Trade-offs and Social Cost Estimates: Focus on CO2 and Contrail Cirrus

Governments worldwide are confronted with a need to curtail emissions of greenhouse gases to achieve the global climate targets outlined in the Paris Agreement. While carbon dioxide (CO₂) remains the primary greenhouse gas targeted in climate policies, it is important to address emissions of non-CO₂ forcers.

Trade-offs between CO₂ and other climate forcers are often determined based on Global Warming Potentials (GWP). An alternative approach is to use climate-economic approaches, estimating the social cost of different forcers. Here we focus particularly on aviation CO₂ emissions and contrail cirrus. Specifically, we explore how the social cost of contrail cirrus can be estimated using a revised version of the integrated assessment model DICE. We analyze contrail forcing from a flight-specific model that considers their spatio-temporal variability. Further, DICE has been revised in particular with respect to the geophysical model (being based on the emulator FaIR 2.0.0), and also with changes in the parameterization of the discounting and damage functions. Additionally, we examine how the social cost of short-lived forcers (contrail cirrus) and long-lived emissions (CO₂) is influenced by the discount rate and the future temperature pathway.

Concerning spatio-temporal variability, we observe that both energy forcing and the social cost of contrail cirrus are strongly dependent on flight specific conditions, including as a strong diurnal variability. Furthermore, we find that the comparison between the social costs of contrail cirrus and the social cost of CO₂ depends very strongly on the discount rate and the climate path the economy is following. This follows from the fact that the climate impacts of contrail cirrus are short lived, making their social cost less contingent than CO₂ on how future climate impacts are valued through the discount rate, and correspondingly less affected by the long-term changes in global mean surface temperature. 

We also explore the additional insights gained from analyzing the ratio of the social cost of contrail cirrus to the social cost of CO₂, beyond the information provided by analyzing the corresponding GWP values. Finally, the potential policy implications of the variability of the social cost of contrail cirrus are discussed.