Abstract: Contrail cirrus – ice clouds forming in aircraft exhaust - are estimated to account for 2% of global anthropogenic climate impact from all sources. Understanding how contrail impacts are affected by changes in aircraft technology and aviation practices, such as the timing and location of flights, is essential for developing effective mitigation strategies but relevant information is limited. Furthermore, investigations of contrail impacts have almost exclusively focused on single years, with no single consistent study of how contrail impacts have changed over time. This has made it difficult to interpret differences in impacts between studies of different years.

In this study, we investigate how the radiative forcing (climate impact) of contrails has evolved from 1980 to 2019. We simulate contrails sampled from an inventory of this period using the Aircraft Plume Chemistry, Emissions, and Microphysics Model (APCEMM). This model captures the dynamics of contrail formation, coagulation of solid and liquid particles, particle settling, and particle growth and evaporation. We use ERA5 reanalysis data to estimate prevailing atmospheric conditions and analyze results with particular emphasis on annual changes in the location, timing, and composition of emissions.

We find that, from 1980 to 2019, total contrail impacts have increased but the impact per unit of contrail distance has decreased. This is due to a combination of technological and policy factors. We discuss how falling soot emissions appear to have reduced contrail optical depth but not lifetime while increasing flight coverage over Europe has a greater impact than the increases over Asia. However, we also find that trends in these impacts are highly sensitive to the criteria used to determine whether a contrail is likely to have evaporated.

This work implies that differences between prior studies of contrail properties may be driven in large part by differences in contrail inventory. Our work also suggests that a small number of additional factors should be considered alongside total flight distance to understand how future changes in aviation are likely to affect its climate impacts.