Contrails and Cirrus Clouds from Aviation
Clouds can have either a cooling or a warming effect: they can cause warming by trapping long-wave (infrared) radiation from the Earth, but also cool it by reflecting short-wave (visible and ultraviolet) solar radiation back into space. Overall, however, clouds caused by air-travel emissions are considered to have a net warming effect.
Contrails are linear ice clouds formed in the wake of aircraft, which, when persistent, can result in the formation of cirrus cloud cover. Aircraft emissions trigger condensation of ambient water vapor into ice crystals in the atmosphere. Contrail formation and persistence depends on flight altitude and the temperature and humidity of the air through which a plane flies; thus contrail and cirrus formation is seasonally dependent.
Approximately 10-20% of all jet flights occur in air masses with a humidity level sufficient to cause contrails. In 1992, contrails were estimated to cover about 0.1% of the Earth’s surface on an annually averaged basis, with larger regional values. According to the IPCC’s most likely scenario, coverage is expected to grow to 0.5% by 2050. Contrails are short-lived and have an overall warming effect that is similar to thin, high clouds. Warming effects of contrails are different during the day than at night. During the day, contrails trap infrared radiation (a warming effect) and reflect solar radiation (a cooling effect). At night, only infrared radiation is trapped and re-emitted downward. The warming effect of contrails is therefore stronger at night. It is important to note that because contrails are short-lived, formed in areas of high air traffic density, and can affect existing cirrus clouds, they may cause local or regional climate responses.
Cirrus clouds are composed entirely of ice crystals and occur above ~6 km, covering approximately 30% of the Earth’s surface. Extensive cirrus cloud development has been observed after the formation of persistent contrails. The science on this relationship is still developing, and while cirrus clouds are acknowledged to have a net warming effect, the significance of this effect is still uncertain. The scientific understanding of cloud formation and modification due to air travel is still limited. As the most recent IPCC assessment report notes:
“Because spreading contrails lose their characteristic linear shape, a component of [aviation-induced cloudiness] is indistinguishable from background cirrus” (IPCC, 2007, AR I, section 2.6.3, p 187).
Yet to get an accurate estimate of total warming impacts from aviation, cirrus effects should be included. Although uncertainty remains about the precise nature of aviation-induced cirrus-caused warming, we do know how cirrus clouds form, and that they have a warming impact. According to some researchers, this warming impact could be very significant. For more information, see the ACCRI papers on contrails and aviation, available here.
have an overall warming effect: For an interesting example of how contrails affect surface temperatures, see the study on the effect of the lack of contrails on surface temperatures during the no-fly period following Sept 11th, 2001: "Regional variations in U.S. Diurnal temperature range for the 11-14 September 2001 aircraft groundings: Evidence of jet contrail influence on climate," Journal of Climate, 17: 1123-1134.