This paper evaluates the effect of diurnal variations of sea surface temperature (SST) and wind speed on the surface ocean CO2 partial pressure (pCO2sw) and sea-air CO2 flux. This is carried out using a combination of climatological and remote sensing data. The calculations show that the diurnal heating cycle can drive a diurnal pCO2sw variability which may cause the global ocean uptake of CO2 to be more than twice as large during night than during day. The effect of diurnal wind speed variations on the sea-air CO2 flux is restricted to the tropics. The concurrent variations of SST and wind speed on diurnal time scales bring around covariance terms that may contribute to the monthly mean flux. These were estimated and found to be negligible. Thus, this study validates the use of diurnally averaged fields for computation of sea-air CO2 fluxes.