WEB APPENDIX Modeling Uncertainty in Integrated Assessment of Climate Change: A Multi-Model Comparison

This appendix explains the procedure for developing the pdf for climate sensitivity. The study began by reviewing the five probability density functions for equilibrium climate sensitivity (ECS) used in the IPCC AR5 that draw upon multiple lines of evidence. These are Aldrin et al. (2012), Libardoni and Forest (2013), Olsen et al. (2012), Annan and Hargreaves (2006), and Hegerl et al. (2006). The data underlying these pdfs is compiled for the IPCC AR5 and were shared with us. For each, we examined both the raw pdfs and fitted a log-normal pdf to each based on minimizing the sum of squared deviations. We found that the fit is extremely good for all of them and for most, it is nearly impossible to tell the difference between the log-normal fits and the raw data pdfs. Figure A1 illustrates the lognormal fits to each of these distributions.