Methodology and data sources
Why bothering about the figures behind the calculator? Because every result should be interpreted with the assumptions made during the calculation in mind. The lack of information about the methodology and data is a major issue I encountered while using calculators on the web, often offered by organisations related to sustainable living. If you don’t know how something is calculated, how can you know what the precision is? The lack of reporting on methodology and data sources was the main reason I started working on my own calculator in the first place. On this page, I explain how and why the calculation is done the way I implemented it. I also present some notes on the data I used, the data itself can be found in the excel file you can download by clicking the third button from the right in the gray bottom bar of the calculator.
The calculation is based on the DEFRA guidelines (Department for Environment, Food and Rural Affairs of the UK government). The method is fairly simple: greenhouse gas emissions = activity x emission conversion factor. In our case, the activity is the distance traveled by a certain transport medium and expressed in kilometers. The emission conversion factor gives the emissions of a certain greenhouse gas per unit activity, in our case per kilometer. These emission conversion factors are collected by scientific institutions and some of them publish them to be consulted for free.
Different activities emit different greenhouse gases. There is carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), nitrogen trifluoride (NF3) etc. To make it easier to compare different activities, it is common to use carbon dioxide equivalents (CO2e) as unit to express the global warming potential of a greenhouse gas. It is the amount of CO2 that has the equivalent global warming impact as the mix of greenhouse gases together. Therefore, the units of all conversion factors used in the calculator are kg CO2e/km. For transport modes with a lot of passengers, such as trains or planes, kg CO2e/pkm is used, this is per kilometer and per passenger.
When using emission conversion factors, one should always be aware of two things: the time period for which the factors are valid and the scope. Due to rapid developments in technology and energy markets, it’s important to use the data valid for the time period studied. Secondly, the scope defines what activities are taken into account when calculating emissions. The calculator for example is limited to scope 1: only the direct emissions of the transport itself is measured. We don’t look at the emissions created by, for example, cleaning the plane after the flight.
Notes on the data
Please download the excel file of the calculator and go to the second tab to find all conversion factors listed with their sources and validity date. Below some additional notes.
Car The car profiles are based on the UK car park and are obtained from the Department of Transport. It’s most convenient to use your own emission data for the particular car used if available. The data given by the manufacturer is uplifted by 15% by the calculator because manufacturers’ data is obtained in a test set-up with ideal conditions.
Bus The conversion factors for a bus as a whole are obtained from DEFRA. They have to be divided by the number of people actually using the bus to find the emissions per passenger. For a town bus in Romsey, this was 12.31 and for a long-distance coach bus I estimated an average of 35 passengers myself based on experience.
Train Data on train emissions are obtained from the VTT Technical Research Centre of Finland. Although the data dates back from 2009, it was the best I could find. For full electric trains the energy usage per passenger per kilometer was listed. I therefore had to find the emissions per unit energy and I did so by using DEFRA’s data on the electricity mix in different countries. For Diesel train the emissions were listed in the normal units. For both train profiles, an average occupation rate of 35% is assumed.
Taxi Data on taxi is the same as for cars. The only difference is that the number of kilometers traveled by the passenger is multiplied by two because the taxi is also driving between different places without any passengers.
Plane Again data from DEFRA is used. A difference between domestic, short-distance and long-distance is made to take into account the impact of take-off and the use of jet streams on the overall fuel consumption. There is also the possibility to indicate the class. When using business class you take up more space in the plane and less people can be seated than when the seat had been a normal one.