Publikationen

While the mean age-of-air, the time from entry into the stratosphere to any interior point, can be derived from trace gas observations, the mean residence time, the time from an interior point to its exit, is constrained only through rare events such as volcanic eruptions. Here we show that age-of-air and residence time are not independent but obey a compensation rule: their opposing latitudinal gradients cancel to produce near-uniform mean total transit times at each altitude. This uniformity reveals a previously unrecognised constraint within the Brewer-Dobson circulation, where rapid tropical ascent is necessarily balanced by prolonged interior residence, and vice versa. Exploiting this constraint, we infer global residence time fields directly from age-of-air observations and reproduce the observed residence time of the 2022 Hunga Tonga water vapour plume within published uncertainty ranges. Our framework transforms age-of-air, routinely measured by existing satellite networks, into a continuous observational constraint on stratospheric residence time. This opens a path to monitor whether the acceleration of stratospheric circulation under climate change shortens or prolongs the persistence of high-altitude emissions.

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.