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Dissecting Flight Carbon Calculators


Flying is complex enough, so we try to make purchasing TerraPass flight offsets pretty simple. But if you’ve been searching around trying to get a more accurate handle on your carbon footprint, you may have noticed that lots of online calculators give you different results.

Consumer Reports looked into this issue in a recent article. They compared the results of sixteen calculators for a New York to Los Angeles flight: the total carbon emissions varied by a factor of three. The Consumer Report’s advice to consumers is pithy, but less than satisfying:

Generally, the more explanation it gives and the more info you’re asked to input, the better

So, fasten your seatbelt and I’ll dig in to the question a bit more and try to explain the differences. Please note that for this analysis I’ve corrected a Consumer’s Reports error (tons vs tonnes) on a number of entries.

Your flight details please

Flat earthers aside, let’s agree that NY and LA are in fact 2,467 miles apart (yes, I am prepped for flames on great circle routes). For this distance, the per-mile emissions factors range between .39 lbs and 1.36 lbs per mile.

Why the differences? First, check for how much information is included about the specific flight. Shorter flights emit more per mile than long flights because a disproportionate amount of energy is used in taking off and landing.

Only three calculators account for this (one of them is ours), and among them their adjusted per-mile emissions factors range from 0.36 to 0.40 lbs per mile, a much tighter spread. The others use more general emissions factors, which range from 0.40 to 0.70 lbs per mile.

Of course if we had access to the actual airline data, we could go even farther and include factors such as aircraft type, actual capacity factor, connections, cargo loading, seat type, even your own baggage allowance.

May the force be with you?

The second big difference is something called radiative forcing: an effect that tries to account for the interactions of a plane’s emissions in the stratosphere. Here the culprits are oxides of nitrogen (NOx) whose upper atmosphere chemistries create a warming effect as they produce ozone, and a cooling effect as they reduce methane.

Six of the eleven calculators use it, four simply double the emissions and the other two multiply them by 2.7. Multiplying by the range of per mile emissions factors above, you can quickly see how a wide range of estimates are produced.

TerraPass doesn’t use radiative forcing for two reasons. First, we don’t yet see consensus in the science. Second, every time we ask the main standard setting body, the World Resources Institute (WRI), for guidance, they advise us to avoid using a radiative forcing number until they have settled the science.

We’re not alone, here’s a short list of folks NOT using radiative forcing:

  • EPA’s Greenhouse Gas reports omit it (although they href=””>discuss it as a factor in 2006)

  • California Climate Action Registry General Reporting Protocol
  • DEFRA (like the EPA except they serve tea)
  • The UK Parliament’s environment committee who had this to say:

    There is still no scientific consensus on the [radiative forcing] factor, its size or temporal impact, and recent research indeed points towards its likely inappropriateness for the calculation of climate effects above and beyond those delivered by carbon.

To add further credence to this last little bullet, on a panel I spoke at last week at the Transportation Research Board, both Anja Kollmus from Tufts / SEI and an official from the FAA pointed out that the traditional manner of calculating radiative forcing is inappropriate and that further research is needed.

And that’s why, just like on the project side, we’re going to stick to the standard for now and report aviation emissions with the traditional six greenhouse gases.

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