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Aviation accounts for approximately 2.5% of human-made global CO2 emissions but its impact on climate extends to non-CO2 effects such as contrails and nitrogen oxides (NOx). 

Contrails are high-altitude ice-cirrus clouds which reflect incoming solar radiation and trap outgoing heat. On balance, they have a warming effect, with diurnal, seasonal and geographical variations. The scientific understanding of the non-CO2 climate effects of aviation has grown but significant uncertainties exist in predicting contrail formation and climate impact. No methods exist yet to monitor contrails on a per flight basis or tools to mitigate them at scale.

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IATA and the aviation community, formed by industry, governments, universities, and research institutions are engaging in initiatives to further understand the climate impact of contrails and potential mitigation. Part of the work that IATA is leading is focused on:

  • Increasing the confidence on where contrails might form and what their climate effect could be
  • Equipping aircraft with humidity sensors, performing contrail avoidance trials
  • Researching and testing the non-CO2 effects of Sustainable Aviation Fuels (SAF) and hydrogen

> View IATA’s position on non-CO2 emissions
> FAQ about non-CO2 emissions and contrails (pdf)

Aviation Contrails and their Climate Effect: Tackling Uncertainties and Enabling Solutions    

A report commissioned by the community calls for a strengthening of collaboration between research and technological innovation, coupled with policy frameworks to address aviation’s non-CO2 emissions through more atmospheric data.

Short term (2024-2030), prioritize the reduction of CO2 emissions over uncertainties in contrail detection and climate impact through:

  • Increasing airline participation in sensor programs
  • Continuing scientific research
  • Improving humidity and climate models

Mid-term (2030-2040):

  • Establishing standards for data transmission
  • Continuously validating models
  • Encouraging aircraft manufacturers to include provisions for meteorological observations

Longer-term (2040-2050):  Aircraft should be continuously providing data and the models and infrastructure should be reliable. We will have a more complete understanding of the non-CO2 effects of alternative fuels, with extended mitigation measures.

> Full report: Aviation Contrails Climate Impact - Tackling Uncertainties & Enabling Solutions (pdf)
> Press release: More Data Needed to Understand Contrails Climate Effect & Develop Mitigations

Non-CO2 emissions explained 

Emissions from burning jet fuel consist of carbon dioxide (CO2), water vapor (H2O), nitrogen oxides (NOx), sulphur oxides (SOx), carbon monoxide (CO), soot (PM 2.5), unburned hydrocarbons (UHC), aerosols, and traces of hydroxyl compounds (-OH), most of which are released in the atmosphere at cruise altitudes of 8– 13 km above mean sea level.

When water vapor is released from jet engines at altitude under certain high humidity conditions (ice supersaturated regions) it can condense into exhaust carbon particles as well as into atmospheric aerosols. If the air is sufficiently humid, the water vapor can condense further into crystals and a cloud can be formed. Such clouds, formed from the condensation of exhaust aircraft water vapor, are called condensation trails or contrails.

The main climate change contributions from non-CO2 emissions of aviation come from the formation of persistent contrails and particularly the resulting aviation-induced clouds, as well as from the chemical atmospheric reactions driven by NOx emissions.

While the effect of these emissions has been estimated at an aggregate level, the capacity to accurately measure their climate impact at an airline or individual-flight level is very limited. Furthermore, considerable uncertainties regarding the overall climate effect of these emissions remain.

For nitrogen oxides, the amount of NOx emitted by an aircraft depends primarily on engine design, technology, and operating conditions (idle, take-off, descent, etc.), as well as on the atmospheric conditions (temperature, pressure, and humidity) at which this engine operates. This variability also applies to the formation of contrails, which relies on atmospheric conditions, engine and aircraft design, and fuel composition.

Although contrails are not always formed, their effect depends on whether they are persistent, the location and time of the day at which they are formed, the weather conditions, the combined effect of multiple contrails, and, importantly, whether they have a cooling or warming effect. This makes calculating their net climate effect on a per flight basis extremely complex.