Evolution of atmospheric methane under the global methane pledge: insights from an Earth system model

The global methane pledge (GMP) aims to cut methane (CH₄) emissions across all sectors by at least 30 percent below 2020 levels by 2030, which can thereby provide benefits in air quality and health, as well as in climate, relative to not cutting the emissions. We have used a fully coupled Earth system model (ESM) with interactive CH₄ sources and sinks to study the future levels and trends of global atmospheric CH₄ concentrations under different emission scenarios. Fully coupled simulations have been performed from 1995 to 2050, using multispecies emissions from the ECLIPSE V6b emissions database supplemented by new anthropogenic methane emissions estimates for Current Legislation (CLE), Maximum Feasible Reduction (MFR) and Global Methane Pledge (GMP) from IIASA/GAINS to simulate the future evolution of CH₄ levels. In the baseline CLE scenario, global anthropogenic CH₄ emissions increase from 298 Tg in year 2000 to 335 Tg in 2015, then continues to increase to 430 Tg in 2050 under CLE. Under MFR, anthropogenic CH₄emissions first drop to 240 Tg in 2030, then slightly decrease to 220 Tg in 2050, while under the GMP scenario, they first drop to 300 Tg in 2030, then slightly increase to 320 Tg in 2050.

Preliminary results show that the interactive simulation slightly underestimates the observations on average by 2% between 1995-2022. All scenarios show an increase in the global CH₄ concentrations, from 1.8 ppm in the present-day CH₄ to 1.9 ppm (6%) in 2050 in the MFR scenario, 2.2 ppm (22%) in the CLE scenario, and 2.1 ppm (17%) in the GMP scenario. In addition, while anthropogenic CH₄ emissions decrease, all simulations predict increasing wetland CH₄emissions, by up to 10% in 2050 compared to 2020. Corresponding atmospheric CH₄ lifetimes also increase in all simulations from 8.4 years in 2020 to lowest 8.5 years in CLE, 9.2 years in MFR, and 9.4 years in GMP. The increasing CH₄ lifetime and concentrations in all scenarios despite reductions in emissions highlights that 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 due to other species such as CO and VOCs. In addition, missing sinks in ESMs such as halogens chlorine can lead to less chemical removal and longer lifetime compared to the box model.

We will further present the impact of these scenarios on the global surface temperatures and evaluate if the GMP will achieve its goal by 2050. However, preliminary results, compared with the recent 2021 AMAP SLCF assessment, suggest that despite the reduction in emissions, the atmospheric global CH₄ levels simulated in the present study may not fulfil the larger goals of the GMP such as decreasing global CH₄ concentrations and avoiding a 0.2°C warming by 2050 relative to 2020. However, reductions in emissions can still be achieved, which can lead to benefits in air quality and health. This work was accomplished through the Reduc(h4)e project funded by the Nordic Council of Ministers-and contributes to ongoing AMAP assessment work.