Global aviation contrail climate effects from 2019 to 2021

Abstract. The global annual mean radiative forcing (RF) attributable to contrail cirrus is comparable to the RF from aviation’s cumulative CO₂ emissions. Here, we simulate the global contrail climate forcing for 2019–2021 using reanalysis weather data and improved engine emission estimates along actual flight trajectories derived from Automatic Dependent Surveillance–Broadcast telemetry. Our 2019 global annual mean contrail net RF (62.1 mW m^-2) is 44 % lower than current best estimates for 2018 (111 [33, 189] mW m^-2). Regionally, the contrail net RF is largest over Europe (876 mW m^-2) and the US (414 mW m^-2), while the RF over East Asia (64 mW m^-2) and China (62 mW m^-2) are close to the global mean value because fewer flights in these regions form contrails as a result of lower cruise altitudes and limited ice supersaturated regions in the subtropics due to the Hadley Circulation. Globally, COVID-19 reduced the flight distance flown and contrail net RF in 2020 (-43 % and -56 % respectively vs. 2019) and 2021 (-31 % and -49 % respectively) with significant regional variation. Around 14 % of all flights form a contrail with a net warming effect, yet only 2 % of all flights account for 80 % of the annual contrail energy forcing. The spatiotemporal patterns of the most strongly warming and cooling contrail segments can be attributed to flight scheduling factors, aircraft–engine particle number emissions, tropopause height, background cloud and radiation fields, and albedo. Our contrail RF estimates are most sensitive to corrections applied to the global humidity fields, followed by assumptions on the aircraft-engine particle number emissions, and is least sensitive to radiative heating effects on the contrail plume and contrail-contrail overlapping. Accounting for the sensitivity analysis, we estimate a 2019 global contrail net RF of 62.1 [34.8, 74.8] mW m^-2.