The Paul Crutzen Publication Prize is awarded annually by the journal Atmospheric Chemistry and Physics for outstanding publications. Starting this year, the shortlist of publications for the prize will be co-published. The publication containing the findings on the climate impact of hypersonic air traffic had made it onto this "shortlist" consisting of six publications.
The idea of covering long distances (Fig. 1) in a few hours (an order of magnitude shorter compared to current civil aviation) seems fascinating for the aviation industry as it is associated with promising business models. However, the constantly advancing climate change requires a drastic reduction of climate impact also in civil aviation. In addition to improving aircraft and operational efficiency, new innovative concepts are considered. In this context, the drastic reduction in travel time through significantly higher flight speeds and the associated potential increase in climate impact were investigated. Different aircraft concepts for hypersonic transport were assessed regarding their technology options and environmental impact in the EU projects STRATOFLY und MORE&LESS.
To make such approaches viable for the future, two different aircraft (ZEHST und LAPCAT) were examined in more detail to better understand their climate impact. Both concepts use liquid hydrogen as fuel, subject to a climate-neutral production. The aircraft speed is Mach 5 (~6000 km/h) resp. Mach 8 (~9500 km/h) and the cruise altitude is 25 resp. 35 km, i.e. in the stratosphere, far above daily weather, while emitting about 18 resp. 21 Tg of water vapor per year at this altitude (1 Tg = 1 gigaton).
Fig. 2: Atmospheric water vapor increase from emissions from potential hypersonic fleets. The natural water vapor background is around 4000-5000 ppbv (=4-5 water vapor particles per 1 million air particles). Results of two climate chemistry models (above/below) for two different aircraft concepts (right/left) are shown (from Pletzer et al. 2022; CC BY 4.0).
Two different climate-chemistry models were used to investigate the effect of these water vapor emissions on the atmosphere (Fig. 2). It shows that the dry stratosphere becomes significantly more humid due to the emissions from these aircraft and that the accumulation of water vapor is significantly higher at higher altitudes. These results are pretty startling, since at these altitudes water vapor is subject to fast photochemical depletion and accumulation should be low. The models do confirm this depletion, but the recombination of the loss products leads to water vapor again, and at the same time an increase in methane oxidation is found, which also leads to the production of water vapor. These chemical processes (Fig. 3) overcompensate the original photochemical water vapor depletion at these high altitudes. This clearly showed that flight altitude is a crucial factor for water vapor accumulation in the stratosphere, having a significant influence on the climate impact of potential hypersonic fleets. A more detailed investigation has shown that the climate impact is about 10 to 20 times higher than that of a representative subsonic aircraft. These important findings are used in the EU project MOREandLESS to significantly minimize the climate impact by choosing lower flight altitudes and other fuels.
Fig. 3: Illustration of the increase of recombination of chemical trace gases into water vapor for two different hypersonic aircraft concepts (from Pletzer et al. 2022; CC BY 4.0).
 Zero Emission High-Speed Transport
 Long-Term Advanced Propulsion Concepts and Technologies
Press release of the TU Delft
ACP Paul Crutzen Publication Award