You have probably heard environmentalists make countless claims about the danger aviation poses to the environment. But is flying really so much worse for the environment than other forms of transport? And if so, why?
Well, the answer to the first part of this question is a simple and resounding yes. Flying is always the least sustainable way to get from A to B. The question of why is where things start to get a little more complicated.
Unequal Distribution of Emissions
For background, aviation is currently responsible for around 5% of global anthropogenic radiative forcing (human-caused climate change). While this seems like a relatively low number at first, it takes on a whole new meaning when you consider how many people are actually responsible for these emissions.
In 2018, approximately 11% of people travelled by air, with at most 4% of those people taking international flights. Best estimates are that more than 80% of the world’s population has never set foot on a plane. Kind of puts the 5% of total emissions into perspective right?
Here’s the kicker; over 50% of all the emissions produced by aviation (2.5% of all GHG emissions) are emitted by just 1% of the world population. Individual users of private planes can contribute as much as 7,500 tonnes of carbon dioxide per year to the atmosphere just as a result of their flying.
To put that into perspective, someone from Ethiopia emits on average 0.1 tonnes of CO2 per year. In other words, it would take an average Ethiopian 75,000 years to emit as much CO2 as some individuals emit every year just by flying in a private jet.
The scary thing is that more and more people are adopting this unsustainable lifestyle. CO2 emissions from aviation have increased by an average of 2.6% every year for the last 25 years. In fairness, a lot of this is people who can now afford to fly for the first time. However, a much bigger proportion is people who could already fly but are now flying even more.
People often underestimate the impact flying has on the environment. Did you know, for example, that taking one return flight from Dublin to New York emits about the same amount of Greenhouse Gases (GHGs) as you would save by doing 20 years of recycling!
The GHG emissions are far from being the only problem here. Aviation also has a massive effect on air quality. It is estimated that around 16,000 people die prematurely each year as a result of air pollution from planes. Of course this is very hard to estimate, and the figure is likely much higher.
The impact of aviation on air quality is much higher during the takeoff and landing phases of the flight. This is because airports are usually situated very close to big cities, and the plane is flying close to the ground during these phases.
The GHG emissions impact, on the other hand, is at its highest when the plane is also at its highest. In fact, 90% of the impacts of flying per unit of fuel burned are attributable to ‘cruising’ at high altitudes.
What Makes Flying so Bad for the Environment?
This brings us nicely to the second part of our question; why exactly is flying so much less sustainable than other forms of transport? As we mentioned at the beginning of the article, this is the bit where things get a lot more complicated.
The key players in this are contrails, CO2, water vapour, Nitrogen Oxides (NOx), sulphates and black carbon. Each of these elements interacts with the environment in different ways, and some have different effects depending on the altitude at which they are emitted.
We will start with contrails. The first thing to do here is to explain what they actually are. Contrails are the white fluffy lines left behind a plane when the conditions are right. They are made up of ice particles which form around soot (or black carbon) which has been ejected from the plane’s exhaust. This is why contrails only form when the air around the plane is very cold and wet. It shouldn’t need to be said, but contrails do not contain chemicals to control your mind or surreptitiously vaccinate you.
It is exceedingly difficult to figure out the effect that these trails have on the climate. It is one thing to figure out the effect of the linear trails themselves, but what happens when they start spreading out and mixing with natural clouds? Once a contrail has spread out, it transitions into what is known as a contrail-cirrus.
Believe it or not, contrails and contrail-cirrus are likely to have the single biggest impact on the climate of any of the things listed above. More even than the CO2! And that is true despite them not hanging around for very long. Contrails can last from a few seconds to a whole day, depending on a number of complex factors. While they are around, though, they can have a real impact.
Think of it like this. If you create clouds (which is essentially what contrail-cirrus are), then they will simultaneously reflect radiation from the sun back into space (reducing warming) and reflect radiation coming from the ground back to earth (increasing warming). If the contrail exists during the day, then, the warming impact roughly evens out.
If they are present at night, however, the ‘outgoing’ radiation from the earth is trapped at a much higher rate than the ‘incoming’ radiation from the sun is reflected. That means that the net effect of contrails is overwhelmingly one of warming. Small changes to flight paths which avoid areas where contrails are likely to form could really help, then, to reduce the impact of aviation! This is a low-hanging fruit which should be plucked post-haste.
Nitrogen oxides are any molecules made up of some combination of nitrogen and oxide. Yes, this does include nitrous oxide (N2O), also known as laughing gas (or whippets to the kids). In this case, however, we are talking about Nitrogen Dioxide (NO2) and Nitric Oxide (NO). The impact of nitrogen oxides (NOx) from aviation on the climate is an even more complicated topic than even contrails.
NOx contributes to ozone build up in the atmosphere, which is a potent greenhouse gas. However, it also breaks down methane into carbon dioxide, which has a much lower global warming potential. It also produces nitrate aerosols, which cool the atmosphere.
This simultaneous warming and cooling effect cancels out some of the climate impact of NOx from aviation emissions. However, the net effect is again one of warming. What’s more, the effect of NOx on air quality and human health is much larger. One study which monetized the impacts estimated that 94% of air quality impacts from aviation are driven by NOx emissions!
An issue with reducing NOx emissions is that there is a tradeoff with CO2 emissions. If you increase the operating temperature in the engine, you increase the efficiency of the fuel burn and as a result reduce the CO2 emissions. However, the higher temperatures also lead to an increase in NOx emissions.
Another complicating factor is that the relationship between fuel burn and NOx emissions is non-linear. It depends not only on the combustion temperature in the aircraft, but also on the background concentrations of NOx already present in the atmosphere.
This effect is so strong it seems that whether the NOx emissions result in a net warming or net cooling effect depends on the existing concentration of NOx in the atmosphere. To hugely simplify a very complicated question, if there is little NOx in the atmosphere, then adding more reduces warming. If there is already a good bit, then adding more increases warming. As NOx concentrations rise as a result of aviation, then, the impact of those emissions becomes more and more powerful.
One final thought about the emissions from aviation is that they have a longer ‘residence time’ in the atmosphere since they are emitted at high altitudes directly into the stratosphere or troposphere. This means that the gases or particles stay in the atmosphere longer and therefore do more damage to the environment.
I hope it is clear at this point that quantifying the impacts from aviation on the environment is not as simple as it might first appear. This highlights the fact that the atmosphere is not yet fully understood, and we should therefore be extremely careful about what we pump into it. You never know what effect it might turn out to have.
For all this complicated chemistry and physics, there is only really one thing that the average person needs to know, and that is that the impact of flying is much much higher than the impact of all other modes of transport. It simply takes a lot of energy to propel an 80-tonne plane to 35,000 feet. As we know, energy cannot be created or destroyed, so the fuel to generate this energy has to come from somewhere.
Try to avoid flying whenever possible! Trains and ferries may take longer to get you where you need to go, but they will be more enjoyable and have a much lower impact on the environment and the future of our young people.