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, ΔT_2x. If ΔT_2x is less than the lower bound given by the Intergovernmental Panel on Climate Change (IPCC), 1.5°C, then AICC may not be a serious problem for humanity. If ΔT_2x is greater than the upper bound given by the IPCC, 4.5°C, 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 ΔT_2x. We find that as a result of natural variability and uncertainty in the climatic radiative forcing, the 90% confidence interval for ΔT_2x is 1.0°C to 9.3°C. Consequently, there is a 54% likelihood that ΔT_2x lies outside the IPCC range.