The variability of surface temperature simulated by a global climate model with a simple mixed-layer ocean is analyzed. The simulated diurnal and seasonal ranges of temperature are compared with observation, as is the day-to-day and interannual variability of temperature. The qualitative changes in these quantities due to doubling atmospheric carbon dioxide concentration are also presented. The simulation of the seasonal cycle of surface temperature has a cold bias in much of the extratropics, including central Europe, even allowing for the difficulties in comparing grid-box surface temperatures with station temperature at screen height. The simulated diurnal range of temperature for present-day climate is similar to that observed, though the diurnal cycle in the model in midlatitudes is generally less than observed. On doubling C02, the diurnal range over land decrease by 0.3°C whereas mean temperatures increase by 6.3°C (global average over land). In CO2-doubling experiments with a one-dimensional radiative-convective model, atmospheric absorption by carbon dioxide and water vapor increases, reducing the solar heating at the surface, and surface evaporation increases faster with temperature than the transfer of sensible heat (due to the Clausius-Clapyeron relation), both of which tend to reduce the diurnal cycle. However, in the three-dimensional model, the diurnal cycle increases substantially where the snow line recedes, where the land surface becomes drier, or where there are substantial decreases in cloud cover. The diurnal cycle of surface temperature decreases where sea ice is replaced by open water because of the increase in thermal inertia of the surface. The simulated patterns of interannual standard deviation of surface temperature are in general aqreement with observations, except in high latitudes in winter, where the model values are larger than the observed, and over the tropical oceans where model values are smaller then observed. The changes on doubling CO2 are generally small and not statistically significant. There are, however, reductions along the sea-ice margins in winter and increases in some regions of northern midlatitudes in summer. On doubling C02, the general patterns of diurnal ranges and daily standard deviations of surface temperature change little, even though the changes in mean temperature am substantial and significant.