A new assessment by a group of UK climate scientists into potential solutions to limit non-CO2 emissions produced by aircraft concludes there is no ‘silver bullet’ and concerted efforts and commitments are needed to better understand the complex chemistry trade-offs involved before making policy decisions. The findings, published in the Royal Society for Chemistry’s journal, Environmental Science: Atmospheres, are the result of a two-year study by Manchester Metropolitan University (MMU), Imperial College London and the universities of Oxford and Reading. Aviation is responsible for around 2.5% of global CO2 emissions caused by human activity but due to the amount of non-CO2 emissions it produces, the sector is responsible for around 4% of the increase in global mean temperatures. The effects have been known for many years but the science remains inconclusive, although recent trials claim contrail avoidance technology and route planning, plus the use of sustainable aviation fuels, may provide a partial solution.

However, the new paper’s lead author David Lee, Professor in Atmospheric Science at MMU and a lead author on landmark papers on aviation and the climate, cautions that reducing the impact of non-CO2 emissions on the climate is not straightforward. “Practically all routes forward with conventional liquid hydrocarbon fuels involve trade-offs, mostly at the expense of emitting more CO2, whether it be technological or operational efforts,” he said.

“These trade-offs and uncertainties mean that there are no simple silver bullets or low-hanging fruit to solve the problem. What is often forgotten is that while the non-CO2 climate impacts of, for example, an individual flight are short lived, a substantial proportion of the emitted CO2 persists for a very long time, literally tens of millennia. This means it is a difficult balancing act if reducing non-CO2 emissions leads to an increase in CO2 emissions.”

Added co-author Keith Shine, Regius Professor of Meteorology and Climate Science at the University of Reading: “Given the many uncertainties in the size of aviation non-CO2 climate effects, it is premature to adopt any strategy that aims to decrease non-CO2 climate effects but, at the same time, risks increasing CO2 emissions. We must be mindful that aviation affects local air quality as well as climate. Sometimes measures that improve one will be to the detriment of the other.”

Aviation non-CO2 emissions that affect climate include nitrogen oxides (NOx), aerosol particles (soot and sulphur-based) and water vapour. Aerosols and water vapour have small direct radiative effects but are also involved in the formation of contrails and contrail cirrus, currently the largest non-CO2 effect on  climate, but which comes with large uncertainties. According to the paper, these non-CO2 effects on climate are quantified by scientists with low confidence, compared to that of CO2, which is quantified with high confidence.

Reducing the occurrence of persistent contrails through navigational avoidance of cold ice-supersaturated regions has currently very low confidence as a mitigation measure, says the paper, because of the challenges in making accurate meteorological forecasts on the time and space scales required for operational implementation on an individual flight basis. “Robust statistics are needed to assess whether diversions would have avoided contrails, and these are not yet available,” it adds. The evidence also remains weak for the use of SAF in reducing contrail cirrus forcing, says the paper.

“It is clear that there is an appetite amongst some stakeholders for non-CO2 mitigation of aviation effects on climate, but we have serious reservations over recommending definitive courses of action until there is better quantification of the actual effects, and further studies of the trade-offs between non-CO2 reductions versus potential CO2 increases,” conclude the authors. “It is realised that this represents a serious barrier to technology development and policy making but there are no short cuts, and the underlying danger is of either nugatory and expensive efforts which are not easily reversed or making matters worse in terms of the total climate effect of aviation.”

The researchers argument for more work to be performed on the complex trade-offs “in order to urgently search for solutions” has been recognised by the UK government, which recently announced, through the Natural Environment Research Council, a £10 million ($12.5m) research programme to help inform policy decisions in this area.

“This is a very welcome and much needed development by the government,” said Professor Lee. “Some of our previous research that was used by the IPCC (Intergovernmental Panel on Climate Change) has vitally informed the government on the scale of the problem. We have endeavoured to keep the UK Department for Transport informed of our work while we prepared this assessment, to inform the shape of future research needed.”

Photo: University of Reading

Christopher Surgenor

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An academic analysis of 12 roadmaps for decarbonising the global aviation industry finds they place a heavy reliance on biogenic fuels in the medium-term and synthetic e-kerosene in the longer term but largely omit a number of fundamental problems with sustainable aviation fuels, says a new science-led paper. Realising these roadmaps could require 9% of global renewable electricity and 30% of sustainably available biomass by 2050, with significant energy losses as SAF production is energy intensive and comes with a risk of clean energy displacement. Additional effects omitted in most roadmaps relate to the decadal time lags in re-sequestering biocarbon in the case of forest biomass and the impact of non-CO₂ emissions. With competition over land and scarce clean energy, along with the high economic and political investments required, there is a question as to whether the use of SAF is a worthwhile climate action, say the authors.

The challenge for aviation is to abate CO₂ emissions that in 2019 totalled around 1 billion tonnes (Gt), while at the same time aiming to grow a sector against an agreed target of net zero emissions by 2050. The total climate impact of air travel is also larger than CO₂ alone due to other emissions, in particular nitrogen oxides, water vapour and soot.

To date, the industry’s climate mitigation strategy has involved market-based mechanisms such as emissions trading and carbon offsetting, although offsetting does not represent absolute emissions reductions. Step-change technologies such as hydrogen-powered planes and battery-powered short-haul flights will lead to GHG reductions but at a small scale given the limited contribution of short flights to overall emissions. This reality means that fuel cells and battery technologies play no role in aviation decarbonisation roadmaps up to 2050 for long-haul flights, where 5% of flights over 4,000 km make up 40% of fuel use.

The long-haul network will only survive in its current form with liquid hydrocarbon fuels, says the paper, which focuses on the SAF pathways related to biomass, waste and power-to-liquid (PtL) e-kerosene, plus their resource requirements.

“A key challenge is to replicate the long-term natural geological processes that produce fossil hydrocarbons,” it says. “Non-fossil primary energy requires significant processing to be turned into ‘final energy’ of SAF that delivers the ‘useful energy’ for flying. With energy becoming increasingly valuable due to universally declining ratios of ‘energy return to energy invested’, it is necessary to be strategic regarding where to invest primary energy. At each stage of the SAF production process, energy is ‘lost’ as waste heat (second law of thermodynamics).”

The real-world availability of clean primary energy at present and for the foreseeable future is limited, argues the paper. “In terms of achieving global decarbonisation, clean energy, just like land, represents a scarce resource. SAF is only one amongst many potential uses.”

The biomass and clean energy requirements of SAF production therefore necessitate a perspective on the availability of these resources beyond aviation. The 2019 global primary energy supply of 612 Exajoules (EJ) was dominated by fossil fuels (84.3%) and delivered final energy of 435 EJ, the difference being losses from the conversion. That same year, the total supply of renewable plus nuclear electricity was 36 EJ, meaning substantial investment is required to grow the clean electricity sector by 2050. Estimates for electricity generation in 2050 range from 224 EJ from renewable sources plus 20 EJ from nuclear.

The aviation final energy demand in 2050 in the roadmaps varies between 15 EJ and up to 30 EJ, although greater amounts of primary energy are required to produce this final energy, with one roadmap calculating that 16 EJ required by aviation in 2050 will demand 28 EJ primary energy. “To synthesise the assumptions in the roadmaps, it appears that broadly, aviation could require 20 EJ of electricity and 15 MJ from biomass,” says the paper. “These would represent 9% of 224 EJ global renewable electricity and 30% of available 50 EJ biomass.

“Clearly, food, municipal and industrial waste volumes will only deliver a fraction (5 EJ in total) of energy, meaning that energy crops (5-10 EJ) and agricultural (10-12 EJ) and forest residues (10-20 EJ) become crucial. The land use implications are substantial.”

Hypothetically, to produce 15 EJ – the total biomass requirements for aviation – from sugar cane would require 125 million hectares of land, larger than the land area of South Africa. For comparison, renewable electricity installations deliver 470-1070 gigajoules (GJ) per hectare and year.

Hydrogen is required for some biogenic SAF processes as well as for PtL, although estimates of how much hydrogen will be available in 2050 were found by the study to vary. In addition, for PtL it is necessary to capture CO₂.

All inputs bring a risk of displacement. FOGs (fats, oils and greases) are already a valuable commodity and diverting feedstock to SAF could lead to replacement by palm oil, vegetable oil or even fossil fuels. E-kerosene can lead to displacement when electricity used in the process is not additional to existing efforts of decarbonising the energy sector.

Examining the carbon benefits of using bioenergy requires an understanding of the global carbon cycle, including carbon stocks and flows, and consideration of the size, longevity and stability of the stock. From an ecological perspective, for example, “there is no such thing as ‘residue’ biomass in a forest ecosystem as all biomass, living or dead, is part of the total ecosystem carbon stock,” says the paper. The assumption made in aviation roadmaps that all biomass is carbon neutral is therefore invalid, it adds. As well, the assumption that SAF emissions will be 80% lower than fossil jet fuel rests on a small number of feedstocks, which have not yet been deployed industrially or are limited in volume.

The paper notes that although with high uncertainties, all forms of SAF contribute to non-CO₂ warming, increasing the overall climate impact of aviation relative to other sectors.

“Against this precarious backdrop and given the physical constraints on land and clean energy, the amount of SAF required to support aviation growth lacks critical and systemic assessment of feedstocks. Airlines have adopted a strategy that is dependent upon rapid and sustainable expansion of SAF because it is the only technical solution to maintaining long-haul flights,” say the authors.

“Whilst technologically feasible, as evidenced in a small number of plants, the production of SAF at scale and the simultaneous minimisation of unintended consequences have yet to be demonstrated. A major constraint is that it is not only aviation but the whole energy system – still largely dependent on fossil fuel – that needs to decarbonise within the next decade or two.”

To reach net zero, roadmaps invoke carbon capture and storage (CCS) technologies, for example bioenergy with carbon capture and storage (BECCS), to remove ongoing residual emissions once all feasible decarbonisations strategies have been deployed but this is high risk, thermodynamically costly and societally untested, warns the paper.

“Clearly, land dedicated to long-lived eco-system carbon sinks is a superior mitigation strategy compared to its use for bioenergy and should be prioritised where possible,” it says. “So the message that ‘net zero is not enough for 1.5C’ is a critical one in the SAF debate. SAF production competes for land area dedicated to nature-based removal, but it also competes – in the case of e-kerosene – with all forms of carbon capture and storage. Both biogenic and PtL derived SAF are designed with the purpose of combustion, thereby releasing GHGs into the atmosphere.

“The implications of SAF usage as a counterfactual to decarbonisation and permanent carbon removal is widely ignored and rarely acknowledged in aviation roadmaps.”

The authors add: “Clearly the societal goal is not to achieve net zero of one single sector but to maximise our chances of averting catastrophic climate impacts. If decarbonising one sector undermines the opportunity of transitioning other parts of the global socio-economic system, then questions need to be asked as to how the allocation of scarce resources should be prioritised. Understanding the consequences of one sector’s climate action on the ability to achieve collective mitigation goals is crucial.

“Given the economic and political investments required, the question is whether SAF really reduces atmospheric concentrations of CO₂ compared with a business-as-usual case of fossil fuel usage. In other words, is it a worthwhile climate action?

“This paper provides insights into some of the trade-offs and risks, including competition over land and scarce clean energy, and the answer will differ for different countries.”

The authors of the paper, ‘Implications of preferential access to land and clean energy for Sustainable Aviation Fuels’, are:

Susanne Becken, Professor of Sustainable Tourism, Griffith Institute for Tourism, Griffith University, Australia. She is a member of the Air New Zealand Sustainability Advisory Panel and member of the Independent Advisory Group of Travalyst.
Brendan Mackey, Director, Griffith Climate Action Beacon, Griffith University.
David S. Lee, Professor of atmospheric science, Aviation and Climate Research Group Leader, Manchester Metropolitan University, UK. He is a member of the UK Jet Zero Council, a co-rapporteur of ICAO’s Impacts and Science Group, and a member of the UK CAA’s Environmental Sustainability Panel.

Christopher Surgenor

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Pre-pandemic, aviation was responsible for nearly one gigaton of CO2 emissions annually, around 2.4% of the global total, although through its additional non-CO2 impacts it contributes around 4% to human-induced global warming, a share that is increasing. A paper published in Environmental Research Letters suggests emissions produced by the aviation industry must be reduced each year if they are not to increase warming further, otherwise the sector could consume up to one-sixth of the remaining budget to limit warming to 1.5°C by 2050. The UK researchers behind the study developed a simple technique for quantifying the temperature contribution of historical aviation emissions, including both CO2 and non-CO2 impacts. They then modelled a number of potential post-Covid industry recovery scenarios and their climate impacts through to 2050, together with the potential effects of the industry scaling up the use of low-carbon fuels. The authors show that the only way to ‘freeze’ the temperature increase from the sector is to decline emissions by about 2.5% per year. However, they found a 90% mix of carbon neutral fuels by 2050 could achieve a similar outcome, with no further temperature increase from the sector.

Aviation is projected to cause a total of about 0.1°C of warming by 2050, half of it to date and the other half over the next three decades, should aviation’s pre-Covid growth resume. The industry would then contribute a 6%-17% – so up to one-sixth – share of the remaining 0.3-0.8°C to not exceed 1.5-2°C of global warming. Under this scenario, the reduction due to Covid-19 to date is small and is projected to only delay aviation’s warming contribution by about five years as it is the cumulative emissions that matter, found the researchers of the study, ‘Quantifying aviation’s contribution to global warming’, from the University of Oxford, Manchester Metropolitan University and the NERC National Centre for Earth Observation.

“Covid has reduced the amount people fly, but there is little chance for the aviation industry to meet any climate target if it aims for a return to normal,” warned Milan Klöwer, a climate physics researcher at the University of Oxford who led the study.

To estimate aviation’s contribution to current and future anthropogenic global warming, the researchers analysed the total climate forcing, taking both CO2 and non-CO2 (those aircraft emissions of water vapour, NOx, sulphur and soot at altitude that have accounted for more than 50% of aviation-induced warming) effects into account. A large fraction of the increase in atmospheric CO2 naturally stays for centuries, hence why recent emissions of CO2 alone do not drive global warming, but the accumulative emissions. The accumulated carbon emissions of aviation for the period 1940-2019 are 33 GtCO2, equivalent to the historic emissions of Canada and about 2% of the world’s cumulative CO2 emissions. By contrast, most non-CO2 effects vanish within a year, the exception being the negative forcing from methane. Taking both CO2 and non-CO2 effects into account, the total aviation-induced warming up to 2019 is about 4% of the almost 1.2°C that the planet has warmed so far, found the researchers.

They have designed four scenarios to capture a possible future of global aviation to 2050:

Scenario 1: No pandemic – Assumes there had been no Covid-19 pandemic and a continuous growth in air traffic CO2 emissions of about 3% per year.
Scenario 2: Back to normal – Assumes a post-Covid recovery for 2021-2024 at 16% annual growth and 3% thereafter, so the pre-Covid level is reached in 2024.
Scenario 3: Zero long-term growth – Assumes a 13% annual growth for the 2021-2024 recovery period, zero growth thereafter and about 90% of the pre-Covid level is reached in 2024.
Scenario 4: Long-term decline – Assumes a 10% annual growth for the recovery period but a 2.5% decline thereafter, with air traffic levels about 50% lower in 2050 compared with 2019, similar to the pandemic year 2020.

The second scenario, ‘back to normal’, leads to the 6% to 17% aviation share of the remaining budget to stay within the 1.5-2°C limit. Without policy intervention, this contribution will continue to increase beyond 2050, say the authors. The recovery period will delay aviation-induced warming, reducing it by about 10% in 2050, but future annual growth will have a much greater impact than Covid, which is projected to only delay the warming contribution of aviation by about five years should the pre-Covid growth resume.

In the third ‘zero long-term growth’ scenario, aviation-induced warming will keep rising over the next decades, as the CO2 emissions continue to accumulate and start to dominate over the non-CO2 effects. Interestingly, points out the paper, if global aviation were to decline by about 2.5% a year (scenario 4), even with no change in the current fuel mix or flight practices, the impacts of the continued rise in accumulated CO2 emissions and the fall of non-CO2 climate forces would balance each other, leading to no further increase in aviation-induced warming with immediate effect.

Said co-author Professor David Lee of Manchester Metropolitan University: “One of the important nuances is that the non-CO2 impacts, like the formation of contrails and cloudiness, have been thought to dominate the total impact: this is true at present, but it’s not widely understood in the stakeholder community that if you take care of CO2, the non-CO2 fraction actually decreases in importance, even more so with sustainable aviation fuels that generate fewer contrails. This emphasises the importance of tackling aviation’s CO2 emissions.”

The study designed two additional scenarios to look at the potential impact on aviation-induced warming of the introduction of low/zero-carbon fuels (bio or synthetic):

Scenario 5: 55% zero-carbon fuels by 2050 – Assumes an air traffic growth following the ‘back to normal’ scenario but with a 3% increase of zero-carbon fuels per year from 2024.
Scenario 6: 90% zero-carbon fuels by 2050 – Makes a similar assumption to scenario 5 but with an annual 5.8% increase in zero-carbon fuels. The increased CO2 emissions from increased air traffic are therefore overcompensated, lowering annual CO2 emissions over time.

While for scenarios 5 and 6 with zero-carbon fuels the CO2 emission indices are respectively lowered, the non-CO2 climate forcings continue to scale with annual fuel consumption (regardless of the carbon neutrality of the fuel mix). However, due to fewer soot particles in bio or synthetic fuels, contrail formation is predicted to be reduced by low-carbon fuels.

The authors conclude that scenario 5 would reduce aviation’s contribution to global warming insufficiently to be sustainable, nor will it stop the non-CO2 effects from increasing, and only scenario 6 would limit aviation-induced warming.

“Any growth in aviation emissions has a disproportionate impact, causing lots of warming,” explained co-author Professor Myles Allen, University of Oxford. “But any decline also has a disproportionate impact in the other direction. So the good news is that we don’t actually need to all stop flying immediately to stop aviation from causing further global warming – but we do clearly need a fundamental change in direction now and radical innovation in the future.”

While solutions such as moving to alternative fuels present a clear pathway to minimising warming, said Dr Simon Proud of the National Centre for Earth Observation and RAL Space, there are actions that could be taken right now, such as more efficient air traffic control. “A ban on fuel tankering – where aircraft carry more fuel than they need, and hence burn extra fuel, to save the cost of refuelling at the destination – would reduce CO2 emissions in Europe alone by almost one million tonnes,” he also suggests.

In the paper, the authors recommend that those using carbon footprint calculators and add a multiplication factor to include the non-CO2 effects of aviation in a simplified way, should use a factor of 2.6, on the assumption of a 3% continuous growth in aviation. In general, though, multiplication factors are scenario and time-dependent, and therefore should be used with caution in carbon footprint calculations, they say.

“Nevertheless, for all scenarios the ‘warming footprint’ of aviation is at least twice as large as its carbon footprint in the coming decade, clearly highlighting that non-CO2 effects are non-negligible to assess the contribution of aviation to global warming,” they conclude.

A substantial component of aviation’s climate impact is presently caused by non-CO₂ effects, most notably contrail formation; if weather conditions are right, these can evolve into so-called contrail cirrus. A recent assessment of the drivers of climate change due to aviation estimated the present-day climate effect of contrail cirrus was 65-70% greater than that of CO₂, albeit with a significantly greater uncertainty. Clearly, if that contribution could be mitigated, it would be highly desirable. One idea that is gaining attention is that flights could be diverted to avoid contrail formation regions – we refer to this as ‘navigational avoidance’. A recent Royal Aeronautical Society meeting characterised navigational avoidance as a “low hanging fruit”. Low hanging fruit may be superficially attractive, but only if that fruit is ripe. It is our contention that, in reality, many years’ research is needed to establish whether it is viable, write climate scientists Keith Shine and David Lee.  Rather than decreasing aviation’s climate impact, premature implementation of the strategy risks increasing it. Beyond this, we have no robust mechanism for confidently verifying its efficacy.

To understand the issues, we need to step back to some fundamentals, and distinguish between contrail types (see Figure 1). Initial contrail formation is relatively easy to predict, from knowledge of the temperature and humidity of the engine exhaust and the temperature and humidity of the surrounding air. As the engine exhaust mixes with the surrounding air a familiar, but peculiar, phenomenon can occur. When two unsaturated air masses mix together, they can form a saturated air mass, allowing a cloud to form; the physics is similar to what causes us to see our breath on a cold day. In the case of contrails, one difference is that soot particles in the exhaust provide the particles on which water vapour can condense.

Such so-called ‘condensation nuclei’ are abundant in the lower atmosphere but much less so at cruise altitudes, a fact important to our later discussion. Another difference is that the droplets almost instantaneously freeze, due to the cold temperatures. Just as the clouds from our breath dissipate quickly, so contrails often only last a few seconds. As the engine exhaust mixes with the surrounding air, it is diluted to such an extent that the cloud particles evaporate (or strictly sublimate from ice to vapour). These short-lived contrails are insignificant from a climate perspective.

Less commonly, but still quite frequently, the surrounding air at cruise altitude is ‘ice saturated’. This means it contains enough water vapour that, if there were sufficient quantities of the right kind of nuclei (‘ice nuclei’), a cloud would form. However, in the clean air at cruise altitudes there are often not enough such particles, and such regions are called ‘ice supersaturated regions’ (ISSR). If an aircraft flies through an ISSR, and conditions are right to form short-lived contrails, then the ice particles in contrails are ideal sites for the ‘excess’ water vapour in the air to condense on. Instead of sublimating, the contrail grows and persists. Line-shape or persistent contrails are a frequent sight over, for example, the UK. A useful definition is they are contrails that persist after the aircraft causing them is no longer visible and this occurs on a timescale of a few tens of minutes.

Persistent contrails can further spread out so that, to the naked eye at least, they appear little different to natural high-altitude cirrus clouds and can persist for many hours. We call these (together with the persistent contrails) ‘contrail cirrus’. It is these that contribute to aviation’s climate effect. A complication is that ISSRs are primed for natural cirrus formation and the formation of contrail cirrus acts to slightly reduce the amount of natural cirrus that would otherwise form. This is an area of active ongoing research but recent calculations indicate that this effect reduces the effective climate impact of contrail cirrus by more than half; this is taken into account in the recent estimates of aviation’s total climate impact.

Although navigational avoidance of ISSRs is superficially an attractive route to mitigate the climate impact of aviation, this is where several confounding factors come in. First is the complex way that contrail cirrus influence climate. There is strong evidence that their overall effect is to contribute to global warming, but this overall impact is the sum of two opposing processes, as shown in Figure 1. Contrails reflect incoming sunlight back to space which, on its own, would cause a cooling effect. The size of this cooling depends on the amount of sunlight available and so varies with time of day, time of year and location. Contrails also absorb infrared energy emitted by the surface and atmosphere; much as greenhouse gases such as CO₂ do, this has a warming influence which, on average, dominates the cooling caused by reflection of sunlight. Whether an individual contrail warms or cools, though, depends not only on the sunlight available at the time of its formation, but that available over its lifetime.

Second, ISSRs are patchy both in the vertical and horizontal. Typical horizontal extents are a few 100 km and depths are typically a km and often much less (cf Schumann/Heymsfield paper). The general weather conditions that cause ISSRs are understood (crudely, they can arise when air is lifted rather slowly, so are often observed when a weather front moves in). However, this does not necessarily mean that the current generation of weather prediction models can predict ISSRs with sufficient accuracy to plan flight routes to avoid them with any confidence. A recent study that compared the best estimate of ISSR occurrence generated by weather forecasting systems with more precise measurements from research aircraft showed little skill. In any operational route planning system that implemented contrail avoidance, forecasts from these models would have to be used, and these forecast ISSRs would be even less reliable.

An alternative approach that has been suggested is sometimes called ‘tactical avoidance’ in which pilots react to contrail formation by, for example, changing their altitude. But on current understanding it would require guesswork to know whether the contrails being formed are short-lived or persistent or whether the new route is any less likely to lead to contrail formation.

And this is the big problem. Flight planners aspire to fly the minimum-fuel route, given the predicted wind conditions, as they are both faster and cheaper for airline operators. Of course, there are confounding factors that prevent this, for example the need to preserve safe distances between aircraft or the perversities in ATM charges, as highlighted in a recent GreenAir Commentary article. But anything other than the minimum-fuel route leads to increased CO₂ emissions, including when a flight is diverted from a minimum-fuel route, ostensibly to avoid ISSRs. This immediately leads to questions about how much extra CO₂ emission is justified to avoid the contrail, and there is no straightforward scientific answer. It depends on various choices such as the balance between near-term and longer-term climate change – this is because while contrails last only a few hours, a substantial part of the CO₂ emitted persists for decades and centuries, and about 20% for millennia.

Ultimately these choices must be made by policymakers rather than scientists. Worse still, if deficiencies in the weather forecast mean that the new longer route also leads to contrail formation, or the original route would not, in the event, have led to contrail formation, more CO₂ is emitted but possibly with no reduction in the contrail climate effect (see Figure 2 – with thanks to Dr Emma Klingaman for the original idea). These are bad outcomes.

We can only touch on the many governance issues. How would airlines gain credit for re-routing to avoid predicted contrails if we cannot be confident that the re-route has even reduced the overall climate impact of a flight? And would credit be given for causing what turned out to be a cooling contrail?

To be absolutely clear, we are not saying that navigational contrail avoidance is a lost cause. Ongoing and future research, including improvements in weather forecasting systems, may establish it as a viable strategy but we assess that it will be many years before this can be confidently established. What we are against is any suggestion that this is a strategy ripe for implementation now. Indeed, we consider that it would be a gamble, and a gamble where one is never sure of the outcome and has the potential to make matters worse.

One attraction of navigational avoidance is that it could mitigate aviation’s climate impact with the present-day fleet. We suggest that more positive near-term mitigation strategies with this fleet include minimising ATM inefficiencies and continuing to explore the potential of lower carbon footprint fuels such as biofuels and power-to-liquid e-fuels, manufactured from sustainable energy sources. These fuels have a low to zero aromatic content (single and double carbon ring structures, essentially an impurity in fossil-fuel iso-paraffin kerosene). These aromatic compounds are the building blocks for soot formation, on which the ice crystals nucleate. Recent in-flight measurements behind a test aircraft burning such low aromatic content fuels at cruise levels have shown reduced soot and ice crystal number concentrations. Initial modelling has shown that such an approach reduces the warming effect of contrail cirrus as a co-benefit of reducing aviation’s carbon footprint.

Photo: University of Reading

About the authors

Keith Shine (k.p.shine@reading.ac.uk) is Regius Professor of Meteorology and Climate Science at the University of Reading, UK. He has worked on the impact of aviation on climate, and the impact of climate change on aviation, since 2006. Keith and colleagues in the Department of Meteorology have worked on several national and European projects in these areas.

David Lee (d.s.lee@mmu.ac.uk) is Professor of Atmospheric Science at Manchester Metropolitan University, UK, and specialises in the effects of aviation on climate. David has participated in WG1 and WGIII Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC) since the IPCC’s 1999 Special Report, ‘Aviation and the Global Atmosphere’. He has also contributed to UNEP Emissions Gap Reports and is a technical advisor to the UK Department for Transport for aviation and climate-related issues, working within the International Civil Aviation Organization’s Committee on Aviation Environmental Protection (ICAO-CAEP), co-leading the Impacts and Science Group. David and his team at Manchester Met have participated in many European projects relating to aviation and climate.

Views expressed in Commentary op-ed articles do not necessarily represent those of GreenAir.