The importance of ‘extremely unlikely’ events: tail risk and the costs of climate change

In assessing the risks associated with climate change, ‘tail risks’ (low‐probability extreme events) often play a much larger role than their probability alone might indicate. There are three main reasons for this: the linear relationship between sensitivity and warming; the convexity of the damage function; and the concavity of the utility function. Ignoring the upper tail of the distribution of possible outcomes will result in serious underestimates of the social cost of carbon dioxide (CO₂) emissions and of the socially optimal price for emissions.