Atmospheric Evidence of CO 2 Fertilization Enhancing Arctic CO 2 Seasonal Cycle Amplitude
Lei Hu, Stephen A. Montzka, John B. Miller, Pieter P. Tans, Aleya Kaushik, Jin Ma, Maarten Krol, Marine Remaud, Kenneth Schuldt, Sylvia E. Michel, Colm Sweeney, Yanlan Liu, Isaac Vimont, Andrew Crotwell, Bradley Hall, Min Chen, Dan YakirAbstract
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.