Long-term decline of global atmospheric ethane concentrations and implications for methane

Abstract

After methane, ethane is the most abundant hydrocarbon in the remote atmosphere. It is a precursor to tropospheric ozone and it influences the atmosphere's oxidative capacity through its reaction with the hydroxyl radical, ethane's primary atmospheric sink. Here we present the longest continuous record of global atmospheric ethane levels. We show that global ethane emission rates decreased from 14.3 to 11.3 teragrams per year, or by 21 per cent, from 1984 to 2010. We attribute this to decreasing fugitive emissions from ethane's fossil fuel source--most probably decreased venting and flaring of natural gas in oil fields--rather than a decline in its other major sources, biofuel use and biomass burning. Ethane's major emission sources are shared with methane, and recent studies have disagreed on whether reduced fossil fuel or microbial emissions have caused methane's atmospheric growth rate to slow. Our findings suggest that reduced fugitive fossil fuel emissions account for at least 10-21 teragrams per year (30-70 per cent) of the decrease in methane's global emissions, significantly contributing to methane's slowing atmospheric growth rate since the mid-1980s.

Atmospheric methane isotopic record favors fossil sources flat in 1980s and 1990s with recent increase

“…In particular, two recent studies using the tracer ethane have linked a sustained drop in ethane mixing ratios to a decrease in fugitive fossil fuel-based CH 4 emissions by 10-30 Tg y −1 since the 1980s (6,7). This top-down result differs with bottom-up emissions inventories that show increasing or flat fugitive fossil fuel CH 4 emissions (8).…”

“…It is particularly difficult to reconcile the results here with the assertion of decreasing (10-30 Tg y −1 ) natural gas and oil fugitive emissions based on falling ethane mixing ratios (6,7). This scenario was selected as more likely than decreasing microbial emissions to explain the stabilization of CH 4 emissions in a recent review synthesis (3).…”

The global methane budget 2000–2012

“…Fourier transform infrared (FTIR) measurements at fixed locations also provide methane column observations . Dlugokencky et al, 1994), the Advanced Global Atmospheric Gases Experiment (AGAGE, Prinn et al, 2000;Cunnold et al, 2002;Rigby et al, 2008), the Commonwealth Scientific and Industrial Research Organisation (CSIRO, Francey et al, 1999) and the University of California Irvine (UCI, Simpson et al, 2012). The data are archived at the World Data Centre for Greenhouse Gases (WDCGG) of the WMO Global Atmospheric Watch (WMO-GAW) programme, including measurements from other sites that are not operated as part of the four networks.…”

“…AGAGE uses an independent standard scale (Aoki et al, 1992), but direct comparisons of standards and indirect comparisons of atmospheric measurements show that differences are below 5 ppb (WMO RoundRobin programme). UCI uses another independent scale that was established in 1978 and is traceable to NIST (Simpson et al, 2012) but has not been included in standard exchanges with other networks so differences with the other networks cannot be quantitatively defined. Additional experimental details are presented in the Supplement from Kirschke et al (2013) and references therein.…”

“…The partition of regional emissions by processes remains very uncertain today, waiting for the development or consolidation of measurements of more specific tracers, such as methane isotopes or ethane, dedicated to constrain the different methane sources or groups of sources (e.g. Simpson et al, 2012;Schaefer et al, 2016;Hausmann et al, 2016).…”