A carbon footprint is a "measure of the impact human activities have on the environment in terms of the amount of greenhouse gases produced, measured in units of carbon dioxide".^[1] It is meant to be useful for individuals, nations and organizations to conceptualize their personal (or organizational) impact in contributing to global warming. A conceptual tool in response to carbon footprints are carbon offsets, or the mitigation of carbon emissions through the development of alternative projects such as solar or wind energy or reforestation. The carbon footprint is a subset of the ecological footprint, which includes all human demands on the biosphere.

The carbon footprint is a measure of the exclusive global amount of carbon dioxide (CO₂) and other greenhouse gases emitted by a human activity or accumulated over the full life cycle of a product or service (see Wiedmann and Minx, 2008).

The life cycle concept of the carbon footprint means that it is all-encompassing and includes all possible causes that give rise to carbon emissions. In other words, all direct (on-site, internal) and indirect emissions (off-site, external, embodied, upstream, downstream) need to be taken into account.^[2]

Normally, a carbon footprint is expressed as a CO₂ equivalent (usually in kilograms or tonnes), which accounts for the same global warming effects of different greenhouse gases (UK Parliamentary Office of Science and Technology POST, 2006). Carbon footprints can be calculated using a Life Cycle Assessment (LCA) method, or can be restricted to the immediately attributable emissions from energy use of fossil fuels. In both cases however, as the term usually indicates the amount of emissions generated through the actions of people, what is important is not only the total amount of energy use, but also how the energy was produced in the first place (e.g. from fossil fuels or renewable resources).

An alternative definition of the carbon footprint is the total amount of carbon dioxide attributable to the actions of an individual (which includes emissions through their energy use, but other unforeseen emissions as well) over a period of one year. This definition underlies the personal carbon calculations. The term owes its origins to the idea that a footprint is what has been left behind as a result of the individual's activities. Carbon footprints can either consider only direct emissions (typically from energy used in the home and in transport, including travel by cars, airplanes, rail and other public transport), or can also include indirect emissions (including CO2 emissions as a result of goods and services consumed). Bottom-up calculations sum attributable CO2 emissions from individual actions; top-down calculations take total emissions from a country (or other low-level entity) and divide these emissions among the residents (or other participants in that entity).

Reducing a carbon footprint

The carbon footprint can be efficiently and effectively reduced by using a Carbon Diet or applying the following steps:

• Life Cycle Assessment (LCA) to accurately determine the current carbon footprint
• Identification of hot-spots in terms of energy consumption and associated CO₂-emissions
• Where possible, changing to another electricity company to switch to buying electricity from renewable sources (from wind turbines, solar panels or hydroelectrical plants -or- from nuclear power plants
• Optimization of energy efficiency and, thus, reduction of CO₂-emissions and reduction of other GHG emissions contributed from production processes
• Identification of solutions to neutralize the CO₂ emissions that cannot be eliminated by energy saving measures. This last step includes carbon offsetting; investment in projects that aim at the reducing CO₂ emissions, for instance tree planting.

Kyoto Protocol, carbon offsetting, and certificates

Carbon dioxide emissions to air (and the emissions of other GHG's) are almost exclusively associated with the conversion of energy carriers like natural gas, crude oil, etc.

The Kyoto Protocol defines legally binding targets and timetables for cutting the greenhouse-gas emissions of industrialized countries that ratified the Kyoto Protocol. Accordingly, from an economic or market perspective, one has to distinguish between a mandatory market and a voluntary market. Typical for both markets is the trade with emission certificates:

• Certified Emission Reduction (CER)
• Emission Reduction Unit (ERU)
• Verified Emission Reduction (VER)

Criticism of the term

Criticism of the concept of a carbon footprint is generally based on disagreement with one or more of the following assumptions usually underlying the calculation of a carbon footprint:

• That carbon emissions are a significant cause of global warming.
• That human activity is a significant cause of these emissions.
• That it is possible to attribute all or most emissions to particular individuals.
• That individual initiative is necessary because market forces or legislation will not be powerful and timely enough.
• That each individual should therefore calculate and attempt to reduce his share of carbon emissions.
• Sometimes, that each person should be given as a target an equal share of emissions, or some other share.

Criticisms derived from rejection of these assumptions may therefore include:

• That other causes, such as methane emissions, are more important.
• That human activity is not as significant a cause as natural processes such as vulcanism or solar radiation.
• That many emissions cannot reasonably be attributed to any individual. E.g. should emissions from commuting be attributed to commuters or consumers of what they produce?
• That market forces or political action will correct human activity in sufficient time.
• That population growth invalidates the calculations,
• That one cannot limit everyone to equal emissions, for example those in urbanized societies may be unable to avoid some emissions, while less-developed countries may not have the technology to mitigate others.

Another criticism relates to ambiguity of the term carbon footprint, which can either also include greenhouse gasses other than carbon dioxide, or can alternatively assess only carbon dioxide itself. For a range of consumables, the non-CO₂ greenhouse gasses like methane, nitrous oxide, or CFCs (in CO₂-equivalents) make up a non-negligible part of the global warming impact. An appropriate term according to the more encompassing definition could be climate footprint.

Carbon Labelling

A carbon label, which shows the life cycle carbon emissions or carbon footprint embodied in a product in bringing it to the shelf, was introduced in the UK in March 2007 by the Carbon Trust. The label is closely linked to a collaboration between The Carbon Trust and The British Standards Institute. The label is intended to comply with a new British Standard, PAS2050, ([4]) and is being actively piloted by The Carbon Trust and various industrial partners ([5]).
Examples of products which already feature this carbon footprint label include Walkers Crisps, a smoothie product from Innocent Drinks, and a shampoo product produced by Boots Group.

Age-related carbon footprint

A number of studies have calculated the carbon footprint of organisations and nations. One UK (2007) study examined age-related carbon emissions based on expenditure and consumption. The study found that on average people aged 50-65 years have a higher carbon footprint than any other age group. Individuals aged 50-55 years old have a carbon footprint of approximately 13.5 tonnes/capita per year compared to the UK average of 12 tonnes.^[3]