Estimate of the climate impact of cryoplanes

The climate impact of cryoplanes (i.e. hydrogen-powered aircraft) is estimated in terms of radiative forcing (RF). We compare two scenarios: the conventional (i.e. kerosene-powered aircraft) scenario assumes a growth of the fleet until 2015 and a constant fleet thereafter. In the cryoplane scenario, the whole conventional fleet is instantaneously replaced by cryoplanes in 2015 and does not change thereafter. The resulting direct and indirect effects of aircraft emissions of H2O (direct RF and RF due to contrails) and NOx (RF due to O-3 production and CH4 destruction), as well as aviation-induced atmospheric CO2 are investigated for both scenarios. Whereas in case of conventional air traffic the components CO2, O-3, CH4 and contrails cause a similar RF, in case of cryoplanes, contrails are by far the most climatic relevant component. In 2015, total RF due to the cryoplane scenario is larger than total RF due to the conventional scenario. This changes with time and in 2100 the climate impact due to the cryoplane scenario is smaller. Due to large uncertainties regarding the optical properties of cryoplane contrails, the RF of contrails, and the impact of aviation on 'natural' cirrus, our present knowledge is not sufficient to decide whether a substitution of the conventional fleet by a fleet of cryoplanes is of environmental benefit. Further research is necessary in order to facilitate a more precise approach to this question. (C) 2001 Editions scientifiques et medicales Elsevier SAS.