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. Globally, the impact of imposing a doubled stomatal resistance in the present-day climate is an increase in the annually averaged surface air temperature of 0.13 K and a reduction in total precipitation of −0.82%. If both the atmospheric C02 content and the stomatal resistance are doubled, the global response in surface air temperature and precipitation are +2.72 K and +5.01% compared with +2.67 K and +7.73% if CO2 is doubled but stomatal resistance remains unchanged as in the usual “greenhouse” experiment. Doubling stomatal resistance as well as atmospheric CO2 results in increased soil moisture in northern midlatitudes in summer.