1 Explaining carbon and ecological footprints

1.1 Carbon footprints

A carbon footprint is a measure of how much someone is contributing to the gases that contribute to global climate change. More scientifically, it is the amount of anthropogenic carbon dioxide (CO₂) emissions (those resulting from or produced by human beings) attributable to an individual or a household or an organisation, generally resulting from their direct or indirect use of energy. Although we talk about a ‘carbon footprint’, it would be more accurate to talk about a ‘carbon dioxide footprint’. A carbon footprint is normally calculated in tonnes of carbon dioxide equivalent (tCO₂e) but occasionally footprints are calculated in tonnes of carbon equivalent (tC). You will need to multiply by 44 and divide by 12 to convert from tonnes of carbon to tonnes of CO₂ equivalent. In this unit we may talk interchangeably about carbon emissions, carbon dioxide emissions and CO₂ emissions.

Carbon footprints may also include other gases that contribute to global climate change – the socalled ‘greenhouse gases’ (GHGs). The most common of these is methane (CH₄), but they also include nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride (SF₆). These other gases tend to be produced from agriculture or industrial processes. Most are much more powerful contributors to global warming than CO₂. Water vapour (H₂O) is also a significant contributor to global warming but, as its concentration varies little with time, it is not considered to be an anthropogenic greenhouse gas, except from aircraft vapour trails (see section 2.3).

However, the largest contributor to global warming is carbon dioxide itself, which is produced from burning fossil fuels such as coal, oil or gas. As well as the direct use of fossil fuels, people's carbon footprints normally include the use of electricity (where CO₂ is emitted at the power station) and as a result of collective travel, such as on trains, buses and aircraft.

For more information on carbon footprints, look at greenhouse gases on Wikipedia.