Global inventory of doubly substituted isotopologues of methane (Δ13CH3D and Δ12CH2D2)

Measurements of methane (CH4) molecules containing two rare isotopes ( 13CH3D and 12CH2D2), also termed doubly substituted or ‘clumped’ isotopologues, have the potential to provide two additional isotopic dimensions to help investigating mechanisms producing the recent global trends and CH4 budget over decadal timescale. In this work, we summarise the current state of research on doubly substituted CH4 isotopologues, with an emphasis on compiling results of all relevant work. The database comprises 1475 records compiled from the literature published until April 2025 (https://dx.doi.org/10.5285/51ae627da5fb41b8a767ee6c653f83e6). For field samples, 40% of records were sourced from natural gas reservoirs, while microbial terrestrial (e.g., agriculture, lake, wetland) samples account only for 12.5%. Lakes samples contribute 75% to collected microbial terrestrial samples. There is limited or no representation of samples coming from significant microbial CH4 sources to the atmosphere, like wetlands, agriculture and landfill. To date, laboratory experiments were mostly focused on microbial (28% of samples from laboratory experiments) and pyrogenic (15%) methanogenesis or anaerobic (16%), and aerobic (8%) CH4 oxidation, and only a single contribution to studies of photochemical oxidation via OH and Cl (5%). The distinct ranges of Δ 13CH3D and Δ 12CH2D2 values measured in these studies suggests their potential to improve our understanding of atmospheric CH4. This work provides an overview of the major gaps in measurements and identifies where further studies should be focussed to enable the highest immediate impact on understanding global CH4.