1.1 Carbon dioxide and other greenhouse gases

CO₂ emissions are used as the basis for the carbon footprints because it is by far the main contributor to the enhanced greenhouse effect from human activity (for example 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 CO₂ equivalent (in kilograms or tonnes CO₂e) in terms of their global warming effect compared to CO₂ 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 activities 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 than CO₂ emissions, their emissions together add about 17% to the UK’s contribution, and about 25% globally, to the enhanced greenhouse effect (BEIS, 2023; Ritchie, 2020). 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, whenever possible the total CO₂ equivalent (CO₂e) will be used.