Anthropogenic perturbations to atmospheric methane reflected in Greenland firn air clumped isotope measurements

Uncertainties regarding the origin of rising atmospheric methane levels underscore gaps in our understanding of the global methane cycle. The clumped isotopic composition (Δ13CH3D and Δ12CH2D2) of methane has recently been developed as an additional tracer to constrain atmospheric methanesources and sinks (1, 2). We present a novel perspective by reconstructing the clumped isotopic composition of atmospheric methane dating back to the early 1990s, based on large-volume firn air samples from the Greenland ice cap. Our measurements indicate that atmospheric ∆12CH2D2 around 1993 was 10 ± 2 ‰ lower than in the mid-2020s.We used a two-box atmospheric model to investigate the drivers of the observed ∆12CH2D2 evolution. Emission fluxes for 1980–2024 were taken from an inversion constrained by atmospheric CH4, δ13C, and δD observations (3). However, sensitivity experiments show that this large ∆12CH2D2 increase cannot be explained by recent changes in methane source composition, as the globally averaged source ∆12CH2D2 varies by less than 1 ‰ over recent decades. Instead, the signal reflects the long-term source-sink disequilibrium effects (4). Owing to the longer atmospheric lifetime of 12CH2D2 relative to other methane isotopologues, Δ12CH2D2 responds slowly to perturbations and records changes in the methane budget over multi-decadal to centennial timescales, leading to a temporal lag relative to changes in methane concentration and bulk isotopic ratios.Extending the simulations back to 1200 CE shows that accelerating methane emissions during industrialisation progressively drove atmospheric Δ12CH2D2 to lower values, reaching a minimum of ~40 ‰ in the late 20th century. The subsequent slowdown in methane growth after 1990 allowed partial re-equilibration, leading to the observed increase in Δ12CH2D2.Our results demonstrate that Δ12CH2D2 uniquely records the anthropogenic perturbation of the global methane cycle and suggest that the firn air samples measured in this study capture the lowest Δ12CH2D2 values of the past millennium. M. Sivan, T. Röckmann, C. van der Veen, M. E. Popa, Extraction, purification, and clumped isotope analysis of methane (Δ13CDH3 and Δ12CD2H2) from sources and the atmosphere. Atmos. Meas. Tech. 17, 2687-2705 (2024).M. A. Haghnegahdar et al., Tracing sources of atmospheric methane using clumped isotopes. Proceedings of the National Academy of Sciences 120, e2305574120 (2023).B. Dasgupta et al., Global Methane Emission Estimates from a Dual-Isotope Inversion: New Constraints from δD-CH₄. EGUsphere 2025, 1-21 (2025).P. P. Tans, A note on isotopic ratios and the global atmospheric methane budget. Gl. Biogeochem. Cycles 11, 77-81 (1997).