Industry Insights

Understanding and Addressing Aviation’s Non-CO₂ Climate Impacts

April 22, 2024

Airlines for America (A4A) and our members are committed to addressing climate change and working to achieve net-zero carbon emissions by 2050. Over the years, U.S. airlines have dramatically improved fuel efficiency and reduced emissions by investing billions in fuel-saving aircraft and engines, innovative new technologies, cutting-edge route optimization software and sustainable aviation fuels (SAF). Going forward, we are focused on achieving our 2050 goal through increased near-term actions, including increasing the production and use of sustainable aviation fuels.

In addition to achieving net-zero carbon emissions, our commitment to climate and sustainability includes improving the understanding and addressing the climate impact of non-CO₂ emissions. While CO₂ is the only significant greenhouse gas emitted from aircraft, other non-CO₂ impacts from aviation, including from the formation of contrails, also affect climate.

Contrails are trails of water vapor emitted from aircraft engines that condense and freeze around suspended particles at high altitude under certain atmospheric conditions. As can be observed when looking up at airplanes in the sky, contrails are not formed on all flights, and instead only when certain atmospheric conditions, including temperature and humidity levels, are present. Research suggests that aviation-induced cloudiness, or cirrus clouds caused by persistent contrails from aircraft, is estimated to be the largest contributor to warming by non-CO₂ emissions, but with large uncertainties in the magnitude of the warming. A4A recognizes that contrails have an overall warming effect and supports action to address contrail-related warming, including further research to reduce scientific uncertainty.

Further research is needed to improve understanding of the warming and cooling effects of contrails, including the climate impact relative to CO₂ emissions, and to reduce the current levels of uncertainty. Additional research is needed to better understand the atmospheric conditions, locations, and times of day and year that are most conducive to contrail formation.  As we increase efforts to better understand and address non-CO₂ emissions, it is essential to proceed carefully, particularly where actions could result in increased CO₂ emissions.  

The following initiatives are some examples of the activities being carried out by A4A members:

• Air Canada and Hawaiian Airlines, through the In-Service Aircraft for a Global Observing System (IAGOS), equipped long-haul aircraft with instruments to measure atmospheric composition, including humidity.
• American Airlines with Breakthrough Energy and Google Research tested the ability to identify contrail forming regions in the atmosphere and whether pilots could avoid contrail formation in flight when supplied with prediction data.
• Delta Air Lines with the Massachusetts Institute of Technology (MIT) Laboratory for Aviation and the Environment are building tools to predict contrail forming regions and experimentally validate contrail avoidance and resulting environmental impact, thus informing possible operational mitigation tools and technology.
• Southwest Airlines with General Electric (GE) Research are developing a real-time, in-flight prediction system for persistent contrails lasting more than five hours. Also with Google Research, conducting a series of desktop trials with Google’s contrail prediction model to evaluate alternatives to planned routes that would have created contrails.
• United Airlines with Boeing, NASA, DLR, FAA, and GE Aerospace are conducting test flights to measure how SAF affects contrails and non-CO₂ emissions.

A4A advocates for a data-driven, fact-based approach to addressing the climate impact of non-CO₂ emissions from aviation. Priority should be placed on understanding climate science, reducing uncertainties, identifying knowledge gaps, addressing those gaps, and evaluating the options for reducing contrail-related warming – and we are committed to these efforts.

Learn more here.