Attempts are made to estimate the temperature changes resulting from increasing the present CO2 concentration by the use of: (a) a one-dimensional radiative convective equilibrium model and, (b) a simplified three-dimensional general circulation model. The following assumptions are made in the 3-D model: a limited computational domain, an idealized topography, zero surface heat capacity, no heat transport by ocean currents and an annual mean insolation. In general, the CO2 increase raises the temperature of the model troposphere, whereas, it lowers that of the model stratosphere for both the 1-D and 3-D models. It is found that the tropospheric warming is somewhat larger for the 3-D model as compared with that obtained from the 1-D radiative convective equilibrium model. In particular, the increase of surface temperature in the 3-D model in high latitudes is magnified due to the recession of the snow boundary and the thermal stability of the lower troposphere which limits convective heating to the lowest layer. It is also found that increasing the CO2 concentration significantly increases the overall intensity of the hydrologic cycle of the 3-D model. However, this does not necessarily imply the increase of wetness everywhere in the model region. In particular, the sign of wetness change depends upon the geographical location within the model domain.