Climate impact of contrail cirrus from hydrogen combustion aircraft

One possibility for reducing the climate impact of aviation is to transition to aircraft powered by hydrogen combustion. Hydrogen combustion leads to zero CO 2 exhaust emissions and represents a potential major step toward reduced climate impact, although the non-CO 2 effects (primarily contrail cirrus) remain uncertain. In this study, we simulate the climate impact, in terms of energy forcing, of contrail cirrus from hydrogen combustion aviation, using a modified version of the Contrail Cirrus Prediction model (CoCiP). With no soot in the exhaust, contrail ice particles mainly form on ambient aerosols entrained into the plume and on lubrication oil droplets in the exhaust. The formation of ice particles is modelled using an emulator developed from a theoretically based microphysical contrail formation model. Following the Schmidt-Appleman criterion, hydrogen combustion enables contrail formation at lower altitudes and higher temperatures than fossil jet fuel. However, we find a significant reduction in contrail energy forcing. This result holds across a wide range of assumptions, with a global average reduction of about 66 % using our base case assumptions on ambient aerosols, lubrication oil properties, droplet size distribution, emission index and energy efficiency of hydrogen aircraft. We conclude that hydrogen aircraft not only eliminate CO 2 emissions in the exhaust, but may also reduce the climate impact of contrail cirrus, depending on engine design for lubrication oil handling. However, we acknowledge that the modelling approach has limitations and uncertainties. Before firm conclusions can be drawn about the contrail cirrus effects of hydrogen-combustion aircraft, further studies are needed. These should include more realistic hydrogen aircraft models, alternative meteorological and microphysical models, and analyses of real-world flight patterns. In particular, measurements of lubrication-oil emissions from hydrogen aircraft under cruise conditions, as well as measurements of ice crystal number concentrations in hydrogen-aircraft contrails, would be valuable for constraining the present analysis.