Estimates from two 2-D (latitude-height) chemical transport models of changes in tropospheric ozone concentrations since pre-industrial times were used to calculate the radiative forcing due to these changes. The ozone changes are from Cambridge (CAMB) and the U.K. Meteorological Office (UKMO) models. The global and annual mean forcing using the UKMO changes was 0.3 Wm-2 and was 0.5 Wm-2 using the CAMB changes. The UKMO ozone changes are typically 60% of those from CAMB and this entirely accounts for the different forcing. Although the uncertainty is large, the calculations continue to support the case that tropospheric ozone changes make a substantial contribution (about 15%) to the total greenhouse gas radiative forcing. It is also shown that ignoring cloud and the effects of stratospheric adjustment in the radiative forcing calculations leads to an over-estimation of the forcing by about 66%, for the global average. ¿ American Geophysical Union 1996