The increasing trend in warm-season net CO2 uptake outweighs that in cold-season emissions from the Alaskan Arctic tundra under a warming climate

The amplified Arctic climate warming has boosted both vegetation growth and ecosystem respiration over the Arctic tundra. Studies have revealed that the most severe Arctic climate warming has occurred during the cold season, enhancing cold-season soil heterotrophic respiration and thus releasing a significant amount of CO₂. Cold-season contributions of CO₂ emissions might largely offset the warm-season net uptake. However, most process-based terrestrial models, e.g., current land models of the Earth System Models (ESMs), poorly represent cold-season CO₂ emissions, making the estimations and predictions of regional-scale annual CO₂ budget over the Arctic tundra highly uncertain, or even contradictory. We first demonstrate that the Energy Exascale Earth System Model (E3SM) land model (ELMv1) reproduces the significant cold season CO₂ emissions from the Alaskan Arctic tundra as reported by two independently observation-constrained datasets (e.g., Commane et al., 2017; Natali et al., 2019). We then analyze the historical and predicted trends in both warm-season net CO₂ uptake and cold season net CO₂ loss under current (1950 - 2017) and future high-emission scenario (i.e., RCP 8.5) climate (2018 - 2100). We found that the warming trend of cold-season (Sep. to May) mean soil temperature is three times that of the warm-season (Jun. to Aug.) mean temperature, i.e., 0.06 °C yr^-1 vs. 0.02 °C yr^-1. Results further demonstrate a larger sensitivity of total warm-season NEP to soil temperature (24.9 gC m^-2 °C^-1) than the temperature sensitivity of cold-season total CO₂ loss (6.8 gC m^-2 °C^-1), mainly due to additional impacts on growing season CO₂ uptake, including changes in VPD, solar radiation, etc. Consequently, although cold season CO₂ emissions show a significant increasing trend of 0.64 gC m^-2 yr^-1 from 1950 to 2017, the warm season net CO₂ uptake has a larger positive trend of 0.74 gC m^-2 yr^-1. Under the RCP 8.5 scenario, the increasing rate of net CO₂ uptake during the warm season is more than twice the rate of cold season emissions from 2018 to 2100, i.e., 1.32 gC m^-2 yr^-1 vs. 0.50 gC m^-2 yr^-1, making this critical tundra ecosystem an annual CO₂ sink with a significant increasing trend at 0.87 gC m^-2 yr^-1. We conclude that with continued warming, the Alaskan Arctic tundra will continue to sequester carbon through the 21^st century.