Objective estimation of the probability density function for climate sensitivity

The size and impacts of anthropogenically induced climate change (AICC) str ongly depend on the climate sensitivity, DeltaT(2x). If DeltaT(2x) is less than the lower bound given by the Intergovernmental Panel on Climate Change (IPCC), 1.5 degreesC, then AICC may not be a serious problem for humanity. If DeltaT(2x) is greater than the upper bound given by the IPCC, 4.5 degre esC, then AICC may be one of the most severe problems of the 21st century. Here we use a simple climate/ocean model, the observed near-surface tempera ture record, and a bootstrap technique to objectively estimate the probabil ity density function for DeltaT(2x). We find that as a result of natural va riability and uncertainty in the climatic radiative forcing, the 90% confid ence interval for DeltaT(2x) is 1.0 degreesC to 9.3 degreesC. Consequently, there is a 54% likelihood that DeltaT(2x) lies outside the IPCC range.