We study contrail formation behind future fuel cell-based aircraft fueled by hydrogen. Simulations show that, under certain conditions, small droplets can form via a process not occurring in current plumes. This can increase ice crystal numbers in contrails, boosting their climate impact. The effect is strongest in cold air and for high plume mixing rates with air. We suggest to decrease the mixing rate and exhaust humidity by technical design to reduce the climate impact of such aircraft.

Model simulations were used to study all air traffic over Northern Europe during 16 days in 2023, following the European Union's planned framework (using CoCiP and aCCFs) for monitoring aviation’s non-CO₂ climate effects. Small changes in flight altitude reduced the climate impact of contrails by 48 % while increasing the fleet-wide fuel use by only 0.4 %. The changes had only limited effects on air traffic density and aircraft spacing, showing that climate-optimized flight planning is feasible.

Sustainable aviation fuels (SAFs) reduce the climate effect of contrail-induced cirrus cloudiness (CiC). We assess the climate-optimal SAF distribution for flights departing from Copenhagen Airport in 2030. We determine the potential climate benefit of 6 % available SAF by allocating it to flights with climatologically largest CiC climate effect to fuel use ratio. Targeted SAF use can here increase the CO2e benefit by 60 kt and enhance the CiC share in climate benefit from 20 % to almost 50 %.

This study presents a multi-physics Eulerian framework for simulating the long-term evolution of aircraft contrails, a major contributor to aviation-induced warming. The model integrates advection, diffusion, ice-particle growth, turbulent settling, and habit evolution through a system of PDEs. Computationally efficient and calibratable against observations, it is well suited for large-scale contrail and radiative forcing simulations.

Commercial flights usually follow routes that minimise travel time and fuel use, but these routes are not always best for the climate. Using simulations of 4,000 flights forming condensation trails over northern Europe in 2023, we show that small altitude changes can reduce climate impact by 11 to 16% for only 1.2% more fuel. The benefits of avoiding warming from condensation trails outweigh the additional climate effect from nitrogen oxides and water vapour emissions.

Aviation organisations have set varied climate goals, but misalignment, ambiguity and internal inconsistencies hinder real progress. This paper addresses this by proposing a comprehensive definition of a climate neutral air transport system: “air transport system of which the climate effects of all its greenhouse gases and non-CO₂ effects throughout the entire life cycle of each element of the system is balanced”.

Contrails are thin ice clouds that may form behind aircraft, are responsible for about half of aviation's climate impact, and are hard to detect and predict. Using data from two flight test campaigns, we developed a tool that can be used with a camera mounted on aircraft to detect contrail formation and predict persistence in real time, with camera placement strongly affecting results. Using only this tool and onboard cameras, this lower-cost approach could help reduce aviation's climate impact.