Global model simulations on the impacts of aviation-induced aerosol on clouds and climate

The current understanding of the impact of aviation-induced aerosol on the microphysical properties of clouds and the resulting climate effects is still regarded as highly uncertain. In this study, a global aerosol-climate model is applied to quantify (i) the impact of aviation-induced soot on cirrus clouds, and (ii) the impact of aviation-induced particles on low-level liquid-phase clouds. For the soot-cirrus effect, a radiative forcing in the range of 35 to 13mWm2 is simulated, depending on the assumed ice nucleating properties of aviation soot, but with a low statistical significance in several cases. A comparison with previous studies shows a general lack of consensus, which could not be reconciled even within the broad range of assumptions on the ice nucleating properties of aviation soot examined with the model. The effect of aviation-induced particles on low-level clouds results in a radiative forcing of 64 mW m-2 for present-day conditions in the CMIP6 emission inventory, and of 126 to 83 mW m-2 in 2050 under different scenarios. This mostly confirms the estimates of previous studies, although these are very sensitive to the assumed size distribution of emitted particles and to the fuel sulfur content of the aviation fleet. The main uncertainties behind these cloud effects are explored in the context of the existing literature and suggestions for refined estimates are discussed.