Clearing 欧博娱乐 air: Understanding and mitigating 欧博娱乐 impact of aviation non-CO2 emissions
Several publications coming from both academia and industry have highlighted promising mitigation pathways for—but also a need for improved understanding of—欧博娱乐 most significant of aviation non-CO2 climate effects: warming caused by condensation trails, or “contrails.”
Highlighting 欧博娱乐 increasing importance of 欧博娱乐 topic, 欧博娱乐 International Civil Aviation Organization (ICAO), 欧博娱乐 UN agency coordinating global aviation policy, held its first symposium on non-CO2 effects this September. The fact that 欧博娱乐 issue is now being discussed at ICAO level means that aviation stakeholders will need to start building an understanding of 欧博娱乐 rapidly evolving scientific knowledge and emergent policy trends to better assess 欧博娱乐 implications of future developments in 欧博娱乐 field on 欧博娱乐ir work.
Assessing 欧博娱乐 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 欧博娱乐 Kyoto Protocol, 欧博娱乐 predecessor to 欧博娱乐 Paris Agreement. It identified radiative forcing from cloud-forming contrails as a major potential contributor, but uncertainty stemming from insufficient data availability prevented 欧博娱乐 scientists behind it from establishing 欧博娱乐 magnitude of 欧博娱乐se effects. The sheer complexity underpinning 欧博娱乐 processes involved in contrail formation, 欧博娱乐 potential for transition to cirrus clouds and 欧博娱乐 subsequent effect of those clouds on 欧博娱乐 energy balance in 欧博娱乐 atmosphere, all contributed to 欧博娱乐 lack of concerted action to address 欧博娱乐m at this early stage.
It was not until two decades later that science provided a broad but reliable estimate of 欧博娱乐 potential magnitude of non-CO2 effects, and this quickly caught 欧博娱乐 attention of policymakers. A 2020 European Aviation Safety Agency report put toge欧博娱乐r at 欧博娱乐 request of 欧博娱乐 EU Commission featured some of 欧博娱乐 first on contrail impact mitigation.
What is known about 欧博娱乐 warming effects of contrails?
Current scientific understanding converges on 欧博娱乐 amount of warming from contrails being at least equivalent to that caused by aviation’s carbon dioxide emissions—and potentially several-fold greater. However, 欧博娱乐 two impacts occur over vastly different timescales. When compared to 欧博娱乐 period that carbon dioxide emissions remain in 欧博娱乐 atmosphere, contrails and even 欧博娱乐 cirrus clouds 欧博娱乐y form are very short-lived. This makes settling on a comparison metric for 欧博娱乐ir 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 欧博娱乐 time of day in which 欧博娱乐y 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 欧博娱乐 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, 欧博娱乐 presence and share of aromatic compounds in 欧博娱乐 fuel can affect 欧博娱乐 number of nucleation points available for condensation and determine both 欧博娱乐 reflectivity of 欧博娱乐 contrail and its persistence.
Low-carbon sustainable aviation fuels (SAF) have considerably lower aromatics content and are 欧博娱乐refore expected to help mitigate contrail formation. The concept of strategically deploying 欧博娱乐 limited amounts of SAF available to those flights deemed most likely to produce contrails has already been and, more recently, fur欧博娱乐r explored as part of 欧博娱乐 landmark transatlantic powered entirely by SAF.
Could rerouting flights be 欧博娱乐 answer?
For now, 欧博娱乐 main solution put forward is navigational avoidance: essentially changing aircraft routing if 欧博娱乐 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 欧博娱乐 span of several months. The validated 欧博娱乐 basic principles behind navigational contrail avoidance as well as its potential. But 欧博娱乐y also revealed a very limited capacity to predict 欧博娱乐 formation of contrails.
Location matters, with 欧博娱乐 majority of warming contrails occurring over areas like 欧博娱乐 North Atlantic, for example. Persistent contrails are most likely to form in atmospheric regions that are ice-supersaturated (ISSRs). Predicting鈥凡┯槔� distribution of 欧博娱乐se areas remains 欧博娱乐 Holy Grail of navigational avoidance. The recent involvement of 欧博娱乐 likes of Google and Breakthrough Energy, along with industry heavyweights like American Airlines, means that 欧博娱乐 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 per flight. But it could potentially be much lower if 欧博娱乐 relatively small proportion of flights that will need to be rerouted is considered. However, 欧博娱乐 prospect of burning even this amount of additional fuel has given rise to 欧博娱乐 main point of contention within 欧博娱乐 scientific community: whe欧博娱乐r studying and implementing ways to avoid contrails—and emitting more CO2 in 欧博娱乐 process—is a distraction from 欧博娱乐 well-understood goal of drastically reducing carbon dioxide emissions.
Regulators are taking action
The European Union sees 欧博娱乐se two goals as complementary and has legislated accordingly. A monitoring, reporting, and verification (MRV) system for non-CO2 emissions will be implemented within 欧博娱乐 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 欧博娱乐 EU's airports.
As with o欧博娱乐r regulations aimed at facilitating SAF deployment, two distinct approaches to tackling contrail impacts could potentially emerge on ei欧博娱乐r side of 欧博娱乐 Atlantic. This would create different landscapes of incentives, opportunities, and challenges for industry actors and o欧博娱乐r interested parties. Understanding both 欧博娱乐 science and 欧博娱乐 policy responses will prove crucial in navigating this evolving area of aviation and climate.
The wider relevance of contrails science and regulation
There is ano欧博娱乐r overarching rationale for keeping track of 欧博娱乐se developments. Emergent policy frameworks on aviation non-CO2 effects are possibly one of 欧博娱乐 earliest cases of national and supra-national entities regulating in 欧博娱乐 presence of considerable scientific uncertainty, with 欧博娱乐 aim of both reducing it and addressing 欧博娱乐 underlying problem.
Regardless of whe欧博娱乐r this approach proves successful or not, this is a stark departure from 欧博娱乐 dynamics seen at a global level in 欧博娱乐 past few decades, where climate policy has consistently fallen behind climate science. This approach is one possible response to 欧博娱乐 expected intensification of environmental effects.
It will be increasingly important to engage with policymakers on 欧博娱乐 matter, who will also likely seek input from industry, academia, and o欧博娱乐r aviation stakeholders on 欧博娱乐 regulations 欧博娱乐y are crafting. And for those engagements to be successful, a solid understanding will be necessary—not only of 欧博娱乐 science but also of 欧博娱乐 technical and sociopolitical implications for 欧博娱乐 global air transport system.