₂ for RCP4.5 were between 1.9% and 15.4% increase in NPP per 100-ppmv CO₂ before saturation, which is generally consistent with syntheses of free-air exchange CO₂ enrichment (FACE) experiments (mean of 13% globally) (29). Thus, there is the need for FACE experiments in the northern permafrost region to better constrain model responses to enhance atmospheric CO₂ in the region. In this study, the model with the least sensitivity (TEM6) was the only model in the sensitivity analysis for which C uptake was limited by plant N dynamics.