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.

Lessening of non-CO₂ aviation impacts on climate is often accompanied by an increase of fuel consumption and emissions. Planning mitigation actions requires balancing the desired climate benefit against inevitable simultaneous climate damage. Due to uncertainty, statistical methods are needed to estimate the failure risk of concrete mitigation measures. This paper presents the required mathematics and assumptions, and points to research needs and alternative risk-aware strategies.

Aviation non-CO₂ climate effects could be reduced through rerouting using Climate Change Functions (CCFs) and its surrogate aCCFs. This study expands CCFs regionally and seasonally and enables a comparison with aCCFs. ACCFs simplify complex processes but reproduce magnitudes and most gradients, though they underestimate variability compared to detailed CCF simulations and limitations to certain altitudes were revealed. The present study promotes future development of refined and extended aCCFs.

This randomized control trial provides evidence for the efficacy of airline-led contrail avoidance to reduce aviation's climate impact. By integrating ML forecasts into standard flight planning, dispatchers routed flights to avoid warming contrails. Satellite validation showed an 11.6 % overall reduction in contrails, increasing to 62 % for flights strictly following the optimized paths, all with no significant fuel penalty.

Operational strategies such as eco-efficient flight routing have potential to reduce aviation’s climate effect. A collaborative workflow integrating aviation weather forecasting, flight planning, air traffic control, and climate benefit assessment was developed and tested in D-KULT. Innovative developments demonstrate substantial progress on how to identify alternative trajectories but also highlight remaining challenges, including uncertainties in weather forecast and non-CO₂ climate effects.