Evaluating Fuel-Climate Tradeoffs in Contrail Avoidance

Abstract

Contrails, the line-shaped clouds that can form behind airplanes, have been estimated to be a major contributor to aviation-induced climate change. Operational contrail avoidance via flight re-routing may be an effective and efficient mitigation approach due to the geometry of contrail-forming regions. Contrail avoidance strategies will result in fuel burn penalties and, consequently, additional climate warming from carbon dioxide emissions; therefore, the climate benefit from avoiding contrails must be evaluated against the carbon dioxide penalties. Prior work has estimated the fuel burn and climate tradeoffs associated with contrail avoidance by focusing on a small set of routes or weather conditions, targeting only specific regions of the world, focusing on minimizing contrail length without quantifying the contrail impact, limiting deviations to horizontal or fixed altitude changes, or not using a fuel-optimal baseline for comparison. In this work, we evaluated the fuel-climate tradeoffs on a large scale by considering global coverage of flights with a full-year simulation accounting for daily and seasonal variation in meteorological conditions. We applied full altitude optimization of flight trajectories, focusing mainly on two contrail avoidance strategies: avoiding only nighttime or avoiding all contrails. The net climate impact of these strategies was evaluated by simulating individual contrail plumes and their radiative forcing impact, comparing trajectories to a fuel-optimal baseline in order to properly isolate the additional fuel requirements of contrail avoidance. We found that nearly 100% of contrail length can be avoided using vertical re-routing exclusively for a fuel burn penalty of 1.3—1.4% on the flights that perform maximum contrail avoidance. However, since only a fraction of the flights need to perform contrail avoidance, the fuel burn penalty averaged over the entire fleet was found to be 0.5% to avoid all contrails and 0.16% to avoid just the nighttime ones. A 5% limit on the fuel burn penalty per flight reduced the avoided contrail length to 97%, whereas limiting the per-flight fuel burn penalty to the mean value of 1.4% limited the reduction in contrail length to 70%. Regarding the climate impact of contrail avoidance strategies, we found that on the flights that applied contrail avoidance, the net climate impact was reduced by 93% for the sample avoiding all contrails and 92% for the sample avoiding only nighttime contrails. We found that on flights that formed contrails the energy forcing of contrails was an order of magnitude larger than that of carbon dioxide for a time horizon of 100 years. In terms of the overall net climate impact across the fleet, i.e., including flights without contrail avoidance, we found that by avoiding all contrails on flights that form them, the entire fleet’s net climate impact was reduced by 80%. On the other hand, by avoiding only nighttime contrails, the entire fleet’s net climate impact was reduced by 28%. Therefore, it is best to avoid all contrails unless avoidance decisions can be made on a per-contrail basis. The flight-by-flight distribution and the seasonal variation of the fuel-climate tradeoffs were also analyzed.