Refining methane emission trends using CH4 isotopes modelling for 1990-2020

Methane (CH₄) abatement has emerged as a top priority for climate action due to its short atmospheric lifetime and high global warming potential. However, our understanding of the changes in CH₄ emissions from various sources remains limited, leading to enduring questions about their contributions to the observed changes in atmospheric CH₄ levels over the past three decades. The isotopic composition (δ¹³C-CH₄) fills the gap as they provide information on origins and fates of atmospheric CH₄ since different sources show characteristic signatures and sink processes make the effects of specific fractionations.

This study integrates the CH₄ and δ¹³C-CH₄ in MIROC (version 4)-based atmospheric chemistry-transport model (MIROC4-ACTM) to identify major source sectors that are driving atmospheric CH₄ levels from 1985 to 2020. The simulations are prepared by using diverse data sources, including emission inventories (EDGARv6 and GAINSv4), KIE values (for OH), chlorine fields, and region-specific source signatures. Based on reconstructions of the temporal trends, latitudinal, and vertical gradient of observed atmospheric CH₄ and δ¹³C-CH₄ using the forward simulations, we show (1) emission reductions from oil and gas exploitation (ONG) since the 1990s stabilized the atmospheric CH₄ growth rate in the late 1990s and early 2000s, and (2) emissions from farmed animals, waste management, and coal mining contributed to the increase in CH₄ since 2006. Our findings support neither the increasing ONG emissions reported by the EDGARv6 inventory during 1990-2020 nor the large unconventional emissions increase reported by the GAINSv4 inventory since 2006. Total fossil fuel emissions remained stable from 2000 to 2020, most likely because the decrease in ONG emissions in some regions offset the increase in coal mining emissions in China. Our findings advance understanding of historical CH₄ emission trends and thus support effective future strategies to enhance the global effort to reduce CH₄ emissions.