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).