2 The scope of a carbon footprint

2.1 Boundary

Before calculating a carbon footprint, you have to decide on the boundary and the scope.

The boundary simply describes or limits what is being measured: it can be a single person, a household or an organisation. If you choose to calculate your carbon footprint on an individual basis, you may need to divide your home's energy consumption by the number of occupants. You may also want to bring in the CO₂ emissions associated with business travel. It is easiest to look at emission from cars on the basis of ownership. So, if you have a car, include all the fuel use (and associated emissions) from that car, but you could split emissions relating to a car that is shared with a partner, for example. It is often easier to look at the total carbon footprint of a household, but you will then need to bring in your partner's travel emissions, too.

At the domestic or individual level, people usually only consider carbon dioxide emissions, and not those from the other GHGs mentioned in section 1. But, even then, you will need to decide how to calculate indirect emissions arising from your use of services – notably electricity supply and public transport, including air travel. In the UK, DEFRA publishes a table each year showing CO₂ emissions associated with electricity. This gives two main values: an average for grid electricity; and a longer-term value for ‘marginal electricity’ – typical emissions per unit of electricity (kWh) for extra capacity brought online when demand rises or falls (that is, when people switch something on or off). However, it doesn't take into account differing emissions associated with different suppliers: for example, you may pay extra for a green tariff or buy your electricity from a company such as Good Energy or Ecotricity, which source most of their supply from renewable sources. Even among the main suppliers, some claim lower than average emissions because of purchases from nuclear power stations or a high proportion of efficient combined-cycle gas turbines. DEFRA's view is that you should use the grid average figure because it is not possible to differentiate the actual electricity that comes down the wires to your home or business.

Collectively, the selection of which indirect emissions to measure is referred to as the scope. At a corporate level, there are three internationally recognised scopes under the Greenhouse Gas Protocol explained on their website. They are described in the next section.