Stomatal conductance shapes the exchange of water and carbon of vegetated land surfaces. Previous studies have demonstrated that optimized stomatal functioning that maximizes productivity provides a realistic description of how stomata operate. Here I investigate the role of optimum stomatal functioning for the sensitivity of terrestrial productivity and land surface climate to concentrations of atmospheric carbon dioxide (pCO ₂). I conduct sensitivity simulations with a coupled vegetation-climate system model with different values of maximum stomatal conductance at different prescribed levels of pCO ₂. The optimum in stomatal conductance shifts to lower values with increasing pCO ₂, which is consistent with observed sensitivities of stomatal density of leaves. If this change in optimum conditions is not taken into account, the climate sensitivity shows (1) a general underestimation of terrestrial productivity under altered pCO ₂, and (2) different sensitivities of key climatic variables to pCO ₂. The climate sensitivity of land temperature for a doubling of pCO ₂ ranges from ΔT = 2.7 K to ΔT = 3.2 K, depending on whether stomata adapt optimally or not at all. These results demonstrate that the assumed ability of vegetation to adapt to its environment can have important consequences for the simulated climate system sensitivity to pCO ₂.