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=”http://www.epa.gov/otaq/climate/420r06003.pdf”>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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  1. Jimi - January 23, 2008

    Given uncertainties in calculating costs, I’m much more concerned about how well my money is spent than a few dollars more or less.
    Are there reliable agencies who study, regulate, analyze the number of outfits like TerraPass who run carbon replacement programs?

  2. Hank Pierce - January 23, 2008

    Tom Arnold;
    I enjoyed your article but I do not think that it addresses the problem. On a long flight there is more opportunity to cover up the emmissions from Jet Planes. The real problem is in taking off. Consider the weight of the aircraft and the horsepower it nees to gain altitude at 35000 feet. The craft probably uses as miuch fuel to gain altitude that it uses to fly the rest of the route. This means that there is going to be higher emmiissions in the area of takeoff. I read some of these figures years ago about the fuel used to gain altitude. This was not a north american report it was Russian and the fuel required to gain altitude was in the specs if a list of their aircraft. I have asked the question at Vancouver International YVR and they smile and say nothing.
    Vancouver International is increasing its traffic more and more every year. It’s business and their attitude in my opinion is that business supercedes everything.

  3. Tom Arnold - January 23, 2008

    Jimi: yep, there are. Check out our Project Listing pages for more info.
    Hank: you’re totally right — emissions in the LTO cycle (landing takeoff) are about 10% of the total emissions and have big local impacts. This is also why short flights have bigger per mile emissions factors — the fixed cost of getting the aircraft in the air is spread over a shorter flight distance.
    These are all good reasons why videoconferencing, trip recycling (combine trips, do work + vacation)or trains make a lot of sense.

  4. Rob - January 29, 2008

    The round-number I’ve always used for “LTO” is “about 30 minutes of flying” — I got this from a commercial pilot.
    I don’t know if this jives with terrapasses’ numbers.

  5. Kate - January 29, 2008

    Thank you for clarifying a complicated set of issues. Is there any plan to establish national, or even international, guidelines for CO2 flight emissions?

  6. Tom Arnold - January 29, 2008

    Hi Kate:
    Yes there is, although unfortunately, we’re not there yet.
    Internationally, the emissions inventories are controlled by the IPCC, which is studying this issue. A thorny additional issue is that many international flights get stuck in accounting la la land — so called bunker fuels that are not official counts of the country’s emissions.
    Nationally, if aviation is going to be included in a cap and trade system, then we have to figure out just exactly how to account for this. I know FAA is working on this, and the EPA is aware that this is a key uncertainty in emissions.

  7. Anonymous - February 5, 2008

    Hi guys,
    I think I found a programming error in your calculator. It seems it calculates multiple short haul flights as a single long haul flight. For example, if I input LAX-LAS 5 times, I get 2,356 miles and 1,507 pounds CO2. However, when I input LAX-LAS and increase the number of trips to 5, I get 2,356 miles and 1,060 pounds.

  8. carl myhill - April 26, 2008

    The science on radiative forcing now seems settled. Check out the recent book by the former chief scientific adviser to the uk government (Sir David King – a professor of chemistry at Cambridge). He suggests the effect is a multiplier of 3 (Monbiot quotes 2.7). So, it’s not a few dollars out. Your calculator is reporting only one third of the “CO2 equivalent” produced from flying. About time to fix it I would say.

  9. Tom Arnold - April 27, 2008

    Hi Carl:
    We’re actively tracking this issue, and the most recent research from FAA and others suggest that 2.7 number is actually not the right way to look at things. 2.7 is simply the average of four studies in the 90’s, and now scientists are proposing a different methodology.
    We’re expecting a good survey paper out in a few weeks, which we’ll blog on. Thanks for your patience, we do want to get this right, but we want to do it with the science community’s support.
    In the meantime, our store system easily allows you to purchase more, just increase the quantities associated with your flight offsets.

  10. Carl Myhill - April 28, 2008

    Hi Tom,
    Huge thanks for your reply.
    You are suggesting that the science I quoted is a bit out of date and that the Federal Aviation Authority has some better research. Do you think they may have a vested interest in coming up with a better number? I do.
    Which are the studies you are referring to which give rise to the 2.7 figure? Even with data collected in the 1990s, as you suggest, since most of the commercial air fleet dates back to the 1970s I don’t expect much has changed since then.
    The science is clearly complex and slightly uncertain but it should be noted that the 2.7 figure comes from the IPCC 1999 report ‘Aviation and the Global Atmosphere’.
    Sir David King, former Chief Scientific Advisor to the UK government and a professor of Chemistry at Cambridge University puts this at “up to 3x”. I think he’s probably in a good position to understand the science and has no vested interest in mis-reporting on the issue. He quotes his source as The Stern Review p388 box 15.6 and the IPCC WGIII chapter 5.
    David King also explains, quite clearly, what Radiative Forcing is all about. I’m sure he won’t mind if I quote him directly. “Aviation is directly responsible for about 700 million tonnes of carbon dioxide each year, which is just 1.6 per cent of the global greenhouse emissions. However, molecule for molecule the emissions count much more than they would on the ground because planes are very efficient at causing greenhouse warming. High-altitude deliveries of nitrogen oxides (which form ozone another greenhouse gas), as well as the water in contrails that can go on to form cirrus clouds, together enhance the direct effect of carbon dioxie by up to a factor of three”. (David King and Gabrielle Walker, 2008, ‘The Hot Topic’).
    As you can see, I think the science community has long been in support of understanding the Radiative Forcing effect of carbon released from planes. It seems that what you are really saying is that you don’t want to move on this without the aviation industry being in support of the figures.
    As an individual, sure, I can increase my carbon offsets with you. My object is that each and every one of the estimates you currently give to people are severely wrong.
    We need some bold moves to tackle climate change within the 20 year window we have to do something about it. I hope you will be bold, read the science, and fix your calculator (and in the process explain why). One of your charts currently makes flying a better option than driving – that is just misleading on a number of levels.

  11. Tom Arnold - April 28, 2008

    Hey Dave:
    Great discussion, so keep it coming. We are exploring of all these issues, but I am sure we can learn more. We do know there a quite a few supporters of the “2.7 view” as well as those that think it should remain 1.0, and those that think it should be much higher than 2.7 (because 2.7 is a number derived from several studies).
    When we first looked at this we actually though it would appear a little self-serving if we boosted the number without support from the science community. And in the US, the main authority is the World Resources Institute (WRI), which as recently as a few weeks ago still recommends a RF factor of 1. Still if your someone that believes that the number is 2.7 well, our calculator is less than satisfactory.
    So we do hear you loud a clear, though and are exploring the issue this summer and awaiting an influential paper on this issue. Thanks for your interest and attention, and I hope to have you in the discussion on any changes we make.

  12. carl myhill - April 28, 2008

    Who thinks the number should remain at 1.0? Can you please supply references.
    You may know that the IPCC, whose 2.7 RF figure is widely recognized, recently won a Nobel Prize for its work on Climate Change along with Al Gore. Are you really suggesting that the science of the IPCC is not good enough for Terrapass even though it has been good enough to gain a Nobel Prize?
    Your position on this really does make me wonder who is behind terrapass.

  13. Tom Arnold - April 28, 2008

    Hey Carl. For one, WRI (as cited above or look in the tools section of ghgprotocol.org).
    FWIW, even though the IPCC conducted this research, aviation emissions are not assigned this number in the national inventories, nor is RF implemented in any trading systems. So it remains theoretical at this moment. We’re not opposed at all to implementing RF (it would probably be good for business) we just want WRI, IPCC, and some other observers to actually agree on a both a methodology and a number.
    As for your conspiracy theory, you may be new to TerraPass, but read along in the blog, and you’ll quickly find us to be genuine well-meaning climate nerds, rather than the Dr. Strangelove image you have in your head.
    Finally, if you want to rally against anything, rally against the fact that a huge portion of aviation emissions are *excluded* from national inventories as “bunker fuels”. Because no-one settled the ownership of these emissions they are the unmanaged hot-potato of emissions under Kyoto.

  14. Carl - April 29, 2008

    I agree that leaving aviation fuels out of accounting and indeed, outrageously, out of taxation is an issue, but it is a separate one which has nothing to do with calculating the carbon equivalent emissions from air travel.
    I still find it odd that you choose to ignore this. You could, for example, show the actual carbon emission figure and next to it, an estimate of the carbon equivalent figure. You could explain that your view is that the science isn’t there yet but if you conservatively estimate it at 1.9 (in line with some recent research) instead of ignoring it completely it would be better than nothing. Do you really believe, as an eco-nerd, that the number is 1.0?
    Please excuse the conspiracy theories but when you tell me you don’t accept IPCC figures and are waiting on the FAA figures, well, it seems quite suspicious.

  15. Joe - April 30, 2008

    Thanks so much for engaging in this discussion.
    I wonder if you can address the huge differences in costs of your flight offsets compared to some others. I am trying to determine the most sensible way to purchase flight offsets for myself and my company.
    I have been looking at the Green-e Energy certified offset programs, particularly those of the Bonneville Environmental Foundation. On their website, at https://www.greentagsusa.org/GreenTags/ their calculator gives a much higher cost than TerraPass.
    For example, offsets for 40,000 miles of flying costs $74.25 from TerraPass but $726 for BEF wind power offsets. And the BEF solar power offsets cost twice as much as the wind power offsets!
    Now, it is obvious that some of this is due to the “2.7” issue discussed above, because BEF uses 1.36 lbs CO2 per passenger mile, not the .40 lbs per mile that TerraPass uses. But there is still a factor of 3 or so to account for.
    Is this due to the fact that your farm and landfill gas projects are much cheaper than clean energy? And if the prices of offsets vary so much by project, how do you think about whether some kinds of offsets are more appropriate for some kinds of energy use (like home electricity versus flying)?

  16. Tom Arnold - April 30, 2008

    Hi Joe:
    You’re right its both price and the use of radiative forcing at other vendors. Our pricing is currently $10.91 per metric ton. The example you are talking about is $20 per MWh, and a carbon content of 1500 lbs per MWh. That works out to $29.39 per metric ton, or about three times the cost. I don’t know why the cost is that high, but the vendor should be able to explain.
    In general, we don’t believe that specific types of offsets should be matched to different uses — carbon is carbon and a farmer’s cow power engine lowers emissions no matter what you are emitting.
    Carl: A few points before we beat this to death.
    1) I promise we are not ignoring it, and are actively watching it, reading the research and trying to make an informed resolution of the issue. We’re also blogging it, and trying to discuss where we are right now.
    2) The Kyoto treatment of aviation emissions is relevant because they also chose to ignore the IPCC study. Maybe they are not right, but its a data point along the way, and worth considering.
    3) I like your idea, and its worth exploring, which is what we are currently doing. We’re just not ready to make any change this second, and we are going to wait for a landmark paper coming out shortly. Sorry if we are slow, but your comments are valuable and have definitely added to our internal discussions.

  17. Carl - May 2, 2008

    Thanks Tom.
    Coincidentally I was at a talk this week by one of the scientists at the IPCC who has a share in the Nobel prize (now the chief scientific advisor of DEFRA). Unfortunately his talk was broad so I could not put this issue to him.
    You don’t quote many references so it is hard to follow back through your comments but Kyoto was in 97 and the IPCC report was 99 so I am a bit confused about what you mean. It’s a shame the US and Australia have still not ratified Kyoto.
    It does seem that there is increasing consensus about using the revised figure of 1.9 for RF which came from a 2005 study ‘Aviation radiative forcing in 2000: and update on IPCC (1999)’ (http://www.ingentaconnect.com/content/schweiz/mz/2005/00000014/00000004/art00013 )
    In fact, DEFRA, in their ‘ draft Code of Best Practice for Carbon Offset
    Providers’ are now recommending the 1.9 RF number be used for flight (they describe this as “in line with the best scientific evidence”:
    (this is an update to your article’s comments about DEFRA)
    I guess my primary concern with the wait and see approach is that, we have just 20 years to do something about CO2 emissions before it gets out of control and it becomes too late to do anything about it (ref: David King’s book ‘The Hot Topic’ which is generally optimistic).
    Us Eco nerds need to act fast and adapt to the science as more information becomes available. We all know the number for RF is not 1.0. The best science now says 1.9. why not accept that, plug it in to the calculators and move on to the next issue?
    Thanks for the discussion.

  18. Tom Arnold - May 2, 2008

    So Carl, I hear you loud and clear, and your comments have given us lots to think about.
    I can’t promise anything, but we are working on this, and as soon as I have an update I will blog. Thanks for the dialog and pushing on this issue.

  19. roger - September 23, 2008

    hi Tom
    besides radiative forcing, the other factor that varies dramatically is an adjustment for space used per passenger ie class economy, business 1st. some make no adjustment others use afactor 3 for 1st class