Impact of host climate model on contrail cirrus effective radiative forcing estimates

Abstract. Aviation is currently estimated to contribute ~3.5 % of the net anthropogenic effective radiative forcing (ERF) of Earth's atmosphere. The largest component of this forcing comes from contrail cirrus (also with a large associated uncertainty of ~70 %), estimated to be two times larger than the contribution from aviation CO₂ emissions. Here we implement the contrail parameterisation previously developed for the USA NCAR (National Center for Atmospheric Research) Community Atmosphere Model (CAM) in the UK Met Office Unified Model (UM). By using for the first time the same contrail parameterisation in two different host climate models, this work investigates the impact of key features of the host climate model on quantifying contrail cirrus radiative impacts. We find that differences in the background humidity (in particular ice supersaturation) in the two climate models lead to substantial differences in simulated contrail fractions, with UM values being two to three times as large as those from CAM. We also find contrasting responses in overall global cloud fraction due to air traffic, with contrails causing increases and decreases in total cloud fraction in the UM and in CAM, respectively. The different complexity of the two models’ cloud microphysics schemes (i.e. single and double-moment cloud schemes in the UM and CAM, respectively) results in significant differences in the simulated changes in cloud ice water content due to aviation. When accounting for the difference in cloud microphysics complexity, we estimate the contrail cirrus ERF of the year 2018 to be 40.8 mWm⁻² in the UM and 60.1 mWm⁻² in CAM. While these two estimates are not entirely independent, they indicate a substantial (i.e. factor of ~2) uncertainty in contrail cirrus ERF from differences in the microphysics and radiation schemes of the two host climate models. We also find a factor of 8 uncertainty in contrail cirrus ERF due to existing uncertainty in contrail cirrus optical depth. We suggest that future work on the contrail cirrus climate impact should focus on better representing the microphysical and radiative contrail characteristics in different climate models and on improved observational constraints.