Calculating your CO2 emissions: A Practical Example

In my previous entry, I wrote that CO2 emissions are the sole metric that you should be tracking if you want to make changes in your life that help mitigate your environmental impact. Despite the abundant material you can find online about it, the calculation of this number does not seem straightforward. Your specific circumstances typically don’t fit the assumptions behind these sources of information that promise to help you. Worst of all, they almost never tell you how they arrive at their estimates.

This entry is a follow-up to my previous one: Your CO2 emissions: The number you should track in 2020

My suggestion is simple: Do not try to get your exact CO2 emissions, but rather a ballpark figure that is good enough for a first approximation. Starting from your baseline, which should be the average emission per capita in the country where you live, add the CO2 contribution that you perceive as the most significant in your life. The key step is to be slightly more precise in that second element that you add to your baseline.

Regarding the baseline, my colleague Ghassane Bentahar made me realize that I should weight the per capita emissions with the time that I spend in each country: 85% in the UK and 15% in Colombia. The Wikipedia page List of countries by carbon dioxide emissions per capita shows that the most recent estimates are 5.6 tons for the UK and 1.8 for Colombia. Thus, my baseline is 85% x 5.6 + 15% x 1.8 = 5 tons of CO2.

Let’s move on now to my most severe contribution to CO2 emissions which, as I said before, most likely is due to my regular intercontinental flights. I’m usually doing 4 trips between London and Bogota every year, so I want to be more precise about my emissions in each of them.

I could rely on the many online carbon emission calculators to estimate that impact. They take as input the origin and destiny of your flight, and they tell you how many kilograms of CO2 are associated with your trip.

The beautifully designed shameplane.com website is one of them, and in an instant, it tells me that each passenger on a one-way London-Bogota trip emits 1.6 tons of CO2. Shocking! In just one trip, it seems I emit almost as much CO2 as an average Colombian does throughout the whole year.

However, if you know me, you will probably know that I never accept a number without double-checking it. I took the challenge of recreating this calculation, and what I found is that the considerations behind the estimation of CO2 emissions are fascinating. They give you a much more in-depth, richer perspective of the problem than if you just accept the output of one of those online calculators.

In the next three sections, I will explain my calculation in some detail. If you don’t like numbers, you can skip to the section “Comparing to Online Calculators“, but still you may want to quickly skim some of the following paragraphs: you may find something interesting, or even spot something I am doing wrong!

Basic CO2 emission calculation per flight

My calculation of the CO2 emissions per person in a one-way London-Bogota flight starts by estimating the emissions of the whole plane. It uses three inputs:

  • The type of aircraft: Avianca, the Colombian airline, operates a direct flight between London and Bogota which is the one I typically choose. It uses the Boeing 787-800 Dreamliner for this trip (the type of aircraft always appears in the safety card).
  • The amount of aviation fuel consumed per hour: The Boeing 787-800 consumes 4.9 tons of fuel per hour. You can goggle that information, but just in case, here is a link with the fuel consumption of various aircraft.
  • The flight duration: The flight between London and Bogota takes 10 hours. The captain always announces the airborne time before taking-off.

With this information, I calculate that the plane will consume 4.9 tons/h x 10 h = 49 tons of aviation fuel on that trip.

This figure is in line with the one you get when following the ICAO Carbon Emissions Calculator Methodology. It needs the distance between London-Bogota, which is 4,571 nautical miles (that is, 8,467 kilometers), to which you add a correction factor of 68 nautical miles (that is, 125 kilometers) to include the emissions flown in excess of the distance between the two cities, stacking, traffic, and weather-driven conditions. The ICAO Fuel Consumption Table indicates that a Boeing 787 consumes 52 tons of fuel in a trip of 4,639 nautical miles, a figure slightly higher than the 49 tons I got before. To be conservative, I will use the higher number.

Now that I have the amount of fuel burned by the plane, I need to convert it into emitted CO2: I multiply the amount of fuel by a conversion factor of 3.15, so the total CO2 emitted for the London-Bogota trip is 164 tons of CO2.

There is one last headache: While in the air, planes release other chemicals besides CO2 that also contribute to global warming, an effect known as radiative forcing. There is still uncertainty in the scientific community on how significant is the greenhouse impact due to this effect. However, a recent document that reviewed the latest publications in the topic suggests that a factor of 2 should be multiplied with the direct CO2 emissions to calculate the total global warming potential of aviation services (notice that other factors have been proposed, ranging from 1 to 2.7). By including the radiative forcing, the total impact of the one-way London-Bogota flight to global warming is 164 x 2 = 328 tons of CO2-equivalent.

That figure is enormous: That one flight emits 328 giant balloons of CO2, like the ones I described in my previous entry!

Chemical interlude

I got the 3.15 conversion factor from fuel to CO2 from an aviation handbook, but I found quite puzzling that the weight of CO2 released in the trip was three times larger than that of the burned fuel. To understand this effect, I called my friend Jorge Posada, an expert Chemist, who explained to me the chemical reaction, in which a molecule of octane in the fuel reacts with oxygen in the air to produce CO2 and water.

A stoichiometric analysis shows that:

1 molecule of C8H18 (octane) + 12.5 molecules of O2 (oxygen)

produce:

8 molecules of CO2 (carbon dioxide) + 9 molecules of H20 (water).

That is, for each molecule of octane, you end up creating eight molecules of CO2. But in terms of molecular mass, octane is heavier than CO2 by a factor of 2.6 (octane weights 114 g/mol while CO2 weights 44 g/mol), so you end up creating 8/2.6 = 3.08 grams of CO2 for each gram of C8H18.

The final factor is slightly higher, because the aviation fuel is a blend of various types of octanes and has some impurities, and the concentration of oxygen in the atmosphere is lower at the altitude at which the airplane flies, making the chemical reaction not 100% efficient.

How much per passenger?

So far the calculations have been pretty straightforward, but to split the 328 tons of CO2 among the 250 passengers you need to make a few assumptions in the following three items:

•    Business-class vs Economy: Should the attribution of CO2 be equally split among all the passengers? Or should those in Business class have a higher share, as they occupy more space?

•    Empty seats: Should you take into account when the plane is not flying at full capacity?

•    Non-passengers cargo:  Should you take into account that some of the freight transported by the aircraft does not belong to the passengers?

The seat map of the Boeing 787-800 shows that the 28 Business class seats occupy an area where you could fit 126 Economy class seats, which means that the Business/Economy ratio is 4.5. This ratio is not far from the price differential between the two classes, as typically it is three times more expensive to fly in Business than Economy. Some methodologies apply a factor of 2, but I have not found any rationale for that number. Hence I will assume that the emissions of flying in Business class are 3 times the ones of flying Economy class.

Avianca is the only carrier operating a direct flight between London and Bogota, so not only that means that they can charge whatever price they want, but also the flights are almost always at full capacity. I asked a flight attendant who has covered that route many times how many empty seats she would typically see in that flight, to which she answered that on average only 10, and never more than 25. Thus, assuming a 90% capacity seems reasonable. Still, if you want to be more conservative, you could use the Passenger Load Factor in the ICAO Carbon Emissions Calculator Methodology, which for the Europe-South America route is 82%.

The last consideration is that airplanes typically transport commercial freight that does not belong to the airborne passengers. The ICAO methodology shows that for the Europe-South America route, 77% of the carrying weight is for passengers and their belongings, and the other 23% is cargo.

Assuming I fly Economy, the final calculation of my share of CO2 emissions is:

328 tons CO2 x 77%/ (90% x [222 Economy seats + 3 x 28 Business seats]) = 0.92 tons.

Comparing to online calculators

I said before that shameplane.com calculated in 1.6 tons of CO2 my emissions for the London-Bogota flight, but I wanted to check other online calculators to see whether they all give the same number. What I found was a wide range of estimations (some calculators give you the option of including radiative force or not, but others don’t state how they handle it):

My estimations were significantly lower: 0.46 with no RF, and 0.92 when including RF. My calculations are in line with ICAO, but they say nothing about RF. The closest one is AtmosFair, but they use a 2.9 factor for RF, while I use a factor of 2. AtmosFair is the only one that allows you to specify the type of aircraft from a wide variety of options (it is so granular that you can even choose between a Boeing 787-800 and a Boeing 787-900, both from the Dreamliner fleet). In contrast, the other sites just pick one for you automatically.

Putting things in perspective

Managing your environmental impact is an exercise in self-awareness. It forces you to think deeply about the way you live your life, and the decisions you make regularly. However, I believe that the only meaningful way in which you can make some progress in being friendlier with the planet is by becoming conscious about your CO2 emissions. This number should be ingrained in your mind, just as your weight or your salary is.

Until the day comes when there is a flashy app in your mobile that accurately tracks your CO2 emissions daily, we will need to rely on broad approximations. My suggestion is to use the average CO2 emissions in your country as a baseline, and then add the emissions from the activity you do that is more intense for the environment, which you recognize is furthest from the average.

In my case, the baseline is 5 tons (weighting the time I spent in the UK and Colombia), to which I add 8 x 0.92 = 7.4 tons from four intercontinental round-trips, so, for now, I will assume that my total emissions per year are 12.4 tons of CO2. There is some double-counting, as the baseline already assumes the average Briton and Colombian makes a few flights every year, but that is not a critical consideration for the moment.

Knowing that my emissions are 12.4 tons of CO2 per year – at least on a first approximation – has been an eye-opener. From here, I could refine more the calculation, making it more precise, but my rough estimate should be good for the moment: What I want to do now is to take some actions.

If you are someone who flies often and wants to talk about your carbon footprint or exchange ideas about how to handle your CO2 emissions, drop me a line or leave a comment. I am no expert on this topic, but I sure would love to hear your views and extend this into a broader conversation. After all, climate change is a collective problem, and it will need all of us to have a chance of solving it.

This is the fifth entry on my series on Climate Change:

  1. I am not proud of flying (but I am not ashamed of it either) – Part 1
  2. I am not proud of flying (but I am not ashamed of it either) – Part 2
  3. Billions of Humans need Trillions of Trees
  4. Your CO2 emissions: The number you should track in 2020
  5. Calculating your CO2 emissions: A Practical Example

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