File:Contrails 2029 study acp-19-8163-2019-f06.jpg

English: Absolute difference (modelisation) in visible optical depth of plane contrails at 200 hPa between scenario C2050-T50 and C2050-T50M due to soot reductions. Dotted regions are significant. (in a study published 2019)

In the scientific paper you can read : "Reduced soot emission and improvement in propulsion efficiency
A reduction in the initial contrail ice particle number by 50 % leads to a strong decrease in the climate impact of contrail cirrus reducing global radiative forcing for the year 2050 by 14 % from 160 to 138 mW m−2 (Table 1). A smaller number of initial ice crystals can grow faster assuming a constant amount of ambient water vapour available for deposition, leading to an earlier and larger sedimentation loss of ice crystals (Bier et al., 2017), and therefore to a decrease in contrail cirrus optical depth, lifetimes and radiative forcing (Burkhardt et al., 2018). The decrease in contrail cirrus radiative forcing for the year 2050 is caused by a decrease in contrail cirrus optical depth of up to 30 % (Figs. 4e, f and 6) and by a decrease in contrail cirrus coverage (Fig. 4d). The changes in radiative forcing are largest over the South East Asia–India area where sedimentation plays a greater role due to the larger amount of water vapour available for deposition. Over Europe the effect is slightly larger than over the US–Mexico area. This is because of its location downwind of the North Atlantic flight corridor where contrail cirrus coverage is strongly influenced by the lifetime of the contrail cirrus originating over the Atlantic. The smallest impact of the reduction in initial ice crystal numbers on contrail cirrus radiative forcing among the four studied regions can be found over the US–Mexico area (Fig. 4c) where contrail cirrus coverage mainly consists of young contrails. The impact of soot reductions is smaller than estimated in Burkhardt et al. (2018), who found that a 50 % reduction in soot emissions causes a 20 % reduction in contrail cirrus radiative forcing for air traffic in the year 2006. The difference in sensitivity may be caused by the change in air traffic volume and pattern. Contrail cirrus radiative forcing is nonlinearly dependent on the initial ice crystal number (Burkhardt et al., 2018). This means that reducing initial ice crystal numbers in an increased air traffic environment has a smaller impact on contrail cirrus radiative forcing than for current air traffic since an abundance of contrail cirrus ice crystals will still exist even if nucleation rates are reduced.

The increase in propulsion efficiency and the change in water vapour emissions (Sect. 2.3) have no significant impact on contrail cirrus radiative forcing. Persistent contrail formation probability around 250 hPa is slightly increased only in the tropics (Fig. 5b), which has no significant impact on the global radiative forcing due to contrail cirrus".