1. To estimate greenhouse gas emissions, and in particular CO2, we calculate the distance between the two airports, and deduce the amount of kerosene needed.
2. Then, we add a fixed quantity of kerosene to take into account takeoff and landing.
3. This mass of fuel oil is multiplied by a fixed factor (from 3.1) to estimate the amount of CO2 emitted
4. Then, this quantity is distributed over the average number of passengers per flight, depending on the weight corresponding to the planes's class. To infer everyone's share.
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Aircraft emissions are not limited to CO2: burning kerosene also produces nitrogen oxides (NOx), which are greenhouse gases, too. In addition, the aircraft produces these gases at a high altitude, where they have a greater environmental impact.
Trees do not absorb NOx, but it is possible to estimate the radiative forcing of NOx, which means the approximate equivalent amount of CO2 that would have to be absorbed to counterbalance the effect of NOx can be calculated.
To be thorough, therefore, a multiplicative factor - generally estimated at a factor of 2 - is applied to the CO2 emitted on its own to estimate the total amount of CO2 that must be absorbed to compensate for all the greenhouse gases emitted by the flight.
Take, for example, a short flight of around 1000 kms - which will last about 2 hours. First of all, a fixed factor of about 95 kms must be added to this distance to take into account evasive actions and tactical manoeuvres upon approach.
To complete this flight, the aircraft will consume 2.7 kg of fuel oil per km, or about 2.95 tons of kerosene for the trip itself, to which 1.1 tons for take-off and landing must be added, as well as for taxiing between runways and terminals. This gives us a total of 4.05 tons.
This consumption of fuel oil produces CO2 - up to 3.1 kg for each kg of fuel consumed. Therefore, in our example, the aircraft will produce a carbon footprint of 12 tonnes of CO2, of which 75% is produced on the route itself and 25% for the landing and take-off phases (or LTO).
When it comes to short flights, we consider about 7% of the weight is used for cargo (this figure increases to 26% for long haul flights), which leaves us with 93% left-over for passengers, or about 11 tonnes.
With an average of 150 seats on the type of aircraft used (for short flights), and an average occupancy rate of about 82%, we obtain an average of 123 passengers. If we divide this evenly over all passengers, we obtain an estimate of 90 kg of CO2 emitted per passenger.
To be fairer, a multiplicative factor is applied to take into account the fact that business class and first class occupy a larger usable area, and therefore generate a proportionally larger share of CO2.
In the case of long haul flights, the calculation method follows the same pattern, but takes into account different variables, such as the amount of kerosene required for take-off, consumption per km, and the number of passengers in the aircraft.