Climate impact of contrail cirrus from hydrogen combustion aircraft

Abstract. To mitigate the climate impact of aviation, combustion of hydrogen as a fuel is one possible future pathway. Hydrogen combustion leads to zero carbon emissions in the exhaust, representing a major step toward climate neutrality, although the non-CO₂ effects, primary 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).

Without soot in the exhaust from hydrogen combustion, contrail ice particles instead form on ambient aerosols entrained into the plume and on lubrication oil droplets in the exhaust. The formation of ice particles are modeled using an emulator developed from a theoretically based microphysical contrail formation model.

Following the Schmidt Applemann 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, including different oil particle size distributions and properties, with a global average reduction of about 70 % using our base case assumptions. We conclude that hydrogen aircraft not only eliminates CO₂ emissions but may also substantially reduce the climate impact of contrail cirrus, although the reduction and magnitude depend on engine design for lubrication oil handling.