Future CH₄ as modelled by a fully coupled Earth system model: prescribed GHG concentrations vs. interactive CH₄ sources and sinks

Abstract We have used the NASA Goddard Institute for Space Studies (GISS) Earth system model GISS-E2.1 to study the future budgets and trends of global and regional CH 4 under different emission scenarios, using both the prescribed GHG concentrations as well as the interactive CH 4 sources and sinks setup of the model, to quantify the model performance and its sensitivity to CH 4 sources and sinks. We have used the Current Legislation (CLE) and the maximum feasible reduction (MFR) emission scenarios from the ECLIPSE V6b emission database to simulate the future evolution of CH 4 sources, sinks, and levels from 2015 to 2050. Results show that the prescribed GHG version underestimates the observed surface CH 4 concentrations during the period between 1995 and 2023 by 1%, with the largest underestimations over the continental emission regions, while the interactive simulation underestimates the observations by 2%, with the biases largest over oceans and smaller over the continents. For the future, the MFR scenario simulates lower global surface CH 4 concentrations and burdens compared to the CLE scenario, however in both cases, global surface CH 4 and burden continue to increase through 2050 compared to present day. In addition, the interactive simulation calculates slightly larger O 3 and OH mixing ratios, in particular over the northern hemisphere, leading to slightly decreased CH 4 lifetime in the present day. The CH 4 forcing is projected to increase in both scenarios, in particular in the CLE scenario, from 0.53 W m −2 in the present day to 0.73 W m −2 in 2050. In addition, the interactive simulations estimate slightly higher tropospheric O 3 forcing compared to prescribed simulations, due to slightly higher O 3 mixing ratios simulated by the interactive models. While in the CLE, tropospheric O 3 forcing continues to increase, the MFR scenario leads to a decrease in tropospheric O 3 forcing, leading to a climate benefit. Our results highlight that in the interactive models, the response of concentrations are not necessarily linear with the changes in emissions as the chemistry is non-linear, and dependent on the oxidative capacity of the atmosphere. Therefore, it is important to have the CH 4 sources and chemical sinks to be represented comprehensively in climate models.