on natural fossil CH₄ emissions by directly recording the ¹⁴C-free fraction of atmospheric CH₄.

Our ¹⁴CH₄ reconstruction does not allow accurate quantification of the post-1950 fossil CH₄ budget, owing to relatively poor constraints on the interfering nuclear ¹⁴CH₄ sources. Previous work used atmospheric δ¹³CH₄ measurements to quantify the global fossil CH₄ source in recent decades³, but relied on inventory-based assessments to constrain the natural geological component. We combine our ¹⁴CH₄-based constraint on natural geological emissions (1.6 Tg CH₄ yr⁻¹) with δ¹³CH₄-based constraints on the total fossil source (following the same one-box model approach as ref. ³; Supplementary Information section 11) to estimate recent anthropogenic fossil CH₄ emissions. This approach yields 177 ± 37 Tg CH₄ yr⁻¹ (1σ) for anthropogenic fossil CH₄ emissions during 2003–2012. Our estimate is 22% higher than the previous estimate of 145 ± 23 Tg CH₄ yr⁻¹ (1σ) over the same interval³, and 33–55% higher than the range of bottom-up estimates (114–133 Tg CH₄ yr⁻¹; ref. ²). We note that our δ¹³CH₄-based calculation uses an updated value for the CH₄ sink isotopic fractionation (Supplementary Information section 11); if we use the same value as ref. ³, the anthropogenic fossil source estimate is 194 ± 34 Tg CH₄ yr⁻¹ for the same time period.

Our results indicate that bottom-up inventories strongly underestimate CH₄ emissions from fossil fuel extraction, distribution and use. A study using both ground-based facility-scale measurements and verification from aircraft sampling found that US oil and natural-gas CH₄ emissions (largely from the production and gathering industry segments) are ~60% higher than those reported by the US Environmental Protection Agency²⁸, one of the primary data sources used in bottom-up inventories². If we consider a scenario in which the global bottom-up emissions of fossil CH₄ from the oil and natural-gas industries (79 Tg CH₄ yr⁻¹; ref. ²) are similarly underreported by 60%, this would amount to unreported emissions of ~47 Tg CH₄ yr⁻¹, which is in agreement with the fossil CH₄ emission shortfall that we identify in the current generation of bottom-up inventories (44–63 Tg CH₄ yr⁻¹). Our results imply that anthropogenic fossil CH₄ emissions now account for about 30% of the global CH₄ source and for nearly half of anthropogenic emissions, highlighting the critical role of emission reductions in mitigating climate change⁹,¹⁰.
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**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 1–8 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.

**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. Valdelonga 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.

**Author 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.

**Competing interests** The authors declare no competing interests.

**Additional information**

**Supplementary information** is available for this paper at https://doi.org/10.1038/s41586-020-1991-8.

**Correspondence and requests for materials** should be addressed to B.H.

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