Distinct Efficacy of Regional Methane Emissions in Affecting Global and Regional Concentrations: A Modeling Study with Interactive Chemistry and Emission Tags

The newly developed emission-driven capability of CESM2 allows methane concentrations to be more realistically simulated based on emissions, transport, and atmospheric chemistry, rather than being prescribed as lower boundary conditions as in most previous global Earth system modeling studies. This capability enables flexibility in investigating the role of spatially resolved methane sources and sinks. In this study, we conduct emission-driven simulations for 2014–2019, with the full chemistry coupling of methane and improved representation of short-lived halogen (SLH) species, and with a novel tagging scheme to track the regionally resolved emissions from three representative regions in the Northern Hemisphere (Asia, Europe, and North America). Our simulations reveal that methane emissions from different regions exhibit distinct efficacy in increasing global and regional methane concentrations. Specifically, in the first year, a unit of methane emitted at higher latitudes (e.g., Europe) is about 30% more effective than the global average at increasing global methane levels, primarily because it originates in, and is then transported over, regions with much longer local lifetimes due to the latitudinally varying distribution of OH. Moreover, despite methane's decade-long lifetime, regional concentrations close to the sources respond faster than the global mean concentration, especially during the first few years after emission changes. Our findings highlight the importance of accounting for local differences in chemical sinks and methane lifetimes when assessing future climate responses to methane emissions and their mitigation.