Clearing the air: Understanding and mitigating the impact of aviation non-CO2 emissions
As the air transport sector continues its journey towards decarbonization, there is an emergent consensus that carbon dioxide emissions are not the only mechanism by which it affects the climate. Several publications coming from both academia and industry have highlighted promising mitigation pathways for—but also a need for improved understanding of—the most significant of aviation non-CO2 climate effects: warming caused by condensation trails, or “contrails.”
Highlighting the increasing importance of the topic, the International Civil Aviation Organization (ICAO), the UN agency coordinating global aviation policy, held its first symposium on non-CO2 effects this September. The fact that the issue is now being discussed at ICAO level means that aviation stakeholders will need to start building an understanding of the rapidly evolving scientific knowledge and emergent policy trends to better assess the implications of future developments in the field on their work.
Assessing the impact of contrails
The existence of non-CO2 effects was acknowledged by ICAO as far back as 1999 in a special report after air transport was left out of the Kyoto Protocol, the predecessor to the Paris Agreement. It identified radiative forcing from cloud-forming contrails as a major potential contributor, but uncertainty stemming from insufficient data availability prevented the scientists behind it from establishing the magnitude of these effects. The sheer complexity underpinning the processes involved in contrail formation, the potential for transition to cirrus clouds and the subsequent effect of those clouds on the energy balance in the atmosphere, all contributed to the lack of concerted action to address them at this early stage.
It was not until two decades later that science provided a broad but reliable estimate of the potential magnitude of non-CO2 effects, and this quickly caught the attention of policymakers. A 2020 European Aviation Safety Agency report put together at the request of the EU Commission featured some of the first policy proposals on contrail impact mitigation.
What is known about the warming effects of contrails?
Current scientific understanding converges on the amount of warming from contrails being at least equivalent to that caused by aviation’s carbon dioxide emissions—and potentially several-fold greater. However, the two impacts occur over vastly different timescales. When compared to the period that carbon dioxide emissions remain in the atmosphere, contrails and even the cirrus clouds they form are very short-lived. This makes settling on a comparison metric for their warming potential more difficult.
Not all contrails persist long enough to significantly affect Earth’s energy balance, and not all of those that do have a net warming effect. Their optical properties and the time of day in which they form play an important role in this regard.
Contrail formation and persistence involve several complex processes, each with its own set of variables. Those involving interactions with the atmosphere are harder to anticipate and measure. By contrast it’s much easier to characterize those related to technical aspects like fuel composition and emission properties. For example, the presence and share of aromatic compounds in the fuel can affect the number of nucleation points available for condensation and determine both the reflectivity of the contrail and its persistence.
This is where the main technology for decarbonizing aviation has relevance for addressing its non-CO2 impacts as well. Low-carbon sustainable aviation fuels (SAF) have considerably lower aromatics content and are therefore expected to help mitigate contrail formation. The concept of strategically deploying the limited amounts of SAF available to those flights deemed most likely to produce contrails has already been proposed in academia and, more recently, further explored as part of the landmark transatlantic Flight100 powered entirely by SAF.
Could rerouting flights be the answer?
For now, the main solution put forward is navigational avoidance: essentially changing aircraft routing if the flight is deemed likely to produce persistent contrails. The first trial involving scheduled air traffic was conducted in European airspace in 2021 and involved calculated altitude changes for thousands of flights over the span of several months. The results validated the basic principles behind navigational contrail avoidance as well as its potential. But they also revealed a very limited capacity to predict the formation of contrails.
Location matters, with the majority of warming contrails occurring over areas like the North Atlantic, for example. Persistent contrails are most likely to form in atmospheric regions that are ice-supersaturated (ISSRs). Predicting the distribution of these areas remains the Holy Grail of navigational avoidance. The recent involvement of the likes of Google and Breakthrough Energy, along with industry heavyweights like American Airlines, means that the knowledge gap is rapidly being narrowed as large-scale trials leverage more detailed operational data and growing computational capacity to refine prediction models.
The potential fuel burn increase to avoid contrails is estimated to average 1.4% per flight. But it could potentially be much lower if the relatively small proportion of flights that will need to be rerouted is considered. However, the prospect of burning even this amount of additional fuel has given rise to the main point of contention within the scientific community: whether studying and implementing ways to avoid contrails—and emitting more CO2 in the process—is a distraction from the well-understood goal of drastically reducing carbon dioxide emissions.
Regulators are taking action
The European Union sees these two goals as complementary and has legislated accordingly. A monitoring, reporting, and verification (MRV) system for non-CO2 emissions will be implemented within the EU Emissions Trading Scheme from 2025. It will take effect alongside ReFuelEU Aviation, its flagship regulation to reduce carbon dioxide emissions by mandating an increasing supply of SAF at the EU's airports.
As with other regulations aimed at facilitating SAF deployment, two distinct approaches to tackling contrail climate impacts could potentially emerge on either side of the Atlantic. This would create different landscapes of incentives, opportunities, and challenges for industry actors and other interested parties. Understanding both the science and the policy responses will prove crucial in navigating this evolving area of aviation and climate.
The wider relevance of contrails science and regulation
There is another overarching rationale for keeping track of these developments. Emergent policy frameworks on aviation non-CO2 effects are possibly one of the earliest cases of national and supra-national entities regulating in the presence of considerable scientific uncertainty, with the aim of both reducing it and addressing the underlying problem.
Regardless of whether this approach proves successful or not, this is a stark departure from the dynamics seen at a global level in the past few decades, where climate policy has consistently fallen behind climate science. This approach is one possible response to the expected intensification of climate change effects.
It will be increasingly important to engage with policymakers on the matter, who will also likely seek input from industry, academia, and other aviation stakeholders on the regulations they are crafting. And for those engagements to be successful, a solid understanding will be necessary—not only of the science but also of the technical and sociopolitical implications for the global air transport system.