Estimating the Climate Efficacy of Contrail Cirrus on Surface Temperature

So far, the various components contributing to the global climate impact of aviation were mostly quantified and ranked on the basis of radiative forcings. However, regarding for example the Paris Agreement, the temperature change at Earths surface is the main target parameter, for which radiative forcing is only a proxy. A specific climate sensitivity parameter for contrail cirrus has, so far, not been established.Here we close this gap for contrail cirrus with specially designed global climate model simulations, equipped with a coupled mixed layer ocean to determine the corresponding surface temperature change. For the first time the climate sensitivity and efficacy parameters were calculated for contrail cirrus. The efficacy of contrail cirrus to warm the Earths surface is found to be 62% smaller compared to CO2 in the effective radiative forcing framework. That means that radiative forcings of same magnitude result in a much weaker surface warming for contrail cirrus than for CO2. The origin of the reduced efficacy can be explained in detail by analyzing the related Feedback processes. An opposing response of the natural clouds (even in sign), with decreasing resp. increasing low- and mid-level clouds in case of CO2 resp. contrail cirrus was found to be the main reason. In addition, a more negative Lapse-Rate Feedback was found for contrail cirrus, originating from a non-homogeneous vertical warming, with the largest temperature increase directly below contrail cirrus and decreasing in strength toward Earths surface.The reduced contrail cirrus efficacy substantially affects contrail cirrus mitigation concepts when using climate metrics based on surface temperature change (e.g. GTP or ATR). Therefore, re-routing aiming for contrail cirrus reduction, but leading to an increased fuel consumption and additional CO2 emissions might be much less effective than currently assumed.