Despite considerable technological advances, aviation impacts on global climate are significant and may constitute a future constraint on the continued growth of air travel. The most important but least understood component in aviation climate impact assessments are contrails, which form as line-shaped ice clouds (linear contrails) and transform into irregularly shaped ice clouds (contrail cirrus) in favorable meteorological conditions. No reliable best estimate of the contribution of contrail cirrus to climate change exists, but statistical evidence from cirrus trend analyses suggests a potentially large contribution. This article reviews the scientific knowledge and key problems regarding the modeling of the life cycle of contrail cirrus (including linear contrails), their global climate impact, and the validation of model simulations with suitable observational datasets. The prerequisites for global modeling of contrail cirrus, such as the representation of ice supersaturation and the processes governing contrail cirrus evolution as well as improvements in the cloud schemes regarding cirrus, are discussed. Recommendations are given for avenues of research to ensure that future decisions aimed at mitigating the climate impact of contrails and contrail cirrus are based on increasingly sound scientific knowledge.
Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Wessling, Germany