The transport sector emerges regularly as the one of the, maybe even the, grandest climate change challenge. It is difficult to decouple transport from fossil fuels and in particular oil as an input when planes, cars and ships run on it at a large-scale. I personally believe that people are not nearly as aware as they could be how their life is related to this massive transport flux around the world. Even if you are a person that puts emphasis on sustainable food, clothing and walking instead of driving means you still are to a high degree dependent on that transport system as goods and materials for literally anything are moved around the globe. Use a computer and on top of that Amazon to order articles and you directly reinforced value chains that span the globe and use fossil fuels to transport minerals to electronic manufacturers and your Amazon delivery to your door. Step beyond your suburb front yard and use the public good “street” to walk or ride a bicycle and you are making use of materials (tar and cement) that have been processed and moved around the world by the power of fossil fuels.

That is not to blame anyone but to argue that our modern world is to large extents modern because we have learned to move stuff around at a massive scale. Oil made that possible. Then again in my research I have to be really careful to get the transport sectors right in terms of how much energy and economic value I associate with them or in other words how energy intensive they are. They are big, fundamental to society and ruin the environment. Three reasons to check my numbers.

Use a computer and on top of that Amazon to order articles and you directly reinforced value chains that span the globe and use fossil fuels to transport minerals to electronic manufacturers and your Amazon delivery to your door.

I recently computed the energy footprint of households all over the world for aviation and automobile travel. Next, I also computed their energy footprint intensities by putting the footprint in relation to the money spend on aviation or automobiles. Some of my energy intensities for the latter are higher? Can that be? Let us have a look at the data and then make an informed back-of-the-envelope calculation. Colleagues and me used the travel route Budapest to Leeds as an example. Here the data that we need to compute energy intensity of traveling that route:

That gives 3L/100 km * 1600km = 48 L per passenger for that trip and 48L*37.5 MJ/L = 1800 MJ. Current one way ticket prices hover at around 100 £ for an averagely priced airline as British Airways (Google flights check one way ticket 21/02/2019). In dollars that is with current exchange rates around 130$. That gives us an energy intensity of 1800MJ/130$ ~ 14 MJ/$. In my data I attain a mean over many countries and all flight distances of 12.1 MJ/$. So that is not far off.

How about traveling that same route by car alone (which is what many people do daily)?

Let us assume a very efficient car drive of 5L/100 km which then makes 80L consumed for a distance of 1600 km. Current Diesel has an volumetric energy density of 35 MJ/L yielding 2800 MJ for that trip https://en.wikipedia.org/wiki/Energy_density. Let’s assume current diesel is around 1.3 £ per liter this month. https://www.theaa.com/driving-advice/driving-costs/fuel-prices

That makes 104£ or in Dollars ~ 135$ for that trip. All in all 2800MJ/135$ ~ 21MJ/$. That is considerably more energy intensive under these assumptions. And considering emissions?

Diesel CO2 emission per liter: 2.65kg/L https://de.wikipedia.org/wiki/Dieselkraftstoff

Kerosene CO2 emissions per liter: 2.76kg/L https://de.wikipedia.org/wiki/Kerosin

Given that a reasonable average for a plane flying that distance is a capacity of 150 passengers that would make roughly 48L * 150 = 7200 L kerosene use https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft. Assuming that the efficiency here is somewhat overestimated for a rather short distance flight still so let us assume 10 000 L most importantly we get the order of magnitude right. 10 000 L * 2.76 kgCO2/L = 27600 kgCO2. That dividing again by 150 persons makes 184 kg/CO2 per person.

In terms of road we attain roughly 2.65kgCO2/L*80L = 212 kgCO2/per person. For persons who wonder know how that can be less CO2 emitted by plane per person than by car:

applied to my case study yields roughly 160 kg CO2 per person for the flight of 1600km too.

  • We have to conclude that long distance by car, sitting alone in one car, is not better than air travel. That does not mean that air travel is not a problem.
  • That air travel is considered a lot worse in terms of CO2 emissions of course comes from the fact that the average German person only travels 14000 km per car in the whole year which is roughly 38 km a day which makes on a day to day basis this makes only roughly 2L per day or roughly 5.4 kg CO2 a day which is nothing against one flight of 1600 km distance which can easily happen in one day. Nations other than Germany travel way less by car.

However to top the emissions from 14 000 km average car travel which is around 700L Diesel * 2.76 or ~ 2000 kg CO2, one would need to conduct at least 11 flights a year of that 1600km range.

I consider myself in recent times a frequent flyer but all in all I do not fly more than 10 times a year this kind of distance on average. In some years way less. So I would have around the same CO2 footprint as some average German business commuter in their Audi A6 or VW Passat.

The point here is not to defend aviation as something that has no environmental impact. However recently I witness an alarm attitude because of air travel and many people consider it to be the worst of the worst impacts. At this point it is important to mention that energy intensity is not a sole measure of environmental impact nor is CO2 emissions alone. Airplanes emit other exhaust gases such as nitric oxides and water vapour at quite a high altitude. Short-lived but high impact gases that contribute to the greenhouse gas effect. See these two studies:

https://pubs.acs.org/doi/pdf/10.1021/es9039693

http://folk.uio.no/gunnarmy/paper/sausen_mz05.pdf

Land footprints and Material footprints play an important role too. But which one is worse here aviation or road?

Clearly another danger of the aviation sector is that it is still growing exponentially or maybe even just beginning to do so. Find these presentation slides by Researchers in Manchester. https://www.transportenvironment.org/sites/te/files/Aviation%20CO2%20emissions%20in%20the%20context%20of%20the%20Paris%20Agreement_Dr.%20John%20Broderick.pdf. If I find the time I am going to elaborate on possible growth prospects in aviation and road transport in another blog post too. I have to control for those aspects anyway either.

The road sector growth might has more or less saturated in OECD countries but is still on the rise in emerging economies. This is why I find a one dimensional fixation on aviation is not justified. At the moment the absolute and world-wide amount of oil used in road transport is more than 6 times greater than the amount used in aviation. ~ 50% of all oil used goes into the road sector, only about 8% to aviation as of 2016 https://www.iea.org/Sankey/#?c=World&s=Final%20consumption

Finding sustainable travel solutions in urban areas, between cities and other settlements is at least of as great importance as flying less. One last time, these back-on-the-envelope computations are no extensive research on the environmental impacts of flying and cars. But they give me and maybe the reader a first feeling for what order of magnitude the energy intensities and impacts are to be expected in. Despite seeing aviation as only as terrible as road is, the perspective can be shifted and tell that the car based system is as bad as aviation! It is way too inefficient and has probably an extreme degree of land, material and emission footprints. I know it is amazing that humanity is even capable of producing such (often even good looking) high-tech machines at a speed and scale as if they were Lego. However despite being a prime example for the creative and outstanding productive capabilities of humankind, the car and road system surely is a prime example for its destructive capabilities either…

I know it is amazing that humanity is even capable of producing such (often even good looking) high-tech machines at a speed and scale as if they were Lego. However despite being a prime example for the creative and outstanding productive capabilities of humankind, the car and road system surely is a prime example for its destructive capabilities either…

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What about trains and trams? (Public transport)

We make the assumption of average lines that stop more often as it is the case also in typical urban environments. This KTH study by Lukaszewicz and Andersson 2009 (https://www.kth.se/polopoly_fs/1.118554.1550156367!/Menu/general/column-content/attachment/GT%20Energy%20consumption%20slutl.pdf) gives out 78 Wh per passenger km for a train with max. 200 km/h and two stops and 50 Wh per passenger km for a high speed train ~ 250 km/h and no stops. So let us assume 70 Wh per passenger km for a train which is a generous assumption I guess given that typical urban transport usually stops much more often and technology world-wide is not as developed on average as in Sweden. 70 Wh is 252000 J or 252 kJ which again is 0.252 MJ per kilometre and per 100 km it makes 25.2 MJ. Given our 1600km Budapest to Leeds example above that would make 25.2 MJ/100km * 1600km i.e. roughly 400 MJ per passenger per that trip. That is 4.5 times more efficient already than going by plane and 7 times more efficient than by car on your own. And there is a bonus of course, many trains run on electricity already instead of fossil fuels meaning the energy is potentially clean. But let’s check the energy intensity for such a hypothetical train ride. In the Swedish study they talk about a ride from Stockholm to Gothenburg which is ~ 450 km. According to go euro ticket prices go from everything between 30 $ to 100 $. 30$ would be incredibly cheap compared for example to German prices for where I paid regularly 100 € for around 350 km back when I lived there. But with a customer card I think you can cut this down quite a bit to 50 € maybe. So let us assume for 450 km 50$ considering that in many other countries in the world train tickets are a lot cheaper (in India we went ~ 1000km for I guess 3000 rupees which is like 42 dollars only). Anyway, 50$ per 450 km makes ~ 180$ for 1600km (by taking 1600km/450km * 50 $). That gives an energy intensity of 400 MJ/180$ or 2.2 MJ/$ so like 6 times less than flying and maybe 10 times less than by car alone.

Find below a summary for the estimates in this post, find a summary of sources used either to check claims and calculations and let me know if you find based on other assumptions other numbers (They have to have the potential though to be better assumptions). I am open to revision. Also note that is a crude order of magnitude estimating and these numbers can vary a lot for specific regions or routes travelled.