A two-dimensional transport model constrained to measured surface CO2 concentrations was used to simulate the spatial and temporal variation of CO2 in the atmosphere for the period from 1975 to 2004. We find that the amplitude, phase and shape of the CO2 seasonal cycle vary as a function of both altitude and latitude. Cross tropopause exchanges, especially the downward branch of the Brewer-Dobson circulation, which brings stratospheric air to the upper troposphere at middle and high latitudes, change the CO2 concentration and seasonal cycle in the extra-tropics. The model results match recent aircraft measurements of CO2 in the upper troposphere (Matsueda et al., 2002) remarkably well. We conclude that upper tropospheric CO2 volume mixing ratios will provide a valuable tool for validating vertical transport. The implications of the CO2 variation caused by the stratosphere-troposphere exchange for remote sensing of CO2 are discussed.