Airplanes emit CO2 and contribute to climate warming. A lesser-known effect comes from contrails and the associated cloudiness, which also contributes to warming. The total warming effect of air traffic is therefore about twice as much as the effect of just the emitted CO2.
What are contrails?
Contrails usually form between 8 kilometers and 13 kilometers altitude, where airplanes fly at cruising altitude. The air is cold enough to leave a trail of condensation as ice particles. This is the same process as when you exhale on a cold winter day and create a cloud. A jet engine does this too. Warm, moist air comes out of the engine and mixes with the cold surrounding air to create a (ice) cloud.
Humidity
Most contrails dissipate relatively quickly. This has to do with the humidity of the air in which the airplane is flying. However, there are also days when the humidity of the atmosphere is sufficient for these contrails to persist for several days. Then it becomes a different story.
Wind shear is also important. This is a change in wind direction and speed with altitude. Thus, contrails can spread out like a kind of fan into a continuous field of high cloudiness, which would not exist under natural conditions. These fields of high cloudiness caused by contrails can sometimes last for days.