Microbial driver of 2006–2023 CH ₄ growth indicated by trends in atmospheric δD–CH ₄ and δ ¹³ C–CH ₄

Methane (CH₄) is the second most important greenhouse gas and has been rising following a brief period of stabilization from 1999 to 2006. Determining the cause of this rise is critical for reducing emissions and predicting future climate sensitivity. The carbon and hydrogen stable isotopic composition of atmospheric CH₄ is controlled by variability in isotopically distinguishable emission categories and fractionating sink processes. While most studies using atmospheric δ¹³C-CH₄ data suggest a dominantly microbial source for recent CH₄ growth, this understanding is not uniform, and uncertainties remain [S. Schwietzke <i>et al.</i>, <i>Nature</i> <b>538</b>, 88-91 (2016), S. Basu <i>et al.</i>, <i><i>Atmos. Chem. Phys.</i></i> <b>22</b>, 15351-15377 (2022), J. Thanwerdas, M. Saunois, A. Berchet, I. Pison, P. Bousquet, <i>Atmos. Chem. Phys.</i> <b>24</b>, 2129-2167 (2024)]. Here, we present a harmonized global measurement record of atmospheric δD-CH₄ and estimate emissions from 1999 to 2022 with global isotope mass balance calculations using both carbon and hydrogen isotopic ratios. We conduct thorough uncertainty analyses to separate absolute magnitude and emission trend uncertainties and find with high confidence that trends in δ¹³C-CH₄ and δD-CH₄ observations are both consistent with an entirely microbial emission driver of the post-2006 CH₄ rise, while fossil fuel emissions have remained relatively stable.