Climate impact of Contrail Cirrus: From conventional and Effective Radiative Forcings to Surface Temperature Change

Abstract

Global temperature change and climate sensitivity in response to an external radiative forcing are known to be modified by radiative feedbacks. The net impact of global aviation on climate originates from CO2 as well as from non-CO2 emission components. So far, calculations of conventional and effective radiative forcings suggest contrail cirrus to make the largest contribution. Here, we present results from general circulation model studies indicating that this ranking might change if contrail cirrus and CO2 emissions are determined from coupled atmosphere-ocean simulations to directly calculate the surface temperature change. A set of simulations with fixed sea-surface temperature to derive the conventional and effective radiative forcings and a second set of interactive ocean simulations were performed for contrail cirrus and CO2. Resulting climate sensitivity parameters of both forcers indicate an exceptionally low efficacy for contrail cirrus to induce the Earth's surface warming. If combined with recent radiative forcing best estimates for air traffic, 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 feedback analysis allows to trace the causes of the remarkably small efficacy of contrail cirrus back to their physical origin. For both rapid radiative adjustments and slow feedbacks, the natural cloud feedback 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.