Global Fossil Methane Emissions Constrained by Multi‐Isotopic Atmospheric Methane Histories

Abstract The global CH 4 budget of sources and sinks is highly uncertain, particularly the emissions from specific sources such as fossil fuels (FF) or agriculture. Here, we estimate plausible global CH 4 source and sink scenarios using historical observations and simulations of atmospheric CH 4 mole fraction and its stable isotopic ( δ 13 C‐CH 4 , δ D‐CH 4 ) and radiocarbon (Δ 14 C‐CH 4 ) composition, combining constraints from all these tracers for the first time. We employ a one‐box model along with a Monte Carlo particle filter technique, explicitly exploring the impact of each isotopic constraints and uncertainties in prior CH 4 source and sink parameters on posterior sectorial source fractions. We find our posterior anthropogenic FF emissions at the global scale are 30% lower than previous isotope‐based studies. Our analysis suggests previous δ 13 C‐CH 4 ‐based studies are potentially biased because the current database‐derived estimate of the global mean biogenic δ 13 C‐CH 4 source signature is too low and/or current sink‐weighted total carbon kinetic isotope effect is underestimated. We find modern atmospheric Δ 14 C‐CH 4 data constrains lower global FF emissions after 1980s, which is contrary to the most recent finding that utilized atmospheric Δ 14 C‐CH 4 data, but supported by an independent estimate of global nuclear 14 CH 4 emissions. Our multi‐isotopic constraints align with CH 4 ‐only inversion results, while reducing their uncertainties with greater robustness against different prior emission scenarios. We find strong constraints not only on FF emissions but also other key sources and sinks, showing that long‐term multi‐isotopic observations are critical for refining the global CH 4 budget and developing effective CH 4 emission mitigation strategies.