I Poll, Contrail control - what are we waiting for?, Aerospace, January 2025, p 20

Efforts continue to restrict the global mean temperature rise to 1.5°C, but aviation is unique because this is due to contrails as well as CO2. Even though contrails occur at about 35 000 ft, the typical altitude for civil aviation, avoidance by variation in height means an extra fuel burn of no more that 0.1%, as distinct from up to 10% from changes horizontally.