Global climate modelling of contrails cirrus from current and alternative fuel aircraft

Contrail cirrus is estimated to be responsible for the largest and the most uncertain aviation effective radiative forcing (ERF) term. With the aviation sector’s commitment to reach net-zero emissions by 2050, there is a stringent need to better understand and constrain this term. A key challenge in reducing its associated uncertainty comes from the very limited number of climate models able to simulate contrail cirrus.

In this work we present results from two new contrail cirrus parameterisations for the UK Met Office Unified Model (UM), one based on the existing contrail scheme within the Community Atmosphere Model (CAM) and the other based on the prognostic contrail scheme developed for the ECHAM model. We find substantial differences in the simulated contrail coverage (up to a factor of 3) and radiative forcing (up to a factor of 8) caused by model differences in ice supersaturation and cloud microphysics schemes, together with existing uncertainty in contrail cirrus optical depth. Using the CAM model, we also quantify the change in the contrail cirrus climate impact due to switching to alternative fuels, such as sustainable aviation fuel (SAF), liquid hydrogen, and fuel cells. We find that the use of liquid hydrogen and fuel cells will likely lead to a substantial increase (up to 70%) in contrail cover compared to kerosene and SAF. However, our simulations indicate that despite this increase in coverage, the reduction in aerosol emissions associated with alternative fuels will lead to an overall reduction in contrail cirrus ERF.

We suggest that future work should focus on better constraining contrail cirrus optical properties (in particular for alternative fuels), and on improved representation of ice supersaturation and contrail microphysical processes in models.