Air traffic has been growing exponentially over the last decades and thus plays an important role for future climate mitigation strategies. Contrail cirrus is regarded to be the largest contributor to aviation induced climate impact on the basis of radiative forcing estimates, exceeding aviation's CO2 emissions. While previous studies revealed a reduced efficacy of linear contrails in forcing the global mean surface temperature, that has not yet been analyzed for the far more important contrail cirrus.
In the context of the present thesis, various climate model simulations were performed in order to gain a better understanding about the climate impact of contrail cirrus. The simulations were conducted with the ECHAM5 and EMAC climate models, equipped with a state-of-the-art contrail cirrus parameterization, and were further analyzed by feedback analysis after the partial radiative perturbation method in order to determine the rapid radiative adjustments and slow feedbacks.
First, the climate impact of contrail cirrus was assessed on the basis of radiative forcings, determined by fixed sea surface temperature simulations. The effective radiative forcing of contrail cirrus turned out to be largely reduced with respect to the conventional radiative forcings. In contrast, the effective radiative forcing of a CO2 increase simulation, with a similarly sized conventional radiative forcing, deviates less strongly. If directly compared to those CO2 reference experiments, the contrail cirrus effective radiative forcing is reduced by 45 % (58 %) for the EMAC (ECHAM5) model, which already indicates a reduced climate impact of contrail cirrus. The simulations were further analyzed by feedback analysis to determine a full set of consistent rapid radiative adjustments. A negative natural cloud adjustment due to a reduction of natural cirrus cover was found to be the main origin for the reduced effective radiative forcing of contrail cirrus.
To determine the actual climate impact, the global mean surface temperature change, induced by contrail cirrus, was directly simulated with the EMAC model coupled to an interactive ocean. For the first time, the climate sensitivity and efficacy parameters of contrail cirrus could be determined. Overall, the surface temperature change due to contrail cirrus turns out to be considerably smaller than for a CO2 increase simulation with a similarly sized conventional radiative forcing. The efficacy parameter, based on the effective radiative forcing, is 0.38, deviating significantly from the theoretically expected value of 1. In terms of conventional radiative forcing, the efficacy parameter is as low as 0.21. Therefore, the global mean surface temperature change induced by contrail cirrus is clearly weaker than suggested by, both, the effective and the conventional radiative forcing. Feedback analysis identified changing low and mid level clouds, in comparison to the CO2 reference case, as main reason for the small efficacy parameter.
In conclusion, there is new and distinct evidence that the use of radiative forcing in aviation climate impact assessments may lead to substantial overestimation of the relative importance of contrail cirrus for aircraft induced global warming. Nevertheless, as several key findings of the present thesis are unique, confirmation from other climate models, equipped with an adequate contrail cirrus parameterization, is regarded as highly desirable
