1.1 Carbon dioxide and other greenhouse gases
CO2 emissions are used as the basis for the carbon footprint because it is by far the main contributor to the enhanced greenhouse effect from human activity (mainly burning fossil fuels, clearing forests and making cement). Sometimes only the mass of CO2 is counted in the carbon footprint. However, as noted earlier, for a complete carbon footprint the other human-generated greenhouse gases are converted into a CO2equivalent (in kilograms or tonnes CO2e) in terms of their global warming effect and added to the footprint. The complete footprint is then expressed as kilograms or tonnes CO2e per year.
The two main non-CO2 greenhouse gases associated with human activity are methane (CH4), mainly from cattle belching, manure spreading and decomposition, wet rice growing and decomposing waste, and nitrous oxide (N2O), mainly from nitrogen fertilisers and industrial processes. One tonne of methane has the equivalent global warming potential of 21 tonnes of CO2, while one tonne of nitrous oxide is equivalent to 310 tonnes of CO2. But because the amounts of methane and nitrous oxide released are much smaller, their emissions together add about 16% to the UK’s contribution, and about 25% globally, to the enhanced greenhouse effect (ONS, 2015; IPCC, 2015). This means that other greenhouse gases should not be ignored when measuring a carbon footprint, although to simplify calculations they often are.
In this course, you will use the total CO2 equivalent (CO2e) effect of carbon dioxide plus other greenhouse gases whenever possible, but where only information on CO2 is available, when discussing carbon footprints, only CO2 emissions will be used.