Isotopic constraints on the decadal trends of global methane emissions favor increasing fossil fuel emissions over recent decades

Abstract:

Despite concerted effort in recent years to understand the changing budget of atmospheric methane (CH₄) there remains considerable uncertainty on the trends and magnitudes of individual methane sources over decadal scales. Using new measurements of atmospheric methane isotopes from an archive of air sampled at Cape Meares Oregon (45°N, 124°W) combined with existing data we performed a time-dependent retrieval of methane fluxes spanning nearly twenty-five years. The inversion was able to reproduce CH₄ and δ¹³C-CH₄ successfully at nearly every site. δD-CH₄ data was well-simulated by δ¹³C-CH₄ optimized emissions up until year 2000, after which the simulated δD significantly exceeded measured data. The inversion estimates a ~30 Tg CH₄ increase in fugitive fossil fuel emissions since 1985 with the highest growth rate occuring after year 2000. This result is consistent with some bottom-up estimates but is not consistent with recent estimates based on atmospheric ethane and other inverse studies. The model also estimates an overall decrease in biomass burning emissions since 1985 with most of the decrease attributed to C3 vegetation. These results are robust to over 40 sensitivity tests where changes to model parameters and input data were made. If the inversion is forced using a fugitive fossil fuel scenario consistent with recent ethane measurements, emissions from waste sources (e.g. landfills) greatly exceed estimates from bottom-up inventories suggesting this scenario is not consistent with the isotope record.