A carbon footprint is a measure estimating the total contribution of some unit (an activity, a company, a country) to global warming. A carbon footprint not only includes the impact of carbon on the climate, but also the impact of all other GHGs. It is called a carbon footprint because the effect of each GHG is converted into the equivalent in terms of carbon dioxide, the main GHG emitted.
The different greenhouse gases each have a different Global Warming Potential (GWP) calculated in relation to the warming impact of CO2 over a certain period of time, usually 100 years. Two factors influence the Global Warming Potential of a GHG, namely its energy absorption capacity and the length of time that it remains in the atmosphere. For example, methane (CH4) has a GWP of 25. This means that each tonne of CH4 is equivalent to 25 tonnes of CO2 (see Table 1-1).
Global Warming Potential for the main GHGs emitted by human activity
Source: Environnement Canada, Global Warming Potentials, April 17, 2015.