By Luca Ferrari
A couple of numbers
Air travel tends to be a hot topic in climate debates and for several good reasons. Aeroplane’s contributions to the global greenhouse gas (GHG) emissions might seem modest with respect to other sectors. After all, in 2016, it was just 1.9%, while just the road transport sector accounted for 11.9% of the total GHG emissions. But how many people in the world drive a car? And how many people fly on a plane every year? By looking at it from a different perspective, things change considerably. As shown in the graph below, one passenger travelling for one kilometre on a plane can emit GHG up to 255 g of CO2 equivalent. Considering that the same passenger could emit from 6 to 41 g of CO2 by using a train, this makes air travel one of the forms of transport with the highest carbon footprint for passengers.
Source: Our World In Data
Let’s have a look at that a bit more in detail. In 2019, aviation was responsible for the emissions of 920 million tonnes of CO2, a 30% increase from 2013. 85% of these emissions come from passenger transport, while the rest from cargo transport.
On a global scale, CO2 and other GHG contribute to climate change. However, these are not the only emissions to consider. Aeroplanes emit several other toxic gases such as carbon monoxide, nitrogen oxides, and particulate matter by burning fossil fuels. These pollutants can lead to cardiocirculatory and respiratory problems. The problem worsens with the proximity of airports, where the concentration of dangerous gases is very high. Also, in countries like the USA, small aircraft still use leaded fuel, despite being phased out years ago for other vehicles. Analyses of lead emissions from aviation underlined a significant relationship with blood lead levels, especially for children. This kind of pollution could cause health issues and damage nerves, kidneys, and the cardiovascular system.
A matter of inequality
But how many people fly every year? In 2019 the number of flights peaked at 39 million before the Covid-19 pandemic. While the number might seem high, access to air travel is restricted to a limited number of people. Research led by prof Stefan Gössling of Lund University suggests that only 11% of the world's population travels by air, with around 4% flying internationally. Because of infrastructure availability and high costs, the share of the low-income population that travels by plane is a minority of the total number of passengers.
Share of flying population adjusted for non-flying share of population. Source: Gössling and Humpe (2020).
On the other hand, around 10% of the most frequent fliers account for 40% of all flights. The sharp inequality is evident, with 1% of the world population responsible for 50% of all air travel emissions.
Air transport demand distribution in the USA. Source: Gössling and Humpe (2020).
Such inequality also arises when we consider the geographical distribution of air travel. Just ten counties, the USA, China, United Kingdom, Japan, Germany, United Arab Emirates, India, France, Spain, and Australia, account for 60% of air travel emissions. However, primarily non-flyers bear the high environmental and health toll involved in flying.
What can you do as a private person?
What can we do to reduce the impact of air travel? Just saying “don’t take the plane” might seem trivial but reducing the demand for air travel is a powerful way to curb emissions. Networks like Stay Grounded advocate for a rapid reduction in aviation. Beneficial actions include shifting modes of transportation. Several short-distance flights could be replaced by buses or trains emitting considerably less. Furthermore, the Covid-19 pandemic showed us that by working from home and using modern telecommunication systems, many travels and flights for work-related reasons are now obsolete and can be replaced by teleconferencing. Another option can be incentivising the local economy to reduce long-distance freight transport and travel.
What can the industry do?
While all these steps are helpful and essential, reducing the number of flights to zero is impossible. People will always need to move goods and travel across continents, and we will still need aeroplanes to do that in the foreseeable future. Improving efficiency has the potential to reduce emissions considerably. However, between 2013 and 2019, air traffic increased four times faster than fuel efficiency. To further increase efficiency, airlines should put more effort into emissions disclosure. Air travel is often perceived as if every passenger emits the same amount of CO2, but this is not the case. There are significant differences in emissions based on the length of the route, the type of plane and the seating class. It is already possible to select flights based on their carbon emissions, for example, by using the dedicated Google tool. By improving emissions disclosures, airlines could make these tools even more powerful. Some airlines also offer travellers the possibility to compensate for their emissions voluntarily. Carbon offsetting is considered an important method to reduce aviation emissions, and it has the potential to improve. However, it does not prevent the burning of fossil fuels, so it should be considered a second option.
Is the private sector moving forward?
The voluntary mitigation actions described are essential, but at the moment, a proper climate policy addressing air travel is lacking. Current carbon pricing could be improved and be more equitable by considering the emissions differences of different types of flights. Furthermore, policy agreements cover only a part of aviation emissions.
Alternative fuels offer interesting prospects for the future. As the table shows, the options for aviation are more limited than for the other means of transport. However, companies are already working on several solutions. Biofuels are already an option, even if they currently account for less than 0.1% of the total fuel used. Furthermore, they also come with environmental issues like extensive land use and biodiversity loss.
Power-to-Liquid (PtL) fuels can also power aircraft in a sustainable way. Through the Fisher-Tropsch (FT) process it is possible to convert a mixture of carbon monoxide and hydrogen into hydrocarbons, that can be used as synthetic jet fuels. At the moment the production comes at high costs and low scale, but given its potential, companies like Airbus are moving towards this alternative.
The sun also represents a compelling alternative. The company Synhelion, born in 2016 as a spin-off of ETH, set the goal of using solar energy to produce innovative fuels. Their innovative technology allows exploiting solar heat to convert CO2 into synthetic fuel called solar fuel, composed of carbon monoxide and hydrogen. This technology is particularly interesting because, in principle, there are very few differences between solar fuels and fossil fuels. This allows us to use solar fuels to power every type of engine without changing existing infrastructures. Currently, the company is focusing on aeroplane fuel, and they plan to start producing in 2023. By 2030, they plan to supply enough fuel to cover half of Switzerland’s plane fuel consumption. Recently, a research group from ETH managed to use solar energy to synthesise kerosene from water and CO2, in a solar tower near Madrid. While this was still an experiment, it represents a relevant first step in making solar fuels competitive.
Heart Aerospace is currently working on another way to exploit the sun’s power to fly. Their goal is to create a fully-electric, 19-passenger airliner with an autonomy of 400 km. The project is interesting since short-distance flights have the highest GHG emissions. Electric vehicles also drastically reduce operating costs and provide quiet travel. The company aims to fly its first plane in 2026 to cover short routes like island hopping or domestic flights in the USA and the Scandinavian countries.
Although the work of these companies is fascinating and promising, their goals are quite distant in time. While we all hope to be able to fly cheaply and with zero emissions soon, now there is no such possibility. Until solutions like these will be scalable to cover global demand, our best way to reduce air travel emissions is to avoid flying when possible, use environmentally friendly transportation, and try to compensate for our impact when the aeroplane is the only feasible choice.
About the author:
"My name is Luca Ferrari, and I am a research assistant at Politecnico di Milano, Italy. I recently obtained my MSc in Environmental Engineering from the same institution. I currently work on air quality models, climate policy, and decision-making under uncertainty.
I have always been deeply interested in climate change. I focused my thesis and my research work on different aspects of this challenge. However, while the research work is vital, it is not enough to address the climate crisis. It is as much a political problem as it is a scientific one. That is why spreading awareness is so important. It is necessary to achieve serious climate goals through policies and individual actions. For this reason, I decided to try and do my part in informing about climate change."