Interpreting Changes in Global Methane Budget in a Chemistry‐Climate Model Constrained With Methane and Isotopic Observations

Abstract The continuous increase in atmospheric methane (CH 4 ) concentrations over the past few decades has become a major concern due to its strong role as a greenhouse gas contributing to climate change. In this work, we investigate the changes in the global methane budget using a global chemistry‐climate model constrained with methane and its isotopic observations. We apply spatially‐resolved isotopic signatures to better constrain the methane sources and include methane‐hydroxyl radical (OH) feedback to better represent methane sinks and lifetime in the model. While anthropogenic activities are found to be mainly responsible for the methane increase since the 1980s, the increasing OH trend simulated by the model plays a critical role in the global methane evolution. We find the observed post‐2006 shift of δ 13 CH 4 can be explained by increases in 13 C‐depleted agricultural and waste emissions in the tropics, coupled with decreasing 13 C‐enriched biomass burning emissions and an increasing OH trend. We also find post‐2006 emission increases in energy and agriculture sectors are large enough to offset the increasing sinks (due to increasing OH), and therefore are shown to contribute to the post‐2006 renewed methane growth. With CH 4 ‐OH feedback included in the model, the results show an increasing sensitivity to emission increases on methane concentrations and lifetime. Our study underscores the importance of OH in the global methane evolution. Neglecting changes in OH could potentially lead to misinterpreting emission changes with respect to the long‐term observations of methane and δ 13 CH 4 .