Contrails inside cirrus clouds predominate with uncertain climate impact

The climate impact of aviation contrails and flight rerouting to avoid persistent contrails is currently under discussion. Proposals focus on avoiding humid areas needed for contrail formation, but neglect that they are often located inside of cirrus clouds, which will change the contrail climate effects significantly. Our analysis of seven years of humidity observations by instrumented passenger aircraft shows that only 20–30% of air masses in major flight areas can develop persistent contrails, of which more than 83% are covered by cirrus. Thin, subvisible cirrus, which contribute 5–15% to all cirrus, amplify the warming effect of contrails, while in thicker, visible cirrus warming is only slightly enhanced, or sometimes even reversed to cooling. We suggest using combined cirrus and humidity forecasts to exclude areas with visible cirrus from flight rerouting, jointly with a thorough assessment of contrail vs. CO ₂ climate effects, considering the interaction of persistent contrails and cirrus.