A Holistic View of Climate Sensitivity

The notion of climate sensitivity has become synonymous with equilibrium climate sensitivity (ECS), or the equilibrium response of the Earth system to a doubling of CO 2 . But there is a hierarchy of measures of climate sensitivity, which can be arranged in order of increasing complexity and societal relevance and which mirror the historical development of climate modeling. Elements of this hierarchy include the well-known ECS and transient climate response and the lesser-known transient climate response to cumulative emissions and zero emissions commitment. This article describes this hierarchy of climate sensitivities and associated modeling approaches. Key concepts reviewed along the way include climate forcing and feedback, ocean heat uptake, and the airborne fraction of cumulative emissions. We employ simplified theoretical models throughout to encapsulate well-understood aspects of these quantities and to highlight gaps in our understanding and areas for future progress. ▪ There is a hierarchy of measures of climate sensitivity, which exhibit a range of complexity and societal relevance. ▪ Equilibrium climate sensitivity is only one these measures, and our understanding of it may have reached a plateau. ▪ The more complex measures introduce new quantities, such as ocean heat uptake efficiency and airborne fraction, which deserve increased attention.