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

Le résumé fourni par la source

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, NOx, 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 NOx and H2O effects. This paper highlights three points: First, while 93% of contrail-optimised flights emit more NOx (on average +2.5%), they emit it at lower, less climate-sensitive altitudes, so only 65% of flights exhibit an increased NOx 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 NOx 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 NOx 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 NOx (and H2O) effects identified here for contrail-optimised trajectories remains robust.

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