Black carbon aerosol particles from fossil fuel combustion are good absorbers of solar radiation and hence exert a positive radiative forcing, reinforcing the warming due to anthropogenic increases in CO2 and other greenhouse gases. However, it is unclear how the climate sensitivity to black carbon aerosol forcing compares with the sensitivity to greenhouse gas forcing. Here we investigate this question using the HadSM4 configuration of the Hadley Centre climate model, extended by the addition of interactive black carbon and sulphate aerosol schemes. The results confirm earlier suggestions that the climate sensitivities are not necessarily similar and indicate that the black carbon sensitivity may be weaker. Possible reasons for this are explored by studying several feedback mechanisms operating in the model.