From the Report, Chapter 10.5.2:

If the conditions are suitable, emissions of soot and water vapour can trigger the formation of contrails (Kärcher 2018), which can spread to form extensive contrail-cirrus cloud coverage. Such cloud coverage is estimated to have a combined ERF that is about 57% of the current net ERF of global aviation (Lee et al. 2021), although a comparison of cirrus cloud observations under pre- and post-COVID-19 pandemic conditions suggest that this forcing could be smaller (Digby et al. 2021). Additional effects from aviation from aerosol-cloud interactions on high-level ice clouds through soot (Chen and Gettelman 2013; Zhou and Penner 2014; Penner et al. 2018), and lower-level warm clouds through sulphur (Righi et al. 2013; Kapadia et al. 2016) are highly uncertain, with no best estimates available (Lee et al. 2021).

The 2 papers listed which quantify the effect:

• Lee, D.S. et al., 2021: The contribution of global aviation to anthropogenic climate forcing for 2000 to 2018. Atmos. Environ. , 244, doi:10.1016/j.atmosenv.2020.117834. Link to paper
• Digby, R.A.R., N.P. Gillett, A.H. Monahan, and J.N.S. Cole, 2021: An Observational Constraint on Aviation-Induced Cirrus From the COVID-19-Induced Flight Disruption. Geophys. Res. Lett. , 48(20) , e2021GL095882-e2021GL095882, doi:10.1029/2021GL095882. Link to paper