Understanding their impact
As well as CO2, flying generates other emissions that can also have an impact on our climate.
One of these is water vapour, which is emitted by aircraft engines when they burn fuel. This water vapour occasionally condenses into the white, cloud-like lines that can sometimes be observed in the sky. These are made of ice crystals and are known as condensation trails - or contrails.
The formation of contrails and the way that they behave is influenced by many factors such as the weather, humidity in the air, season, time of day, flying altitude, engine type and fuel composition. Sometimes they only last for a few seconds but if the air is humid and cold enough, they can persist for several hours and spread across large sections of the sky.
The climate impact of individual contrails is difficult to assess. Sometimes they have a cooling effect, especially during the day, when they tend to block the sun’s radiation from reaching the Earth’s surface. At night, however, they can prevent heat from escaping from our planet, which causes warming. The latest scientific consensus is that overall, they tend to have a warming effect.
Research
The aviation industry is actively contributing to scientific research and participating in operational trials and other studies, with a view to improving our understanding of when contrails will form. This is proving challenging, with the studies returning high numbers of false positives and false negatives.
It is very difficult to forecast where they will develop with enough precision to enable flight plans to be altered to avoid their formation. It is also hard to predict whether their climate impact would justify any additional fuel burn and CO2 emissions related to their avoidance.
Multiple efforts are underway to better understand the science behind the formation of contrails and to identify ways of avoiding their formation through the use of alternative fuels, revolutionary combustion technology and operational improvements. Despite the uncertainties, aviation is continuing to work on potential solutions to help avoid contrail formation.
Industry strategy
To address contrails, the aviation industry supports continued academic and technical research into:
- understanding how they form and their effect
- developing humidity sensors that can be used regularly on flights
- understanding the impact of alternative fuels
- other non-CO2 impacts such as nitrous oxides and oxides of sulphur.
The industry also supports the establishment of large-scale operational trials on their avoidance. Industry does not support:
- establishing regulatory measures while there are still scientific uncertainties
- adopting contrail avoidance measures before trials have fully assessed all the potential trade-offs
- creating carbon credits from contrail avoidance
- deliberately creating contrails that might have a cooling impact.
For more information, including next steps for governments, download the Briefing Paper on Climate Action: Addressing Contrails.