[A-CG34] Projection and detection of global environmental change

[ACG34-07] Future projections of atmospheric methane loss using the MIROC-ES2L-CHEM earth system model.

*Takashi Sekiya¹, Tomohiro Hajima¹, Kengo Sudo^2,1, Yugo Kanaya¹, Michio Kawamiya¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Graduate School of Environmental Studies, Nagoya University)

Keywords:Methane, Atmospheric Chemistry, Earth system model

Short-lived climate forcers (SLCFs) including methane, ozone, and aerosols, have much shorter lifetimes than carbon dioxide, and their reductions are expected to contribute to short-term climate mitigation for achieving the Paris’ Agreement goals. Methane is a potent greenhouse gas, and its accurate projections require understanding the global methane budget. One of the remaining largest uncertainties in the global methane budget is the atmospheric chemical loss (Saunois et al., 2020), which is mainly determined through the reaction of methane with hydroxyl radical (OH). OH is a key species in tropospheric chemistry that are influenced by air pollutants and climate change. Diverse changes in OH are projected among different mitigation scenarios and climate models (Voulgarakis et al., 2013). In this study, we assess future changes in OH using the Model for Interdisciplinary Research on Climate, Earth System version 2 for Long-term simulations (MIROC-ES2L; Hajima et al., 2020) coupled with CHASER atmospheric chemistry module (Sudo et al., 2002) (hereinafter, MIROC-ES2L-CHEM), and compare them to the Aerosols and Chemistry Model Intercomparison Project (AerChemMIP) models. Using MIROC-ES2L-CHEM, tropospheric mass-weighted mean OH abundance was projected to be decreased by 9.0% from the present day (2005–2014) to the future (2046–2055) under the SSP3-7.0 scenario, which was a larger reduction than the AerChemMIP multi-model mean of 6.5±1.2%. Over the northern midlatitudes, future decreases derived from MIROC-ES2L-CHEM (by 14.2%) were larger than those in the AerChemMIP models (by 5.6±2.8%), while changes over Amazon ranged from -13.0% to 13.7% among different models. Furthermore, the sensitivity simulation for 2015–2055, prescribed by the present-day sea surface temperatures (SST), confirmed that OH decreases due to SLCF emission changes (by 15.0%) were partially cancelled out by OH increases due to climate change (by 6.0%). These emission-induced deceases and climate-induced increases in OH were larger than those in the AerChemMIP models (by -9.5±1.3% and 3.8±1.5%, respectively). These inter-model differences can partially be attributed to methane-OH feedback (feedback factor of 1.46 in MIROC-ES2L-CHEM in comparison to the AerChemMIP models’ estimates of 1.34–1.36), and climate—chemistry interactions through biogenic volatile organic compounds (BVOCs) emissions and lightning NOx production. Overall, these results suggest that further investigations are required to elucidate drivers of future changes in OH.