Can we achieve significant climate mitigation by optimising only for contrails?

Abstract. Contrail-avoidance is a widely studied measure to reduce the non-CO₂ climate effect of aviation. However, some mitigation gain is likely to be compromised through increased emissions (CO₂, NO_x, and H₂O) and their climate effect. Here, we analyse the impact of contrail-optimised flights on the overall net climate benefit. We evaluate more than 4,000 flights with large contrail impact traversing Northern Europe in 2023, each with two trajectory options: the filed trajectory submitted to the network manager and a contrail-optimised trajectory, allowing only vertical deviations and a maximum fuel penalty of 2%. Our model setup is based on the European Unions's non-CO₂ monitoring, reporting and verification (MRV) framework, modelling contrails with the contrail cirrus prediction model (CoCiP) and employing the algorithmic climate change functions (aCCFs) for NO_x and H₂O effects. This paper highlights three points: First, while 93% of contrail-optimised flights emit more NO_x (on average +2.5%), they emit it at lower, less climate-sensitive altitudes, so only 65% of flights exhibit an increased NO_x climate effect (+1.1%, measured in efficacy-weighted global warming potential over 100 years, EGWP100), with considerable spatial and daily variability. Second, we find a risk of only 2% that the net climate benefit is not achieved when optimising for contrails alone, whereas using forecast weather data poses a far greater risk, causing optimisation to fail in ~15% of cases. The net climate benefit, however, is reduced from −16.5% to −11.1%, measured in EGWP100, when including NO_x and H₂O effects in the evaluation. Third, we find that the results are consistent to the choice of climate metric, with net climate benefit rates of −11.1%, −12.4%, and −16.2% obtained for EGWP100, the average temperature response over 100 years (ATR100), and EGWP20, respectively. We conclude that savings attributed to contrail-avoidance under a hard constraint on extra fuel and considering flights with large contrail impact only, exceed the NO_x and H₂O penalties. We do, however, recommend further research that accounts for model uncertainties, in order to determine whether the clear dominance of contrail over NO_x (and H₂O) effects identified here for contrail-optimised trajectories remains robust.