Carbon footprint

Measure of the amount of greenhouse gas (GHG) emission involved in human activity. It is expressed in terms of tonnes of carbon dioxide equivalent produced in a unit of time. (Carbon dioxide equivalent is the mass of carbon dioxide gas that has the same warming effect in the Earth's atmosphere as the given GHG emission.)

The term ‘footprint’ is based on an analogy with ‘ecological footprint’, a measure of resource use that is expressed in hectares, units of physical area. A full ‘life-cycle assessment’ of a carbon footprint in relation to, say, a car component will calculate the total GHG emissions involved in extracting raw materials, in manufacturing the part, in using it, in recycling or disposing of it, in transporting materials at every stage, and so on.

The carbon footprints of various activities are regularly published by the European Union and the governments of the USA, the UK, and other leading nations. Leading the lifestyle of a US citizen for one year has a carbon footprint of about 23 tonnes of carbon dioxide equivalent, compared with about 11 tonnes for the average European, and about 4 tonnes for an African or Asian. (These figures take account of the effects of deforestation, which is a major contributor to GHG emission, but whose magnitude is very uncertain.) The concept of the carbon footprint is used in evaluating the implications of activities for global warming, and for evaluating offsetting activities.