Stronger Arctic amplification produced by decreasing, not increasing, CO2 concentrations

Arctic amplification (AA), referring to the phenomenon of amplified warming in the Arctic compared to the warming in the rest of the globe, is generally attributed to the increasing concentrations of carbon dioxide (CO _2 ) in the atmosphere. However, little attention has been paid to the mechanisms and quantitative variations of AA under decreasing levels of CO _2 , when cooling where the Arctic region is considerably larger than over the rest of the planet. Analyzing climate model experiments forced with a wide range of CO _2 concentrations (from 1/8× to 8×CO _2 , with respect to preindustrial levels), we show that AA indeed occurs under decreasing CO _2 concentrations, and it is stronger than AA under increasing CO _2 concentrations. Feedback analysis reveals that the Planck, lapse-rate, and albedo feedbacks are the main contributors to producing AAs forced by CO _2 increase and decrease, but the stronger lapse-rate feedback associated with decreasing CO _2 level gives rise to stronger AA. We further find that the increasing CO _2 concentrations delay the peak month of AA from November to December or January, depending on the forcing strength. In contrast, decreasing CO _2 levels cannot shift the peak of AA earlier than October, as a consequence of the maximum sea-ice increase in September which is independent of forcing strength. Such seasonality changes are also presented in the lapse-rate feedback, but do not appear in other feedbacks nor in the atmospheric and oceanic heat transport processeses. Our results highlight the strongly asymmetric responses of AA, as evidenced by the different changes in its intensity and seasonality, to the increasing and decreasing CO _2 concentrations. These findings have significant implications for understanding how carbon removal could impact the Arctic climate, ecosystems, and socio-economic activities.