Abstract
Abstract. Harmonized time series of column-averaged mole fractions of atmospheric methane and ethane over the period 1999–2014 are derived from solar Fourier transform infrared (FTIR) measurements at the Zugspitze summit (47° N, 11° E; 2964 m a.s.l.) and at Lauder (45° S, 170° E; 370 m a.s.l.). Long-term trend analysis reveals a consistent renewed methane increase since 2007 of 6.2 [5.6, 6.9] ppb yr−1 (parts-per-billion per year) at the Zugspitze and 6.0 [5.3, 6.7] ppb yr−1 at Lauder (95 % confidence intervals). Several recent studies provide pieces of evidence that the renewed methane increase is most likely driven by two main factors: (i) increased methane emissions from tropical wetlands, followed by (ii) increased thermogenic methane emissions due to growing oil and natural gas production. Here, we quantify the magnitude of the second class of sources, using long-term measurements of atmospheric ethane as a tracer for thermogenic methane emissions. In 2007, after years of weak decline, the Zugspitze ethane time series shows the sudden onset of a significant positive trend (2.3 [1.8, 2.8] × 10−2 ppb yr−1 for 2007–2014), while a negative trend persists at Lauder after 2007 (−0.4 [−0.6, −0.1] × 10−2 ppb yr−1). Zugspitze methane and ethane time series are significantly correlated for the period 2007–2014 and can be assigned to thermogenic methane emissions with an ethane-to-methane ratio (EMR) of 12–19 %. We present optimized emission scenarios for 2007–2014 derived from an atmospheric two-box model. From our trend observations we infer a total ethane emission increase over the period 2007–2014 from oil and natural gas sources of 1–11 Tg yr−1 along with an overall methane emission increase of 24–45 Tg yr−1. Based on these results, the oil and natural gas emission contribution (C) to the renewed methane increase is deduced using three different emission scenarios with dedicated EMR ranges. Reference scenario 1 assumes an oil and gas emission combination with EMR = 7.0–16.2 %, which results in a minimum contribution C > 39 % (given as lower bound of 95 % confidence interval). Beside this most plausible scenario 1, we consider two less realistic limiting cases of pure oil-related emissions (scenario 2 with EMR = 16.2–31.4 %) and pure natural gas sources (scenario 3 with EMR = 4.4–7.0 %), which result in C > 18 % and C > 73 %, respectively. Our results suggest that long-term observations of column-averaged ethane provide a valuable constraint on the source attribution of methane emission changes and provide basic knowledge for developing effective climate change mitigation strategies.
Highlights
Methane (CH4) is the second most important anthropogenic greenhouse gas and responsible for about 20 % of global warming since preindustrial times (Kirschke et al, 2013)
We demonstrate that long-term observations of column-averaged ethane within the NDACC Fourier transform infrared (FTIR) framework provide a valuable constraint on the source attribution of methane emission changes
We present harmonized time series of column-averaged dry-air mole fractions of methane and ethane for Zugspitze (47◦ N, 11◦ E) and Lauder (45◦ S, 170◦ E), representative of high northern and southern latitude background conditions
Summary
Methane (CH4) is the second most important anthropogenic greenhouse gas and responsible for about 20 % of global warming since preindustrial times (Kirschke et al, 2013). Recent work provides evidence that there are likely two dominant contributors to the recent methane increase (Kirschke et al, 2013; Nisbet et al, 2014), namely increasing emissions from (i) tropical and boreal wetlands driven by precipitation and temperature anomalies (Dlugokencky et al, 2009; Bousquet et al, 2011) and (ii) growing exploitation of fossil fuels (natural gas, oil, and coal) (e.g., Bergamaschi et al, 2013; see references in the paragraph). Bader (personal communication, 2015) suggest that the recent methane increase is dominated by anthropogenic emissions from increased fossil fuel extraction Important in this context is the strong increase in US oil and natural gas production starting in the mid2000s (Moore et al, 2014; Wang et al, 2014), which is expected to continue through 2040 (US Energy Information Administration, 2014).
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