Current increase in atmospheric CO2 is expected to modify both climate and plant function, thereby impacting plant structure and gas exchange. We investigate the effects of doubled CO2 on leaf area index (LAI) and evapotranspiration (ETR) using a global vegetation model for present-day and doubled-CO2 conditions. The model assumes that adaptation of plants to the local climate leads to an equilibrium LAI, which depends on resource availability (minimizing water stress, canopy carbon cost and self-shading). The model compares reasonably well with remote sensing estimates of LAI. Four doubled-CO2 simulations are designed to investigate the role of climate, CO2-induced stomatal closure, enhanced photosynthesis, and a combination of these effects. These simulations show that plant physiological responses to doubled CO2 are potentially more important than climate changes for LAI, and equally important for ETR. In addition, even the sign of the simulated changes in LAI and ETR varies with the assumptions on plant responsiveness to CO2. A reduction of stomatal conductance moderates or cancels the water losses caused by a warmer and drier climate, but photosynthesis stimulation counteracts this stomatal effect, especially in the mid-to-high latitudes, because of enhanced LAI. Experimental evidence of LAI and ETR response to CO2 has been reviewed and compared to the different simulations. On the basis of this confrontation we argue that plant response to CO2 doubling may have a relatively small net impact on global ETR and may cause a moderate increase of LAI. Tree stomata may be less responsive to CO2 than was previously assumed, and stimulated plant growth partly cancels the water savings caused by stomatal closure. Understanding the responses of plant canopies to CO2 is therefore critical for land surface hydrology in a CO2 rich world.