Bumping against a gas ceiling

The adoption of physical thresholds as a ceiling for permitted climate chan ge sidesteps contentious issues such as: policy cost, impact valuation, dis counting and equity. In this paper I offer some reflections on the concept of tolerable climate change. I also use an integrated climate assessment mo del (ICAM-3) to demonstrate how uncertainties in our understanding of socio economic and earth systems reduce the probability of success in keeping cli mate change within a pre-defined tolerable range. Finally, I explore the im plications of socioeconomic thresholds for welfare loss in pursuit of a cli mate policy (e.g., tax rebellions). Crossing such regional socioeconomic th resholds will lead to local failures to pursue climate change mitigation po licies - increasing the probability of straying beyond the tolerable window of global climate change. Given various uncertainties and the dynamics of the socioeconomic and the earth systems, the odds of success in staying wit hin a climate change window of Delta T less than or equal to 2 degrees C, a nd Delta T/yr less than or equal to 0.015 degrees C are estimated to be no higher than 25% over the next century. A risk-risk tradeoff approach appear s to hold promise, but while adoption of a larger window of tolerance incre ases the probability of success, it also opens the window specification cri teria to contention.