The report's respective ratio of 1 passenger car vs one long haul plane are 40% versus 110%:

Image 1 (DEFRA):
car = 171 plane = 195 (110%)

Image 2 (EPA):
car = 4096 plane = 1760 (40%)

If the us uses petrol, it should be the other way around if anything

A car exhaust is at 50cm altitude, versus a plane at 10km altitude. The atmospheric lifetimes of the heavy fumes should therefore be different in the EPA's 100 year atmospheric lifetime indices.

Why do the reports have a ratio of 40% vs 110 % GWP CO2 equivalent for a car vs a plane?

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EPA source:

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  • 4
    $\begingroup$ Also: would it be possible to link the reports ? $\endgroup$
    – user20217
    Apr 9, 2020 at 13:40
  • 1
    $\begingroup$ Please do some homework first. If you dig into both reports you will find definitions of what exactly they are measuring. Add that to the question if you think it does not adequately explain the difference. $\endgroup$
    – Jan Doggen
    Apr 9, 2020 at 13:45
  • $\begingroup$ @jan Actually the difference is very easy: CO2 equivalent is the standard measure of all compiled sources of emissions: CO2 diffuses readily into the atmosphere, whereas heavier gases have an atmospheric life of a few years if released from 8 km high, and less from ground level. Therefore the measures are very easy to define: CO2 equivalent including the atmospheric lifetime of all emissions. The homework is not trivial in this instance, it necessitates a science pub estimating 8km high emissions. $\endgroup$ Apr 9, 2020 at 14:15
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    $\begingroup$ This post needs a table putting UK and their corresponding EPA estimates right against each other in the same row. Right now, I don't see where you're coming up with the 200% figure. $\endgroup$
    – Spencer
    Apr 9, 2020 at 15:18
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    $\begingroup$ Can you add your GWP calculations for the 50% and 200% to your question. There are a lot of numbers in the EPA table and BBC figure, and it's difficult to know which ones you're looking at. $\endgroup$
    – Deditos
    Apr 9, 2020 at 18:07

2 Answers 2


I've looked into this briefly and noticed a few things that are worth pointing out. First, your EPA table looks like it was based on Table 8 of this EPA pdf:


There’s a note in the table that,

Air Travel factors from 2017 Guidelines to Defra / DECC's GHG Conversion Factors for Company Reporting. Version 1.0 August 2017.

So, with a lovely bit of irony, those EPA air travel numbers are taken from the UK/DECC/Defra, which are available here:


Note that they’ve translated the names of the air travel distance categories so they make sense in the US market:

  • US short-haul (< 300 mi) = UK domestic (UK to UK)
  • US medium-haul (300 - 2300 mi) = UK short-haul (< 3700 km)
  • US long-haul (> 2300 mi) = UK long-haul (> 3700 km)

Taking some numbers from US short-haul/UK domestic as an example, the CO2 emissions are,

0.14002 kgCO2e/pass km = 140.02 gCO2e/pass km = 225.3 gCO2e/pass mi

which is just basic units conversion and agrees with your EPA table and the EPA pdf. The same is true for the CO2 emissions in the other distance categories. Note that the EPA are using the numbers for the CO2e emissions without radiative forcing feedbacks from water vapor, contrails, NOx, etc., which is the dark blue portion of the bars in your plot from the BBC. You should be comparing that portion of the emissions, not the full bar including the light blue.

There's a further problem with the CH4 and N2O values in your EPA table. Taking the same example as above but for CH4 emissions,

0.00006 kgCO2e/pass km = 0.06 gCO2e/pass km = 0.097 gCO2e/pass mi = 0.0039 gCH4/pass mi

This is correct in the EPA pdf, but has been mis-transcribed in your EPA table as 3.9 gCH4/pass mi. Oops! someone got confused between grams and kilograms, and they've made the same mistake in the CH4 and N2O entries for all modes of transport. They’ve then used this to calculate the overall GWP column, so their values are oversensitive to CH4 and N2O:

GWP = 225*1 + 3.9*25 + 7.2*298 = 2468.1

The correct calculation is,

GWP = 225*1 + 3.9e-3*25 + 7.2e-3*298 = 227

The bottom line is that I think the GWP column in your EPA table is nonsense and the aircraft emissions factors have the same source (albeit the BBC are probably using more recent UK emission factors). The respective CO2 emissions for cars in the sources I've linked to aren't wildly different (297 gCO2/mi in UK, 343 gCO2/mi in US) and for motorcycles are almost identical (183 gCO2/mi in UK, 189 gCO2/mi in US).

  • $\begingroup$ that's awesome! It's what environmental scientists learn to work with and indeed 2400 CO2 equivalent in one report and 227 in another was difficult to understand. Thanks, so the EPA is similar, just without acknowledging contrails and radiative forcing. $\endgroup$ Apr 11, 2020 at 9:41
  • $\begingroup$ @com.prehensible Yes, the UK methodology notes that there's a lot of uncertainty in the RF feedback effects and they're not required for UNFCCC reporting, so it makes sense that the EPA use the non-RF feedback numbers. The UK is choosing to include it to reflect the upper bound of aviation impacts. $\endgroup$
    – Deditos
    Apr 11, 2020 at 12:16

The unit is different.

The British data is in gram per passenger per kilometer travelled, while the US data is in gram per passenger per mile traveled.

1 mile = 1.609 km

So if you take the long haul flight, 102 g/km (UK) = 164 g/mi, which is almost similar to the US figure of 166 g/mi.

  • $\begingroup$ The ratios are also different, regardless of the units: The US chart states that cars are twice the GWP of planes, and the UK states that cars have half the GWP of planes. The EPA chart suggests that N2O factor from a car and a plane is about the same, despite the plane sending the N2O from 5-10 km altitude, and the car from 50cm. The EPA suggests that the atmospheric lifetimes of fumes from 50cm and 8km are the same? it's weird. Either way $\endgroup$ Apr 9, 2020 at 15:58
  • $\begingroup$ @com.prehensible ["Nitrous oxide has an atmospheric lifetime of 110 years."]( theconversation.com/…). It will be mixed throughout the atmosphere in a much shorter time. $\endgroup$ Apr 10, 2020 at 0:52
  • $\begingroup$ there is another bigger unit problem, the number of assumed passengers is different in both estimates, you can't use cars as a comparable unit of measure like that. the fact that the average US car is drastically less efficiency also matters. $\endgroup$
    – John
    Apr 10, 2020 at 3:01
  • $\begingroup$ @Keith thanks for the heads up, I'll read about that. my geography teacher told us that nitrogen precipitates fairly easily and also falls to the ground near roadsides. $\endgroup$ Apr 10, 2020 at 6:51
  • $\begingroup$ @John hey thanks, Indeed the US probably includes trucks together with cars, it would account for 5 vs 6.7 L/100, so the graph ratios are only different by 75 to 110 using that re-alignement. $\endgroup$ Apr 10, 2020 at 6:53

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