Recent decreases in fossil-fuel emissions of ethane and methane derived from firn air

Journal name:
Nature
Volume:
476,
Pages:
198–201
Date published:
DOI:
doi:10.1038/nature10352
Received
Accepted
Published online

Methane and ethane are the most abundant hydrocarbons in the atmosphere and they affect both atmospheric chemistry and climate. Both gases are emitted from fossil fuels and biomass burning, whereas methane (CH4) alone has large sources from wetlands, agriculture, landfills and waste water. Here we use measurements in firn (perennial snowpack) air from Greenland and Antarctica to reconstruct the atmospheric variability of ethane (C2H6) during the twentieth century. Ethane levels rose from early in the century until the 1980s, when the trend reversed, with a period of decline over the next 20years. We find that this variability was primarily driven by changes in ethane emissions from fossil fuels; these emissions peaked in the 1960s and 1970s at 14–16 teragrams per year (1Tg = 1012g) and dropped to 8–10Tgyr−1 by the turn of the century. The reduction in fossil-fuel sources is probably related to changes in light hydrocarbon emissions associated with petroleum production and use. The ethane-based fossil-fuel emission history is strikingly different from bottom-up estimates of methane emissions from fossil-fuel use1, 2, and implies that the fossil-fuel source of methane started to decline in the 1980s and probably caused the late twentieth century slow-down in the growth rate of atmospheric methane3, 4.

At a glance

Figures

  1. Ethane mixing ratios in firn air at three sites, and the atmospheric histories derived from these measurements.
    Figure 1: Ethane mixing ratios in firn air at three sites, and the atmospheric histories derived from these measurements.

    af, Results for Summit (a, b), South Pole (c, d) and WAIS-D (e, f). Filled and open squares in a, c and e show measurements with estimated uncertainties (error bars, ±2s.e.) (Supplementary Data). Solid lines in a, c and e show modelled firn profiles for the three respective sites, with the five different atmospheric histories (solid lines) shown in b, d and f. These five atmospheric histories were obtained by inverse modelling of the measurements with five different boundary conditions at to that are identified by different colours, each representing a different pre-industrial mixing ratio for ethane. The data points in the top 35m (Summit and South Pole) and 40m (WAIS-D) of the firn are subject to the effects of seasonal variations in surface ethane levels (Supplementary Information) and are ignored during inverse modelling (open squares in a, c and e). The inversions were forced with contemporary annual-mean surface mixing ratios of 1,460p.p.t. at Summit and 215p.p.t. at South Pole and WAIS-D (red triangles in a, c and e) (Supplementary Information). The South Pole atmospheric history is overlaid on the independently derived WAIS-D atmospheric histories for comparison (dashed line in f). The inversions are sensitive to assumptions about the pre-industrial ethane levels before 1950, 1940 and 1910 at Summit, WAIS-D and South Pole, respectively (shaded areas), implying that only the inversion results for the later years are valid atmospheric histories.

  2. Ethane source emissions and the resulting atmospheric histories.
    Figure 2: Ethane source emissions and the resulting atmospheric histories.

    ad, Historical global ethane emissions (a, b) and the resulting atmospheric histories (c, d) are derived with the two-box model to be consistent with the site-specific atmospheric histories based on firn-air results (Fig. 1). Fossil-fuel emission histories were developed by considering five different biomass-burning cases. In the first four cases, biomass-burning emissions are fixed at 1, 2, 3 and 4Tgyr−1 (black, purple, green and blue lines, respectively; ad), and the fossil-fuel emissions are varied to minimize the χ2 value of the fit to the firn-air-based ethane atmospheric histories (black squares in c and d) from both the high northern latitudes and high southern latitudes (HNL and HSL) during 1950–2000, and from HSL only during 1900–1940. We included 2010 in the box model optimizations by assuming the atmospheric ethane levels remained constant at the mixing ratios used as surface tie-points used in firn inversions (Fig. 1). As a fifth case, the fit to the atmospheric histories is optimized by allowing both the fossil-fuel and the biomass-burning emissions to vary (thick red lines). Variable biomass burning is considered only for the period since 1950, because atmospheric histories from both hemispheres are needed to constrain the partitioning between fossil-fuel and biomass-burning emissions. Emissions from biofuel burning are fixed at the historical estimates (dashed black line; a) in all five cases2. The atmospheric lifetime of ethane (Methods Summary) is also the same in all five cases. The hemispheric-average atmospheric histories are also shown for the fifth case (dashed red lines; c and d), in which fossil-fuel and biomass-burning emissions are both varied. Ethane levels in HNL and HSL are obtained from Northern and Southern hemispheric (NH and SH) levels using the following ratios for fossil-fuel, biofuels and biomass-burning emissions: HNL/NH: 1.6, 1.4, 1.0; HSL/SH: 0.7, 0.7, 0.7. Values differ from 1.0 owing to the latitudinal distribution of each type of emission (Supplementary Information).

  3. Ethane and methane emissions from fossil fuels, biofuels and biomass burning.
    Figure 3: Ethane and methane emissions from fossil fuels, biofuels and biomass burning.

    a, Ethane-based fossil-fuel emission histories (left y-axis) for fixed biomass burning (solid grey lines) and variable biomass burning (solid black line) cases, compared with bottom-up methane emission estimates from fossil fuels (coloured lines, right y-axis) based on the following sums of emissions: blue, gas flaring, gas supply and coal mining1; red, fossil-fuel consumption, fossil-fuel production and industrial2; green, oil, gas and solid fuels (data from http://edgar.jrc.ec.europa.eu/). The oil and gas only methane emissions from EDGAR4.1 (see URL in previous sentence; green dashed line) and gas flaring and gas supply methane emissions from ref. 1 (blue dashed line) are also shown. The trends in the ethane-based histories are relatively insensitive to assumptions about biomass burning. None of the bottom-up methane fossil-fuel emission inventories display a persistent decline during 1980–2000 that would be consistent with the ethane-based emission histories. The scales of the left and right y-axes are set at a ratio of 8 because it suits the presentation well and is consistent with b. b, Ethane emissions (left y-axis) from biomass burning (dashed black line) and biomass-burning+biofuels (solid black line) for the variable biomass-burning case in the two-box model compared with methane emissions (right y-axis) attributed to the total burning product from ref. 1 (blue line)1 and with EDGAR-HYDE 1.4 (red line)2. The scales of the left and right y-axes are set at a ratio of 8, which is a measurement-based estimate of methane/ethane emission ratio (MER) from biomass burning30. Our results and the bottom-up methane inventories are consistent in how hydrocarbon emissions from biomass burning changed over time and suggest an MER of 8 on a Tg per Tg basis, in agreement with the earlier estimates30. Our estimate of ethane biomass-burning emissions from the variable biomass-burning case is also in good agreement with some independent estimates of historical ethane emissions from biomass burning16 (green diamonds, left y-axis) and a total burning emissions estimate (biomass burning+biofuels) for present day6 (red triangle, left y-axis).

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Author information

Affiliations

  1. Department of Earth System Science, University of California, Irvine, California 92697, USA

    • Murat Aydin,
    • Kristal R. Verhulst,
    • Eric S. Saltzman,
    • Donald R. Blake,
    • Qi Tang &
    • Michael J. Prather
  2. Department of Physics and Astronomy, Bowdoin College, Brunswick, Massachusetts 04011, USA

    • Mark O. Battle
  3. Earth System Research Laboratories – Global Monitoring Division, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, USA

    • Stephen A. Montzka

Contributions

M.A.: firn-air sampling, ethane analysis in firn air and surface-air flasks, firn-air modelling, two-box modelling, box-model inversions, manuscript preparation. K.R.V.: ethane analysis in firn air and surface-air flasks, firn-air modelling, two-box modelling, firn-air and two-box model inversions, manuscript improvements. E.S.S.: firn-air modelling, two-box modelling, firn-air and two-box model inversions, manuscript improvements. M.O.B.: firn-air sampling, firn-air modelling, manuscript improvements. S.A.M.: halocarbon measurements in firn air to constrain firn processes, NOAA surface air samples, manuscript improvements. D.R.B.: ethane measurements in surface air, manuscript improvements. Q.T.: CTM modelling, manuscript improvements. M.J.P.: CTM modelling, manuscript improvements.

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The authors declare no competing financial interests.

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Supplementary information

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  1. Supplementary Information (2M)

    This file contains Supplementary Text and Data, additional references, Supplementary Figures 1-10 with legends and Supplementary Tables 1-2.

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  1. Supplementary Data (10K)

    This file contains the Firn air ethane data.

Additional data