CO2 equivalences for short-lived climate forcers

Abstract

With advancing climate change there is a growing need to include short-lived climate forcings in cost-efficient mitigation strategies to achieve international climate policy targets. Tools are required to compare the climate impact of perturbations with distinctively different atmospheric lifetimes and atmospheric properties. We present a generic approach for relating the climate effect of short-lived climate forcers (SLCF) to that of CO2 emissions. We distinguish between three alternative types of metric-based factors that can be used to derive CO2 equivalences for SLCF: based on forcing, activity and fossil fuel consumption.We derive numerical values for a wide range of parameter assumptions and apply the resulting generalised approach to the practical example of aviation-induced cloudiness. The evaluation of CO2 equivalences for SLCF tends to be more sensitive to SLCF specific physical uncertainties and the normative choice of a discount rate than to the choice of a physical or economic metric approach. The ability of physical metrics to approximate economic-based metrics alters with changing atmospheric concentration levels and trends. Under reference conditions, physical CO2 equivalences for SLCF provide sufficient proxies for economic ones. The latter, however, allow detailed insight into structural uncertainties. They provide CO2 equivalences for SLCF in short term strategies in the face of failing climate policies, and a temporal evolution of CO2 equivalences over time that is noticeably better in line with cost-efficient climate stabilisation.