A 21‐Year Global Daytime Satellite Climatology of Cirrus Cloud Cover and Its Links to Upper‐Tropospheric Conditions and Aviation Over Europe and the North Atlantic

ABSTRACT Cirrus clouds play a key role in the Earth's radiation budget, yet their long‐term global variability and trends remain uncertain due to limitations in satellite detection of optically thin ice clouds. Here we present a 21‐year (2003–2023) global cirrus cloud climatology, with particular attention to the effects of upper‐tropospheric thermodynamics and aviation activity over Europe and the North Atlantic. Cloud detection was performed using MODIS/Aqua radiometer data with an adapted All Tests Consolidation (ATC) cloud detection algorithm developed in our previous work, complemented by ERA5 reanalysis of temperature and humidity fields and OpenSky aircraft traffic records. Results show that cirrus clouds cover on average 39.5% of the Earth, peaking in tropical convective regions where cirrus cloud fractions reach ~80%. Long‐term trends indicate slight increases in polar regions (0.80%/10 years) and decreases over midlatitudes and the tropics (−0.66% to −0.25%/10 years), reflecting spatially heterogeneous climate‐driven changes in moisture availability. Cirrus occurrence is strongly linked to colder and moister upper‐tropospheric conditions, with aviation further modulating variability. Overall, cirrus cloudiness emerges as a product of both natural atmospheric variability and anthropogenic influences. A reduction in cirrus cover was observed over Europe during the winter season of 2020, coinciding with decreased air traffic during the COVID‐19 pandemic, with a magnitude of approximately −3 pp.