Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions

Abstract

Atmospheric methane (CH 4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era 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 debate 2,3. 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 century 4,5. 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 year 6,7. Geological emissions were less than 15.4 teragrams CH 4 per year at the end of the Pleistocene, about 11,600 years ago 8 , 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 reductions 9,10. 14 CH 4 emissions from nuclear power plants 4,5. 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 timescales 14 , 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,

The Global Methane Budget 2000–2017

“…The main bottom-up global inventory datasets covering anthropogenic emissions from all sectors (Table 1) are from the United States Environmental Protection Agency (USEPA, 2012), the Greenhouse gas and Air pollutant Interactions and Synergies (GAINS) model developed by the International Institute for Applied Systems Analysis (IIASA) (Gomez Sanabria et al, 2018;, and the Emissions Database for Global Atmospheric Research (EDGARv3.2.2; Janssens-Maenhout et al, 2019) compiled by the European Commission Joint Research Centre (EC-JRC) and Netherland's Environmental Assessment Agency (PBL). We also used the Community Emissions Data System for historical emissions (CEDS) (Hoesly et al, 2018) developed for climate modelling and the Food and Agriculture Organization (FAO) dataset emission database (Tubiello, 2019), which only covers emissions from agriculture and land use (including peatland and biomass fires).…”

A Structured Approach for the Mitigation of Natural Methane Emissions—Lessons Learned from Anthropogenic Emissions

“…The isotope 14 C is rather steadily produced by the interaction of cosmic radiation with atmospheric nitrogen. As 14 C decays with a half-life of~5730 years [34] and dead organic materials stop incorporating 14 C, the 14 C concentration is used for so-called radiocarbon dating but can be also used to differentiate among methane sources [35]: 14 C has completely decayed in fossil (thermogenic) methane, whereas biogenic and pyrogenic methane sources typically processing recent carbon sources have 14 C levels similar to the atmospheric CO 2 [36]. (CH 4 ) showing a possibility to categorize methane emissions according to the emission pathway.…”

“…Further progress to reduce the uncertainty has been made with radioactive isotopic methane measurements. Hmiel et al [35] measured 14 C levels of 11,660-year-old atmospheric methane in an ice core. They found that natural CH 4 emissions from geologic ( 14 C-free) sources were highly overestimated in the literature and that current models therefore underestimated the amount of current anthropogenic methane emissions from fossil fuels by 33% over the upper uncertainty threshold [13,35].…”

“…Hmiel et al [35] measured 14 C levels of 11,660-year-old atmospheric methane in an ice core. They found that natural CH 4 emissions from geologic ( 14 C-free) sources were highly overestimated in the literature and that current models therefore underestimated the amount of current anthropogenic methane emissions from fossil fuels by 33% over the upper uncertainty threshold [13,35]. When future inventories adapt the higher fossil fuel emissions, top-down models will have to adapt the amount of natural methane emissions as well, which could decrease the high uncertainty and help with Problem (13), due to now clearer emission inventories and improved inverse modeling approaches.…”

Background heterogeneity and other uncertainties in estimating urban methane flux: results from the Indianapolis Flux Experiment (INFLUX)

“…Recent studies of urban CH 4 emissions in California indicate that the California Air Resources Board (CARB) inventory tends to underestimate the actual CH 4 urban fluxes, possibly due to fugitive emissions from NG infrastructures in urban environments (Wunch et al, 2009;Jeong et al, 2016Jeong et al, , 2017. The accuracy and precision of atmospheric estimates of urban CH 4 emissions are limited by available atmospheric observations (Townsend-Small et al, 2012), potential source magnitude variability with time (Jackson et al, 2014;Lamb et al, 2016), errors in atmospheric transport modeling (Hendrick et al, 2016;Deng et al, 2017;Sarmiento et al, 2017), and complexity in atmospheric background conditions (Cambaliza et al, 2014;Karion et al, 2015;Heimburger et al, 2017). In this work, detailed analysis of urban CH 4 mole fractions is performed for the city of Indianapolis to better understand the aforementioned uncertainties of urban CH 4 emissions.…”