Abstract
The adoption of physical thresholds as a ceiling for permitted climate change sidesteps contentious issues such as: policy cost, impact valuation, discounting and equity. In this paper I offer some reflections on the conept of tolerable climate change. I also use an integrated climate assessment model (ICAM-3) to demonstrate how uncertainties in our understanding of socio-economic and earth systems reduces the probability of success in keeping climate change within a pre-defined tolerable range. Finally, I explore the implications of socio-economic thresholds for welfare loss in pursuit of a climate policy (e.g. tax rebellions). Crossing such regional socio-economic thresholds will lead to local failures to pursue climate change mitigation policies - increasing the probability of straying beyond the tolerable window of global climate change. Given various uncertainties and the dynamics of the socio-economic and the earth systems, the odds of success in staying within the climate change window of T<2°C, and T/yr<0.015°C are estimated to be no higher than 25% over the next century. A risk-risk tradeoff approach appears to hold promise, but while adoption of a larger window of tolerance increases the probability of success it also opens the window specification criteria to contention.