This talk by Prof Stephan Lewandowsky is part of the fortnightly BRIDGE seminar series.

Abstract

Uncertainty forms an integral part of climate science, and appeals to uncertainty are often used in public debate to argue against mitigative action. To date, the scientific response to those arguments has been relatively muted. I present an analysis of the impact of climate uncertainty that is robust to a range of assumptions. The analysis is conducted at an ordinal level, that is at the level of questions such as “greater than” or “less than.” I show that increasing uncertainty is necessarily associated with greater expected damages from warming, provided the function relating warming to damages is convex. This constraint is unaffected by subjective or cultural risk-perception factors, it is unlikely to be overcome by the discount rate, and it is independent of the presumed magnitude of climate sensitivity. The analysis also extends to "second-order" uncertainty; that is, situations in which experts disagree. Greater disagreement among experts increases the likelihood that the risk of exceeding a global temperature threshold is greater. Likewise, increasing uncertainty requires increasingly greater protective measures against sea level rise. This constraint derives directly from the statistical properties of extreme values. Any appeal to uncertainty thus compels a stronger, rather than weaker, concern about unabated warming than in the absence of uncertainty.