Atmospheric Evidence of CO2 Fertilization Enhancing Arctic CO2 Seasonal Cycle Amplitude

The Arctic has experienced the largest increase in atmospheric CO₂ 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 CO₂ SCAs have increased at a relative rate similar to atmospheric CO₂ abundance over the past five decades, suggesting a likely strong role of CO₂ fertilization. Using a tagged-tracer transport experiment and long-term atmospheric observations of CO₂, carbonyl sulfide (COS), and the ¹³C:¹²C ratio of CO₂ (${\delta }^{13}C$), we quantified high northern latitude (HNL, 53°–90°N) ecosystem process changes. Results suggest that HNL terrestrial ecosystems drive Arctic CO₂ 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$\sigma$) %, consistent with CO₂–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 $6.3\pm 2.6$ (1$\sigma$) %. Furthermore, ${\delta }^{13}C$ observations suggest the growing-season intercellular-to-ambient CO₂ ratio in HNL ecosystems remained constant. Integrating these estimates, we estimate growing-season GPP in HNL ecosystems averaged 33 ± 3 (1 $\sigma$) PgC yr⁻¹ from 2000 to 2020 and increased by 12 ± 2 (1$\sigma$) % over this period, driven primarily by CO₂ fertilization.