Seminar: Chunhui Zhan

Increasing atmospheric CO2 concentration influences the carbon assimilation rate and stomatal conductance of plants, thereby affecting global carbon and water cycles. However, detecting these physiological effects in observational data is challenging, due to natural variations and confounding factors (e.g., warming) that overshadow the effects of increasing CO2 in real-world ecosystems. To address these challenges, we first use a process-based land surface model to investigate the emergence of increasing CO2 effects on carbon and water cycles. We develop a statistical method to detect the change in CO2 concentration at which the long-term changes due to CO2 exceeds the interannual variability. We find that the signal in gross primary production (GPP) emerges at a relatively small CO2 increase (ΔCO2 ~ 20 ppm since 1901). Notably, the increasing CO2 effect emerges earlier in sites where plant productivity is not limited by climatic constraints, and earlier in forest-dominated ecosystems compared to grass-dominated ones.