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I took this self-survey of CO2 emissions run by the German Environment Agency.

It computes for example that an economy class seat from London to San Francisco and back – about 22 hours flight – accounts for 5.28 tonnes of CO2 equivalents (CO2eq or CO2e).

Calculator screenshot

For comparison,

  • the average CO2-emission in 2017 per capita has been 9.7 tonnes in Germany according to this Wikipedia article – but that excludes other greenhouse gases.

  • The International Civil Aviation Organization gives only 0.762 tonnes CO2 emission for the same flight. The methodology describes only CO2 emitted when the fuel is burnt.

Calculator output

Is the German Environment Agency's estimate for CO2eq from such a return flight reasonable?

  • 9
    You understand that one site is giving pure CO2 and the other CO2e numbers. You understand that they aren't the same. You understand there is no conflict. So what is the specific claim that you are doubtful about? – Oddthinking Jun 25 at 8:44
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    It would make far more sense to ask this on Earth Science, where the domain experts hang out. – EnergyNumbers Jun 25 at 10:03
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    @Oddthinking My question is whether the Environmental office is exaggerating its interpretation. If they are correct then the most damaging behavior for the climate I have is flying. I could save less than 10% by improving my apartment for 30,000$, or by getting rid off my car, but if I stopped flying it would be more than 50% less without any other effort. And I know people that fly 4 of 5 times as often as I do!! I have not perceived that other media are presenting flying as the most harmful behavior for the climate a private person can have. Therefore my question. – KlausN Jun 25 at 11:14
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    @KlausN: "I have not perceived that other media are presenting flying as the most harmful behavior for the climate a private person can have." -- That is strange, because the headlines seem to be full of exactly that discussion recently. Greta Thunberg taking the train to Davos instead of flying prompted the recent round, but it's already been around for a while before that. – DevSolar Jun 25 at 12:37
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    Why don't you ask if the estimate from the International Civil Aviation Organization is reasonable? The ICAO surely has a vested interest in claiming that flights aren't too damaging. – Eric Duminil Jun 26 at 8:37
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Water vapor is indeed the most important greenhouse effect contributor.(Ref. 1)

It is usually not showing up in climate change discussions because mankind basically has no way of directly affecting the amount of water vapor in the air, either positively or negatively, as anything we could or could not do is utterly dwarfed by natural processes. (As opposed to CO2.)(Ref. 2)

There is one major difference, though. Airplanes. They put significant amounts of CO2, and more importantly, water vapor, directly where it has the most significant impact: High up in the atmosphere. And they are the only source doing that. When US air traffic was grounded for three days following 9/11, the effects were immediately measurable.(Ref. 3)

CarbonIndependent.org Aviation sources elaborates in some detail on how its CO2 calculator does its calculation, arriving at 250 kg CO2e per flight hour (about the same value that the GEA calculator gives you -- 250kg/h * 22h = 5500kg ), then going on to quote Professor David Lee, Director, Centre for Air Transport and the Environment (CATE), Manchester Metropolitan University:

Whilst it is incorrect to multiply CO2 emissions by the RFI, it is clear from the foregoing that aviation's effects are more than that of CO2. Currently, there is not a suitable climate metric to express the relationship between emissions and radiative effects from aviation in the same way that the global warming potential does but this is an active area of research. Nonetheless, it is clear that aviation imposes other effects on climate which are greater than that implied from simply considering its CO2 emissions alone.

The site then goes on to state (emphasis theirs, but especially note the last line):

According to this statement, it is incorrect to multiply CO2 emissions by the RFI, but it is also incorrect to ignore them. For the purposes of a calculator, some decision must be taken until further evidence is available, and the Carbon Independent calculator will for the time being multiply aviation CO2 emissions by a factor of 2.0. The practical consequences are in fact minor as regards informing people on the easiest way to reduce their carbon footprint; whatever the size of the factor used, the easiest way for most people who fly to reduce their carbon footprint will be to cut back on flying.


So, without going into actual number crunching and arguing about the "correct" CO2e calculation (which does not exist), the Environment Agency's calculation seem legit, and does correspond to the broad opinion in climate research.

(1): http://www.geo.utexas.edu/courses/387H/PAPERS/kiehl.pdf

(2): https://www.bpb.de/apuz/30101/klimawandel-einige-fakten

(3): http://www.greenmedinfo.com/blog/artificial-weather-revealed-post-9-11-flight-groundings


Further references:

  • Comments are not for extended discussion; this conversation has been moved to chat. – Sklivvz Jun 29 at 20:46
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Values from International Civil Aviation Organization cannot be correct.

Warning : very short calculation ahead. I hope it's alright for Skeptics.

One way to check if the results are plausible is to calculate the CO2 emissions in grams per passenger-km.

  • CO2 emissions for round-trip : 762 kgCO2 / passenger = 762200 gCO2 / passenger
  • Total distance for round-trip is : 2 * 8652km = 17304km
  • Specific CO2 emissions : 762200 / 17304 = 44 g gCO2 / (km * passenger)

According to this IPCC report, the direct CO2 emissions for passenger aircrafts are between 95 and 250 gCO2 / (km * passenger) :

ipcc_wg3_ar5_chapter8 figure 8.6

Source:

Sims R., R. Schaeffer, F. Creutzig, X. Cruz-Núñez, M. D’Agosto, D. Dimitriu, M. J. Figueroa Meza, L. Fulton, S. Kobayashi, O. Lah, A. McKinnon, P. Newman, M. Ouyang, J. J. Schauer, D. Sperling, and G. Tiwari, 2014: Transport. In: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Page 610, Figure 8.6

44 gCO2 / (km * passenger ) would correspond to an efficient train or coach, but seems to be well outside the range of any current airplane.

How does the ICAO calculate these values?

I have no idea how the ICAO could even come close to 44 gCO2 / (km * passenger).

Even with those best-case assumptions:

it doesn't seem to be possible to get any value under 60 gCO2 / (km * passenger). Modifying any of the assumptions in order to calculate more realistic values would increase the amount of released CO2.

UPDATE : The methodology is described in this PDF, I'll try to reproduce the results. (Thanks @Nat!)

Note: I'll investigate the values given by German Environment Agency later. Showing that the values from the ICAO are wrong doesn't say anything about those given by the GEA.

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    I think the "calculation" you did there is totally acceptable. Basically you just converted the given numbers for comparison with the IPCC report. – DevSolar Jun 26 at 11:46

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