Contrail cirrus radiative forcing for future air traffic
Abstract. The climate impact of air traffic is to a large degree caused by changes in cirrus cloudiness resulting from the formation of contrails. Contrail cirrus radiative forcing is expected to increase significantly with time due to the large projected increases in air traffic. We use ECHAM5-CCMod, an atmospheric climate model with an online contrail cirrus parameterization including a microphysical two-moment scheme, to investigate the climate impact of contrail cirrus for the year 2050. We take into account the predicted increase in air traffic volume and changes in propulsion efficiency and emissions, in particular soot emissions, and the modification of the contrail cirrus climate impact due to anthropogenic climate change. Contrail cirrus radiative forcing increases by a factor of 3 from 2006 to 2050, resulting from the increase in air traffic volume and a slight shift of air traffic towards higher altitudes. Large increases in contrail cirrus radiative forcing are expected over all of the main air traffic areas but relative increases are largest over South-East Asia/India and Eastern China/Japan. The projected upward shift of air traffic attenuates contrail cirrus radiative forcing increases in the mid-latitudes but reinforces it in the tropical areas. Climate change has an insignificant impact on global contrail cirrus radiative forcing. Of the emission reductions it is the soot number emission reductions by 50 % that lead to a significant decrease in contrail cirrus optical depth and coverage, leading to a decrease in radiative forcing by approximately 15 %. The strong increase in contrail cirrus radiative forcing due to the projected increase in air traffic volume cannot be compensated for by the decrease in initial ice crystal numbers due to reduced soot emissions and by improvements in propulsion efficiency.