Newswise — A team of researchers from the Paul Scherrer Institute PSI and ETH Zurich has conducted an extensive analysis to determine the feasibility of achieving climate neutrality in air traffic by 2050. Their findings indicate that simply replacing fossil-based aviation fuel with sustainable synthetic alternatives will not be sufficient. To attain climate neutrality, a reduction in air traffic volume is imperative. The researchers' results are being published today in the journal Nature Communications.
The European Union, in line with the target set by the European Parliament in 2021, aims to achieve climate neutrality by 2050. Switzerland shares this objective. As the aviation sector is responsible for approximately 3.5 percent of global warming, it is expected to contribute significantly to these goals. Aircraft emissions generate two to three times more greenhouse gases per passenger or freight kilometer compared to other transportation sectors. Recognizing this, the International Civil Aviation Organisation (ICAO) and numerous airlines have pledged to eliminate CO2 emissions entirely by 2050 or become climate neutral.
In their recent study, scientists at PSI and ETH Zurich have evaluated the feasibility of these objectives and the methods by which they can be accomplished. "Defining the meaning of zero carbon or climate neutrality is crucial," explains Romain Sacchi, one of the lead authors of the study from PSI's Laboratory for Energy Systems Analysis. Viola Becattini, co-author from ETH Zurich, adds that considering only the CO2 emitted by aircraft during flight falls significantly short. Given the projected growth in air traffic, the researchers' calculations indicate that by 2050, CO2 emissions from aircraft alone will account for only around 20 percent of their overall climate impact. To achieve climate neutrality for the aviation sector as a whole, it is imperative to ensure that not only flying but also fuel production and the entire aviation infrastructure have no further adverse effects on the climate.
Nonetheless, the study findings suggest that the climate objectives currently pursued in flight operations will not be sufficient to achieve this goal by 2050. "While advancements in engines, environmentally friendly fuels, and carbon capture and storage (CCS) technologies show promise, they alone will not enable us to attain climate neutrality," remarks Marco Mazzotti, Professor of Process Engineering at ETH Zurich. "In addition to these measures, a reduction in air traffic is necessary."
Non-CO2 effects play a major role
In their study, Sacchi and Becattini explored various scenarios and shed light on important factors. They found that while considering the climate impact of aviation infrastructure, such as aircraft manufacturing, airport construction, and operations, is necessary, it accounts for a relatively small portion of the overall impact until 2050 and beyond. The primary contributors to climate change are the emissions from flying itself and the production of aviation fuel, which aligns with previous knowledge.
What was previously less understood is the significance of the so-called non-CO2 effects, which go beyond CO2 emissions. The majority of the greenhouse effect caused by aviation is not solely attributable to the carbon released from burning aviation fuel. It is also influenced by the release of particulate matter (such as soot) and nitrogen oxides, which react in the atmosphere to form methane, ozone, water vapor, and the condensation trails that contribute to the formation of cirrus clouds in the upper atmosphere. "Many previous analyses and 'net-zero' commitments have overlooked these factors," highlights Romain Sacchi. "Or they have not been accurately quantified."
Expressing emissions and effects in terms of CO2 equivalents is a common practice for assessing the overall balance. However, Marco Mazzotti points out that the existing methods and values used for these calculations have proven inadequate. To address this, the researchers adopted a more precise approach. Their methodology considers a crucial distinction among the factors: non-CO2 effects, known as "short-lived climate forcers" (SLCFs), have a shorter lifespan compared to CO2.
While around half of emitted carbon dioxide is absorbed by forests and oceans, the remaining half persists in the atmosphere for thousands of years, acting as a long-lasting greenhouse gas. On the other hand, methane has a significant climate impact but decomposes within a few years, and contrails and associated clouds dissipate within hours. The challenge arises from the fact that as air traffic increases, the production of SLCFs accumulates instead of dissipating quickly. Consequently, these SLCFs exert a substantial greenhouse effect over extended periods, adding to their overall impact.
This situation can be likened to a bathtub with both the drain and tap open. As long as the inflow from the tap exceeds the outflow from the drain, the bathtub continues to fill up until it eventually overflows. Similarly, the increasing production of SLCFs due to air traffic leads to their cumulative impact and longer-lasting greenhouse effect.
Climate-friendly fuel alone does not achieve the goal – but it helps
"However, this analogy also highlights a crucial lever that is within our control: the volume of air traffic," emphasizes Romain Sacchi. "By reducing the frequency of flights, essentially closing the tap instead of opening it, we have the ability to cool the atmosphere and significantly diminish the greenhouse effect caused by aviation." This doesn't imply that we need to completely halt air travel. The study's calculations indicate that to achieve climate neutrality in aviation by 2050, air traffic needs to decrease by 0.8 percent annually, alongside underground storage of carbon dioxide, if we continue using fossil fuels. This would bring the volume of air traffic down to about 80 percent of the current level by 2050. However, if we successfully transition to more environmentally friendly fuels based on renewable electricity, a reduction of 0.4 percent per year would be sufficient.
The study also examined these new fuels in more detail. Researchers worldwide are striving to replace conventional petroleum-based engines, utilizing electric batteries, fuel cells, or direct combustion of hydrogen, similar to developments in road transport. However, the energy density available in these alternatives is currently only suitable for small aircraft operating on short routes or, in the case of hydrogen, for medium-sized planes on medium-haul flights. Nevertheless, large aircraft undertaking long-haul flights exceeding 4,000 kilometers contribute the most to global air traffic and greenhouse gas emissions in the aviation industry.
Synthetic aviation fuel has pros and cons
Furthermore, the implementation of propulsion technologies based on electricity or hydrogen for the aviation industry is still in its early stages and not yet ready for widespread adoption. Therefore, the industry is placing great hope in Sustainable Aviation Fuel (SAF). This synthetic aviation fuel has the potential to replace petroleum-based aviation fuel without requiring extensive modifications to turbines and aircraft.
SAF can be produced through a production cascade process, where CO2 is extracted from the air using air capture technology, and hydrogen is derived from water through electrolysis. "If all the necessary processes are powered by renewable energy sources, SAF can be considered nearly climate-neutral," explains Christian Bauer from the PSI Laboratory for Energy Systems Analysis, who participated in the study. "This reduces our dependence on fossil fuels." Another advantage of SAF is its lower production of short-lived climate forcers (SLCFs), which would otherwise necessitate capturing equivalent amounts of CO2 from the air and storing them underground. This is particularly crucial as CO2 storage capacity is limited and not exclusively allocated to the aviation industry.
Air tickets three times more expensive
However, SAF does come with certain drawbacks. Its production requires significantly more energy compared to conventional aviation fuel, primarily due to the energy-intensive process of hydrogen production through electrolysis. Moreover, energy is lost at each step of the production process, including air capture, electrolysis, and synthesis. This increased reliance on electrical power translates to the consumption of additional resources like water and land. Additionally, SAF is expensive to produce, not only in terms of electricity but also due to the costs associated with carbon capture and electrolysis plants. As a result, SAF is typically four to seven times more costly than conventional aviation fuel. In essence, while the widespread adoption of SAF enables carbon-neutral aviation, it also requires more resources and incurs higher costs. Consequently, air travel would need to become even more expensive than it already is in order to meet climate targets.
Romain Sacchi notes, "Currently, passengers have the option to pay a few extra euros to make their flight appear carbon neutral by investing in climate protection. However, this practice can be considered as greenwashing since many carbon offset measures are ineffective. To fully offset the actual climate impact, tickets would need to cost approximately three times their current price."
Viola Becattini adds, "Such a significant price increase would substantially reduce the demand for flights and bring us closer to the goal of climate neutrality." Moreover, as SAF production scales up, it is expected to become more cost-effective and efficient over time, positively impacting its carbon footprint. The study accounts for such dynamics, including the shifting electricity mix used in SAF production, differentiating it from many other analyses.
"In conclusion, there is no single solution to achieve climate neutrality in aviation by 2050," asserts Sacchi. "We cannot proceed with business as usual. However, by rapidly and efficiently developing infrastructure for underground CO2 storage and SAF production, alongside reducing our demand for air travel, we have the potential to succeed."
RSS