The Implications of Intermediate Stop Operations on Aviation Emissions and Climate

Among the various transport modes aviation's impact on climate change deserves special attention. Due to typical flight altitudes in the upper troposphere and above, the effect of aircraft engine emissions like e.g. water vapour, nitrogen oxides and aerosols on radiative forcing agents is substantial. The projected doubling of aircraft movements in the next 15 years will lead to an increase of aviation's impact on climate and requires immediate mitigation options. Besides technological measures also new operational strategies are widely discussed; one of these concepts which has been subject of several studies in the past is Intermediate Stop Operations (ISO). It is based on the idea to reduce the stage length of flights by performing one or more intermediate landings during a mission. Here, we analyse the ISO concept by combining different models, which include a realistic traffic simulation taking into account operational constraints and ambient conditions, like e.g. wind, the calculation of engine emissions and the integration of a climate response model. We analyse the ISO concept for today's worldwide aircraft fleet, including its influence on global emissions distributions as well as the impact on climate change by taking into account CO2 and non-CO2 effects, arising from contrail-cirrus, water vapour and nitrogen oxide emissions. We show in agreement with earlier findings that due to shorter flight distances the amount of fuel burnt over the mission can be reduced by roughly 5% on average globally. For the first time, we quantify the climate impact of ISO, where the flight trajectory is optimised for fuel use and the aircraft is not redesigned for the ISO procedure. We find an increased warming effect, which arises from nitrogen oxide and water vapour emissions, which are released at higher cruise altitudes and which over-compensate reduced warming effects from CO2 and contrail-cirrus. However, we expect a climate impact reduction for ISO even with existing aircraft, avoiding the higher flight altitude in the first flight segment and hence reducing the fuel savings. Thus, climate impact benefits could be achieved if lower fuel savings were acceptable. Moreover, this negative climate impact is found for the particular case of introducing ISO using the current wide-body fleet. It does not necessarily apply to the adoption of ISO using aircraft redesigned for a shorter range.