## 4 Discussion and Conclusions
Our decision to investigate future-emission based scenarios is reasoned by the aggregated analysis of the flight plan and the focus on long-term impact of an introduction of the ISO concept. By doing so, we can also incorporate the forecasted developments of the air transport sector [3; 40]. It needs to be considered that the estimation of climate response, especially for non-CO\({}_{2}\)-emission-induced effects in terms of RF and the resulting ATR, underlie large uncertainties [1; 5]. However, it has been shown that an assessment of mitigation options including variation in cruise altitudes can reliably be assessed with AirClim when referring to relative differences of the climate impact [30]. Moreover, we focused on an investigation of the general potentials of the climate-optimised ISO concept by providing the methodology and investigated the differences between fuel and climate-optimised set-ups. In this context, we concentrated our quantitative analysis on the parameters fuel consumption, emission quantities, route configurations and climate impact. Hence, a comprehensive analysis of the ISO concept's impact on the stakeholders of the air transport system is excluded. This comprises an investigation of changes in direct operating costs for the airlines operating on long-haul routes. Through higher fuel consumption and longer flight times in the climate-optimised case, these costs are expected to rise as well as CO\({}_{2}\) compensation expenses and possibly higher navigation and landing fees through intermediate stops. In addition, ISO is also expected to affect passengers' acceptance, as longer flight times increase travel duration and additional take-offs and landings reduce travel comfort. Additionally, local air quality and noise pollution are expected to deteriorate at selected refuelling airports. Besides these disadvantages, increasing the number of cycles can raise stress for flight crew and accident rates with the higher number of required take-offs and landings. Furthermore, lateral and vertical shifts of routes leads to increased traffic densities in certain areas and thus higher efforts for air-traffic management. The resulting implications for the stakeholders, such as airlines, airports, air traffic management, and passengers, require further investigation. The same holds true for a comparison of the achieved results with other measures, such as a sole adjustment of flight levels, to reduce the climate impact or climate-optimised trajectories. Furthermore, an optimisation of the concept considering economic implications as well as airline networks and expanding the concept with a replacement of aircraft optimised for shorter routes should be subject to further research following this study. All in all, we confirm the climate mitigation potential of ISO if the right set-up is selected. We contribute to current state of research by first, providing a methodology to characterise climate-optimised ISO, including selection of intermediate refuelling airport and flight altitudes that significantly contribute to a reduction in the climate impact. Second, we estimate the climate mitigation potential of ISO in terms of ATR100 reduction of up to 40% for long-haul flights compared to the non-stop reference case. Third, we compare the results from the climate-optimised ISO set-up with its fuel-optimised counterpart. In this course we find that climate-optimised ISO routes are on average conducted at lower latitudes and altitudes. Furthermore, climate-optimised ISO severely impacts non-climate parameters, such as fuel burn and flight time so that stakeholder interests are affected in course of implementation. To address this, eco-efficient operation set-ups can be derived that reduce fuel consumption and ATR at the same time. Conceptualisation, F.L., C.M.W. and Z.L.Z. ; methodology, K.D., F.L., C.M.W. and Z.L.Z. ; software, K.D., F.L., C.M.W. and Z.L.Z. ; validation, K.D., F.L., C.M.W. and Z.L.Z. ; data curation, C.M.W. and Z.L.Z. ; writing--original draft preparation, Z.L.Z.