Effective radiative forcing of aviation-induced aerosols in present-day and future climate simulated with UKESM

Current best estimates suggest that aviation contributes around 5% to global warming, with a substantial fraction arising from non-CO₂ effects. While contrail cirrus dominates these non-CO₂ impacts, aerosol-cloud interactions have also been suggested to play an important role. However, aviation-induced aerosol-cloud interactions remain poorly quantified, limiting our ability to inform future aviation climate mitigation policies. Improving our understanding of aviation-induced aerosol-cloud interactions is therefore essential, particularly in the context of ambitious mitigation targets and emerging alternative fuels.

Here, we quantify the effective radiative forcing (ERF) from aerosol–radiation interactions (ARI) and aerosol-cloud interactions (ACI) driven by aviation emissions using the UK Earth System Model (UKESM). Three-dimensional emissions of H₂O, SO₂, soot, and NO_x are constructed based on fuel consumption data from two independent aviation inventories: the Aviation Environmental Design Tool (AEDT) and the Aviation Integrated Model (AIM), and are implemented within the UKCA chemistry-aerosol framework. Atmospheric chemistry, aerosol microphysical processes, and cloud microphysical processes are simulated with UKCA and its GLOMAP component.

A present-day simulation for 2018 indicates a net aviation aerosol ERF (ARI + ACI, SW + LW) of -24 mW m^-2 when using AEDT-based emissions, relative to a control simulation without aviation emissions. This forcing is dominated by ACI (-22 mW m^-2), with shortwave and longwave contributions of -17 and -5 mW m^-2, respectively, while ARI contributes only -3 mW m^-2. Very similar results are obtained using AIM-based emissions, yielding a net ERF of -27 mW m^-2. These values are large enough to offset approximately half of the best-estimate ERF from contrail cirrus reported by Lee et al. (2021). 

Changes in cloud macrophysical properties such as cloud fractions and liquid water paths remain difficult to detect above internal variability, whereas cloud microphysical responses are clearly visible. Cloud droplet number concentrations in the upper troposphere increase by up to ~15% over northern mid- to high-latitude regions, primarily driven by sulphate-liquid cloud interactions. Interactions between soot and ice clouds, including contrail cirrus, are not yet fully represented and will be addressed in future work by coupling UKESM with more sophisticated cloud microphysics model, CASIM.

Ongoing simulations extend this analysis to 2050 under multiple fuel scenarios, including conventional fuels, sustainable aviation fuel (SAF), and liquid hydrogen. While growing aviation demand amplifies both warming and cooling effects under conventional fuels, reduced aerosol emissions from alternative fuels are expected to weaken aviation-induced aerosol cooling effect. Results from these future scenarios will also be presented.