Increasing levels of atmospheric CO2 will not only modify climate, they will also likely increase the water-use efficiency of plants by decreasing stomatal openings. The effect of the imposition of “doubled stomatal resistance” on climate is investigated in off-line simulations with the Biosphere–Atmosphere Transfer Scheme (BATS) and in two sets of global climate model simulations: for present-day and doubled atmospheric CO2, concentrations. The anticipated evapotranspiration decrease is seen most clearly in the boreal forests in the summer although, for the present-day climate (but not at 2 × CO2), there are also noticeable responses in the tropical forests in South America. In the latitude zone 44°N to 58°N, evapotranspiration decreases by −15 W m−2, temperatures increase by +2 K, and the sensible heat flux by +15 W m−2. Soil moisture is often, but less extensively, increased, which can cause increases in runoff. The responses at 2 × CO2 are larger in the 44°N to 58°N zone than elsewhere. Globa... Abstract Increasing levels of atmospheric CO2 will not only modify climate, they will also likely increase the water-use efficiency of plants by decreasing stomatal openings. The effect of the imposition of “doubled stomatal resistance” on climate is investigated in off-line simulations with the Biosphere–Atmosphere Transfer Scheme (BATS) and in two sets of global climate model simulations: for present-day and doubled atmospheric CO2, concentrations. The anticipated evapotranspiration decrease is seen most clearly in the boreal forests in the summer although, for the present-day climate (but not at 2 × CO2), there are also noticeable responses in the tropical forests in South America. In the latitude zone 44°N to 58°N, evapotranspiration decreases by −15 W m−2, temperatures increase by +2 K, and the sensible heat flux by +15 W m−2. Soil moisture is often, but less extensively, increased, which can cause increases in runoff. The responses at 2 × CO2 are larger in the 44°N to 58°N zone than elsewhere. Globa...