Predicting CO2 and CH4 fluxes and their seasonal variations in a subarctic wetland under two shared socioeconomic pathway climate scenarios

The Arctic is undergoing a shift toward a warmer and wetter climate. Recent experiments indicate that the carbon balance of subarctic wet tundra is sensitive to both summer warming and deeper snow. However, few studies have combined experimental data with process-oriented models to predict how the terrestrial carbon cycle will respond to future climate change. Here, we use CoupModel, a process-oriented model, to investigate CO₂ and CH₄ dynamics in a subarctic wet tundra ecosystem under two contrasting climate change scenarios over the 21^st century. Our findings show that the model successfully reproduced the treatment effects of warming on CO₂ and CH₄ fluxes comparing to measurements from control, open top chambers and snow addition plots. For 2014–2020, the studied ecosystem functioned as a minor source of CH₄ and a neutral balance of CO₂, resulting in the overall greenhouse gas emissions of 10.5 ± 79.1 g CO₂-eq m^-2 yr^-1. The calibrated model was used to predict CO₂ and CH₄ fluxes and their seasonal variations under future climate scenarios. By 2100, a warmer climate could enhance the mean annual sink strength of CO₂ to 10.7 g C-CO₂ m^-2 yr^-1 under SSP126 (Shared Socioeconomic Pathway 1 and the radiative forcing level of 2.6 W m^-2) and 26.2 g C-CO₂ m^-2 yr^-1 under SSP585 (Shared Socioeconomic Pathway 5 and the radiative forcing level of 8.5 W/m²). However, increasing trends in the CH₄ budget were marginally small. The negligible response of CH₄ emission can be mainly explained by insignificantly wetter climate and limited soil C stock. For the radiative balance of the ecosystem, CO₂-equivalent flux of methane offset 78% of CO₂ sink in SSP126, and 31% in SSP585. Overall, the subarctic wet tundra transitions from being a source to a sink for greenhouse gases, excluding N₂O.