We have performed new calculations of the radiative forcing due to cha
nges in the concentrations of the most important well mixed greenhouse
gases (WMGG) since pre-industrial time. Three radiative transfer mode
ls are used. The radiative forcing due to CO2, including shortwave abs
orption, is 15% lower than the previous IPCC estimate. The radiative f
orcing due to all the WMGG is calculated to 2.25 Wm(-2), which we esti
mate to be accurate to within about 5%. The importance of the CFCs is
increased by about. 20% relative to the total effect of all WMGCT comp
ared to previous estimates. We present updates to simple forcing conce
ntration relationships previously used by IPCC.