Understanding the impact of emission on aviation is a complex, scientific challenge that requires extensive expertise and careful analysis. Non-CO₂ aviation emissions, which include water vapour (H2O), nitrogen oxides (NOx), particulate matter (PM), sulphur oxides (SOx), carbon monoxide (CO), and unburned hydrocarbons (UHCs), have a net warming effect on the global climate. Unlike CO₂ emissions, which have been widely studied and addressed, the full climate effects of non-CO₂ emissions are more complex and less understood. Each component interacts with the atmosphere in distinct ways, such as the formation of contrails and changes in ozone and methane levels, resulting in varied radiative forcing effects. Understanding and mitigating these impacts is crucial, as they can lead to both warming and cooling of the climate, depending on numerous factors:
- Radiative forcing: Radiative forcing assesses the difference between the amount of energy entering the atmosphere and the amount of energy leaving it. If more radiation enters than exits the result is a warming impact known as positive radiative forcing. Conversely when more energy leaves it results in cooling referred to as negative radiative forcing.
- NOx emissions: Nitrogen oxides (NOx) emissions impact the concentrations of other greenhouse gases, such as ozone and methane. Although there is significant uncertainty, nox emissions are likely to contribute to a net warming of climate. Increased ozone at cruise altitudes likely leads to a positive forcing or warming, while other chemical reactions related to increased NOx may cause a negative radiative forcing or cooling. This complexity makes it challenging to evaluate the overall climate impact of NOx.
- Contrails: Contrails form when water vapor from a cruising aircraft condenses and rapidly freezes into ice crystals. At the engine exhaust source of an aircraft, particulate emissions mainly consist of ultrafine soot or black carbon emissions. Such particles are called “non-volatile” (nvPM). While ice crystals can form solely with water droplets, more commonly they form as the result of water vapor on these nvPM particles. The radiative forcing from contrails depends on their persistence and spread. Many uncertainties remain. For example, contrails formed during the night likely have a positive warming radiative forcing while those formed in the day could have a net negative or positive forcing around the world.
ICAO initiated the next step in the international community’s efforts to reduce air transport’s impact on the climate: addressing the non-CO2 emissions from international aviation. While carbon dioxide (CO₂) emissions have long been the focus of international agreements and industry efforts, the sector’s non-CO₂ climate impacts have remained uncertain. ICAO’s landmark Symposium on Non-CO₂ Aviation Emissions, held from 16-18 September 2024, drew some 800 attendees both in-person and virtually and focused on topics such as contrail formation, nitrogen oxides and particulate matter, striving to understand and enhance the climate science, potential mitigation measures and policy options. All of the discussions that took place during this event are available on ICAO TV for on-demand viewing.
“We’re entering a new era of climate change solutions in aviation, and ICAO is firmly committed to deepening knowledge on this subject, exploring innovative solutions, and forging partnerships” said Salvatore Sciacchitano, ICAO Council President. “This symposium is about more than awareness; it’s about charting a course for meaningful action on a problem that requires more scientific understanding.”
The event brought together a unique coalition of atmospheric scientists, aerospace engineers, policy makers, and airline operators. Participants represented States, United Nations system organizations, international and regional organizations, industry and related associations, research and development institutions, and academia. This interdisciplinary approach yielded rich discussions on potential mitigation strategies, from altering flight paths to avoid contrail formation to developing cleaner-burning jet fuels.
During the event, ICAO organized an exhibition on the latest initiatives associated with non-CO2 emissions and debuted an educational display installation and a series of videos aimed at addressing common misconceptions about non-CO₂ emissions. To maintain the momentum, ICAO also introduced a new tracking tool for non-CO₂ emissions initiatives.
“This symposium comes at a critical juncture in aviation’s climate journey,” said Juan Carlos Salazar, ICAO Secretary General. “By addressing non-CO₂ emissions alongside our existing decarbonization efforts, we’re taking a more holistic approach to lead aviation to a more sustainable future.”
“It is clear that the path forward will require not only technological innovation but further consideration of how we measure and mitigate the full scope of aviation’s climate impact” said Secretary General Salazar. “The impressive wealth of information presented during the event from the best scientists, technical experts, industry and academia highlights the importance for ICAO to facilitate continuous cooperation in this area.”
