Can we achieve significant climate mitigation by optimizing 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 compromised through increased emissions (CO2, NOx, and H2O) and their climate effect. Here, we analyse the impact of contrail-optimized flights on the overall climate impact mitigation, focusing on the contrail-NOx trade-off.

Our dataset includes 4112 flight-trajectory pairs, each containing the trajectory filed with the network manager and the vertically adjusted contrail-mitigation trajectory (<2% fuel penalty). They cover routes across Northern Europe and were sampled on 16 systematically selected days in 2023. We use an aircraft performance model (BADA 3.16) to estimate fuel flow and apply the Boeing fuel flow method and the ICAO emissions databank to derive NOx emissions. The climate effects of CO2, NOx-related O3 and CH4 as well as H2O are calculated using the aCCF model. The contrail climate impact is calculated with the CoCiP model.

Our results show that while 95.9% of contrail-optimized flights emit more NOx (on average +2.5%), they emit it at lower, less climate-sensitive altitudes, so only 64.7% (GWP100) to 67.9% (ATR20) of flights exhibit an increased NOx climate effect. Spatial and daily variability is considerable: oceanic airspaces show small increases in NOx climate effects, while land regions display larger deviation, including strong reductions. On days with fewer rerouted flights, the NOx climate effect is driven mainly by emissions. On days with more rerouting, it becomes more sensitive to meteorology and tropopause height.

Our ongoing research explores the impact of NOx and H2O climate effects on the overall mitigation gain. Initial results indicate that most contrail-optimized flights still achieve a net climate benefit, meaning that the savings due to contrail avoidance outweigh NOx and H2O penalties. Further analysis will examine this issue.