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

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Abstract

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 20 years. 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 (1 Tg = 1012 g) and dropped to 8–10 Tg yr−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.

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Figure 1: Ethane mixing ratios in firn air at three sites, and the atmospheric histories derived from these measurements.
Figure 2: Ethane source emissions and the resulting atmospheric histories.
Figure 3: Ethane and methane emissions from fossil fuels, biofuels and biomass burning.

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Acknowledgements

We thank T. Sowers, M. Drier and ICDS drillers for support during firn-air sampling and drilling in the field, and T. Sowers for discussions during preparation of the manuscript. We thank M. Bender and J. Severinghaus for 15N measurements in firn air, S. Meinardi for ethane measurements in surface flasks, and P. Tans and P. Lang for firn-air CO2 data. This work was supported by the National Science Foundation (grants ANT-0739598, ANT-0440602, ANT-0440509, ARC-0520460) and NASA (grant NAG58935).

Author information

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.

Correspondence to Murat Aydin.

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Supplementary Information

This file contains Supplementary Text and Data, additional references, Supplementary Figures 1-10 with legends and Supplementary Tables 1-2. (PDF 2094 kb)

Supplementary Data

This file contains the Firn air ethane data. (XLS 10 kb)

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