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This paper contributes to the normative literature on mitigation and adaptation by framing the question of their optimal policy balance in the context of catastrophic climate risk. The analysis uses the WITCH integrated assessment model with a module that models the endogenous risk of experiencing an economic catastrophe if temperature increases above a certain threshold. We find that the risk of a catastrophic outcome would encourage countries to reduce emissions even in the absence of a coordinated global agreement on climate change and to realign the policy balance from adaptation toward more mitigation. Our analysis also shows that adaptation transfers from and strategic unilateral commitments to adaptation in developed countries appear to provide weak incentives for reducing emissions in developing countries. Thus our first conclusion is that precautionary considerations, rather than the ability to reduce smooth damage increases, justify mitigation as a fundamental policy option. Accordingly, adaptation is needed to cope with the non-catastrophic damages that countries would fail to address with mitigation Our second conclusion is that supporting adaptation in developing countries should be considered primarily as a mean for ensuring equity or improving development, and very marginally as a mitigation incentive.

• jel: C61, D58, Q5