Time series of the instantaneous radiative forcings for main anthropogenic and natural forcing agents from the year 1850 to 2000 are evaluated at the Earth's surface as well as at the tropopause with an atmospheric general circulation model. This evaluation corresponds to a simulation of 20th century climate with a synthetic coupled atmosphere-ocean general circulation model. The evaluation indicates that the positive radiative forcing at the tropopause rapidly increases from 1910 to 1950 and after 1970 principally due to long-lived greenhouse gases, while the negative radiative forcing at the surface sharply increases between 1955 and 1965 mainly due to the aerosol direct and indirect effects. This study suggests that a simultaneous analysis of changing rates of the radiative forcing both at the tropopause and surface can explain tendencies of changes in the surface air temperature.