Radiative forcing and rapid atmospheric adjustments induced by contrail cirrus

The sustainability of worldwide air traffic forms an important issue due to its expected large growth rates in the coming decades. Contrail cirrus is regarded to be the largest contributor to aviation climate impact and thus plays an important role in considerations towards limiting aviation induced climate change. Here, we present results from global climate model simulations, designed to determine the adjusted radiative forcing (RFadj) and the effective radiative forcing (ERF) of contrail cirrus. For a 2050 air traffic scenario a RFadj of 160 mWm-2 was determined, which corresponds to an increase by a factor of more than 3 compared to 2006 values (49 mWm-2) and thus highlights the largely growing impact of air traffic in a future climate. However, as has been indicated by earlier studies, the efficacy of RFadj of linear contrails in forcing surface temperature is significantly reduced and it stands to reason that this might hold for contrail cirrus as well. For this reason we also performed ERF simulations which account for further rapid radiative adjustments in the atmosphere, not included in RFadj, and thus may form a better metric for estimating surface temperature changes. ERF of contrail cirrus is found to be severely reduced by between 50 and 75% (best estimate about 65%), compared to RFadj. In a subsequent feedback analysis the rapid adjustments, which are physically responsible for the reduced ERF, have been determined. A large negative cloud adjustment, due to a decline of natural cirrus cover, is found to be the main driver of the substantial reduction. For a CO2 doubling simulation, the reduction of ERF in comparison to the RFadj is found to be much smaller