Objective estimation of the probability density function for climate sensitivity

The size and impacts of anthropogenically induced climate change (AICC) strongly 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 degreesC, 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 temperature record, and a bootstrap technique to objectively estimate the probability density function for DeltaT(2x). We find that as a result of natural variability and uncertainty in the climatic radiative forcing, the 90% confidence 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.