Both effects could have a potentially high impact on the aviation’s climate impact but are poorly confined by experimental evidence and hence render the major open science questions in aviation climate research. Some model estimates might even tip the originally positive sign of aircraft’s climate impact into a negative one when the aerosol effects are taken into account (Figure 1).While some model studies reveal a very large cooling due to soot-induced cirrus modifications, such effects could not be identified in other studies. Large model uncertainties still result from the representation of aerosols, cirrus clouds, and dynamical forcing. Existing model quantifications of the potentially large cooling effect of aerosol-driven changes of low clouds suffer from uncertain representations of aerosol processing in aircraft plumes, aerosol long-range transport, and aerosol-induced liquid cloud formation. New insights from observations are urgently required to constrain model simulations towards more robust quantifications of the
aerosol climate effects.