Reduced biomass burning emissions reconcile conflicting estimates of the post-2006 atmospheric methane budget

Abstract

Several viable but conflicting explanations have been proposed to explain the recent ~8 p.p.b. per year increase in atmospheric methane after 2006, equivalent to net emissions increase of ~25 Tg CH4 per year. A concurrent increase in atmospheric ethane implicates a fossil source; a concurrent decrease in the heavy isotope content of methane points toward a biogenic source, while other studies propose a decrease in the chemical sink (OH). Here we show that biomass burning emissions of methane decreased by 3.7 (±1.4) Tg CH4 per year from the 2001–2007 to the 2008–2014 time periods using satellite measurements of CO and CH4, nearly twice the decrease expected from prior estimates. After updating both the total and isotopic budgets for atmospheric methane with these revised biomass burning emissions (and assuming no change to the chemical sink), we find that fossil fuels contribute between 12–19 Tg CH4 per year to the recent atmospheric methane increase, thus reconciling the isotopic- and ethane-based results.

Methane Mitigation: Methods to Reduce Emissions, on the Path to the Paris Agreement

“…Fuel loads are also strongly dependent on preceding meteorology (Zubkova et al, ). Satellite data sets may show a recent decline in global burnt area (Andela et al, ), and globally, Worden et al () showed evidence of a decline in biomass burning between the 2001–2007 and 2008–2014 time periods, which may reflect complex socio‐economic changes as much as meteorology, though Forkel et al () found the trend is not significant. Zubkova et al () concluded that changing climate patterns and increased terrestrial moisture facilitated a decline in African fires in the 2002–2016 but pointed out that most African fires are human‐caused and the fire‐climate‐human relationship is complex.…”

“…The post‐2007 growth and isotopic shift has now been sustained for over a decade, but the reasons for the change are still not well understood. Possible explanations for this growth and the concurrent isotopic shift (for isotopic values of sources, see Sherwood et al, ); may include increases in biogenic emissions (especially in the tropics and subtropics), changes in the chemical sinks of methane by atmospheric OH or Cl, increased fossil fuel emissions coupled with declining biomass burning (Nisbet et al, , ; Schaefer et al, ; Schwietzke, Sherwood et al, ; Turner et al, , ; Rigby et al, ; Worden et al, ), more oxidation of methane by methanotrophy in forest soils (Ni & Groffman, ), or, more likely, some combination of all these factors.…”

“…Nisbet et al (, ) used isotopic evidence to infer that fossil fuel emissions have declined as a proportion of the total methane budget, though they could not rule out an increase in absolute terms. But it is also possible (within the uncertainties of the observations) that a decline in biomass burning may be masking a rise in fossil fuel emissions (Worden et al, ).…”

The NASA Carbon Airborne Flux Experiment (CARAFE): instrumentation and methodology

“…This construct provides very little information on the nature and distribution of the land flux or its potential variations. Global CH 4 budgets are similarly under-constrained in detail (e.g., Bousquet et al, 2011;Dlugokencky et al, 2011;Worden et al, 2017).…”

Only the instantaneous global warming potential is consistent with honest and responsible greenhouse gas accounting

“…For instance, Turner et al (2016) concluded on the basis satellite and surface data that there had been a large increase in the methane emissions of the United States over the decade prior to their 2016 study. Worden et al (2017) subsequently traced these increases back to fossil fuel sources. At the same time, it has become increasingly clear that the climate system is deteriorating at the rate outpacing projections, those of the Intergovernmental Panel on Climate Change (IPCC) in particular.…”