Selecting a suitable climate metric for aviation policy and aircraft design

Abstract

A growing body of research has highlighted the major contribution of aviation non-CO2 emissions to anthropogenic climate change. The EU Parliament has expressed interest in regulating aviation non-CO2 emissions and has recently adopted legislation to revise the EU Emissions Trading System (ETS) for this purpose. This regulation requires the use of a climate metric, which relates non-CO2 emissions to their consequences on the climate and on society. However, establishing an adequate equivalence is challenging due to the high uncertainties of aviation non-CO2 emissions, their variability in atmospheric lifetimes and their dependence on emission location and altitude. Furthermore, as a result of their calculation methods, all climate metrics have hidden trade-offs and can inadvertently favour certain aircraft design choices, emission species or effects. The choice of climate metric is thus a key consideration for all stakeholders, because it can influence aircraft design choices and ultimately impact the success of aviation climate policy implementation. There is, however, currently no consensus on which climate metric is most appropriate. In this presentation, we explore the neutrality, temporal stability, compatibility and transparency of existing, physical climate metrics and find the Average Temperature Response (ATR) to be the most suitable for aviation policy and aircraft design. Determining the right time horizon for the ATR remains a challenge, although for certain combinations of climate objectives and emissions scenarios an optimum can be found. We find that the choice of time horizon becomes more important at lower time horizons and that the ATR generally requires larger time horizons to properly account for the atmospheric temperature adjustment. Finally, we find the GWP* to be unstable and potentially misleading to policymakers, and therefore do not recommend its use in aviation policy.