1 Defining a carbon footprint

1.1 Why carbon?

A carbon footprint is a measure of how much an organisation is contributing to the greenhouse gases (GHGs) that cause global climate change. There are six main groups of GHGs: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6). All of these GHGs can arise from industrial processes, although most smaller companies are only responsible for carbon dioxide, which is produced whenever fossil fuels (coal, oil and gas) are burned, generally from the direct or indirect use of energy. For this reason, other GHG emissions are normally converted into a figure for tonnes of carbon dioxide equivalent (tCO2e). Although – perhaps confusingly – we talk about a ‘carbon footprint’, it would be more accurate to talk about a ‘carbon dioxide footprint’. In this unit we talk interchangeably about carbon emissions, carbon dioxide emissions or CO2 emissions and often just use ‘energy’.

The five industrial gases are all more powerful contributors to global climate change than CO2. For example, a kilogram of the refrigerant gas HFC-134 has the same global warming potential as a tonne of carbon dioxide, and some gases – notably SF6 and the perfluorocarbons – are even more damaging. However, most gases are only used under carefully controlled circumstances, so this unit will focus mainly on carbon dioxide as it is by far the most common (and important) contributor to climate change.

1.2 What is to be measured – the boundary