Kenji's Blog

Blog for Dr Kenji Takeda at the University of Southampton. Find out about what he's up to in aerodynamics, aircraft noise, flight simulation and Formula One teaching, research and schools outreach. Also see what's going on in the School of Engineering Sciences at the University of Southampton.

Wednesday, September 24, 2008

Planes, trains and automobiles... which is greenest?

You may have seen our recent BBC South Today feature comparing the environmental performance of trains, planes and automobiles on the Southampton to Edinburgh route. We hope you find it interesting, if you missed it you can read it here, or see the video clip here. Here is the chart that compares total carbon dioxide emissions. for 1, 2 and 4 passengers (click picture to enlarge).

An important point that was not delved into was the issue of non-CO2 emissions from aircraft. This often leads to the application of an 'uplift' factor, where we multiply the CO2 emissions by a fctor of up to 3.7 to take into account other effects. These include Nitrogen Oxides, Methane, contrails and possible increases in cirrus cloud formation. The science related to this is difficult, and there is a good deal of uncertainty about what is the right uplift factor, or whether it is relevant in this context. Why?
Well, carbon dioxide lives in the atmosphere for about 100 years. Other emissions last a few years, months, weeks or even just a few hours, in the case of contrails. The physics of how this actually affects climate is complex, so boiling it down to a single number - usually radiative forcing - is simplistic, but convenient.
So for the analysis we did, we just look at carbon dioxide. CO2 'calculators' online, such as those used for carbon offsetting, add an 'uplift' factor. They claim that what they are telling you is the CO2 you will emit. This is not quite true, it is an 'equivalent CO2' amount. So be careful when you talk about 'carbon footprint' - that means CO2, if it includes non-CO2 effects then make sure that is included for everything you are talking about.
As there is some uncertainty about the climate science, we have chosen to focus just on CO2 here. Clearly, if we apply an uplift factor the numbers change, and this is important.

Another factor is that on this route the train is diesel-powered, not electric. In the UK around 50% of trains are diesel, which is very different to other countries such as France and Japan. Also, electric trains must get their power from somewhere, and again, the UK has a lot of coal, gas and oil-fired power stations. French trains are great, but their electricity is almost all from nuclear power stations - and that's a another environmental discussion altogether.
The message remains, cars emit a lot of CO2, so always share. Use a train if you can, it is still the best environmental option, especially if it is electric not diesel! Flying is an option that should not be excluded, particularly if travelling alone, as its CO2 emissions are not too bad - even when using 'equivalent CO2' with an uplift factor.
If you want to design your own airliner to see how environmentally-friendly it is, visit our website at
For more information on aviation and the environment see


  • At 3:22 pm, Blogger NickB said…

    The researcher ignored the uplift factor because "there is some uncertainty" about the climate science. But on those grounds you would ignore the figures from the manufacturers too. (It's not as if they have an incentive to understate emissions, right?) Because he ignores the uplift factor, people are likely to take away from this research that it's relatively green to fly, which is not at all substantiated by the arguments presented. This is exactly the way the BBC presented it.

    Then there is this: "Flying is an option that should not be excluded, particularly if travelling alone, as its CO2 emissions are not too bad - even when using 'equivalent CO2' with an uplift factor."
    This does not follow because no analysis is presented with the uplift factor. Ignoring that problem, you might as well conclude that taking the train is almost as bad as flying as say that flying is almost as good as taking the train.

    I think it's really inexcusable to ignore the uplift factor and to present findings in this way given how it predictably plays out in the media. It's depressing that people at my university are doing this. I imagine the researcher will censor my comments.

  • At 5:41 pm, Blogger Dr Kenji Takeda said…

    Thanks for your comments NickB. I am not in the business of censorship, as I value everybody's opinions, and yours is certainly very valid. So thanks for commenting here, keep doing it.

    What I am trying to show is difference in CO2 effects. The uplift factor is not CO2, it is other effects. If we take an uplift factor at the bottom end of the scale (x1.2) then flying is not as bad as a car when travelling alone. When we take an uplift factor at the high end (3.7), then the case changes.

    There is a great deal of uncertainty around uplift factors. That is why climate scientists who are defining the European Emissions Trading Scheme for aviation are excluding uplift factors. That does not even take into account that uplift factors are applied as a multiplier to radiative forcing, which does not take into account timescale effects.There is a big debate in climate science as to whether this is correct. IPCC think so, and that is a major scientific body that we can trust, so it does hold a lot of weight. But there are others who think we should use global warming potential, which, if you look at it that way, means the uplift factors are completely different.

    We do not know uplift factors for the other forms of transport, so a comparison with aviation CO2 * uplift would be unfair. Personally I suspect the uplift factors for cars and trains to be pretty small, but even so, we need to compare apples with apples, and oranges with oranges.

    If you would like to read more about uplift factors, then you can read our extensive review of aviation and climate here

    I hope that helps clarify the position. The analysis is comparing CO2 emissions FULL STOP.


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