Anthropogenic emissions of greenhouse gases activate a number of feedback processes in the climate system. Scientists warn that some feedbacks can lead to abrupt and irreversible changes in climate dynamics, so-called tipping points. The threat of tip- ping points plays a major role in demands for aggressive emission reductions and for limiting warming to 2◦C, as agreed upon in the Copenhagen Accord. We extend a benchmark integrated assessment model of climate change to account for three inter- acting, irreversible tipping points: (i) a feedback increasing climate sensitivity, (ii) a feedback reducing carbon sink uptake, and (iii) a tipping point directly aﬀecting eco- nomic damages. Each tipping point is triggered by an imperfectly known temperature threshold. Optimal mitigation policy has to account for the impact of today’s emissions on future carbon stock, temperatures, and damages in diﬀerent possible futures where tipping points may have been triggered at diﬀerent temperature thresholds. Optimal mitigation today also has to incorporate how future decision makers will respond to current actions, observations, and possible tipping. We show that the presence of the three tipping points approximately doubles the currently optimal carbon tax and that the presence of multiple tipping points cuts the optimal peak temperature by approx- imately 1◦C. The eﬀects of the individual tipping points are approximately additive, suggesting that interactions among tipping points are not crucial for policy.
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