Managing Uncertainties in the Equilibrium Climate Sensitivity

Despite the great progress that climate science and our understanding of the climate system have made in the last three decades, significant uncertainties in our knowledge of the climate system and future climate change remain. In this talk I will focus on estimates for one climate variable, the equilibrium climate sensitivity (ECS), and examine how uncertainties in its estimates are treated in the most recent IPCC report AR6. AR6 supplements a framework of probability intervals (adopted from earlier reports) by in addition considering low-likelihood but high-impact “storylines”. I will argue in this paper that including such storylines represents an important improvement compared to prior reports. More specifically, I will argue that (i) in light of the deep uncertainties plaguing estimates of the values of central climate variables seemingly probabilistic assessments of these quantities ought to be read in a coarse-grained way and interpreted within a possibilistic fra!
mework as statements of ranked possibilities; (ii) given the high stakes involved, an appropriate decision framework for climate policy decisions ought to include broadly precautionary considerations that are sensitive to high-impact and catastrophic climate futures; (iii) storylines are a useful tool for probing possible high-impact climate futures.