Is climate change concerning because of its expected damages, or because of the risk that damages could be very high? Climate damages are uncertain, in particular they depend on whether the accumulation of greenhouse gas emissions will trigger a tipping point. In this article, we investigate how much risk contributes to the Social Cost of Carbon in the presence of a tipping point inducing a higher-damage regime. To do so, we decompose the effect of a tipping point as an increase in expected damages plus a zero-mean risk on damages. First, using a simple analytical model, we show that the social cost of carbon (SCC) is primarily driven by expected damages, while the effect of pure risk is only of second order. Second, in a numerical experiment using a stochastic Integrated Assessment Model, we show that expected damages account for most of the SCC when the tipping point induces a productivity shock lower than 10%, the high end of the range commonly used in the literature. It takes both a large productivity shock and high risk aversion for pure risk to significantly contribute to the SCC. Our analysis suggests that the risk aversion puzzle, which is the usual finding that risk aversion has a surprisingly little effect on the SCC, occurs since the SCC is well estimated using expected damages only. However, we show that the risk aversion puzzle does not hold for large productivity shocks, as pure risk greatly contributes to the SCC in these cases.

气候变化是否由于预期的损失而引起关注，还是因为损失可能很高的风险而引起关注？气候损害尚不确定，特别是取决于温室气体排放量的累积是否会触发临界点。在本文中，我们研究了在存在导致更高损害机制的临界点的情况下，有多少风险对碳的社会成本做出了贡献。为此，我们将临界点的影响分解为预期损害的增加加上损害的零平均风险。首先，我们使用简单的分析模型表明，碳的社会成本（SCC）主要由预期的损害驱动，而纯风险的影响仅是二阶的。其次，在使用随机综合评估模型的数值实验中，我们表明，当临界点引起的生产率冲击低于文献中通常使用的范围的上限10％时，预期的损害就占了SCC的大部分。巨大的生产力冲击和高风险规避对于纯风险而言都需要对SCC做出重大贡献。我们的分析表明风险厌恶之谜，这是通常的发现，即风险厌恶对SCC的影响出乎意料地很小，这是因为仅使用预期的损害就可以很好地估计SCC。但是，我们表明，风险厌恶之谜不适用于大规模的生产率冲击，因为在这些情况下，纯风险极大地影响了SCC。