Emissions calculation

Our model for calculating emissions from transportation has been developed by NTM, the Nordic Network for Transport and the Environment, headquartered in Stockholm, Sweden. The model covers most forms of transportation, and performs calculations based on a rich set of parameters.

Air Travel

For air travel, the NTM model takes account of the following factors:

the start and destination airports
the aircraft type (e.g. Boeing 747-400) - so we know available number of seats and engine efficiency
the load factor (i.e. what proportion of seats are expected to be filled) - since we need to know how much of the total emissions are attributable to each passenger.

The user doesn't usually have access to the load factor or the aircraft type, so we industry averages from the ICAO as defined by NTM.

In more detail, the NTM model works as follows:

For multi-hop flights, the emissions of each leg are calculated separately. This provides a more accurate answer than simply looking at the first and last airport, since takeoff contributes disproportionately to emissions.
For each leg, the distance flown is calculated as the sum of the great circle distance between the airports (based on latitude and longitude coordinates).
We then calculate the total emissions of the flight using a fixed value for emissions at takeoff and landing plus a variable value based on the distance calculated in the previous step. The fixed values for takeoff and landing and the per-km value are different for each combination of aircraft type and load factor quartile (e.g. "Boeing 747-400 with load factor in range 50-75%" is one such combination). These fixed values for take-off and landing include the emissions for an extra 100km, as a standard value to allow for hold time (stacking) on arrival.
We then divide the flight's total emissions by the number of passengers, based on the aircraft's capacity and the specified (or default) value for the load factor (cabin factor).

Accordingly, the result at this stage is the amount of carbon dioxide attributable to one passenger on that flight. The total climate impact is greater than this, however, and the final result is calculated in the next stage.

We update the parameters in our model as new information (e.g. an airline's average load factor) becomes available, usually on an annual basis.

Discussion

The flexibility of the NTM model that we use, taking into account aircraft type and load factor, and the fact that it accounts for the extra emissions associated with takeoff, mean that the results can be very accurate when fed with appropriate data. Even when detailed information about aircraft type and load factor is missing, the model will on average provide a very good approximation of the true emissions. However, all we know for sure is that the answer is not 100% accurate for any given flight or passenger. Some of the factors that lead to inaccuracies are as follows:

We do not maintain load factor and fleet composition data for every airline in the world, so we use average values in many cases. The list of airlines for which we have this data is available here. We add new airlines to this list based on how many of our customers specify them and availability of information.
The model takes no account of what class the passenger flies. Business and First Class seats take more room on board, and it might be reasonable to apply a correction factor to such passengers to take account of this.
The model makes a simple assumption about the time spent stacking on arrival, and takes no account of, for example, "green approach" or delayed engine start with taxi-pushback, which some airlines are implementing at certain airports. This can lead to an over-estimation of emissions, perhaps of the order of 1%.
The model takes no account of any flight path deviations from the great circle route. Particularly within crowded airspaces such as Europe and the US, this leads to an underestimation of the total emissions, perhaps of the order of 10%.
The model assumes that all aircraft fly with the type of aviation fuel referred to as Jet A-1. In some areas, Jet A (US) or Jet B (extremely cold climates) are more common, but their greenhouse gas emissions characteristics are generally deemed to be identical to those of Jet A-1.
The model assumes that all aircraft of a given type have the same type of engine. This is not always valid, although all aircraft of a given type will tend to have engines of the same generation and roughly the same fuel efficiency.