Demystifying Aviation’s Non-CO2 Emissions

Looking Beyond Carbon

Dec 12, 2024

The quest for sustainability has taken center stage in the aviation industry. In October 2022, the International Civil Aviation Organization (ICAO) Member States, a United Nations agency made up of 193 countries, signed an international agreement to reach net-zero global carbon emissions by 2050.Though much of the conversation up until now has revolved around carbon emissions, there is a growing interest in the non-CO2 impacts of flight.

While the aviation industry contributes about 2% of man-made global carbon emissions, aviation’s climate impact extends far beyond simply carbon emissions. Interestingly, non-CO2 emissions may represent as much as two-thirds of aviation’s total climate impact. Thus, in its quest for sustainability, the aviation sector is beginning to address the effects of non-CO2 emissions: how to measure them, quantify them, and ultimately eliminate them.

Ikke CO2-relaterede klimaeffekter fra flyvning: Kan vi dog ikke bare gå stille med dørene? - Bevar jordforbindelsen — Transport & Environment Non-CO2 Infographic

What are Aviation’s Non-CO2 Emissions?

Non-CO2 is a comprehensive term for all emissions that are not CO2 (obviously). That sounds pretty straight forward, right? But what exactly are the non-CO2 emissions of aircraft? Practically everything airplanes emit BESIDES carbon.

According to the International Air Transport Association (IATA) the non-CO2 emissions from burning jet fuel consist of water vapor (H2O), nitrogen oxides (NOx), sulfur oxides (Sox), carbon monoxide (CO), soot (PM 2.5), unburned hydrocarbons (UHC), aerosols, and traces of hydroxyl compounds (-OH). Some of these interact to generate persistent contrails and the resulting aviation-induced clouds. The impact of contrails, as well as the atmospheric reactions caused by NOx, are aviation’s main contributors climate change.

Each of these emissions play their own role in the complex relationship between aviation and the atmosphere. The exact impact of each component is still being realized through concentrated research efforts, but the understanding of the overall effects of non-CO2 has expanded greatly in the last decade.

Contrails and Water Vapor

Condensation trails (contrails) are the white streaks in the sky sometimes seen behind aircraft. They are formed when water vapor and particulate matter – such as soot and sulfur – emitted from an airplane’s engine exhaust interact with cold, humid air to create ice crystals. Often, contrails only last for a few seconds and disperse as quickly as they are formed, but occasionally they can linger for hours, becoming the nebulous and nefarious, persistent contrails.

Eliminate Persistent Contrails To Cut Global Warming, Report Says | Aviation Week Network — Aviation Week - Julia Gavin - Alamy Stock Photo

Water vapor is a common byproduct of jet fuel exhaust and, by itself, has a minimal impact on the climate. However, when aircraft fly through ice super-saturated regions (ISSRs) – areas of high humidity – the water vapor freezes and the particulates become ice nuclei, eventually forming those persistent contrails. These contrails trap heat from the sun which would otherwise be released from the atmosphere, warming the planet.

NOx and Other Particles

NOx emissions, similar to contrail formation, depend on the engine design, operating conditions, and atmospheric conditions. Studies on the impact of aircraft NOx have found NOx emissions change the atmospheric concentration of ozone and methane and negatively affect air quality and human health. Essentially, NOx acts as a chemical aide, strengthening or weakening the influence of other greenhouse gases in the atmosphere, making its overt presence from aviation emissions a disruption to natural climatic cycles.

Then there is the particulate matter of soot, sulfur, and hydrocarbons. Soot particles, or black carbon (which sounds like a super villain), are remnants of jet exhaust which act as cloud seeds. According to research done by Michigan Tech, these soot seedlings, along with other hydrocarbons, clump together in humid regions of the atmosphere, change their structure, and eventually form clouds. In cloud form, the villainous black carbon particles absorb and trap heat, having a warming effect on the planet.

Sulfur, or sulfate aerosols, on the other hand, reflects sunlight and has a cooling effect overall, as such, it is being considered for stratospheric aerosol injection. This is the notion of releasing sulfates and other reflective particles into the atmosphere to reflect the sunlight and cool the planet (a topic of discussion for another day). Despite its cooling properties, sulfur also acts as a coating for soot particles, enhancing the formation of contrails.

These particles interact in a delicate microphysical dance which has a marked impact on the climate, the environment, and human health. The intricate moves of this dance are slowly being interpreted by academia to unveil exactly what that impact is and how it works.

Climate Impact of Non-CO2

Estimating the climate impact of non-CO2 has been a complicated challenge, as individually, the respective emissions can have both warming and cooling effects. The shorter atmospheric lifetime of these emissions has made it difficult to measure their Effective Radiative Forcing (ERF), with factors such as the flight’s location, time of day, and time of year playing a significant role. However, there is an emerging consensus in the scientific community that non-CO2 emissions have a net-warming effect overall.

In a 2021 study, researchers dived into the ERF of both CO2 and non-CO2 aircraft emissions and concluded that aviation’s climate impact is three times greater than the previously held rate of CO2 measurements alone. They discovered that contrail cirrus causes the largest positive net-warming followed by CO2 and NOx. Even with sulfate aerosols yielding a net-cooling effect, non-CO2 emissions sum to account for more than half (66%) of aviation’s net ERF.

Future aviation climate impacts: the role of aerosol emissions - NERC Panorama DTP — Science Direct - Lee et. al. 2021

Though research has been rigorous, non-CO2 climate forcing is riddled with uncertainties which need to be addressed to get a fuller understanding of aviation’s climate impact. These uncertainties, stemming from weather forecasting, flight routes, and fuel composition, remain a focus of the ongoing work to improve the data surrounding non-CO2’s climate impact, eliminate non-CO2 emissions, and develop policies which will structure the future of flight.