Concept of risk-aware contrail avoidance strategies

Targeted contrail avoidance consists of rerouting aircraft to minimise the formation of contrails whose warming of the climate system can be much larger than that due to the CO2 emitted for some of the flights. A commonly proposed strategy is to reroute all flights for which the trade-off between additional CO2 emissions and reduction in contrail warming leads to a climate benefit. However, current predictions of contrail climate impact are highly uncertain. In this study, we describe a framework to integrate the risk of unintentionally damaging the climate in the contrail avoidance decision-making process, using the Contrail Cirrus Prediction model (CoCiP) and operational ensemble weather forecasts. A first strategy consists in optimising trajectories around a best estimate of contrail radiative forcing, then using weather and parametric uncertainties to estimate the risk. In that case, 55 % of the reroutings have a higher-than-5 % risk of unintentionally damaging the climate compared to a standard risk-unaware avoidance strategy. This fraction increases to 76 % at the lowest risk tolerance level. However, the reroutings that are the least risky to operate are also those with the highest potential climate benefit, often referred to as “big hits”. Alternatively, accounting for uncertainties from the start of trajectory optimisation allows to mitigate the risk directly when planning the flight. This strategy would even result in a 52 % higher potential climate benefit compared to the risk-unaware avoidance strategy, at the lowest risk tolerance level. Our results thus demonstrate that the risk of unintentionally damaging the climate can and should be included in the decision-making of contrail avoidance, in particular in the context of early adoption policies.