Commercial supersonic aircraft may return in the near future, offering reduced travel time while flying higher in the atmosphere than subsonic aircraft, thus displacing part of the passenger traffic and associated emissions to higher altitudes. For the first time since 2007, we present a comprehensive multi-model assessment of the atmospheric and radiative effect of this displacement. We use four models (EMAC, GEOS-Chem, LMDz–INCA, and MOZART-3) to evaluate three scenarios in which subsonic aviation is partially replaced with supersonic aircraft. Replacing 4 % of subsonic traffic with Mach 2 aircraft that have a NOx emissions index of 13.8 g (NO2) kg−1 leads to ozone column loss of −0.3 % (−0.9 DU; model range from −0.4 % to −0.1 %), and it increases radiative forcing by 19.1 mW m−2 (model range from 16.7 to 28.1). This forcing is driven by water vapor (18.2 mW m−2), ozone (11.4 mW m−2), and aerosol emissions (−10.5 mW m−2). The use of a Mach 2 concept with low-NOx emissions (4.6 g (NO2) kg−1) reduces the effect on forcing and ozone to 13.4 mW m−2 (model range from 2.4 to 23.4) and −0.1 % (−0.3 DU; model range from −0.2 % to +0.0 %), respectively. If a Mach 1.6 aircraft with a lower cruise altitude and NOx emissions of 4.6 g (NO2) kg−1 is used instead, we find a near-net-zero effect on the ozone column and an increase in the radiative forcing of 3.7 mW m−2 (model range from 0.5 to 7.1). The supersonic concepts have up to 185 % greater radiative effect per passenger kilometer from non-CO2 emissions compared to subsonic aviation (excluding contrail impacts).

Hypersonic aircraft flying at Mach 5 to 8 are a means for traveling very long distances in extremely short times and are even significantly faster than supersonic transport (Mach 1.5 to 2.5). Fueled with liquid hydrogen (LH2), their emissions consist of water vapor (H2O), nitrogen oxides (NOx), and unburned hydrogen. If LH2 is produced in a climate- and carbon-neutral manner, carbon dioxide does not have to be included when calculating the climate footprint. H2O that is emitted near the surface has a very short residence time (hours) and thereby no considerable climate impact. Super- and hypersonic aviation emit at very high altitudes (15 to 35 km), and H2O residence times increase with altitude from months to several years, with large latitudinal variations. Therefore, emitted H2O has a substantial impact on climate via high altitude H2O changes. Since the (photo-)chemical lifetime of H2O largely decreases at altitudes above 30 km via the reaction with O(1D) and via photolysis, the question is whether the H2O climate impact from hypersonics flying above 30 km becomes smaller with higher cruise altitude. Here, we use two state-of-the-art chemistry-climate models and a climate response model to investigate atmospheric changes and respective climate impacts as a result of two potential hypersonic fleets flying at 26 and 35 km, respectively. We show, for the first time, that the (photo-)chemical H2O depletion of H2O emissions at these altitudes is overcompensated by a recombination of hydroxyl radicals to H2O and an enhanced methane and nitric acid depletion. These processes lead to an increase in H2O concentrations compared to a case with no emissions from hypersonic aircraft. This results in a steady increase with altitude of the H2O perturbation lifetime of up to 4.4±0.2 years at 35 km. We find a 18.2±2.8 and 36.9±3.4 mW m-2 increase in stratosphere-adjusted radiative forcing due to the two hypersonic fleets flying at 26 and 35 km, respectively. On average, ozone changes contribute 8 %-22 %, and water vapor changes contribute 78 %-92 % to the warming. Our calculations show that the climate impact, i.e., mean surface temperature change derived from the stratosphere-adjusted radiative forcing, of hypersonic transport is estimated to be roughly 8-20 times larger than a subsonic reference aircraft with the same transport volume (revenue passenger kilometers) and that the main contribution stems from H2O.