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Harmonized time series of column-averaged mole fractions of atmospheric\udmethane and ethane over the period 1999–2014 are derived from solar Fourier\udtransform infrared (FTIR) measurements at the Zugspitze summit\ud(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\udincrease since 2007 of 6.2 [5.6, 6.9] ppb yr−1 (parts-per-billion per\udyear) 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\udthat the renewed methane increase is most likely driven by two main factors:\ud(i) increased methane emissions from tropical wetlands, followed by (ii) increased thermogenic methane emissions due to growing oil and natural gas\udproduction. Here, we quantify the magnitude of the second class of sources,\udusing long-term measurements of atmospheric ethane as a tracer for thermogenic\udmethane emissions. In 2007, after years of weak decline, the Zugspitze\udethane time series shows the sudden onset of a significant positive trend\ud(2.3 [1.8, 2.8]  ×  10−2 ppb yr−1 for 2007–2014), while a\udnegative trend persists at Lauder after 2007 (−0.4 [−0.6, −0.1]  ×  10−2 ppb yr−1). Zugspitze methane and ethane time series are\udsignificantly correlated for the period 2007–2014 and can be assigned to\udthermogenic methane emissions with an ethane-to-methane ratio (EMR) of\ud12–19 %. We present optimized emission scenarios for 2007–2014 derived\udfrom an atmospheric two-box model. From our trend observations we infer a\udtotal ethane emission increase over the period 2007–2014 from oil and\udnatural gas sources of 1–11 Tg yr−1 along with an overall methane\udemission increase of 24–45 Tg yr−1. Based on these results, the oil\udand natural gas emission contribution (C) to the renewed methane increase is\uddeduced using three different emission scenarios with dedicated EMR ranges.\udReference scenario 1 assumes an oil and gas emission combination with EMR  =  7.0–16.2 %,\udwhich results in a minimum contribution C  >  39 % (given as lower bound of 95 % confidence interval). Beside this most\udplausible scenario 1, we consider two less realistic limiting cases of pure\udoil-related emissions (scenario 2 with EMR  =  16.2–31.4 %) and pure\udnatural gas sources (scenario 3 with EMR  =  4.4–7.0  %), which result in\udC  >  18 % and C  >  73 %, respectively. Our results\udsuggest that long-term observations of column-averaged ethane provide a\udvaluable constraint on the source attribution of methane emission changes\udand provide basic knowledge for developing effective climate change\udmitigation strategies

atmospheric chemistry and physics

Contribution of oil and natural gas production to renewed increase in atmospheric methane (2007–2014): top–down estimate from ethane and methane column observations