Variability in Atmospheric Methane From Fossil Fuel and Microbial Sources Over the Last Three Decades

Atmospheric measurements show an increase in CH4 from the 1980s to 1998 followed by a period of near‐zero growth until 2007. However, from 2007, CH4 has increased again. Understanding the variability in CH4 is critical for climate prediction and climate change mitigation. We examine the role of CH4 sources and the dominant CH4 sink, oxidation by the hydroxyl radical (OH), in atmospheric CH4 variability over the past three decades using observations of CH4, C2H6, and δ13CCH4 in an inversion. From 2006 to 2014, microbial and fossil fuel emissions increased by 36 ± 12 and 15 ± 8 Tg y−1, respectively. Emission increases were partially offset by a decrease in biomass burning of 3 ± 2 Tg y−1 and increase in soil oxidation of 5 ± 6 Tg y−1. A change in the atmospheric sink did not appear to be a significant factor in the recent growth of CH4. Plain Language Summary Methane is the second most important greenhouse gas and is responsible for approximately 17% of the direct radiative forcing from all long‐lived greenhouse gases. Observations of methane in the atmosphere have shown a dramatic increase from 2007 after a period of relative stability between the late 1990s and early 2000s, but the cause of this increase is still under scientific debate. This study uses atmospheric observations of methane and two related tracers, the isotopic ratio of carbon in methane and ethane, to constrain the sources and sinks of methane over the past three decades. The increase in methane between 2007 and 2014 is likely due to an increase in microbial sources, of 24–48 Tg/y (predominantly natural wetlands and agricultural), as well as fossil fuel sources, of 7–23 Tg/y. In contrast to other recent studies, a reduction in the atmospheric sink of methane was found not to be a significant factor in explaining the recent atmospheric increase. Key Points Microbial and fossil fuel sources of CH4 have both contributed to the observed increase in atmospheric CH4 from 2007 Fossil fuel sources of CH4 decreased significantly between 1990 and 1996 but increased again from the early 2000s Variation in the main atmospheric sink of CH4, oxidation by OH radicals, appears not to have significantly contributed to the recent increase in CH4