Impact of atmospheric radiocarbon and stable isotope measurements on understanding the global CH4 budget over 1850–2015

Measurements of stable isotope ratios of atmospheric CH₄ (δ¹³C-CH₄, δD-CH₄) have been utilized to evaluate contributions of individual CH₄ sources and sinks to global atmospheric CH₄ budget. However, given the uncertainty of both the source isotope signatures and kinetic isotope effects, recent estimates of the global atmospheric CH₄ budget using stable isotope observations are still inconclusive. Radiocarbon measurements (Δ¹⁴C-CH₄) could provide stronger additional constraint on the fossil-fuel CH₄ sources (i.e.,¹⁴C-free), but the uncertainty of ¹⁴CH₄ emissions from nuclear power facilities and a lack of data have limited such utilization. Here we describe a new approach to estimate plausible global CH₄ emissions and sinks scenarios over 1850–2015 using observations and one-box model simulations of atmospheric CH₄, δ¹³C-CH₄, δD-CH₄, and Δ¹⁴C-CH₄. As inputs to the model, we prepare a priori bottom-up CH₄ emission inventories, total atmospheric CH₄ lifetime, source and sink isotope signatures, nuclear power facility database, and atmospheric δ¹³C-CO₂ and Δ¹⁴C-CO₂ observations and their uncertainties. We then run a Monte Carlo simulation of atmospheric CH₄, δ¹³C-CH₄, δD-CH₄, and Δ¹⁴C-CH₄ over the period using the inputs with the uncertainties. By using the observational CH₄ and three isotope constraints, we derive the best combinations of biogenic, anthropogenic fossil-fuel, natural geologic, biomass-burning, and nuclear power facility emissions and total CH₄ lifetime. We find that reconciling CH₄, δ¹³C-CH₄, δD-CH₄, and Δ¹⁴C-CH₄ observations indicates that (1) natural geologic emissions are likely smaller than the recent bottom-up estimate 43–50 Tg CH₄ yr^-1 reported by Etiope et al. (2019), (2) biomass burning and anthropogenic fossil emissions are larger than current bottom-up estimates, and (3) biogenic emissions are somewhat smaller than current bottom-up estimates. Our finding suggests multiple isotope measurements, including Δ¹⁴C-CH₄, have a strong potential to evaluate the current and future bottom-up global CH₄ emission inventories.