“Stop flying! It’s bad for the climate! Take the train or your car instead!” You have probably heard these sentences a million times. Mobility-related greenhouse gas emissions have been steadily on the rise in the last decades, although virtually every mode of transportation has become more efficient at the same time. According to various interest groups the ranking of climate-damaging modes of transportation is clear. No matter if you ask the European Union, NGOs, federal environmental agencies, scientists or activists, the answer seems to be always the same: planes are the worst, followed by cars, trains and buses.

In such comparisons, tailpipe emissions of the respective means of transportation are a common indicator. Thus, only CO2 (or other polluting substances) that is directly emitted during operation is taken into account. The following pictures illustrate some exemplary (and popular) calculations of the environmental impact and/or fuel efficiency of different modes of transport done by different institutions or people.

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Calculations from the German Federal Environment Agency (Umweltbundesamt) (first chart: CO2 emissions in g/PKT; second chart: NO2 emissions in g/PKT;  third chart: particulate matter emissions in g/PKT)

 

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Calculations from the European Environment Agency (energy efficiency progress in transport in the EU)

 

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Calculations from the think tank International Council on Clean Transportation (ICCT) (fuel economy of various US transport modes on 300 to 500 mile trips)

 

Interesting and certainly valid numbers, right? But all those calculations might not be sufficient to draw the complete picture. What many studies neglect are the indirect emissions that are caused by manufacturing and the infrastructure that each means of transportation requires. Think of emissions caused by the manufacturing of cars or the construction and maintainance of highways, bridges, tunnels, parking spaces, rails, train stations, airports, etc.

A study from 2009 by two researchers from the University of California jumps into this gap. The authors attempted to holistically analyze the environmental impact of transportation by also integrating the construction, operation and maintenance of the necessary infrastructure into their calculation. Their results show that in many cases indirect emissions of cars, buses, trains and planes outnumber their respective direct, i.e. tailpipe emissions. On top of that, their approach seriously challenges the ‘traditional’ sustainability ranking of different transportation modes. According to the study, planes emit less CO2 per passenger-kilometer-traveled (PKT) than conventional cars do when all emissions (i.e. indirect and direct) are considered. Moreover, given the average occupancy rate of cars, aircrafts even score better in terms of tailpape emissions per PKT. Buses and trains remain the most climate-friendly mobility choice. But let the numbers speak for themselves:

 

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Energy consumption and GHG emissions of different modes of transportation when taking into account the related infrastructure (from Chester & Horvath, 2009)

 

A similar picture is drawn by the activist blog ShrinkThatCarbonFootprint, which calculated the carbon footprint of different transportation modes by also including emissions caused by the manufacturing of vehicles and production of fuels:

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Calculations from the blog ShrinkThatCarbonFootprint

 

Based on two the studies, one might easily conclude that planes are actually less harmful than everyone thinks they are and that most recommendations of NGOs, governments and activists are nonsense. But wait, it’s not that simple!

First of all, the studies don’t take into account that you usually travel for longer distances when you take the plane. Thus, the absolute emissions caused by an air trip are often much higher than those of a local or long-distance train or bus ride.

Second, the environmental footprint of a transport mode also heavily depends on the occupancy rate. Thus, emissions per passenger may significantly vary. This can be seen in the two figures above: an urban diesel bus in peak times, for example, has a much better carbon footprint than the same bus in off-peak times, i.e. when the occupancy rate is lower. Or take a plane with different kinds of seating: your carbon footprint is much worse when you travel in business class than if you would opt for economy class. This is because many more passengers can be fitted into a plane with economy class seating. A fully occupied car is usually also more environmental-friendly than a full plane.

Third, the authors of the 2009-study employed a so-called life-cycle analysis (LCA). Although this method is a very common approach for assessing environmental footprints, the amount and detail of data required for such an analysis is immense. This, in turn, might lead to distortions or biases in the results. Moreover, the data is only applicable for transportation in the US. A generalization of the results to other countries is therefore not possible.

And finally, the take-home message: there’s no single truth with regard to the sustainability of different means of transportation. It always depends on the perspective taken and the data the assessment is based on. In any case, going by bike is even better than taking the car, bus, train or plane. 😉

 

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