A carbon footprint is the sum total of the impact of a person or thing or activity on the environment measured in tonnes of carbon dioxide (CO2, the major grenhouse gas) released back into the environment.

A personal carbon footprint is the total carbon emissons resulting from the activities of one person, again usually measured in terms of tonnes of CO2 per year.

Why footprint? Well, a footprint is what is left behind as a result of an individual's activities. Carbon footprints can measure only direct emissions from energy used in the home and in transport, or can also include indirect emissions as a result of goods and services consumed. (See the next paragraph).

Products can also be considered to have a carbon footprint. This is considered to be the toal CO2 emissions produced as a result of bringing the product from raw materials to the shelf. This includes emissions produced as a result of manufacturing and transport.

There are lots of carbon footprint calculators on the web which act as a starting point for people wanting to reduce their emissons. You can find plenty by Googling for 'carbon footprint calculator'. You should be aware however that different calculators may give different results for the same inputs. We recommend this one.

All the online calculators are oversimplified however. For example, they won't tell you that watching a plasma television for three hours every day contributes 250kg of carbon to the atmosphere each year while an LCD set is responsible for less than half that number - yet these are the sort of decisions that cumulatively can make a substantial difference. For a more detailed estimate of you carbon emissions covering all aspects of your life we recommend Mark Lynas' Carbon Counter (left)