Abstract; ; The Arctic has experienced the largest increase in atmospheric CO; 2; seasonal cycle amplitude (SCA) since the 1960s. Previous studies primarily attributed this to enhanced regional warming. Here, we show that at numerous locations in the Arctic, atmospheric CO; 2; SCAs have increased at a relative rate similar to atmospheric CO; 2; abundance over the past five decades, suggesting a likely strong role of CO; 2; fertilization. Using a tagged‐tracer transport experiment and long‐term atmospheric observations of CO; 2; , carbonyl sulfide (COS), and the; 13; C:; 12; C ratio of CO; 2; (), we quantified high northern latitude (HNL, 53°–90°N) ecosystem process changes. Results suggest that HNL terrestrial ecosystems drive Arctic CO; 2; SCA increases, with gross primary productivity (GPP) exerting a more dominant impact than ecosystem respiration. Using atmospheric COS measurements and mass balance calculation, we estimated 2000–2020 growing‐season (May–August) average HNL stomatal conductance at 0.024 ± 0.004 mol m; −2; s; −1; . This exhibited a slight negative trend of −4.3 ± 3.8 (1) %, consistent with CO; 2; –induced reductions in stomata observed previously at smaller scales. However, accounting for regional leaf area expansion, leaf‐area‐integrated ecosystem conductance was estimated to be 0.044 ± 0.006 mol m; −2; s; −1; with a small positive trend of (1) %. Furthermore, observations suggest the growing‐season intercellular‐to‐ambient CO; 2; ratio in HNL ecosystems remained constant. Integrating these estimates, we estimate growing‐season GPP in HNL ecosystems averaged 33 ± 3 (1 ) PgC yr; −1; from 2000 to 2020 and increased by 12 ± 2 (1) % over this period, driven primarily by CO; 2; fertilization.;
