The climate impact of contrails from hydrogen combustion and fuel cell aircraft

Aviation has been under increasing pressure in recent years to substantially cut its impact on climate. The International Air Transport Association (IATA) committed in October 2021 to achieve net-zero carbon emissions by 2050. Other similar ambitious targets have been set for aviation, all relying strongly on alternative fuel aircraft such as hydrogen combustion or fuel cells. A significant (i.e. 60%) proportion of the current aviation contribution to global warming is caused by non-CO₂ effects, with contrail cirrus the largest of these effects. Here we perform and analyse the first calculation of the contrail cirrus effective radiative forcing (ERF) for fuel cell powered aircraft and one of the first calculations for liquid hydrogen combustion aircraft, comparing them with estimates for kerosene and sustainable aviation fuel (SAF).

The exhaust mix of the hydrogen combustion in an aircraft gas turbine or hydrogen fuel cell aircraft is different to that of the current hydrocarbon fuels. This leads to a potential climate penalty due to the significant increase in associated water vapour emissions. We find that for liquid hydrogen combustion and fuel cell powered planes, the area of the globe covered by contrails is set to increase substantially (~70%) due to their additional water vapour emissions leading to more regions of the atmosphere becoming susceptible to contrail formation. However, the expected cleaner exhaust and corresponding increase in average contrail particle sizes lead to changes in contrail radiative properties. These changes result in a reduction in contrail cirrus ERF for liquid hydrogen combustion (~25%) and fuel cell (~20%) powered planes, compared to kerosene planes. SAF planes are expected to lead to a slight (~5%) increase in contrail cover, but a decrease (~20%) contrail cirrus ERF, compared to kerosene planes.

In general, our analysis finds that changes in contrail cirrus ERF are relatively modest between fuel types, compared to the overall uncertainty in the ERF estimate itself. Some of this uncertainty stems from our representation of physical processes in contrails and further work is needed to look at the effects of alternative fuels on contrail ice crystal sizes, contrail lifetimes and aerosol-cloud interactions.