Abstract
Condensation trails from aircraft, also called vapour trails or contrails, are the line-shaped clouds visible from the ground produced by exhausts from aircraft engines or changes in air pressure. However, contrails have an unfortunate side effect, creating a thermal blanket across the globe and contributing to global warming.
Recent discourse over the past two years has revealed that aircraft contrails have a major contributory impact to the greenhouse effect - an extremely worrying prospect for the global aviation industry. Although a relative minority of contrails have a warming effect, depending on atmospheric conditions elaborated on below, they have a massively disproportionate impact on our planet's climate.
The influence of warming contrails, though believed to be significant, is not unequivocal – a degree of scientific uncertainty remains with regards to the exact impact on the climate. Many scientists and researchers, in particular the Transport & Environment Campaign Group (T&E), believe that the global warming caused by contrails is greater than the warming caused by aviation’s CO2 emissions. A recent 2024 T&E study stated that the benefits from avoiding warming contrails would always outweigh those from mitigating the climate impact from aviation’s CO2 emissions. Some researchers (e.g. Adam Durant of SATAVIA and D.S. Lee et al.) have even concluded that the climate impact of contrails nearly double the impact of direct CO2 emissions – an annual total of 652 MTCO2e.
Aviation accounts for c. 2.5% of global CO2 emissions and c. 4% of global warming to date. At the upper estimated limit, scientists like D.S. Lee have argued that ‘the sum of aviation CO2 and non-CO2 effects’ have contributed to about 5% of global heating.
In the UK, the climate impacts of the aviation sector are set to double by 2050 despite the industry attempting to achieve net-zero emissions by that time. The 1.5C warming threshold is likely to be breached this decade, and aviation is one of the hardest sectors to tackle due to long aircraft lifespan coupled with both technological and regulatory barriers.
Some of the most promising prospects to reduce climate impact in the aviation industry are sustainable fuel and novel engine technologies. These two factors together could reduce carbon emissions by 80% by 2050. However, there are issues with feedstock supply and competition with other sectors for the required resources. It will likely be decades for the first hydrogen and electric aircraft to enter into service, and so will be inadequate for the necessary emission reduction trajectory.
Avoiding contrail warming reveals the potential for a ‘quick fix’. The T&E study found that 80% of contrail warming globally is caused by just 3% of flights, and contrail avoidance is one of the easiest climate measures to implement at this moment in time. Carlos Lopez de la Osa, T&E’s Aviation Technical Manager, stated earlier this year that ‘The aviation industry is being offered a simple and cheap way to reduce its climate impact…there are very few climate solutions that can be implemented so quickly, at so little cost and with little impact to industry and consumers’.
Aircraft Contrail Formation
Long-lasting contrails, generally known as ‘persistent contrails’, are effectively man-made cirrus clouds which block heat from escaping Earth’s atmosphere and exacerbate the greenhouse effect. Contrails are created when hot water vapour emitted by an engine cools and condenses in the Earth’s atmosphere. In order for an aircraft to form a persistent contrail, the right conditions need to be in place – cold and humid air. Dr Roger Teoh of Imperial College London has deduced that 24% of flights form persistent contrails, with these persistent contrails on average holding a 70% probability of having a warming effect.
Aircraft fly in the troposphere, the lowest layer of Earth’s atmosphere, and ice-supersaturated regions in the troposphere provide ideal cold and humid conditions for persistent contrails to form. Avoiding these regions by altering flight paths or aircraft altitude is known as ‘contrail avoidance’, since contrails formed elsewhere have an appreciably smaller likelihood of persisting or even forming.
Geography and flight latitude therefore have a very strong influence on whether a contrail is warming. As seen in the picture below, more than half of global contrail warming is caused by flights over North America, Europe, and the Northern Atlantic. The largest warming contribution came from evening, night, and winter flights, with heat escaping at a greater rate in lower temperatures.
2019 Global Annual Mean Net Radiative Forcing (RF) of Contrail Cirrus.
Types of Aircraft and Contrail Formation
Modern aircraft emit less CO2 than older, more inefficient models, but the nature of contrails means they contribute more on balance to the greenhouse effect. To avoid aerodynamic drag and burn less fuel, thereby emitting less CO2, modern aircraft typically fly in the upper troposphere at heights above or around 39,000 feet. In comparison, older models of aircraft flew at around 35,000 feet. Because the air is thinner higher in the atmosphere, contrails created by modern aircraft take longer to dissipate and therefore create a longer warming effect.
Private jets create an even more outsized impact. They fly well above 40,000 feet, where the airspace is much clearer, and although they are smaller and use less fuel a 2024 Imperial College London study found they create similar persistent contrails to much larger commercial aircraft.
Requirements for Contrail Avoidance
Contrail avoidance – as Professor Ian Poll succinctly puts it, ‘avoiding more warmers and generating more coolers’ – does not require any new technology or substantial investment. By altering the altitude or flight paths of a small number of flights, to avoid ice-supersaturated regions of the troposphere, contrail avoidance provides an easy and effective solution to help the aviation sector reduce its climate impact.
The main challenge faced is the lack of financial incentives for airlines to implement contrail management strategies. The other limiting factor is practical, concerning how airlines map the atmosphere and their flight paths to avoid ice-supersaturated regions.
Current Contrail Research and Findings
The T&E study showed that because 3% of flights cause 80% of contrail warming, rerouting this small number of flights has the potential to reduce global contrail warming by more than half by 2040. The study further estimated that this would add roughly £3.50 per ticket for a trans-Atlantic flight from Paris to New York – a flight with a high risk of forming persistent warming contrails.
A project led by the University of Cambridge’s Aviation Impact Accelerator (AIA) emphasised earlier this year the importance of contrail avoidance. In 2024, the AIA proposed a five-year plan with four primary goals to help the UK’s aviation sector achieve net-zero emissions by 2030. The first goal, ‘Operation Blue Skies’, focuses on contrail avoidance as a quick, cheap, and immediate measure to reduce aviation’s climate impact. The AIA showed that if implemented correctly, the UK’s aviation industry could reduce its climate impact by up to 40% in an extremely short timeframe.
The AIA and Reviate hosted a talk at COP29 in November to highlight the importance and ‘simple’ solution of contrail avoidance. Although contrail warming was discussed at the climate conference, no significant action was taken with an agreement on climate finance taking priority.
The 2024 Imperial study mentioned above presented another simple step to reduce the lifetime of contrails by reducing the amount of soot emitted from aircraft engines, which is produced when fuel burns inefficiently. Although multiple models have predicted that contrails last longer when formed with older, dirtier, and less efficient engines, the Imperial study was the first to confirm this phenomenon using real-world observations.
Another issue arises when contrails overlap, because of the cumulative effect of the artificial cloud cover. An upcoming study will explore this in more detail and is expected to be published in 2025.
Volker Grewe, of the Institute of Atmospheric Physics in Germany, looked at 800 transatlantic flights and how their flight paths could have been altered to reduce the warming effect from both contrails and CO2 emissions. He deduced that there could have been a 10% reduction in warming for just a 1% increase in operational costs.
Active Contrail Avoidance Measures and Research
On 22 October 2024, Britain’s Civil Aviation Authority (CAA) set out detailed plans to modernise Britain’s airspace for the first time. There has been little to no change since the 1950’s and has been described by the CAA as the ‘biggest shake-up to airspace design in 70 years’. The aim of this reform is to reduce delays, noise pollution, and carbon emissions through planes spending less time in the air.
Chief Executive of Airlines UK, Tim Alderslade, described the CAA’s plan as ‘a critical pathway through which the industry can achieve net zero emissions’. A team of aviation experts will work with domestic airports to improve the way planes operate in UK airspace, with an initial focus on London as the most congested area. It remains to be seen whether the CAA will take a proactive approach towards mitigating the effects of contrails through this revamp. However, given the increasing conversation and debate surrounding aircraft contrails, it is likely that contrail avoidance will at least be discussed.
The aforementioned Dr Teoh is currently testing his findings in an operational setting, through pilot programmes aimed at steering airlines away from ice-supersaturated regions. By avoiding these areas, he hopes to see a marked decrease in persistent contrail formation, and if successful could potentially implement his findings in a large-scale commercial setting.
Ironically perhaps, given Donald Trump’s re-election as President of the United States, American actors have conducted the majority of practical measures concerning aircraft contrails. Professor Steven Barret of MIT has previously said that ‘We can’t efficiency our way to net-zero’. He has further pinpointed decarbonisation, contrail avoidance, and lowering NOx emissions as the three challenges in mitigating aviation’s climate impact. In October this year, at a summit in Britain, he pointed to MIT’s development of a ‘real-time contrail avoidance tool’ which had been trialled with Delta Air Lines as a cost-efficient and effective strategy.
In the US over a six-month time period, a group of American Airlines pilots flew 70 test flights with their flight paths mapped using Google’s AI-based predictions cross-referenced with Breakthrough Energy’s open-source contrail models. The pilots avoided altitudes likely to cause contrails whilst still attempting to avoid substantial detours which would lead to increased flight times. The experiment clearly showed that contrail avoidance was a cost effective and scalable solution to aviation’s climate impact, as the analysis of satellite imagery showed that contrail formation was reduced by 54%. When additional fuel usage was taken into account, the 70 flights suggested that contrails could be avoided at a scale of around $5 – 25/tonCO2e (CO2 equivalent).
Three weeks ago, NASA and General Electric announced a new partnership to study the aviation industry and in particular the effects of contrails on climate change. The first test flights were launched on November 18th, and using LiDAR sensors aims to advance scientific understanding of contrails and their impact.
Perhaps the most significant active development in the field of contrail avoidance is the decision of the EU’s Emissions Trading System (EU ETS). The EU ETS is planning to measure the non-CO2 impacts of aviation from 2025 onwards, and financial penalties are likely to be introduced shortly afterwards. Contrail avoidance will therefore have to be prioritised by European airlines to avoid financial penalties, providing a financial stimulus to be proactive in avoiding persistent warming contrails.
Conclusion
Contrail warming, though poorly understood currently, is a significant contributor towards aviation’s global climate impact. It is practically unique in the fact that contrail avoidance is both cheap and easy to implement on a global scale.
Although reticence is understandable, the recent and increasing discourse concerning aircraft contrails is bringing contrail avoidance into sharper focus each month, and the next step is for North American and European aircraft sectors to take a proactive approach in the fight against the climate crisis.
Sadly, non-CO2 emissions have been neglected as unimportant across many varied sectors in the past decade, and the aviation sector is no exception. Measuring and penalising non-CO2 impacts in addition to standard CO2 emissions is beginning to happen, and when non-CO2 impacts are taken into account airlines and wider industry actors will have to take contrail warming and avoidance strategies into consideration. A lack of financial stimulus is the sole limiting factor at the moment, so the onus is on both airlines and regulators to start enforcing or taking a proactive approach to contrail management.
Moreover, when hydrogen and electric aircraft eventually come into service, contrail warming will no longer be an issue. Electric aircraft will not produce contrails, and although hydrogen aircraft will form contrails at lower altitudes, Dr Alexandru Rap of the University of Leeds has discovered that hydrogen contrails will induce less warming due to different ice crystal properties.
The pressing matter in the aviation industry is thus not more efficient engines or reduced flight times but contrail avoidance. There is a great opportunity to take advantage of the simplicity inherent in contrail avoidance and may nip ‘aviation’s dirty secret’ in the bud just as it comes to the fore of climate conversation.
Sources:
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