[A-CG34] Global Carbon Cycle Observation and Analysis

[ACG34-08] Lower anthropogenic fossil CH₄ emissions inferred from multi-isotopic constraints on the global CH₄ budget

★Invited Papers

*Ryo Fujita^1,2, Heather Graven² (1.Meteorological Research Institute, 2.Imperial College London)

Keywords:methane, isotope, GHGs, carbon cycle, atmospheric chemistry

Stable isotope measurements of atmospheric CH₄ (δ¹³C-CH₄, δD-CH₄) have been utilized to resolve major three CH₄ sources (i.e., biogenic, fossil fuel, and biomass burning sources) in the global CH₄ budget. However, given the uncertainty of both the source isotope signatures and kinetic isotope effects, recent estimates of the global 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 present a new approach to estimate plausible global CH₄ emissions and sinks scenarios over 1750–2015 using historical observations and Monte Carlo simulations of atmospheric CH₄, δ¹³C-CH₄, δD-CH₄, and Δ¹⁴C-CH₄. We utilize a particle filter (or Sequential Monte Carlo filter) approach to optimize the key 19 parameters of global CH₄ sources and sinks. The ensemble members of CH₄ source and sink scenarios, generated by the ensembles of 19 parameter combinations considering their uncertainties, were used to simulate atmospheric CH₄, δ¹³C-CH₄, δD-CH₄, and Δ¹⁴C-CH₄, and then retained when matching the historical observations. Our multi-isotopic model-data analysis suggests that current bottom-up estimates in natural and anthropogenic fossil and natural biogenic CH₄ emissions are too high, whereas those in anthropogenic biogenic and biomass burning emissions are too low. The estimated global total fossil emission is also lower than the recent Δ¹⁴C-CH₄-derived top-down estimates, but an independent estimate of global nuclear ¹⁴CH₄ emissions would rather support our result. To obtain more robust estimates from multi-isotopic model-data analysis, many more global background observations of atmospheric δ¹³C-CH₄, δD-CH₄, and Δ¹⁴C-CH₄, with their inter-laboratory measurement differences reduced, are indispensable to characterize their global representatives.