Understanding the Role of Contrails and Contrail Cirrus in Climate Change: A Global Perspective

Abstract. Globally emissions from aviation affect the Earth’s climate via a complex set of processes. Contrail cirrus and carbon dioxide emissions are the largest factors from aviation in the radiative forcing on climate. Although contrail cirrus enhances the impact of natural clouds on the climate, there are still unresolved questions concerning their characteristics and life cycle. Despite extensive research in studying contrails, significant uncertainties persist. Contrail cirrus encompasses linear contrails and the associated cirrus clouds; these are characterized by ice particle properties, e.g., size, concentration, extinction, ice water content, optical depth, geometrical depth, and cloud coverage. The climate impact of contrails may intensify due to projected increases in air traffic. The radiative forcing from global contrail cirrus has the potential to triple and could reach as much as 160 mW m⁻² by 2050. This projection is based on anticipated growth in air traffic and a potential shift to higher altitudes. The future climate impact of contrail cirrus is influenced by factors like the magnitude and geographical spread in air traffic, advancements in fuel efficiency, the effects from use of alternative fuels, and the effects of the changing climate on the background atmosphere. This study reviews the microphysical processes affecting contrail formation and the aging of contrails and contrail-cirrus. Furthermore, the study explores global observational datasets for contrails, current analyses, and future projections and will aid in evaluating the effectiveness and trade-offs associated with various mitigation strategies. The research highlights gaps in knowledge and uncertainties while outlining research priorities for the future.