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The communities I live in in Germany keep telling everybody that flying should be avoided by all circumstances, because it has a far bigger ecological footprint than any other mode of transportation. But according to this paper, considering other costs than the energy spent to actually get from point A to B, flying and train travel just show the same amount of energy consumption (see figure 1).

Is this study also applicable to German/European trains? And are there other scientific statements on this subject (I didn't find any!, and I don't want to make travelling choices based on one paper from california).

Edit: In this question I only want to discuss travelling options that are viable solutions for inner-European travel, to my regards those are cars, trains (national and international companies) and flying in aeroplanes.

Code Gorilla
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Quantumwhisp
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  • What's the range? that's a huge factor in answering this question? Clearly, it doesn't pay to take a plane up to altitude for a 100km short haul. – Evan Carroll May 02 '17 at 21:16

2 Answers2

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Chester and Hovarth are scientists publishing in their area of expertise. I am not qualified to evaluate their methods. I have to restrict this answer to evidence that is at least as high quality as theirs. Their paper has garnered 315 citations since it was published. Ultimately, other researchers are the real peer review. I skimmed a few of these articles looking for a critical review of Chester and Hovarth's work. Most of the authors seemed to accept Chester and Hovarth's main conclusion, which had nothing to do with your question. Maybe someone willing to put in a massive amount of time and effort can find something. Maybe more work will be published in the future.

Your question appears to correctly restate the claims of the paper.

According to their paper, flying produces fewer greenhouse gas emissions per passenger kilometer traveled (PKT) than driving. A large aircraft produces roughly 125 grams of CO2 equivalent per PKT; A sedan produces roughly 230.

The conclusions of the paper depend on assumptions they made in their methods. They chose to evaluate 747s and 2005 Toyota Camrys. They also chose data about average passenger occupancy for both of these vehicles. I cannot find in their paper exactly what this data said about the average number of passengers in a car. Although these assumptions are a potential weakness in their study, all studies make assumptions. Without simplifying assumptions, you can never get anything done.

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The above figure is an excerpt from a larger figure in the paper. It compares the equivalent CO2 emissions of a Camry and a 747. "The maroon-colored line captures the range in per-PKT emissions at low and high occupancy." This shows that a high occupancy car is better than a low occupancy airplane. Overall the chart says that the average plane is better than the average car.

They also seem to be assuming that there is on average, significantly more than one person in a car. I am skeptical of this. I may be misinterpreting that aspect of their figure.

Other researchers have made broadly similar claims This article, reviews research by several authors, including Chester and Hovarth.

According to an American study by Michael Sivak, "The main finding of that study was that, while flying domestically in the U.S. used to be much more energy intensive than driving, that is no longer the case." This agrees with the claim. Sivak's finding that the emissions from air travel has changed dramatically over time, implies that results done in the US may not be super relevant to the EU. I don't know how European and American air travel is different. Sivak also agrees that a high occupancy car is better than an average airplane.

These articles, take issue with Sivak's analysis. Their critique also applies to Chester and Hovarth. I did not detail read Sivak's work, so I will apply the critiques to Chester and Hovarth's work. They have similar methods, and the same problem.

The critics do not take issue with the basic finding that an average plane ride is better than a low occupancy car ride. The average american car ride, is a low occupancy commute within a city, and that is what Chester and Hovarth based their work on. These trips cannot be replaced with a plane trip. Comparing average car rides to average train rides is an apples to oranges comparison. We should compare average cross country car rides to average plane rides.

For cross country trips the second article reports that cars are better than planes if there are 3 or more people in the car. I am not sure her math is exactly correct, but this seems roughly consistent with the other authors.

Conclusion

I am not sure how to compare the average cross country trip to the average plane ride, but if you have a choice between a car and a plane for a cross country trip:

  1. Choose the plane if you are going by yourself.
  2. Choose the car if you are traveling with 2 or more other people.
BobTheAverage
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  • Good answer, seems to be a economies of scale issue. A plane can just move so many people compared to the energy losses and the small number of people a car can move. – RomaH May 03 '17 at 15:17
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    Your answer seems to be neglecting trains and buses which I expect both to have a smaller GHG footprint for cross country travel. In particular trains use electricity and thus profit from the increasing share of renewable energy sources in the EU energy mix. – Roland May 03 '17 at 19:00
  • I believe the issue with aircraft emissions has more to do with the _location_ of the emissions (i.e. in the upper atmosphere) than with the volume of emissions. This is called "radiative forcing". The upshot is that in the long run, planes lead to less warming, but their emissions cause the planet to warm sooner than emissions from ground vehicles. Check out http://pubs.acs.org/doi/abs/10.1021/es9039693 – John Doucette May 05 '17 at 14:09
  • There is often considerable travel to and from an airport, which you avoid when taking a car. And cars will be more efficient if the engine is warm, and you drive long distances on a motorway at a slightly lower speed. But the real difference is between driver only and driver plus passengers. – gnasher729 May 05 '17 at 16:57
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Update: It seems that although the emissions (I know the question asked about energy, but from a sustainability perspective it might be argued that emissions or at least environmental impact is more important) from construction for railways is more than twice as large as that from the construction of airports, on a passenger-kilometer basis, the emissions from the flights themselves dwarfs those of trains no matter what fuel source are used for the trains. The following table is from Sweden where there is a relatively large portion of hydropower used to power the trains, but it is hard to imagine any fuel mix exceeding those of flights:

Table showing Life Cycle emissions of various modes of transportation, broken down by construction and use

The figure can be found in this report from Swedish IVA, and is based on the freely availble (although in Swedish) calculator found here.

Original post: I am sure you can exceed the flight emissions if you drive single person in e.g. a 2014 Bugatti Veyron 16.4 Grand Sport Vitesse (2.4 liters per 10 km) at very high speeds (fuel economy drops) with frequent hard accelerations and braking.

Sure the infrastructure causes emissions but you also need to consider where the electricity is coming from. This means that for different parts of Europe the calculations will look different (e.g. in Scandinavia during the summer it is all renewables, especially in the spring when snow melts while in Poland which is mostly coal powered you might be better off with an airplane than the local high speed train).

So to answer your question, for certain routes at certain times it is definitely better to take the train. However, for travel inside Poland, for example, there might be a case for flying but I am not aware of anyone having done a full life cycle assessment including infrastructure.

Moreover, with the electrification of transportation, even infrastructure emissions will drop as trucks become electrified while it is hard to imagine airplanes being fossil free in the foreseeable future. This means that if you care about emissions, it is probably better to support alternatives to flights. The Hyperloop is an interesting such development. And you should also support mass deployments of renewables and electrification of transportation to bring down infrastructure emissions.

Uli Alskelung Von Hornbol
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