Improved Simulation of Cold-season Methane Emissions over Alaska's Permafrost with the E3SM Land Model (ELM)

Field observations have shown that cold-season methane (CH₄) emissions contribute a substantial portion to annual carbon emissions in permafrost regions. However, current Earth System Model (ESM) land models generally underestimate cold-season CH₄ production and emission. We hypothesize that the biases result from poor representation of coupled biogeochemical and hydrological processes in permafrost soils, the lack of a reasonable lowland/wetland module that can adequately account for inundated hydro-ecological dynamics, underestimation of snow accumulation due to micro-topographic effects and thus the snow insulation to the ground, among others. Here we use the Energy Exascale Earth System Model (E3SM) land model (ELM) to examine how different hydrological and thermal regimes impact cold-season CH₄ emissions and to address the knowledge gap of the relative contribution of seasonal CH₄ emissions to annual totals over permafrost regions. Simulation results are evaluated against CH₄ measurements from the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) (2013 - 2014) and the Arctic Boreal Vulnerability Experiment (ABoVE) (2015 - 2017) at Alaskan Arctic tundra sites in the continuous permafrost zone. Enhancements to the simulation of cold-season CH₄ emissions were achieved via improving the representation of coupled water and heat transport with phase change in freezing soils. Specifically, improved simulation of the zero-curtain period (i.e., the period when soil temperatures linger around 0°C) results in better simulation of CH₄ production and emission. In addition, incorporation of a lowland module and microtopographic snow redistribution contributes to the improvement of cold-season CH₄ emission predictions. Sensitivity analysis of the CH₄ flux to critical biogeochemical, hydrological, and thermal variables help quantify the uncertainty in current simulation results. We also use ELM to evaluate how CH₄ emissions across permafrost regions would alter under future (warmer and wetter) climate scenarios.