We use an ``optimal detection'' methodology to compare observed near-surface temperature change over the twentieth century with spatio-temporal patterns simulated by a climate model. We find that anthropogenic increases in greenhouse gases and sulphate aerosols are largely responsible for the temperature change observed since 1945, rather than internal climate variability or changes in solar irradiance and volcanic aerosol. We also find that both the early century and late century warming can only be explained when anthropogenic factors are included. Using the strength of the greenhouse gas signal in recent observations, we estimate climate sensitivity to doubling CO$_2$ to be $2.6$K within a 5--95\% uncertainty range of $1.6$--$3.6$K. Our key results appear to be insensitive to details of the analysis procedure, but still depend on the accuracy of model-predicted patterns of change, the observational record and the simulation of internal climate variability