A carbon footprint is historically defined as "the total set of greenhouse gas emissions caused by an [individual, event, organisation, product] expressed as carbon dioxide equivalent."
Greenhouse gases (GHGs) can be emitted through transport, land clearance, and the production and consumption of food, fuels, manufactured goods, materials, wood, roads, buildings, and services.[3] For simplicity of reporting, it is often expressed in terms of the amount of carbon dioxide, or its equivalent of other GHGs, emitted.