by John R. Worden, A. Anthony Bloom, Sudhanshu Pandey, Zhe Jiang, Helen M. Worden, Thomas W. Walker, Sander Houweling & Thomas Rockmann
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.
Introduction
Recent changes in the growth rate of methane1, the second most important greenhouse gas, and important ozone precursor2, could be due to changing anthropogenic emissions in the form of fossil fuel (FF) or agricultural emissions3,4,5,6,7,8. Alternatively, natural wetland methane fluxes in the high latitudes or tropics could be increasing in response to variations in temperature, the water cycle, and/or carbon availability to methanogens9,10,11,12, giving a preview of carbon cycle feedbacks to global warming13. However, determining the relative contributions of anthropogenic, biogeochemical, and chemical drivers of methane trends has been extremely challenging and consequently there is effectively no confidence in projections of future atmospheric methane concentrations. The striking disagreement from several recent studies explaining the changes to atmospheric methane since 20065,6,7,8 is likely due to the assumptions (and extrapolations) involved in attributing source variability to the observed changes in atmospheric methane. For example, surface measurements of CH4 and its isotopic composition suggest a shift of methane sources toward increasing tropical biogenic (BG) sources5,14,15. However, this explanation appears to directly conflict with observations of increasing FF sources that range between 5 and 25 Tg CH4 per year based on ethane/CH4 ratios6,7,8 as well as studies based on satellite-based total column methane measurements16,17. Other studies18,19 show that we cannot rule out inter-annual variations in the hydroxyl radical (OH) chemical methane sink as the cause; however, these studies do not directly show changes in atmospheric OH or provide a mechanistic reason for a change.
Biomass burning (BB) contributes only moderately to atmospheric methane with past estimates ranging from 14 to 26 Tg CH4 per year out of the ~550 Tg CH4 per year budget20,21. The range of BB CH4 emissions estimates is in part due to uncertainties in burnt area estimates, combustion factors, and emission factors22,23,24,25 and to large inter-annual variability (IAV) resulting from substantial regional changes in rainfall due to ENSO26. For example, larger than normal inter-annual changes in atmospheric CH4 in 2006 and likely 1997 can be directly attributed to massive Indonesian peat fires27,28. Estimates based on burnt area suggest a decrease of ~2 Tg per year after 2007 (Global Fire Emissions Database, version 4—GFEDv4s)29 with decreasing burnt area over Africa likely due to better fire management and agricultural practices30 as well as reduced emissions over South America and Indonesia25,31,32. Our study focuses on how changes in biomass burning BB emissions of methane affect our knowledge of the FF and BG components of the atmospheric methane budget.
GFED bottom-up estimates for methane emissions from BB depend on satellite observations of burnt area, vegetation type, combustion efficiency, and amount of burnt biomass29,33. Top-down estimates depend on the combination of observationally constrained total CO flux estimates and in situ or satellite constraints on the CH4/CO ratio25,28(Methods). Because the seasonality and location of fires are typically distinct from other emissions such as biofuels, industry, and transportation, top-down approaches can robustly distinguish biomass burning emissions from other sources based on satellite CO concentration measurements and prior information on burnt-area-based fire emissions estimates25,28,31,32. Here, we combine bottom-up estimates of fire emissions, based on burnt area measurements, with the top-down CO emissions estimates31 (Methods), based on the satellite concentration data and the adjoint of the Goddard Earth Observing System Chemistry model (GEOS-Chem). This approach for quantifying CO and CH4 fire emissions accounts for published uncertainties in the bottom-up estimates and includes empirical estimates of the key factors that contribute to uncertainties in emissions inferred from concentration data such as errors in transport and chemistry, partitioning of CO emissions on the 5 × 4° GEOS-Chem grid cell to FF, fires, or chemical sources31,32, and uncertainties in the CH4/CO emission factors and their IAV. We use satellite and in situ measurements of CH4/CO ratios to evaluate fire-based CH4/CO values and their associated uncertainties (Methods). We then show that biomass burning emissions of methane decreased by 3.7 (±1.4) Tg CH4per year from the 2001–2007 to the 2008–2014 time periods, nearly twice the decrease expected from prior estimates based on burnt area measurements. 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 FFs and BG sources contribute 12–19 Tg CH4 per year and 12–16 Tg CH4 per year, respectively, to the recent atmospheric methane increase, thus reconciling the isotopic- and ethane-based results.
