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

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Abstract
The ongoing debate over the origin of rising atmospheric methane levels highlights gaps in our understanding of its global budget. To our knowledge, we present the first reconstructed 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 per mil lower than in the mid-2020s, likely marking the lowest level over the last millennium. We attribute this shift to the unprecedented anthropogenic perturbation of global methane emissions during the industrial period. The temporal lag in the atmospheric isotopic signature reflects the disequilibrium effect caused by the longer atmospheric lifetime of 12CH2D2 relative to methane (CH4). Moreover, our results suggest that the kinetic isotope effect in the removal of 12CH2D2 by the hydroxyl radical needs to be much smaller than previously estimated. Future changes in atmospheric ∆12CH2D2 will continue to reflect evolving CH4 emission and removal pathways.

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