New Insights Into the Global Methane Budget from Measurements of Atmospheric δD-CH₄

B. Riddell-Young¹, J. Miller², S.E. Michel³, P. Tans⁴, X. Lan^1,2 and H. Moossen⁵

¹Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309; 518-258-4017, E-mail: benjamin.riddell-young@noaa.gov
²NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305
³Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309
⁴NOAA Global Monitoring Laboratory (GML) Retired, Boulder, CO 80305
⁵Max Planck Institute (MPI) for Biogeochemistry, Jena, Germany

Extensive measurements of the ¹³C-to-¹²C ratio of atmospheric CH₄ (expressed as δ¹³C-CH₄) have helped to quantify the role of different sources in the ongoing and rapid increase in atmospheric CH₄ abundance. However, incomplete understandings of the spatiotemporal variability in the δ¹³C-CH₄ of major CH₄ source types and atmospheric sink characteristics both add considerable uncertainty to interpretations of δ¹³C-CH₄. The D-to-H ratio in atmospheric CH₄ (δD-CH₄) is uniquely sensitive to aspects of the CH­₄ budget, but it’s measurement has received significantly less attention. Although several laboratories now make routine measurements of atmospheric δD-CH₄, these data have seldom been used to improve constraints on the global CH₄ budget.

Here, we examine two global, high-resolution datasets of δD-CH₄: One measured by the INSTAAR Stable Isotope Laboratory from 2005 to 2009 using air samples collected within NOAA’s Global Greenhouse Gas Reference Network, and the other measured by the Max Planck Institute from 2011 to present. With these data, we observe robust latitudinal, seasonal, and interannual variability in δD-CH₄. Specifically, we observe a ~10 ‰ latitudinal gradient during NH summer that increases to 18 ‰ during NH winter, reflecting a combination of seasonal variability in the regional strength of microbial emissions and the OH sink. Using an atmospheric one-box model framework, we find that the most recent top-down estimates of the CH₄ budget are consistent with the observed negative trend in δD-CH₄ beginning in 2010 (Figure 1).

Figure 1. Trends in δD-CH₄. (a) Change in δD-CH₄ relative to 2005, when data begins. Global annual mean δD-CH₄ and uncertainty from the GML/INSTAAR dataset (orange). MPI raw data and annual average δD-CH₄ from ALT, with 9.18 ‰ subtracted to represent global annual mean (Blue line and points). Box modeled relative change in δD-CH₄ from 2005 and uncertainty generated using budget estimates from Basu et al., 2022 (light blue) and CarbonTracker-CH₄ (green). (b) Global average CH₄ mixing ratio (NOAA GML).