Aviation emission from fossil fuel-based combustion engines are adversely contributing to global climate change. Based on the 2015 Paris Agreement of limiting global warming to 1.5°C, zero Greenhouse Gases (GHG) are to be emitted by 2050. While aircraft propulsion and aerodynamics technology improvements are achieved, aviation growth and related GHG emissions are outpacing these by 2.5% per year. Continuing the business-as-usual scenario leads to a significant emission gap by 2050. In this thesis, alternative aircraft propulsion concepts such as bio- and synthetic fuels, electric aircraft and liquid hydrogen fuelled aircraft are demonstrated and compared based on assorted criteria. The research methods applied are a quantitative comparison by Levelized Cost of Mobility (LCOM), as well as a qualitative technology selection by Analytic Hierarchy Process (AHP). From a cost perspective, bio- and synthetic fuels are to reduce emission of aviation on short term once carbon costs increase. By 2020, kerosene fuelled combustion engines remain the least cost option. By 2035, first all-electric aircraft with zero emission are ready to be deployed on regional and short routes, however, they are infeasible with current and projected future battery densities on routes >1667 km. From 2050 on, liquid-hydrogen aircraft with close to zero emissions can contribute to lowering GHG emissions, especially on long ranges. Following overall sustainability criteria such as cost, emission, readiness and safety, electric propulsion is the most recommended option to close the emission gap of aviation, followed by bio- and synthetic fuels and hydrogen fuelled aviation.