However, calculating the total carbon footprint is impossible due to the large amount of data required and the fact that carbon dioxide can be produced by natural occurrences. It is for this reason that Wright, Kemp, and Williams, writing in the journal Carbon Management, have suggested a more practicable definition:
"A measure of the total amount of carbon dioxide (CO2) and methane (CH4) emissions of a defined population, system or activity, considering all relevant sources, sinks and storage within the spatial and temporal boundary of the population, system or activity of interest. Calculated as carbon dioxide equivalent (CO2e) using the relevant 100-year global warming potential (GWP100)."
Greenhouse gases can be emitted through transport, land clearance, and the production and consumption of food, fuels, manufactured goods, materials, wood, roads, buildings, and services. For simplicity of reporting, it is often expressed in terms of the amount of carbon dioxide, or its equivalent of other GHGs, emitted.
Most of the carbon footprint emissions for the average U.S. household come from "indirect" sources, i.e. fuel burned to produce goods far away from the final consumer. These are distinguished from emissions which come from burning fuel directly in one's car or stove, commonly referred to as "direct" sources of the consumer's carbon footprint.
The concept name of the carbon footprint originates from ecological footprint,discussion, which was developed by Rees and Wackernagel in the 1990s which estimates the number of "earths" that would theoretically be required if everyone on the planet consumed resources at the same level as the person calculating their ecological footprint. However, carbon footprints are much more specific than ecological footprints since they measure direct emissions of gasses that cause climate change into the atmosphere.
Measuring Carbon Footprints
An individual's, nation's, or organization's carbon footprint can be measured by undertaking a GHG emissions assessment or other calculative activities denoted as carbon accounting. Once the size of a carbon footprint is known, a strategy can be devised to reduce it, e.g. by technological developments, better process and product management, changed Green Public or Private Procurement (GPP), carbon capture, consumption strategies, and others. Several free online carbon footprint calculators exist, with at least one supported by publicly available peer-reviewed data and calculations from the University of California, Berkeley's CoolClimate Network research consortium.
The mitigation of carbon footprints through the development of alternative projects, such as solar or wind energy or reforestation, represents one way of reducing a carbon footprint and is often known as Carbon offsetting.
The main influences on carbon footprints include population, economic output, and energy and carbon intensity of the economy. These factors are the main targets of individuals and businesses in order to decrease carbon footprints. Scholars suggest the most effective way to decrease a carbon footprint is to either decrease the amount of energy needed for production or to decrease the dependence on carbon emitting fuels.
By area 
The Average Carbon Footprint in the United States vs. World The average U.S. household carbon footprint is 48 tons CO2e per year. The single largest source of emissions for the typical household is from driving (gasoline use). Transportation as a whole (driving, flying & small amount from public transit) is the largest overall category, followed by housing (electricity, natural gas, waste, construction) then food (mostly from red meat, dairy and seafood products, but also includes emissions from all other food), then goods followed lastly by services. The carbon footprint of U.S. households is about 5 times greater than the global average, which is approximately 10 tons CO2e per household per year. For most U.S. households, the single most important action to reduce their carbon footprint is driving less or switching to a more efficient vehicle. Cement production and carbon footprint resulting from soil sealing was 8.0 Mg person−1 of total per capita CO2 emissions (Italy, year 2003); the balance between C loss due to soil sealing and C stocked in man-made infrastructures resulted in a net loss to the atmosphere, -0.6 Mg C ha−1 y−1.
Of products 
Several organizations have calculated carbon footprints of products; The US Environmental Protection Agency has addressed paper, plastic (candy wrappers), glass, cans, computers, carpet and tires. Australia has addressed lumber and other building materials. Academics in Australia, Korea and the US have addressed paved roads. Companies, nonprofits and academics have addressed mailing letters and packages. Carnegie Mellon University has estimated the CO2 footprints of 46 large sectors of the economy in each of eight countries. Carnegie Mellon, Sweden and the Carbon Trust have addressed foods at home and in restaurants.
The Carbon Trust has worked with UK manufacturers on foods, shirts and detergents, introducing a CO2 label in March 2007. The label is intended to comply with a new British Publicly Available Specification (i.e. not a standard), PAS 2050, and is being actively piloted by The Carbon Trust and various industrial partners. As of August 2012 The Carbon Trust state they have measured 27,000 certifiable product carbon footprints.
Evaluating the package of some products is key to figuring out the carbon footprint. The key way to determine a carbon footprint is to look at the materials used to make the item. For example, a juice carton is made of an aseptic carton, a beer can is made of aluminum, and some water bottles either made of glass or plastic. The larger the size, the larger the footprint will be.
Of energy 
The following table compares, from peer-reviewed studies of full life cycle emissions and from various other studies, the carbon footprint of various forms of energy generation: Nuclear, Hydro, Coal, Gas, Solar Cell, Peat and Wind generation technology.
− − − − −
|Conc. Solar Pwr||40±15#|
Note: 3.6 MJ = megajoule(s) == 1 kW·h = kilowatt-hour(s), thus 1 g/MJ = 3.6 g/kW·h.
Legend: B = Black coal (supercritical)–(new subcritical), Br = Brown coal (new subcritical), cc = combined cycle, oc = open cycle, TL = low-temperature/closed-circuit (geothermal doublet), TH = high-temperature/open-circuit, WL = Light Water Reactors, WH = Heavy Water Reactors, #Educated estimate.
These studies thus concluded that hydroelectric, wind, and nuclear power always produced the least CO2 per kilowatt-hour of any other electricity sources. These figures do not allow for emissions due to accidents or terrorism. Renewable electricity generation methods, for example wind power and hydropower, emit no carbon from the operation, but do leave a footprint during construction phase and maintenance during operation.
Of heat and various combined heat and power schemes, heat pumps, etc. 
The previous table gives the carbon footprint per kilowatt-hour of electricity generated, which is about half the world's man-made CO2 output. The CO2 footprint for heat is equally significant and research shows that using waste heat from power generation in combined heat and power district heating, chp/dh has the lowest carbon footprint. much lower than micro-power or heat pumps.
Kyoto Protocol, carbon offsetting, and certificates 
Carbon dioxide emissions into the atmosphere, and the emissions of other GHGs, are often associated with the burning of fossil fuels, like natural gas, crude oil and coal. While this is harmful to the environment, carbon offsets can be purchased in an attempt to make up for these harmful effects.
The Kyoto Protocol defines legally binding targets and timetables for cutting the GHG 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:
Mandatory market mechanisms 
To reach the goals defined in the Kyoto Protocol, with the least economical costs, the following flexible mechanisms were introduced for the mandatory market:
The CDM and JI mechanisms requirements for projects which create a supply of emission reduction instruments, while Emissions Trading allows those instruments to be sold on international markets.
- Projects which are compliant with the requirements of the CDM mechanism generate Certified Emissions Reductions (CERs).
- Projects which are compliant with the requirements of the JI mechanism generate Emission Reduction Units (ERUs).
The CERs and ERUs can then be sold through Emissions Trading. The demand for the CERs and ERUs being traded is driven by:
- Shortfalls in national emission reduction obligations under the Kyoto Protocol.
- Shortfalls amongst entities obligated under local emissions reduction schemes.
Nations which have failed to deliver their Kyoto emissions reductions obligations can enter Emissions Trading to purchase CERS and ERUs to cover their treaty shortfalls. Nations and groups of nations can also create local emission reduction schemes which place mandatory carbon dioxide emission targets on entities within their national boundaries. If the rules of a scheme allow, the obligated entities may be able to cover all or some of any reduction shortfalls by purchasing CERs and ERUs through Emissions Trading. While local emissions reduction schemes have no status under the Kyoto Protocol itself, they play a prominent role in creating the demand for CERs and ERUs, stimulating Emissions Trading and setting a market price for emissions.
A well-known mandatory local emissions trading scheme is the EU Emissions Trading Scheme (EU ETS).
New changes are being made to the trading schemes. The EU Emissions Trading Scheme is set to make some new changes within the next year. The new changes will target the emissions produced by flight travel in and out of the European Union.
Other nations are scheduled to start participating in Emissions Trading Schemes within the next few year. These nations include China, India and the United States.
Voluntary Market Mechanisms 
In contrast to the strict rules set out for the mandatory market, the voluntary market provides companies with different options to acquire emissions reductions. A solution, comparable with those developed for the mandatory market, has been developed for the voluntary market, the Verified Emission Reductions (VER). This measure has the great advantage that the projects/activities are managed according to the quality standards set out for CDM/JI projects but the certificates provided are not registered by the governments of the host countries or the Executive Board of the UNO. As such, high quality VERs can be acquired at lower costs for the same project quality. However, at present VERs can not be used in the mandatory market.
The voluntary market in North America is divided between members of the Chicago Climate Exchange and the Over The Counter (OTC) market. The Chicago Climate Exchange is a voluntary yet legally binding cap-and-trade emission scheme whereby members commit to the capped emission reductions and must purchase allowances from other members or offset excess emissions. The OTC market does not involve a legally binding scheme and a wide array of buyers from the public and private spheres, as well as special events that want to go carbon neutral.
There are project developers, wholesalers, brokers, and retailers, as well as carbon funds, in the voluntary market. Some businesses and nonprofits in the voluntary market encompass more than just one of the activities listed above. A report by Ecosystem Marketplace shows that carbon offset prices increase as it moves along the supply chain—from project developer to retailer.
While some mandatory emission reduction schemes exclude forest projects, these projects flourish in the voluntary markets. A major criticism concerns the imprecise nature of GHG sequestration quantification methodologies for forestry projects. However, others note the community co-benefits that forestry projects foster. Project types in the voluntary market range from avoided deforestation, afforestation/reforestation, industrial gas sequestration, increased energy efficiency, fuel switching, methane capture from coal plants and livestock, and even renewable energy. Renewable Energy Certificates (RECs) sold on the voluntary market are quite controversial due to additionality concerns. Industrial Gas projects receive criticism because such projects only apply to large industrial plants that already have high fixed costs. Siphoning off industrial gas for sequestration is considered picking the low hanging fruit; which is why credits generated from industrial gas projects are the cheapest in the voluntary market.
The size and activity of the voluntary carbon market is difficult to measure. The most comprehensive report on the voluntary carbon markets to date was released by Ecosystem Marketplace and New Carbon Finance in July 2007.
Ways to reduce carbon footprint 
The most common way to reduce the carbon footprint of humans is to Reduce, Reuse, Recycle. In manufacturing this can be done by recycling the packing materials, by selling the obsolete inventory of one industry to the industry who is looking to buy unused items at lesser price to become competitive. Nothing should be disposed off into the soil, all the ferrous materials which are prone to degrade or oxidize with time should be sold as early as possible at reduced price.
This can also be done by using reusable items such as thermoses for daily coffee or plastic containers for water and other cold beverages rather than disposable ones. If that option isn't available, it is best to properly recycle the disposable items after use. When one household recycles at least half of their household waste, they can save 1.2 tons of carbon dioxide annually[unreliable source?].
Another easy option is to drive less. By walking or biking to the destination rather than driving, not only is a person going to save money on gas, but they will be burning less fuel and releasing fewer emissions into the atmosphere. However, if walking is not an option, one can look into carpooling or mass transportation options in their area.
Yet another option for reducing the carbon footprint of humans is to use less air conditioning and heating in the home. By adding insulation to the walls and attic of one's home, and installing weather stripping or caulking around doors and windows one can lower their heating costs more than 25 percent. This helps because it reduces the amount of energy needed to heat and cool the house. One can also turn down the heat while they are sleeping at night or away during the day, and keep temperatures moderate at all times. Setting the thermostat just 2 degrees lower in winter and higher in summer could save about 1 ton of carbon dioxide each year [unreliable source?].
The carbon handprint movement emphasizes individual forms of carbon offsetting, like using more public transportation or planting trees in deforested regions, to reduce one's carbon footprint and increase their "handprint."
Furthermore, the carbon footprint in the food industry can be reduced by optimizing the supply chain. A life cycle or supply chain carbon footprint study can provide useful data which will help the business to identify critical areas for improvement and provides a focus. Such studies also demonstrate a company’s commitment to reducing carbon footprint now ahead of other competitors as well as preparing companies for potential regulation. In addition to increased market advantage and differentiation eco-efficiency can also help to reduce costs where alternative energy systems are implemented.
See also 
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- "What is a carbon footprint?". UK Carbon Trust. Retrieved 2009-07-24.
- Wright, L.; Kemp, S., Williams, I. (2011). "'Carbon footprinting': towards a universally accepted definition". Carbon Management 2 (1): 61–72. doi:10.4155/CMT.10.39.
- "The CO2 list (and original sources cited therein)". Retrieved 2011-03-18.
- "Graph of the Average Carbon Footprint of a U.S. Household". Retrieved 4 May 2012.
- Safire, William (2008-02-17). "Footprint". The New York Times. Retrieved 2010-04-28.
- "CoolClimate Carbon Footprint Calculator for U.S. Households and Individuals". Retrieved 4 May 2012.
- "Online supporting data, calculations & methodologies for paper: Jones, Kammen "Quantifying Carbon Footprint Reduction Opportunities for U.S. Households and Communities" ES&T, 2011 (publicly available)". Retrieved 4 May 2012.
- Brown, Marilyn A., Frank Southworth, and Andrea Sarzynski. Shrinking The Carbon Footprint of Metropolitan America. Brookings Institution Metropolitan Policy Program, May 2008. Web. 23 Feb. 2011.
- "Jones, Kammen "Quantifying Carbon Footprint Reduction Opportunities for U.S. Households and Communities" ES&T, 2011, 45 (9), pp 4088–4095 DOI: 10.1021/es102221h". Retrieved 4 May 2012.
- Scalenghe, R., Malucelli, F., Ungaro, F., Perazzone, L., Filippi, N., Edwards, A.C. (2011). "Influence of 150 years of land use on anthropogenic and natural carbon stocks in Emilia-Romagna Region (Italy)". Environmental Science & Technology 45 (12): 5112–5117. doi:10.1021/es1039437.
- "CO2 Released when Making & Using Products". Retrieved 27 October 2009.
- "Footprint measurement". The Carbon Trust. Retrieved 14 August 2012.
- Prof. Bilek, Marcela; Dr. Hardy, Clarence, Dr. Lenzen, Manfred & Dr. Dey, Christopher (2008). "Life-cycle energy balance and greenhouse gas emissions of nuclear energy: A review" (PDF). SLS - USyd - USyd-ISA - pubs - pandora-archive Energy Conversion & Management 49 (8): 2178–2199. Retrieved 2009-11-04.
- Fridleifsson,, Ingvar B.; Bertani, Ruggero; Huenges, Ernst; Lund, John W.; Ragnarsson, Arni; Rybach, Ladislaus (2008-02-11). In O. Hohmeyer and T. Trittin. The possible role and contribution of geothermal energy to the mitigation of climate change (pdf) IPCC Scoping Meeting on Renewable Energy Sources. Luebeck, Germany. pp. 59–80. Retrieved 2009-04-06.
- Hanova, J; Dowlatabadi, H (9 November 2007). "Strategic GHG reduction through the use of ground source heat pump technology". Environmental Research Letters 2 (UK: IOP Publishing). pp. 044001 8pp. doi:10.1088/1748-9326/2/4/044001. ISSN 1748-9326. Retrieved 2009-03-22
- Callick, Rowan. "Nations Split on Route to Reduce Carbon Emissions | The Australian." The Australian | The Australian Homepage | TheAustralian. 02 Mar. 2011. Web. 01 Mar. 2011.
- "Time Magazine: Handprints, Not Footprints ES&T, 2012, 45 (9), pp 4088–4095 DOI: 10.1021/es102221h". Retrieved 4 March 2012.
- Product Carbon Footprint Calculation SGS, Retrieved 04/08/2013
- Wright, L., Kemp, S., Williams, I. (2011) 'Carbon footprinting': towards a universally accepted definition. Carbon Management, 2 (1): 61-72.
- UK Carbon Trust (2008) "Carbon Footprinting".
- Parliamentary Office of Science and Technology POST (2006). Carbon footprint of electricity generation. October 2006, Number 268
- Wiedmann, T. and J. Minx (2008). A Definition of 'Carbon Footprint'. Ecological Economics Research Trends. C. C. Pertsova: Chapter 1, pp. 1–11. Nova Science Publishers, Inc, Hauppauge NY, USA. https://www.novapublishers.com/catalog/product_info.php?products_id=5999, also available as ISA-UK Research Report 07/01 from http://www.censa.org.uk/reports.html.
- World Energy Council Report (2004). Comparison of energy systems using life cycle assessment.
- Energetics (2007). The reality of carbon neutrality.
- Walkers Carbon Footprint. http://www.walkerscarbonfootprint.co.uk/walkers_carbon_trust.html