A complex promise
In 1903, when the Wright brothers made their first test flights with a manoeuvrable engine-powered aircraft on a beach near the small town of Kitty Hawk, North Carolina, it was not yet possible to predict how aviation would develop in the following 120 years. Today flying is commonplace, a comprehensive network of flight paths spans the earth, and nearly every region on our globe – whether near or far – can be reached by plane in one or two days at most. It’s an offer that meets the needs of many people. While IATA, the international umbrella organisation of airlines, estimated passenger numbers at 4.35 billion in 2023, this figure is expected to more than double by 2050, topping 10 billion passengers per year.
Obviously, this will have consequences. The effects on the climate are particularly critical, because in terms of passenger kilometres, air travel places a far greater burden on our climate than other means of transportation. The first thing that comes to mind might be the carbon dioxide (CO₂) emitted by aircraft engines. Yet, as is so often the case, the situation is in fact more complex.
It begins with the booking. Already here, climate-related considerations can play a role. According to the International Energy Agency (IEA), air transportation accounts for more than three percent of all global CO₂ emissions, and its overall contribution to the greenhouse effect is even greater – not least because the atmosphere reacts much more sensitively at the higher altitudes, where aeroplanes fly, than on the ground. A passenger flying economy class roundtrip between Zurich and New York, for example, is responsible for the emission of around three tonnes of carbon dioxide. The amount is roughly double for business-class flights and three times as high in first-class, because of extra space required. Three tonnes is already one-fourth of the average emissions otherwise caused by a Swiss person throughout a whole year. Citizens of our country are particularly frequent flyers: on average, 1.6 flights per person per year. That is two to three times more than people living in neighbouring Germany, Austria, France, and Italy.
Many airlines therefore offer “green” booking options. For example, on a flight to New York, the CO₂ emissions caused can be offset by paying an additional 90 Swiss francs on top of the 800-franc ticket. Some airlines promise that 80 percent of the extra fee will be invested in climate protection projects, and 20 percent go towards purchasing so-called sustainable aviation fuel (SAF). This is sustainable kerosene that is produced without petroleum.
Does that take care of everything? Could air travel be made climate-neutral by compensatory payments on this scale?
“Unfortunately not,” says Thomas J. Schmidt, head of the PSI Center for Energy and Environmental Sciences. In the past year, a study of PSI and ETH Zurich made it clear that climate-neutral aviation is indeed possible – but funding climate protection and SAFs calls for far more than such affordable surcharges. “And it’s not just about higher prices,” Schmidt says.
High ambitions
The aviation industry has set itself the goal of reducing its CO₂ emissions to net zero by 2050 and achieving climate neutrality. In doing so, it is pursuing the same goal that the EU decided on in 2021 and which Switzerland decided by referendum in 2023. The aim is to become climate-neutral overall by 2050, in order to effectively counteract global warming. The big question is whether and how this can be done. Especially in aviation, since aircraft require enormous amounts of energy to take to the air, as well as complex infrastructure and logistics on the ground.
Exactly this question has been investigated by Romain Sacchi and Christian Bauer of the Energy Systems Analysis Lab at PSI, together with Viola Becattini and Marco Mazzotti of the Institute for Process Technology at ETH Zurich. They collected data and forecasts on all the different aspects and calculated various scenarios for developments up to 2050.
“One important question in this is what, exactly, is actually meant by net-zero CO₂ or climate neutrality,” says Romain Sacchi, principal author of the study alongside Viola Becattini. Many calculations only consider CO₂ emissions due to the actual flight. “But that is too limited,” adds Becattini. If air travel continues to grow as it has until now, the calculations indicate that CO₂ emissions due to air travel will actually account for only around 20 percent of the total climate impact by 2050. If we want to make flight operations as a whole climate-neutral, we must take into account not just flying itself, but also fuel production and the overall aviation infrastructure – but above all, other aircraft-related emissions that have an impact on the climate.
The other half of the truth
There’s more to a travelling by air than flying. Just as a car journey requires roads, petrol stations, and parking lots, an aircraft also requires a certain infrastructure – along with complex logistics to provide for the aircraft, crews, and passengers and to enable everyone to take off as punctually as possible. Kerosene, luggage, and catering require transportation; aircraft need to be maintained; terminals, hangars, and other operational buildings have to be cleaned, heated, and illuminated. All this causes additional greenhouse gas emissions.
According to its own data, Zurich Airport, Switzerland’s largest, handled nearly 250,000 flights in 2023, carrying 380,000 tonnes of freight and 29 million passengers. Twenty-seven thousand people work at the site. Its total energy consumption is equivalent to that of a medium-sized city. This has less of an impact on the CO₂ balance in Switzerland, where electricity is generated mainly by hydropower and nuclear plants and less than two percent comes from fossil fuels, than it would in other countries. Nevertheless, electricity production also releases CO₂. In addition, the emissions released during the construction of the entire facility, including the aircraft, must of course be taken into account in the climate balance.
The study by PSI and ETH Zurich does show that the climate impact of the aviation infrastructure needs to be taken into account. Overall, however, it is relatively small, especially over the period until 2050 and beyond. The climate impact of flying itself and the emissions from fuel production are much greater. This is illustrated by the CO₂ balance of Zurich Airport for 2021: the emissions from airport operations were around 30,000 tonnes, while those associated with flights and delivery traffic amounted to 1.6 million tonnes.
The so-called non-CO₂ effects of flying are even more relevant for the climate – as the study showed more clearly than ever before. These effects include soot and other particles released when kerosene is burned; such particles serve as condensation nuclei for clouds. They also include nitrogen oxides that promote the formation of ozone in the air, which in the troposphere acts as a greenhouse gas, as well as water vapour and the resulting condensation trails that can promote the formation of cirrus clouds in the upper atmosphere at an altitude of around ten kilometres.
These wispy clouds made up of ice crystals do not have a cooling effect on the earth’s surface like the lower stratus and cumulus clouds, which reflect incoming sunlight. “Instead, they are quite transparent to sunlight,” says Benjamin Tobias Brem, whose research at the Laboratory for Atmospheric Chemistry of the PSI Center for Energy and Environmental Sciences focuses on the environmental consequences of aviation emissions. “In contrast, the ice crystals reflect the infrared radiation from the earth’s surface back to earth very effectively, thereby causing additional warming.” This warming effect depends on the geographic latitude and height of the clouds. At our latitudes, the warming effect occurs at an altitude of around six kilometres – right where the traffic of big planes on long-haul flights, which are responsible for the majority of aircraft emissions worldwide, is concentrated.
“Altogether, these non-CO₂ factors account for more than half of the climate impact of aviation,” says Christian Bauer. “Up to now, though, they have been ignored in many analyses and net-zero promises. Or not correctly factored in.”
Previous calculations have been imprecise
It is customary to convert such emissions and effects into CO₂ equivalents so as to include them in the overall balance. “But the methods and values used to date have proven to be inaccurate,” says Marco Mazzotti. “We therefore proceeded with greater precision.” The methods used by the researchers take into account one essential difference between the various factors: non-CO₂ effects are much more short-lived than CO₂. In fact, they are known as short-lived climate forcers, or SLCFs.
While around half of the carbon dioxide emitted is absorbed by forests and oceans, the other half remains in the atmosphere for thousands of years, spreading out and behaving as a greenhouse gas. In contrast to this, ozone, for example, is many times more potent than CO₂ as a greenhouse gas, but it breaks down within a few months. Contrails and the resulting clouds vanish in just a few hours. “The problem is that the increasing air traffic means we are constantly producing more SLCFs, so that instead of rapidly disappearing they accumulate. As a result, they exert their enormous greenhouse potential over longer periods of time,” says Viola Becattini. It’s like a bathtub with both the tap and the drain open: as long as the tap lets in more water than the drain lets out, the tub keeps getting fuller – until eventually it overflows.
In the researchers’ scenarios, they considered ways of closing the tap, at least enough so that it only lets in as much as can leave through the drain. Aviation should not emit more CO₂ equivalents overall than will be removed from the atmosphere elsewhere. Only then can flying actually be climate-neutral. This explicitly includes capturing CO₂ from the air and compressing it for climate-safe storage in the ground, much as oil was stored in the ground before we began extracting it. Such carbon capture and storage (CCS) techniques are already in use and are considered an option particularly where a limited CO₂ emissions cannot be avoided. However, the long-term effectiveness and safety of CCS have yet to be proven.
The engine is the most obvious lever for reducing the climate impact of air transport. Researchers around the globe are working on electric or hydrogenpowered aircraft. Such engines would be climate-neutral provided the electricity for charging the batteries or producing the hydrogen is obtained from hydroelectric, wind, or solar power. However, this also requires all upstream production processes – such as the manufacture of batteries – to be carried out using renewable energy.
But even if that were the case, there would still be insurmountable technical hurdles. Batteries currently have an energy density of 250 watt-hours per kilogram. Kerosene delivers 12,000 – almost 50 times higher. A large plane on a long-haul flight, such as a trip to New York, would have to carry so many fully charged batteries that it would be too heavy to take off. Even advances in battery technology are not expected to yield sufficient improvements in the foreseeable future. For the time being, battery-electric power can only be used by small aircraft flying shorthaul routes. These, however, account for less than two percent of greenhouse gas emissions in aviation.
The outlook for hydrogen is similar. Even in its super-cooled, compact, liquid form, hydrogen takes up four times as much volume as kerosene per watthour delivered. Planes would not have to carry more weight, but they would have to provide more volume for the fuel, and volume too is a critical factor in aircraft design. Intensive research is being conducted into the optimum design for aircraft and the supply chain for hydrogen. But that’s the catch: redesigning the aircraft and the infrastructure requires an immense effort – and takes time. Some manufacturers such as Airbus have announced that they will not put the first hydrogen aircraft into service until 2035 at the earliest.
Even these will not be very large (up to around 90 seats) and will be suitable for intra-continental flights rather than long-haul flights. They are aiming for a range of around 2,000 kilometres. That won’t get you to New York. And the fact remains that it is large aircraft on long-haul flights of more than 4,000 kilometres that account for the bulk of greenhouse gas emissions in aviation. The hope, therefore, is the previously mentioned SAF, sustainable aviation fuel.
Air tickets at triple the cost
Widespread use of SAF could certainly bring us much closer to climate-neutral aviation by 2050, but it is more costly in terms of resources and money, especially because producing hydrogen by electrolysis is very energy-intensive.
Which brings us back to carbon offsetting. The PSI and ETH researchers have also costed this out. “A surcharge of a few euros would be pure window-dressing,” says Christian Bauer. “To comprehensively balance out the actual climate impact of travelling by air, a ticket would have to cost around three times as much.” That flight to New York, then, wouldn’t have cost us 890 Swiss francs instead of 800, but closer to 2,400.
“A further advantage of such a sharp price increase would also be that it would certainly reduce demand for flights significantly,” says Viola Becattini. This brings us to the most important message of their study: as promising as the use of SAF, the development of alternative engines, and potential efficiency gains in production, logistics, and transport may be, none of these will get us all the way to our goal, according to the calculated scenarios. At least not by 2050. Because to do so, we would have to offset large amounts of remaining emissions using CCS, that is, capture them from the air and store them in the ground. “The worldwide capacity of this technology is not sufficient to do this,” says Bauer. Especially since CCS facilities are not reserved for aviation alone and have not yet been tested on a large scale.
The bottom line: Ultimately, we need to travel by air less rather than more – there is no other way. Sacchi and Becattini have also calculated how much less. In combination with increased efficiency and CCS within the scope of what is feasible, air traffic would have to decrease by 0.2 percent every year, bringing it to around 95 percent of today’s volume by 2050. Were we to stick to fossil kerosene, we would have to cut back four times as much – to 80 percent.
“How we regulate the number of flights to the extent needed is a question that society and politics must answer,” says Thomas J. Schmidt. “Changes in our behaviour are certainly needed. But such changes are not easy to impose. The number of flights will probably only drop noticeably if they become really expensive.”
