Contribution of Contrail Cirrus to Aviation Induced Radiative Forcing and Surface Temperature Change

Abstract

The net impact of global aviation on climate originates from CO2 as well as from non-CO2 emission components. Recent assessments, based on calculations of conventional and effective radiative forcings, have suggested contrail cirrus to make the largest contribution. Here we provide evidence from a consistent set of scaled sensitivity simulations, which indicates a change of this ranking if the contrail cirrus and CO2 response is determined from a coupled atmosphere-ocean GCM that allows to directly calculate the surface temperature change. Fixed sea surface temperature simulations are used to derive the conventional and effective radiative forcings, while corresponding interactive ocean simulations are used to determine the surface temperature response and climate sensitivity. Resulting climate sensitivity parameters of both forcers indicate an exceptionally low efficacy for contrail cirrus to induce global mean surface warming. If combined with radiative forcing best estimates for air traffic at year 2018 (as given by Lee et al., 2021), the climate impact - in terms of global equilibrium surface temperature change - turns out to be larger for aviation CO2 emissions than for contrail cirrus. An extensive analysis of global radiative feedbacks allows the causes of the remarkably small efficacy of contrail cirrus to be traced back to their physical origin. For both short time-scales (rapid radiative adjustments) and longer time-scales (slow radiative feedbacks), the natural cloud response is found to act quite differently (even in sign) for contrail cirrus and CO2. Together with contributions from a deviating lapse rate feedback it forms the main reason for the low contrail cirrus efficacy to effect surface temperature on the global scale.