Regional and seasonal impact of hydrogen propulsion systems on potential contrail cirrus cover

The decarbonization of air transportation requires novel propulsion concepts in order to replace fossil kerosene powered gas turbines. Within various options, H₂ based propulsion is one of the most promising candidates, at least for regional and short haul routes. However, despite the potential to reduce CO₂ emissions to zero, those aircraft can still have a significant climate impact due to increased contrail formation caused by higher water emission when using H₂ as a propellant. In order to understand potential changes in the climate impact of H₂ powered air traffic, it is crucial to evaluate how the potential for contrail formation and the potential contrail cirrus cover would change under representative atmospheric conditions. To this end, we developed a tool which uses several years of meteorological reanalysis data (ERA-5 and MERRA-2) in combination with contrail formation conditions adjusted to H₂ gas turbine and H₂ Fuel Cell propulsion in order to investigate their regional and seasonal variation. Contrail formation conditions for three different propulsion settings (kerosene gas turbine, H₂ gas turbine and H₂ fuel cell) are calculated to obtain global statistics of potential contrail cover and potential contrail cirrus cover over 12 years. For H₂ based propulsion contrails are more likely to form due to the increased water vapor emission. However, this does not necessarily translate into the climatically relevant potential for contrail cirrus formation. Focusing on three hot spots of regional air traffic, we find that the difference between kerosene and H₂ scenarios has a strong systematic dependency on season, altitude and latitude. Maximum differences in potential contrail cirrus cover are found in the transition region from typically no-contrail to contrail forming conditions at a potential contrail cover around 50%. In contrast, less to no difference in potential contrail cirrus cover is found at very high (close to 100%) or rather low potential contrail cover. This study demonstrates, that the question whether H₂ powered air traffic produces more climate relevant contrail cirrus can not be parameterized by a simple factor but rather strongly depends on the propulsion type, season, region and flight altitude.