– Last Update January 27, 2025 –

How deeply disinformation about aviation has penetrated society became clear in an RTL4 news broadcast on Sunday, December 1.

The item was about the recently opened international airport of Nuuk, the capital of Greenland, and the presenter concluded that item with:

“but flying to a country whose ice caps are melting precisely because we fly so much does sound a bit strange.”

My translation, statement made at 14 minutes into the broadcast

For starters, this places a lot on the shoulders of aviation, being responsible for at most 2% of greenhouse gas emissions. Something certainly needs to be done about that – and it is being worked on – but it does not seem to be very decisive in terms of the effect on the ice caps.

Furthermore, flying is – contrary to what many people think – in itself an extremely energy-efficient way to go places. The only alternative for a visit to Nuuk is a passenger ship, which – despite it’s low speed – uses seven times as much energy per passenger as an aircraft. And therefore also causes seven times as many greenhouse gas emissions per passenger.

Mobility

Reducing -or even abandoning- aviation will backfire if the need for mobility continues to exist. You can of course limit mobility, but the question is if that will be better for society or not. The residents of Nuuk certainly do not think so, as is evident from the last sentence of the article about this item on the RTL4 website:

“One small additional advantage for the islanders: they can now also go somewhere more easily themselves. Very nice, says Iversen, because his wife and he are originally from Denmark. “It was expensive and a very long journey to get from here to Copenhagen. Now we can go there a bit more easily. And maybe visit New York.”

my translation

Linkedin

The above text was initially only posted on Linkedin and that post received, in addition to much support, also the by now usual negative comments. Two of them deserve some extra attention, because they concern facts that some cannot or do not want to believe. The first fact is that passenger ships use so much more energy per passenger than aircraft over the same distance. The second fact is that aircraft are almost miraculously energy-efficient compared to other means of transport.

Seven times

The fact that passenger ships use seven times as much energy per passenger as aircraft comes as a surprise to many people. Yet it is simply a consequence of the laws of nature. The drag that a vehicle has to overcome is directly proportional to the density of the medium it moves in and the square of the speed. The density of water is 2500 times greater than the density of air at the cruising altitude of aircraft.

That density is also the dominant factor for the high energy consumption of a passenger ship. The speed of an aircraft is of course much higher than that of a ship, which leads some people to think that an aircraft will use much more energy per passenger. That is not very strange, but if you do a quick calculation you will see that the effect of speed remains subordinate to the effect of density.

Let’s take a few easy numbers for that calculation. Suppose the cruising speed of a passenger ship is 40 km/h and an aircraft flies 800 km/h. That is twenty times as fast and the effect of speed is quadratic, so that factor is 400 times greater for the aircraft . However, the influence of density is 2500 times greater for the ship and therefore has more than six times as much influence in the formula for drag as the difference in speed.

Source

The energy consumption of passenger ships – unlike the energy consumption of aircraft – turned out to be particularly difficult to determine. Not so strange, because their era ended when the B747 came onto the market in the early seventies. Not because of the climate – that was not yet an issue at the time – but for economic reasons. Because not only does a passenger ship use seven times as much energy per passenger per kilometer as an aircraft , the depreciation per pkm is also ten times as high. Intercontinental transport of passengers by ship was suddenly a thing of the past.

They held out for a while for vacations, as a cruise ship, but that market quickly went to purpose-built ships. A few iconic ocean liners fortunately still live on, albeit as a museum. Such as the SS Rotterdam from 1959 of the Holland America line, with the RMS Queen Elisabeth 2 from 1967 of the Cunard Line as the most persistent. An article about that last ship – in the Guardian of December 20, 2006 – eventually provided the long-sought usage data. This is the quote:

“There are remarkably few figures. But George Marshall of the Climate Outreach Information Network has conducted a rough initial calculation for the Queen Elizabeth II. Cunard says the ship burns 433 tonnes of fuel a day, and takes six days to travel from Southampton to New York. If the ship is full, every passenger with a return ticket consumes 2.9 tonnes. A tonne of shipping fuel contains 0.85 tonnes of carbon, which produces 3.1 tonnes of carbon dioxide when it is burnt. Every passenger is responsible for 9.1 tonnes of emissions. Travelling to New York and back on the QEII, in other words, uses almost 7.6 times as much carbon as making the same journey by plane.”

Disinformation

Those who could not believe this referred to a table from Our World in Data, which supposedly states that an airplane passenger causes more than seven times as much emissions as a passenger on a ship. Exactly the opposite. Fortunately, the link to the table was provided. Checking it was easy and then a number of things stand out. This is that table:

Ferry

First of all. This is not about passenger ships, but about ferries. And specifically about a walk-on passenger. I immediately believe that the number is correct, because a ferry also transports a lot of weight in loaded or unloaded trucks and regular cars. Although I am curious how the allocation was made, but that is besides the point. Apart from that, a ferry is almost always a connection over a relatively short distance. There is no ferry between Copenhagen and Nuuk.

Secondly. The numbers mentioned for aircraft are extremely high. If you look purely at CO2 emissions – i.e. fuel consumption – you will arrive at 60 g rather than 148 g for a long-haul flight and for short-haul it is rather 85 g than 151 g per pkm. Apparently a rather arbitrary multiplication factor has been applied here, which is often the case. See also faq 2 on the FAQ page. Incidentally, the emissions per passenger of the latest generation of aircraft – which are now being put into service by many airlines – are another 20% lower than those of the previous generation. So rather 50 g and 70 g respectively.

HSR infrastructure

Thirdly, the Eurostar’s emissions are extremely low. This is because it runs on electricity from nuclear power stations, but mainly because the emissions from the infrastructure are not included. Nevertheless, the HSR is of course an excellent alternative to flying on this route -London to Paris- with around ten million passengers per year.

However, this does not apply to every HSR connection, because the emissions from the HSR infrastructure must of course be included. For more on this, see the post HSR-syndrome. Moreover, if you do not include the emissions from the HSR infrastructure, you have to halve the emissions from an aircraft for the comparison. Because trains cannot run without rails and aircraft (and birds) cannot fly without their wings developing lift. And for the latter, aircraft and birds use about half of their energy.

Birds and aircraft

The drag of aircraft (and also birds) is somewhat more complicated than that of trains or ships. They not only experience zero-lift drag- the drag of the aircraft when it sits in the airflow in such a way that the wings do not provide any lift – and which of course, just like with trains, cars and ships, increases quadratically with speed. They also experience induced drag. This develops when their wings hang at an angle in the airflow and then develop the necessary lift.

This induced drag decreases quadratically with speed. The total drag is the sum of these two and that means that aircraft (and birds) have an optimal speed and that is -not entirely coincidentally- also their cruising speed. What that speed is depends on the weight. For a bird like the bar-tailed Godwit that is 70 km/h, for an aircraft 800 to 900 km/h. Flying slower gives more drag and therefore costs more energy. Which may not feel right, but in reality it does.

Miraculously energy-efficient

All these effects make flying almost miraculously energy-efficient. How miraculous can be seen when you compare aircraft not only with passenger ships and high-speed trains, but also with cars. For example, to holiday destinations on the Mediterranean, aircraft – which at 800 km/h travel about eight times as fast as a car – use less energy per person than a car with fewer than three (or fewer than four for the latest generation of aircraft) occupants.

A thought experiment shows how efficient this is. If a car were to travel as fast as an aircraft instead of approximately 100 km/h, then that car would experience more than sixty times as much air resistance and therefore also use approximately sixty times as much energy. Incidentally, the efficiency of aircraft is – as explained above – not a miracle but the result of the laws of nature.

Own place

Finally: as is also evident from the report on the new Nuuk airport, aviation is simply infrastructure. Aviation, like every other mode of transport, has its own place in the transport mix.

The different modalities do not have to compete with each other, because they simply complement each other. As a result, where people fly now, that is almost always the best option for the climate as well.


Incidentally, the now classic nonsense frame that you should not fly short distances because of the energy it takes to climb to flight altitude was also brought up in the comments on Linkedin. For an analysis of this disinformation, see the post ‘Knowledge Lost‘. In that message, you can also find the formula for drag, for those who do not have it readily available.

January 27, 2025Thought experiment added under ‘Miraculously energy-efficient’, plus a few editorial changes.