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

Abstract

Atmospheric measurements show an increase in CH 4 from the 1980s to 1998 followed by a period of near-zero growth until 2007. However, from 2007, CH 4 has increased again. Understanding the variability in CH 4 is critical for climate prediction and climate change mitigation. We examine the role of CH 4 sources and the dominant CH 4 sink, oxidation by the hydroxyl radical (OH), in atmospheric CH 4 variability over the past three decades using observations of CH 4 , C 2 H 6 , and δ 13 C CH4 in an inversion. From 2006 to 2014, microbial and fossil fuel emissions increased by 36 ± 12 and 15 ± 8 Tg y À1 , respectively. Emission increases were partially offset by a decrease in biomass burning of 3 ± 2 Tg y À1 and increase in soil oxidation of 5 ± 6 Tg y À1 . A change in the atmospheric sink did not appear to be a significant factor in the recent growth of CH 4 .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.

Very Strong Atmospheric Methane Growth in the 4 Years 2014–2017: Implications for the Paris Agreement

“…A parallel contributory hypothesis is that a decline in biomass burning emissions has masked a strong increase in fossil fuel emissions, given evidence suggesting reduced burning in the 2001-2014 period (Worden et al, 2017). As noted above, Thompson et al (2018) also infer that fossil fuel sources increased in 2006-2014, while biomass burning decreased. Methane from fires, especially in tropical savannas dominated by C4 grasses, is much enriched in δ 13 C CH4 Dlugokencky et al, 2011); thus, pulses of methane from tropical savanna and peatland fires (δ 13 C CH4 around −30‰ to −15‰) have strong leverage on ambient values.…”

“…In this context, Thompson et al () found that OH appears not to have contributed significantly to recent methane growth. Further insight into the OH problem came from Nicely et al () who showed that atmospheric OH is probably currently well buffered, with mean anomalies of 1.6%.…”

An increase in methane emissions from tropical Africa between 2010 and 2016 inferred from satellite data

“…These hypotheses include increased fossil fuel emissions, increased microbial emissions, or some combination of the two allied with other factors (e.g. Schaefer et al, 2016;Hausmann et al, 2016;Worden et al, 2017;McNorton et al, 2018;Thompson et al, 2018). Additionally, we 10 cannot discount a role for a changing OH sink based on CH 4 , isotopic δ 13 CH 4 and methyl chloroform observations Turner et al, 2017).…”

Influence of Atmospheric Transport on Estimates of Variability in the Global Methane Burden

“…This constraint is implicitly used in two-box model studies (Rigby et al, 2017;Sapart et al, 2012., Turner et al, 2017 and multiple-box model studies (Nisbet et al, 2016;Thompson et al, 2018). The inter-hemispheric difference (IHD) in CH 4 abundance has been used to quantify the ratio between Northern and Southern Hemisphere CH 4 emissions or sinks.…”

“…The inter-hemispheric difference (IHD) in CH 4 abundance has been used to quantify the ratio between Northern and Southern Hemisphere CH 4 emissions or sinks. This constraint is implicitly used in two-box model studies (Rigby et al, 2017;Sapart et al, 2012., Turner et al, 2017 and multiple-box model studies (Nisbet et al, 2016;Thompson et al, 2018). Dlugokencky et al (2003) furthermore introduced the metric inter-polar difference (IPD) as the difference between the annual averages of CH 4 from northern (53°N to 90°N) and southern (53°S to 90°S) polar regions, weighted with the inverse sine of latitude.…”