Contrails in a Global Climate Model - Radiative Forcing and Beyond

Abstract

Contrails form an important contribution to the net climate effect from aviation. Early studies of contrail climate impact have been based on regional observations of coverage and optical properties that may be extrapolated to the whole globe and used as input for radiative transfer models. This method has its limitations, as it neglects most of the interaction between contrails and the ambient atmosphere, and can hardly be used for quantifying a global climate impact beyond the radiative forcing. Line-shaped contrails can be parameterised, similar to natural clouds, in the framework of a three-dimensional global climate model (GCM). This approach yields a self-consistent simulation of contrail coverage, optical properties, radiative forcing, and climate response. However, global climate models have systematic errors whose consequences must be identified and included in the assessment of the contrail results. In this talk, an overview is given for GCM results of various properties of line-shaped contrails, including the climate response in terms of changes in mean temperature and diurnal temperature range. We find the GCM simulated contrail climate impact to be substantially less in comparison with what has been claimed from regional observations and empirical model studies. Much more work is needed, however, before the full effect of aviation induced cloudiness, beyond the contribution from line-shaped structures, can be estimated reliably in a GCM framework.