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What's so bad about flying (10 minute read)

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Every now and again, when I feel as if I can take the bad news it gives me, I look at some of the calculators that allow me to understand my own personal contribution to climate change. I know that there are some positives in there – I work from home, so I don’t have any carbon emissions from my commute, and I’ve made a big reduction in the amount of meat that I eat. I’ve also made a real effort on eliminating food waste. But for all the good things I do, the calculators always bring me bad news.

The bad news is centred on one reason, my work. I’ve spent my career working at the boundary of science and policy, and that has meant travel to meetings and conferences. Much of that has gone online since the pandemic, but not all of it. And now that the borders are open, I have a trip coming up. From a work perspective, it’s going to be really worthwhile, and I’m looking forward to it.

But, having spent so much time writing about the science of climate change over the last year, it’s also nagging at my conscience. I know that flying is considered particularly bad in terms of carbon emissions. But I’ve still decided to do the trip.

This got me wondering – air travel is bad for the climate, but how bad? Is there anything we can do about it? What about offsetting emissions?

My questions about air travel made me take a step back and think about flying in general. We barely give it a second thought, because we so often see animals flying. But the prevalence of flight in nature masks a curious fact – flying is an ability which has evolved very rarely.

There are only three groups of animals which are capable of true flight – insects, birds and bats. Any other “flying” animal, such as flying squirrels or flying fish, are, technically, gliding (I’ve linked to some lovely BBC footage of both if you want to see what I mean). Flight is not an ability that has evolved frequently – many sources indicate that it has evolved only a few times, once each for insects, birds and bats, and also for a group of dinosaurs called pterosaurs. Recent evidence suggests that isn’t quite true, and that dinosaurs and, perhaps, birds evolved flight a number of times. But the evolution of flight is still comparatively rare, and all the birds and bats which fly today are descended from a small number of flying ancestors. Even though we see flying insects almost everywhere we go, it’s thought that all insects are descended from a single flying ancestor, hundreds of millions of years ago.

On the other hand, flightlessness in birds and insects is relatively common. New Zealand has sixteen species of flightless birds, and we used to have fifteen more. There are many flightless insects too, such as fleas, lice, many types of stick insect and New Zealand’s wētā. But all flightless birds and insects evolved from ancestors which could fly.

Why would evolution drive birds and insects capable of flight towards flightlessness? Although the reasons are debated, one important reason is the energy cost of flying. If there isn’t a substantial advantage to an animal in having flight, then evolution will lead to that animal losing the ability to fly.

There is, however, a crucial advantage in flying. Although flying for a certain amount of time uses much more energy than running for the same time, flight is faster. For example, a goose can fly more than twenty times as fast as it can run. As a result, flying is a more energy efficient way to cover larger distances.

Does the same economy apply to us when we travel by air?

To answer this question, I used a couple of different carbon emission calculators (Carbon Independent and Toitu Envirocare), as a proxy for energy efficiency. I used the calculators to compare a return trip from Wellington to Auckland. For those outside New Zealand, that’s a journey of just under 500 kilometres, or just over 300 miles, each way. The road is winding in some areas, so it’s not a quick journey. It takes around 10 hours to drive (with a couple of breaks), 12 hours on the Intercity bus (with several stops) and about an hour by plane. There is a train, but it’s quite a limited service, so I’ve left it out of the calculations.

Although there were some considerable differences between the two calculators I used, the basic answer was the same – making the journey by air emitted much more carbon dioxide than travel in a car or bus. In fact, making the journey by air emitted around ten times as much carbon dioxide as making the journey by long-distance bus and two or three times as much as travelling by car. In fact, the figures may be even worse than they first appear, because a hunt around in the assumptions used by the models indicates that the estimated occupancy rate used for aircraft is higher than that used for buses. In other words, the calculators may be making air travel look better than it actually is.

So, for a bird, flying is an energy efficient way of travelling. For a human, it’s not even close.

I can’t do the same calculation for my trip to Europe, because there’s no alternative way to get there. What I did do was use the Carbon Independent calculator to calculate my overall emissions for the year, including that trip to Europe. And, I will admit, I shocked myself. The first thing I noticed was that my total emissions exceeded those for the average American. Then I looked closely. The reason my emissions were so high came down to one thing – my trip. In fact, that single trip was 50% of my total emissions for the year. It’s important for my work, but I’m struggling with just how bad this trip is turning out to be for the climate.

But the bad news doesn’t stop there. The large amount of fuel used isn’t the only reason that flying contributes to climate change. Aircraft also emit nitrogen oxides, which are much more potent greenhouse gases than carbon dioxide, as well as water vapour, sulphur compounds and soot. These gases and particles have complex effects on the chemistry of the atmosphere, and some of these effects lead to warming and others to cooling. Because the effects are complex, it’s hard to know exactly how large a contribution aircraft emissions make to climate change. However, there is some evidence that the nitrogen oxide emissions from aircraft could contribute as much warming as the carbon dioxide emissions.

There is another way that aircraft affect the earth’s climate too – through condensation trails, or contrails, those thin trails of cloud that you sometimes see after an aircraft has flown by.

Photo by Gabriela Natiello on Unsplash

A contrail forms when water vapour condenses around soot particles emitted by an aircraft’s jet engines, and then droplets of water vapour freeze to ice and form high-altitude clouds. Contrails appear so insubstantial that it’s hard to imagine them having an impact – but they do. The ice crystals inside contrails absorb heat in much the same way that greenhouse gases do, making a contribution to global warming. How much of a contribution? That is the really bad news – contrails actually contribute more to climate change than the emission of carbon dioxide and other gases from aircraft.

Both the calculators I used recognised the particular harm of flying as opposed to other forms of transport, and included the additional harm from emissions like nitrous oxide and contrails in their calculations. But that’s not true for all calculators. Air New Zealand’s calculator, for example, gives a carbon footprint for my trip which is half that from Toitu Envirocare.

Is there any good news about flying? What about offsetting emissions, by planting trees or some other activity which helps to reduce climate change? Some airlines offer you this option when you are booking your flights, and there are many websites that offer to calculate your carbon emissions and then sell you offsets.

The idea behind offsetting sounds good – trees absorb carbon dioxide from the atmosphere when they grow, so planting trees can be a way to make up for activities which emit carbon dioxide. Unfortunately, we are emitting too much carbon for offsets to make a significant contribution. For example, one source suggested the average American would need to plant 750-1000 trees to offset their emissions, every year. Using the lower figure of 750 trees, that works out at 249 billion trees, just for one year of emissions. I can’t even begin to comprehend that number. But, planted at the average density of natural forest in the USA, it would take more than 200 million hectares of land, which means the USA would run out of land in about five years.

Nobody is seriously suggesting that we can make up for all of our carbon emissions by offsetting. However, since my trip works out at about two thirds of an American’s annual emissions, it’s clear that offsetting flights isn’t going to be sustainable. (For a more in-depth look at the problem with offsetting by tree planting, I found this article good). There are other criticisms of offsetting too, including the very valid point that it gives us a false sense of security. If we buy offsets, it’s easier to pretend that we can keep on emitting as we have always done, and everything will be alright.

That’s not to say there is no value in donating to projects that are genuinely helping to fight climate change. I certainly think that those who have the privilege of flying should do so. But it won’t make as much difference as avoiding emissions in the first place.

The aviation industry is making plans to use more sustainable fuels, such as biofuels made from waste. But the amount of sustainable biofuel available is limited, and care has to be taken not to encourage use of unsustainable biofuels, which can result in deforestation, increased food prices and even, in some cases, more carbon emissions than fossil fuels. There are plans for aircraft which use hydrogen, but these are still many years away. Improved fuel efficiency can help, but it’s not enough.

There is one area, however, where small changes could make a big difference to aviation’s climate impact – contrails. It turns out that just 2% of flights contribute around 60% of the warming associated with contrails. A study published within the last couple of years suggests that small adjustments to altitude and improvements in aircraft engine quality could eliminate 90% of the harm caused by contrails.

But there is no way around the fact that we need to think about flying less. Certainly, one important lesson from the pandemic is that we can connect with people around the world without leaving our homes. It’s not the same as being there, but flying less is feasible. I have had meetings, attended seminars, done training courses and even attended one conference online. Mostly, these were things that I would have travelled to before 2020 (a few, such as training courses from overseas, I simply wouldn’t have had the opportunity to attend).

If travel is important, alternatives to flying are limited in New Zealand, in comparison to places like Europe. However, in some cases they do exist. My outbound flight is from Auckland, in order to allow me to catch up with my family there for a few days. In the past, I would have flown from Wellington to Auckland without thinking about it. But now, my conscience is nagging me. So I looked into alternatives and found that there is an overnight bus which leaves Wellington in the evening and arrives in Auckland the next morning. In my experience, I sleep quite easily on buses. So, in the interests of the climate, I’ve booked my bus ticket from Wellington to Auckland. It’s only reduced the emissions from my trip by around 4%, and I’m still feeling very conflicted, but it’s a start.

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