Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions

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. Isotopic evidence from ice cores indicates that preindustrial-era geological methane emissions were lower than previously thought, suggesting that present-day emissions of methane from fossil fuels are underestimated.

[1]  J. Barnola,et al.  Reconstructing recent atmospheric trace gas concentrations from polar firn and bubbly ice data by inverse methods , 1997 .

[2]  D. Etheridge,et al.  Reconstructing atmospheric histories from measurements of air composition in firn , 2002 .

[3]  John Yen,et al.  Introduction , 2004, CACM.

[4]  D. Lowe,et al.  The atmospheric cycling of radiomethane and the "fossil fraction" of the methane source , 2006 .

[5]  J. Greinert,et al.  Fate of rising methane bubbles in stratified waters: How much methane reaches the atmosphere? , 2006 .

[6]  D. Etheridge,et al.  Centennial evolution of the atmospheric methane budget: what do the carbon isotopes tell us? , 2006 .

[7]  G. Etiope,et al.  Did geologic emissions of methane play any role in Quaternary climate change , 2008 .

[8]  Quan Hua,et al.  14CH4 Measurements in Greenland Ice: Investigating Last Glacial Termination CH4 Sources , 2009, Science.

[9]  R. Weiss,et al.  CH 4 Measurements in Greenland Ice : Investigating Last Glacial Termination CH 4 Sources , 2009 .

[10]  Giuseppe Etiope,et al.  Microseepage in drylands: Flux and implications in the global atmospheric source/sink budget of methane , 2010 .

[11]  R. Weiss,et al.  Deep air convection in the firn at a zero-accumulation site, central Antarctica , 2010 .

[12]  P. M. Lang,et al.  Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland , 2011 .

[13]  L. Höglund-Isaksson.  Global anthropogenic methane emissions 2005-2030: technical mitigation potentials and costs , 2012 .

[14]  Q. Hua,et al.  Atmospheric Radiocarbon for the Period 1950–2010 , 2013, Radiocarbon.

[15]  Robert W. Howarth,et al.  Methane emissions and climatic warming risk from hydraulic fracturing and shale gas development: implications for policy , 2015 .

[16]  G. Etiope.  Natural Gas Seepage: The Earth’s Hydrocarbon Degassing , 2015 .

[17]  R. Weiss,et al.  Measurements of 14 C in ancient ice from Taylor Glacier, Antarctica constrain in situ cosmogenic 14 CH 4 and 14 CO production rates , 2016 .

[18]  Murat Aydin,et al.  Preindustrial atmospheric ethane levels inferred from polar ice cores: A constraint on the geologic sources of atmospheric ethane and methane , 2016 .

[19]  Pieter P. Tans,et al.  Upward revision of global fossil fuel methane emissions based on isotope database , 2016, Nature.

[20]  V. Brovkin,et al.  The Global Methane Budget 2000–2017 , 2016, Earth System Science Data.

[21]  Stefan Reimann,et al.  Historical greenhouse gas concentrations for climate modelling (CMIP6) , 2016 .

[22]  V. Balaji.  Implications for Policy , 2017 .

[23]  J. Schmitt,et al.  Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4 ice core records , 2017, Proceedings of the National Academy of Sciences.

[24]  D. Valentine,et al.  Rapid rates of aerobic methane oxidation at the feather edge of gas hydrate stability in the waters of Hudson Canyon, US Atlantic Margin , 2017 .

[25]  R. Weiss,et al.  Minimal geological methane emissions during the Younger Dryas–Preboreal abrupt warming event , 2017, Nature.

[26]  M. Prather,et al.  Large changes in biomass burning over the last millennium inferred from paleoatmospheric ethane in polar ice cores , 2018, Proceedings of the National Academy of Sciences.

[27]  H. Graven,et al.  Global and Regional Emissions of Radiocarbon from Nuclear Power Plants from 1972 to 2016 , 2018, Radiocarbon.

[28]  M. Omara,et al.  Assessment of methane emissions from the U.S. oil and gas supply chain , 2018, Science.

[29]  John B. Miller,et al.  Limited contribution of ancient methane to surface waters of the U.S. Beaufort Sea shelf , 2018, Science Advances.

[30]  G. Etiope,et al.  Gridded maps of geological methane emissions and their isotopic signature , 2018, Earth System Science Data.