Contrails in 2023: Their Environmental Impact and Emerging Regulations

Nov 21

The aviation industry has been focusing its efforts on reducing and mitigating the impact of CO2 emissions on climate change, and the exciting new fuels and propulsion technologies to achieve emissions neutrality. However, to be truly emissions-neutral and reduce the complete impact of climate change, the industry must comprehensively consider its entire footprint. For perspective, it is estimated that non-CO2 emissions account for two-thirds of aviation’s total warming impact(1). Those two-thirds of emissions include soot, water vapor, and nitrogen oxide (NOx), to name a few. Contrails, the visible lines left in the wake of aircraft, have gained special attention due to their high environmental impact. This paper explores the formation of contrails, their environmental implications, existing government policies and voluntary disclosures worldwide, and mitigation strategies.

What Are Contrails?

Surprisingly, contrails have been studied since World War II(2). While the military's interest was primarily related to aircraft detectability, the broader environmental and climate impact of contrails has become a central subject for aviation scientific research and study.

Contrails, short for “condensation trails,” are a product of aircraft engine emissions interacting with the atmosphere. Soot and aerosol particles are emitted and serve as the condensation nuclei for ice crystals to form(3). When atmospheric conditions are right, the water vapor emitted at high altitudes and cold temperatures can rapidly freeze into ice crystals around these particles. These crystals leave a visible trail behind the aircraft, which ultimately can coalesce, and create expansive cirrus clouds(1). Because contrails are simply ice condensation, they require the right combination of temperature and humidity to form, or what is known as an “ice supersaturated region (ISSR).” Supersaturation of water is very common in the troposphere, but less likely above the tropopause and in the stratosphere(4). These regions typically have extensive horizontal coverage but may only extend a few thousand feet vertically, forming thin, wide bands(5). Persistent contrails gradually spread into cirrus clouds, known as contrail cirrus.

In most cases, once contrails are created, they will disappear quickly, within a few seconds or minutes. However, if the relative humidity compared to ice is above 100%, contrails can persist for several hours, termed “persistent contrails”, and depending on the winds, can even coalesce with other contrails and cause increased cirrus cloudiness or “aircraft induced cirrus.” (1,4) The conditions for the formation of contrails are slightly different than conditions required for persistence, meaning that not all contrails persist nor will coalesce.

Sizeable Impact on the Environment

At a high level, global aviation is increasing cloudiness through the creation of contrails, especially as they persist, coalesce, and spread(1). However, the impact of a specific contrail and how we relate it to CO2 emissions involves a few additional concepts, such as radiative forcing and atmospheric lifetime.

Radiative forcing is a concept used in climate science to quantify the change in the energy balance of the Earth's climate system. It measures the difference in the amount of energy received from the sun against the amount of energy radiated back into space(6). All greenhouse gas emissions (including nitrous oxides, sulfur gases, carbon monoxide, and soot) have a different radiative force, which can be categorized as either warming or cooling forces, depending on whether they result in more incoming or outgoing energy(1,6). Each of these varying forces needs to be measured in order to understand our full emissions impact.

A visual representation of the warming and cooling impact contrails have based on time of day. Source: Google

Contrails complicate this equation. What makes them unique from other aviation emissions is that the time of day is an important factor in determining their environmental impact. All contrails trap heat that would otherwise be radiated into space back towards earth, warming the environment. However, during the day, high-altitude cirrus clouds can reflect some sunlight away from the earth’s surface, providing an offsetting cooling effect that can counteract the warming impact created. The magnitude of the cooling impact depends on the angle of the light relative to the contrail, with shallow angles providing the most reflection. This means when the sun is low around dawn or dusk, that is when contrails are most likely to have a high cooling effect. During the night, this opportunity for light reflection does not exist, causing contrails at night have a bigger net warming impact. What is important to understand around the time impact of contrails is that it matters what time of day the contrail predominantly persists during, not when the aircraft initially creates the contrail, in terms of defining its overall effect.

While contrails have the potential to warm (trapping energy) and cool (reflecting energy), the global net impact is warming. In fact, a study of aviation’s impact on climate forcing showed that contrails have the greatest warming impact of all aircraft tailpipe emissions(1). This trapping effect also keeps pollutants in the lower atmosphere, influencing local air quality in specific regions.

Another factor to consider particularly when comparing contrails to CO2 is the atmospheric lifetime difference, meaning that different emissions persist in our atmosphere for different amounts of time. For example, carbon remains in the earth’s atmosphere for around 100 years on average, whereas contrails will last 18 hours at the most. It is the persistent contrails (those that last longer than a few minutes) that have the most substantial environmental impact(4). If all aviation activity were to cease, any impact from contrail cirrus clouds would disappear as soon as the last contrail dissipated(3). This makes comparing the relative impact of carbon emissions to that of contrails complex and far from a straightforward task.

In a sense, contrails have a multiplying effect—as other greenhouse gas emissions increase and warm the climate, contrails trap and reflect this warming effect(7). While it is difficult to narrow the exact environmental impact of a specific contrail, it is widely agreed that the overall impact is equal to, if not substantially worse than, CO2. A 2022 Intergovernmental Panel on Climate Change (IPCC) report estimates contrails alone could account for 35% of aviation’s climate impact(8).

Mitigation

It's important to note that the scientific community's understanding of contrails and their effects continues to evolve as more research is conducted and new data becomes available. Contrails are difficult to track because eventually they spread out and cannot be distinguished from typical cirrus clouds. Because contrails need the right combination of temperature and humidity, researchers are studying areas of the troposphere and tropopause where these conditions exist(1). One strategy to eliminate the environmental impact is avoidance: can we identify ice supersaturated regions in the atmosphere and re-route aircraft to prevent contrail creation altogether?

Keen aviation professionals may have immediately identified the issue with this solution: re-routing aircraft can impact fuel efficiency, creating scenarios where avoiding contrail creation may result in a fuel penalty and additional carbon dioxide emissions. However, research indicates that a very small percentage of flights are responsible for most of the contrail-based warming impact on the environment. One study of select flights in Eastern Asia showed that only 2% of flights were responsible for 80% of the total radiative forcing. This could mean that reroutes for a small number of flights could have a big impact. Most times, changing flight altitudes by just a few thousand feet can avoid entering an ice supersaturated region(5).

A map with flights showing areas where warming and cooling contrails have formed — Reviate Contrails Map by Breakthrough Energy *(contrails.org)*

Initiatives like those from Breakthrough Energy (in partnership with Google and American Airlines) and Massachusetts Institute of Technology (MIT) are not only working to forecast and map contrails in real time, but are exploring ways to better understand and quantify these potential tradeoffs. You can visit map.contrails.org to see some of their work that visualizes real contrail impacts from global flight data and classifies the impact relative to a flight’s CO2 emissions.

The goal is to create a tool to provide flight planners and crews with information to fly white the smallest climate impact possible, and several tools are being piloted today(12). After a directive from the White House, the FAA Center of Excellence for Alternative Jet Fuels & Environment (ASCENT) has projects dedicated to identifying contrails, simulating contrail plumes under various weather conditions, minimizing contrail length, and building a forecasting tool to predict ice supersaturated regions for avoidance. Individuals can even support research through the University of Cambridge’s Contrailkit, capturing and uploading photos of contrails in real-time to help build an observer-based dataset. With the help of artificial intelligence and real-world data, these projects are developing models that will enable flight planning around ISSRs.

Another mitigation solution could come in the form of new technologies. As alternative engine propulsion and fuel technologies develop, the “changes in engine fuel efficiency...might change the amount of heat and water emitted in the exhaust plume, thereby affecting the frequency and geographical cover of contrails” (FAA). Without water vapor as a byproduct of fuel combustion, contrail formation should be rare or nonexistent(7).

The industry is hopeful about the role sustainable aviation fuel (SAF) could play. While the tank-to-wake CO2 emissions are the same from drop-in SAF as traditional Jet-A, research shows that the impact on non-CO2 climate impact may be different(3). The increased hydrogen-to-carbon ratio in SAF results in more water vapor but decreases the number of soot particles coming out of the back of an engine—an interesting combination when considering contrail formation. Contrails from SAF will ultimately be different from those formed by burning traditional Jet-A(3). Studies like those from NASA and the German Aerospace Center (DLR) are currently working to understand the impact of SAF on contrail formation, and have indicated exciting preliminary results, but it is too early to validate any claims yet.

Ultimately, the climate impact of aviation emissions depends on the altitude of emission. Business aviation aircraft frequently fly at higher altitudes, often above the tropopause, which results in emissions being directly released into the upper atmosphere. Little data exists, but the expectation is that this will increase the lifespan of those emissions, although it should prevent the formation of contrails. These counteracting effects need to be considered together in order to quantify the total impact aviation has and when developing mitigation options(5).

Regulatory Landscape

While contrail research is developing in real-time, so too are the discussions around implementing regulations to capture the impacts made by contrails. The most recent EU directive states that “aircraft operators shall report once a year on the non-CO2 aviation effects" starting in 2025, including oxides of nitrogen (NOx), soot particles, oxidized sulfur species, and effects resulting from water vapor, such as contrails. The EU will take the data collected and consider whether to integrate contrails and other non-CO2 emissions into the EU Emissions Trading Scheme (ETS) by the end of 2027. The updated mandates within ReFuel EU do not call out contrails specifically but require fuel suppliers to report the “aromatics, naphthalenes, and sulfur content” of fuels. This data could lead to regulation around the amount of aromatics in fuel to mitigate soot formation and encourage SAF adoption(6). While these initiatives are only aimed at operations in the EU, this sets a precedent as the first government to require reporting on non-CO2 aviation emissions.

Governments like the United Kingdom and the United States are actively engaged in pioneering research in this field, but they have not yet taken concrete steps toward regulating contrails. However, research from groups like ASCENT and MIT will be provided to the Committee on Aviation Environmental Protection (CAEP) in upcoming workshops, which assists the International Civil Aviation Organization (ICAO) in formulating new policies and standards. It is crucial to define consistency in the scientific understanding and measurement of contrails before implementing any regulations concerning them.

Conclusion

Understanding the precise environmental consequences of contrails is a complex challenge, but their impact on the environment should not be underestimated. Research findings suggest that contrails, in isolation, have the potential to contribute up to one-third of aviation's overall climate impact. Despite the absence of comprehensive regulatory mandates and the ongoing development of mitigation strategies, it is imperative to closely monitor and address the issue of contrails. The biggest opportunity for that today is by understanding your operation’s baseline impact and frequency of contrail formation. 4AIR has launched the first contrail footprinting effort in business aviation and one of the first pilot programs to mitigate contrail formation through flight planning. As strides are taken to achieve our industry's ambitious 2050 sustainability goals, confronting the environmental implications of contrails is an integral part of the broader effort to reduce aviation's carbon footprint and ensure a more sustainable future for air travel.