Contrails are the traces behind air planes. They consist of water droplets feeded by the water vapor which is emitted by airplane engines. Their existence depends on the water vapor concentration and the temperature in the surrounding higher atmosphere.
To measure the effect of contrails one needs an experiment with and without airplanes. The 9/11 strike followed by a three day interruption of all air traffic in the USA was the unique chance to measure the effects of contrails. The mean temperature was not affected but the temperature variation was reduced by contrails. During the day they shade the incoming daylight, during the night they radiate back the infrared radiation. Contrails reduce the temperature variation.
Such a small effect might affect some species which depend on an appropriate variation of temperatures. Cirrus clouds initiated by contrails might bear a much larger risk of severe influence to weather or climate.