An emission intensity is the average emission rate of a given pollutant from a given source relative to the intensity of a specific activity; for example grams of carbon dioxide released per megajoule of energy produced, or the ratio of greenhouse gas emissions produced to GDP. Emission intensities are used to derive estimates of air pollutant or greenhouse gas emissions based on the amount of fuel combusted, the number of animals in animal husbandry, on industrial production levels, distances traveled or similar activity data. Emission intensities may also be used to compare the environmental impact of different fuels or activities. The related terms emission factor and carbon intensity are often used interchangeably, but "factors" exclude aggregate activities such as GDP, and "carbon" excludes other pollutants.
Estimating emissions 
Emission factors assume a linear relation between the intensity of the activity and the emission resulting from this activity:
Emissionpollutant = Activity * Emission Factorpollutant
Intensities are also used in projecting possible future scenarios such as those used in the IPCC assessments, along with projected future changes in population, economic activity and energy technologies. The interrelations of these variables is treated under the so-called Kaya identity.
The level of uncertainty of the resulting estimates depends significantly on the source category and the pollutant. Some examples:
- Carbon dioxide (CO2) emissions from the combustion of fuel can be estimated with a high degree of certainty regardless of how the fuel is used as these emissions depend almost exclusively on the carbon content of the fuel, which is generally known with a high degree of precision. The same is true for sulphur dioxide (SO2), since also sulphur contents of fuels are generally well known. Both carbon and sulphur are almost completey oxidized during combustion and all carbon and sulphur atoms in the fuel will be present in the flue gases as CO2 and SO2 respectively.
- In contrast, the levels of other air pollutants and non-CO2 greenhouse gas emissions from combustion depend on the precise technology applied when fuel is combusted. These emissions are basically caused by either incomplete combustion of a small fraction of the fuel (carbon monoxide, methane, non-methane volatile organic compounds) or by complicated chemical and physical processes during the combustion and in the smoke stack or tailpipe. Examples of these are particulates, NOx, a mixture of nitric oxide, NO, and nitrogen dioxide, NO2).
- Nitrous oxide (N2O) emissions from agricultural soils are highly uncertain because they depend very much on both the exact conditions of the soil, the application of fertilizers and meteorological conditions.
Published data on energy sources emission intensity per unit of electricity generated 
A literature review of numerous total life cycle energy sources CO2 emissions per unit of electricity generated, conducted by the Intergovernmental Panel on Climate Change in 2011, found that the CO2 emission value, that fell within the 50th percentile of all total life cycle emissions studies were as follows.
|Nuclear||various generation II reactor types||16|
|Solar thermal||parabolic trough||22|
|Geothermal||hot dry rock||45|
|Solar PV||Polycrystaline silicon||46|
|Natural gas||various combined cycle turbines without scrubbing||469|
|Coal||various generator types without scrubbing||1001|
|Energy Intensity (min & max estimate)
|Electric (min & max estimate)
cc:2.35 (2.20 – 2.57)|
oc:3.05 (2.81 – 3.46)
|Hydroelectricity||0.046 (0.020 – 0.137)||15 (6.5 – 44)|
|Conc. Solar Pwr||40±15#|
|Photovoltaics||0.33 (0.16 – 0.67)||106 (53–217)|
|Wind power||0.066 (0.041 – 0.12)||21 (13–40)|
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.
World CO2 intensity in 2009 
In 2009 CO2 intensity in the OECD countries reduced by 2.9% and amounted to 0.33 kCO2/$05p in the OECD countries. The USA posted a higher ratio of 0.41 kCO2/$05p while Europe showed the largest drop in CO2 intensity compared to the previous year (−3.7%). CO2 intensity continued to be roughly higher in non-OECD countries. Despite a slight improvement, China continued to post a high CO2 intensity (0.81 kCO2/$05p). CO2 intensity in Asia rose by 2% during 2009 since energy consumption continued to develop at a strong pace. Important ratios were also observed in countries in CIS and the Middle East.
Carbon Intensity in 2012 
Carbon intensity in Europe 
Total CO2 emissions from energy use were 5% below their 1990 level in 2007. Over the period 1990–2007, CO2 emissions from energy use have decreased on average by 0.3%/year although the economic activity (GDP) increased by 2.3%/year. After dropping until 1994 (−1.6%/year), the CO2 emissions have increased steadily (0.4%/year on average) until 2003 and decreased slowly again since (on average by 0.6%/year). Total CO2 emissions per capita decreased from 8.7 t in 1990 to 7.8 t in 2007, that is to say a decrease by 10%. Almost 40% of the reduction in CO2 intensity is due to increased use of energy carriers with lower emission factors Total CO2 emissions per unit of GDP, the “CO2 intensity”, decreased more rapidly than energy intensity: by 2.3%/year and 1.4%/year, respectively, on average between 1990 and 2007.
Emission factors for greenhouse gas inventory reporting 
One of the most important uses of emission factors is for the reporting of national greenhouse gas inventories under the United Nations Framework Convention on Climate Change (UNFCCC). The so-called Annex I Parties to the UNFCCC have to annually report their national total emissions of greenhouse gases in a formalized reporting format, defining the source categories and fuels that must be included.
The UNFCCC has accepted the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories, developed and published by the Intergovernmental Panel on Climate Change (IPCC) as the emission estimation methods that must be used by the parties to the convention to ensure transparency, completeness, consistency, comparability and accuracy of the national greenhouse gas inventories. These IPCC Guidelines are the primary source for default emission factors. Recently IPCC has published the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. These and many more greenhouse gas emission factors can be found on IPCC's Emission Factor Database. Commercially-applicable organisational greenhouse gas emission factors can be found on the search engine, EmissionFactors.com.
Particularly for non-CO2 emissions, there is often a high degree of uncertainty associated with these emission factors when applied to individual countries. In general, the use of country-specific emission factors would provide more accurate estimates of emissions than the use of the default emission factors. According to the IPCC, if an activity is a major source of emissions for a country ('key source'), it is 'good practice' to develop a country-specific emission factor for that activity.
Emission factors for air pollutant inventory reporting 
National Air Pollution Emission Inventories are required annually under the provisions of the UNECE Convention on Long-Range Transboundary Air Pollution (LRTAP). Emission estimation methods and the associated emission factors for air pollutants have been developed by the EMEP Task Force on Emission Inventories and Projections (TFEIP) and are published in the EMEP/CORINAIR Emission Inventory Guidebook.
Intensity targets 
The U.S. plans to cut carbon intensity per dollar of GDP by 18% by 2012. This has been criticised by the World Resources Institute as this approach does not ensure absolute reductions if GDP grows faster than intensity declines.
From 1990 to 2000, the carbon intensity of the U.S. economy declined by 17%, yet total emissions increased by 14%. In 2002, the U.S. National Environmental trust labelled carbon intensity, "a bookkeeping trick which allows the administration to do nothing about global warming while unsafe levels of emissions continue to rise."
Sources of emission factors 
Greenhouse gases 
- 2006 IPCC Guidelines for National Greenhouse Gas Inventories
- Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (reference manual).
- IPCC Emission Factor Database
- National Inventory Report: Greenhouse Gas Sources and Sinks in Canada.
- United Kingdom's emission factor database.
Air pollutants 
- AP 42, Compilation of Air Pollutant Emission Factors US Environmental Protection Agency
- EMEP/CORIMAIR 2007 Emission Inventory Guidebook.
- Fugitive emissions leaks from ethylene and other chemical plants.
See also 
- Energy intensity
- Carbon footprint
- List of countries by ratio of GDP to carbon dioxide emissions
- Low carbon economy
- Low-carbon fuel standard
- Emission inventory
- Air pollution
- AP 42 Compilation of Air Pollutant Emission Factors
- Emission standard
- Greenhouse gas and Greenhouse effect
- IPCC list of greenhouse gases
- Mobile Emission Reduction Credit (MERC)
- Radiative forcing
- Kaya identity
- Moomaw, W., P. Burgherr, G. Heath, M. Lenzen, J. Nyboer, A. Verbruggen, 2011: Annex II: Methodology. In IPCC: Special Report on Renewable Energy Sources and Climate Change Mitigation (ref. page 10)
- 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.
- Dowlatabadi, H (9 November 2007), "Strategic GHG reduction through the use of ground source heat pump technology", Environmental Research Letters (UK: IOP Publishing) 2: 044001 8pp, Bibcode:2007ERL.....2d4001H, doi:10.1088/1748-9326/2/4/044001, ISSN 1748-9326, retrieved 2009-03-22
- Enerdata Statistical Yearbook 2012.
- Calculation of carbon intensity in 2012
- Energy Efficiency Trends and Policies in the EU 27 Results of the ODYSSEE-MURE project
- This section deals with CO2 emissions from energy combustion published in official inventories from the European Environment Agency. The indicators are not expressed under normal climate conditions (i. e. with climate corrections) to comply with the official definition of CO2 inventories. CO2 emissions of final consumers include the emissions of auto producers.
- Task Force on National Greenhouse Gas Inventories (1996). "Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories". IPCC. Retrieved 19 August 2012.
- "Emission Factor Database – Main Page". IPCC. 2012. Retrieved 19 August 2012.
- "Emission Factors". emissionfactors.com. 2012. Retrieved 19 August 2012.
- White house fact sheet: Earth day 2007 (see section on providing a Realistic growth-orientated approach to climate change
- Tim Herzog (2007-04-27). "China's Carbon Intensity Target". World resources Institute. Retrieved 2010-11-04.
- Fischlowitz-Roberts, Bernie (2001). "Carbon Emissions Climbing". Earth Policy Institute. Retrieved 2006-08-12.
- "National Environmental Trust Special Reports", 2002. Retrieved 2006-08-12.
- Washington Post article with an example of change in carbon intensity
- A Note On Variations in UK Grid Electricity CO2 Intensity with Time
- IPCC Special Report on Emissions Scenarios
- Statistical Energy Review 2012
- World Energy Council:Odyssee Database