Evidence of decadal climate prediction skill resulting from changes in anthropogenic forcing

It is argued that simulations of the twentieth century performed with coupled global climate models with specified historical changes in external radiative forcing can be interpreted as climate hindcasts. A simple Bayesian method for postprocessing such simulations is described, which produces probabilistic hindcasts of interdecadal temperature changes on large spatial scales. Hindcasts produced for the last two decades of the twentieth century are shown to be skillful. The suggestion that skillful decadal forecasts can be produced on large regional scales by exploiting the response to anthropogenic forcing provides additional evidence that anthropogenic change in the composition of the atmosphere has influenced the climate. In the absence of large negative volcanic forcing on the climate system (which cannot presently be forecast), it is predicted that the global mean temperature for the decade 2000-09 will lie above the 1970-99 normal with a probability of 0.94. The global mean temperature anomaly for this decade relative to 1970-99 is predicted to be 0.35 degrees C with a 5%-95% confidence range of 0.21 degrees-0.48 degrees C.