Bogoliubov inner product

2013-12-28T16:48:04Z

80.109.67.61: Correction

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{{Use dmy dates|date=July 2013}}
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[[File:Greenhouse Effect.svg|thumb|350px|right|alt=refer to caption and image description|[[Greenhouse effect]] schematic showing [[energy]] flows between [[outer space|space]], the [[Earth's atmosphere|atmosphere]], and Earth's [[surface of the Earth|surface]]. [[Irradiance|Energy influx and emittance]] are expressed in [[watt]]s per [[square meter]] (W/m2).]]

A '''greenhouse gas''' (sometimes abbreviated '''GHG''') is a [[gas]] in an atmosphere that [[Absorption (electromagnetic radiation)|absorbs]] and [[Emission (electromagnetic radiation)|emits]] radiation within the [[thermal infrared]] range. This process is the fundamental cause of the [[greenhouse effect]].<ref name="IPCC AR4-SYR">{{cite web|url=http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_appendix.pdf|format=PDF|title=IPCC AR4 SYR Appendix Glossary|accessdate=14 December 2008}}</ref> The primary greenhouse gases in the [[Earth's atmosphere]] are [[water vapor]], [[carbon dioxide]], [[methane]], [[nitrous oxide]], and [[ozone]]. Greenhouse gases greatly affect the temperature of the [[Earth]]; without them, Earth's surface would average about 33 °C colder, which is about 59 °F below the present average of {{convert|14|°C|°F}}.<ref>{{cite journal |author=Karl TR, Trenberth KE |year=2003 |title=Modern global climate change |url=http://www.sciencemag.org/cgi/content/abstract/302/5651/1719 | journal=Science |volume=302 |issue=5651 |pages=1719–23 |doi=10.1126/science.1090228 |pmid=14657489|bibcode = 2003Sci...302.1719K }}</ref><ref>{{cite book |author=Le Treut H., Somerville R., Cubasch U., Ding Y., Mauritzen C., Mokssit A., Peterson T. and Prather M.|year=2007 |url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter1.pdf |title=Historical overview of climate change science. In: Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Solomon S., Qin D., Manning M., Chen Z., Marquis M., Averyt K.B., Tignor M. and Miller H.L., editors) |format=PDF |publisher=Cambridge University Press |accessdate=14 December 2008}}</ref><ref name="h2o">{{cite web|url=http://nasascience.nasa.gov/earth-science/oceanography/ocean-earth-system/ocean-water-cycle |title=NASA Science Mission Directorate article on the water cycle |publisher=Nasascience.nasa.gov |date= |accessdate=2010-10-16}}</ref>

Since the beginning of the [[Industrial Revolution]] (taken as the year 1750), the burning of [[fossil fuels]] and extensive clearing of native forests has contributed to a 40% increase in the atmospheric concentration of [[Carbon dioxide in Earth's atmosphere|carbon dioxide]], from 280 to 392.6 [[parts per million]] (ppm) in 2012.<ref name="blasing ghg concentrations"/><ref>The most recent preliminary estimate of global monthly mean {{CO2}} concentration (as of May 2013) is 396.71 ppm: (Ed Dlugokencky and Pieter Tans, NOAA/ESRL ([http://www.esrl.noaa.gov/gmd/ccgg/trends/global.html])</ref> This increase has occurred despite the uptake of a large portion of the emissions by various natural "sinks" involved in the [[carbon cycle]].<ref name="cdiac">{{cite web |url=http://cdiac.ornl.gov/pns/faq.html |title=Frequently asked global change questions |publisher=[[Carbon Dioxide Information Analysis Center]]}}</ref><ref>{{cite web|author=ESRL Web Team |url=http://www.esrl.noaa.gov/gmd/ccgg/trends/ |title=Trends in carbon dioxide |publisher=Esrl.noaa.gov |date=14 January 2008 |accessdate=2011-09-11}}</ref> [[Human impact on the environment|Anthropogenic]] carbon dioxide ({{CO2}}) emissions (i.e., emissions produced by human activities) come from [[combustion]] of [[carbon-based fuel]]s, principally [[wood]], [[coal]], [[oil]], and [[natural gas]].<ref>{{cite book |author=Lindeburgh, Michael R. |title=Mechanical engineering reference manual for the PE Exam |publisher=Professional Publications |location=Belmont CA |year=2006 |isbn=978-1-59126-049-3 }}</ref> Under ongoing greenhouse gas emissions, available Earth System Models project that the Earth's surface temperature could exceed historical analogs as early as 2047 affecting most ecosystems on Earth and the livelihoods of over 3 billion people worldwide.<ref>{{cite journal |last1=Mora |first1=C |title=The projected timing of climate departure from recent variability |journal=Nature |volume=502 |pages=183–187 |year=2013 |doi=10.1038/nature12540}}</ref> Greenhouse gasses also trigger ocean bio-geochemical changes with broad ramifications in marine systems.<ref>{{cite journal |last1=Mora |first1=C. et al. |title=Biotic and Human Vulnerability to Projected Changes in Ocean Biogeochemistry over the 21st Century |journal=Plos Biology |volume=11 |pages=e1001682 |year=2013 |doi=10.1371/journal.pbio.1001682}}</ref>

In the [[Solar System]], the atmospheres of [[atmosphere of Venus|Venus]], [[atmosphere of Mars|Mars]], and [[atmosphere of Titan|Titan]] also contain gases that cause a greenhouse effect, though Titan's atmosphere has an [[anti-greenhouse effect]] which reduces the warming.

{{TOC limit|limit=4}}

==Gases in Earth's atmosphere==
{{Main|Greenhouse effect|Global warming|Carbon dioxide in Earth's atmosphere}}

===Greenhouse gases===
[[File:Atmospheric Transmission.png|right|thumb|alt=refer to caption and adjacent text|Atmospheric absorption and scattering at different [[wavelength]]s of [[Electromagnetic radiation|electromagnetic waves]]. The largest absorption band of [[carbon dioxide]] is in the [[infrared]].]]
Greenhouse gases are those that can absorb and emit [[thermal infrared|infrared radiation]],<ref name="IPCC AR4-SYR" /> but not radiation in or near the visible spectrum. In order, the most abundant greenhouse gases in Earth's atmosphere are:
*[[Water vapor]] ({{chem|H|2|O}})
*[[Carbon dioxide]] ({{CO2}})
*[[Methane]] ({{chem|CH|4|}})
*[[Nitrous oxide]] ({{chem|N|2|O}})
*[[Ozone]] ({{chem|O|3|}})
*[[Chlorofluorocarbon|CFC]]s

Atmospheric concentrations of greenhouse gases are determined by the balance between sources (emissions of the gas from human activities and natural systems) and sinks (the removal of the gas from the atmosphere by conversion to a different chemical compound).<ref name="IPCC_WG1_AR4_Ch7">{{cite web|author=IPCC|work=IPCC WG1 AR4 Report | title = Chapter 7: Couplings Between Changes in the Climate System and Biogeochemistry| url = http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf |format=PDF|accessdate = 11 July 2011|year=2007|publisher=IPCC|page = FAQ 7.1; report page 512; pdf page 14}}</ref> The proportion of an emission remaining in the atmosphere after a specified time is the "[[Airborne fraction]]" (AF). More precisely, the annual AF is the ratio of the atmospheric increase in a given year to that year’s total emissions. For {{CO2}} the AF over the last 50 years (1956–2006) has been increasing at 0.25 ± 0.21%/year.<ref name=Canadell2007>{{cite journal
|author = Canadell, J.G.
|coauthors = Le Quere, C.; Raupach, M.R.; Field, C.B.; Buitenhuis, E.T.; Ciais, P.; Conway, T.J.; Gillett, N.P.; Houghton, R.A.; Marland, G.
|year = 2007
|title = Contributions to accelerating atmospheric {{CO2}} growth from economic activity, carbon intensity, and efficiency of natural sinks
|journal = Proc. Natl. Acad. Sci. U.S.A.
|volume=104
|issue= 47
|pages=18866–70
|accessdate = 15 March 2008
|pmid=17962418
|doi=10.1073/pnas.0702737104
|pmc=2141868
|bibcode = 2007PNAS..10418866C }}</ref>

===Non-greenhouse gases===
Although contributing to many other physical and chemical reactions, the major atmospheric constituents, [[nitrogen]] ({{chem|N|2}}), [[oxygen]] ({{chem|O|2}}), and [[argon]] (Ar), are not greenhouse gases. This is because [[diatomic molecule|molecules containing two atoms of the same element]] such as {{chem|N|2}} and {{chem|O|2}} and [[monatomic]] molecules such as argon (Ar) have no net change in their [[Molecular dipole moment|dipole moment]] when they vibrate and hence are almost totally unaffected by [[infrared|infrared radiation]]. Although molecules containing two atoms of different elements such as [[carbon monoxide]] (CO) or [[hydrogen chloride]] (HCl) absorb IR, these molecules are short-lived in the atmosphere owing to their reactivity and solubility. Because they do not contribute significantly to the greenhouse effect, they are usually omitted when discussing greenhouse gases.

=== Indirect radiative effects ===
[[File:Mopitt first year carbon monoxide.jpg|thumb|200px|alt=world map of carbon monoxide concentrations in the lower atmosphere|The false colors in this image represent levels of carbon monoxide in the lower atmosphere, ranging from about 390 parts per billion (dark brown pixels), to 220 parts per billion (red pixels), to 50 parts per billion (blue pixels).<ref>http://earthobservatory.nasa.gov/Library/RemoteSensingAtmosphere/remote_sensing6.html {{dead link|date=December 2012}}</ref>]]

Some gases have indirect radiative effects (whether or not they are a greenhouse gas themselves). This happens in two main ways. One way is that when they break down in the atmosphere they produce another greenhouse gas. For example methane and carbon monoxide (CO) are oxidized to give carbon dioxide (and methane oxidation also produces water vapor; that will be considered below). Oxidation of CO to {{CO2}} directly produces an unambiguous increase in radiative forcing although the reason is subtle. The peak of the thermal IR emission from the Earth's surface is very close to a strong vibrational absorption band of {{CO2}} (667 cm−1). On the other hand, the single CO vibrational band only absorbs IR at much higher frequencies (2145 cm−1), where the ~300 K thermal emission of the surface is at least a factor of ten lower. On the other hand, oxidation of methane to {{CO2}} which requires reactions with the OH radical, produces an instantaneous reduction, since {{CO2}} is a weaker greenhouse gas than methane; but it has a longer lifetime. As described below this is not the whole story, since the oxidations of CO and {{chem|CH|4}} are intertwined by both consuming OH radicals. In any case, the calculation of the total radiative effect needs to include both the direct and indirect forcing.

A second type of indirect effect happens when chemical reactions in the atmosphere involving these gases change the concentrations of greenhouse gases. For example, the destruction of [[NMVOC|non-methane volatile organic compounds]] (NMVOC) in the atmosphere can produce ozone. The size of the indirect effect can depend strongly on where and when the gas is emitted.<ref name="forsteretal">{{cite book |url= http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-3.html |contribution=2.10.3 Indirect GWPs |title=Changes in Atmospheric Constituents and in Radiative Forcing |series=Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change | publisher=Cambridge University Press |year=2007| author=Forster, P. et al.|accessdate=2012-12-02}}</ref>

Methane has a number of indirect effects in addition to forming {{CO2}}. Firstly, the main chemical which destroys methane in the atmosphere is the [[hydroxyl radical]] (OH). Methane reacts with OH and so more methane means that the concentration of OH goes down. Effectively, methane increases its own atmospheric lifetime and therefore its overall radiative effect. The second effect is that the oxidation of methane can produce ozone. Thirdly, as well as making {{CO2}} the oxidation of methane produces water; this is a major source of water vapor in the [[stratosphere]] which is otherwise very dry. CO and NMVOC also produce {{CO2}} when they are oxidized. They remove OH from the atmosphere and this leads to higher concentrations of methane. The surprising effect of this is that the global warming potential of CO is three times that of {{CO2}}.<ref name=maccartyetal>{{cite web|last=MacCarty|first=N.|title=Laboratory Comparison of the Global-Warming Potential of Six Categories of Biomass Cooking Stoves|url=http://www.scscertified.com/lcs/docs/Global_warming_full_9-6-07.pdf|publisher=Approvecho Research Center}}</ref> The same process that converts NMVOC to carbon dioxide can also lead to the formation of tropospheric ozone. Halocarbons have an indirect effect because they destroy stratospheric ozone. Finally [[hydrogen]] can lead to ozone production and {{chem|CH|4}} increases as well as producing water vapor in the stratosphere.<ref name="forsteretal" />

===Contribution of clouds to Earth's greenhouse effect===
The major non-gas contributor to the Earth's greenhouse effect, [[Cloud forcing|clouds]], also absorb and emit infrared radiation and thus have an effect on radiative properties of the greenhouse gases. Clouds are water droplets or [[ice crystal]]s suspended in the atmosphere.<ref name="kiehl197">{{cite journal| url=http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf| title=Earth's annual global mean energy budget| first=J.T.| last=Kiehl| coauthors= Kevin E. Trenberth| format=PDF|journal=Bulletin of the American Meteorological Society| pages=197–208| volume=78| issue=2| year=1997| accessdate=1 May 2006| doi=10.1175/1520-0477(1997)078<0197:EAGMEB>2.0.CO;2 |archiveurl = http://web.archive.org/web/20060330013311/http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf  |archivedate = 30 March 2006| bibcode=1997BAMS...78..197K}}</ref><ref name="realclimate.org">{{cite web| url=http://www.realclimate.org/index.php?p=142| date=6 April 2005| title=Water vapour: feedback or forcing?| publisher=RealClimate| accessdate=1 May 2006}}</ref>

== Impacts on the overall greenhouse effect ==
[[File:Attribution of individual atmospheric component contributions to the terrestrial greenhouse effect, separated into feedback and forcing categories (NASA).png|right|thumb|alt=refer to caption and adjacent text|Schmidt ''et al.'' (2010)<ref>
{{citation
| author1=Schmidt, G.A.
| author2=R. Ruedy | author3= R.L. Miller | author4= A.A. Lacis
| authorlink1=Gavin Schmidt
| year=2010
| title=The attribution of the present-day total greenhouse effect
| url=http://pubs.giss.nasa.gov/docs/2010/2010_Schmidt_etal_1.pdf
| work=J. Geophys. Res.
| volume=115
| doi=10.1029/2010JD014287
|bibcode = 2010JGRD..11520106S }}, D20106. [http://pubs.giss.nasa.gov/abs/sc05400j.html Web page for paper.]
</ref> analysed how individual components of the atmosphere contribute to the total greenhouse effect. They estimated that water vapor accounts for about 50% of the Earth's greenhouse effect, with clouds contributing 25%, carbon dioxide 20%, and the minor greenhouse gases and [[aerosol]]s accounting for the remaining 5%. In the study, the reference model atmosphere is for 1980 conditions. Image credit: [[NASA]].<ref>
{{citation
| author=Lacis, A.
| date=October 2010
| title=NASA GISS: CO2: The Thermostat that Controls Earth's Temperature
| url=http://www.giss.nasa.gov/research/briefs/lacis_01/
| publisher=NASA GISS
| location=New York
}}
</ref>]]
The contribution of each gas to the greenhouse effect is affected by the characteristics of that gas, its abundance, and any indirect effects it may cause. For example, the direct radiative effect of a mass of methane is about 72 times stronger than the same mass of carbon dioxide over a 20-year time frame<ref name="TableOfWarmingPotentials" /> but it is present in much smaller concentrations so that its total direct radiative effect is smaller, in part due to its shorter atmospheric lifetime. On the other hand, in addition to its direct radiative impact, methane has a large, indirect radiative effect because it contributes to ozone formation. Shindell ''et al.'' (2005)<ref>{{cite journal |doi=10.1029/2004GL021900 |url=http://www.nasa.gov/vision/earth/lookingatearth/methane.html |title=An emissions-based view of climate forcing by methane and tropospheric ozone |year=2005 |last1=Shindell |first1=Drew T. |journal=Geophysical Research Letters |volume=32 |pages=L04803 |bibcode=2005GeoRL..3204803S |issue=4}}</ref> argue that the contribution to climate change from methane is at least double previous estimates as a result of this effect.<ref>{{cite web|url=http://www.nasa.gov/vision/earth/lookingatearth/methane.html |title=Methane's Impacts on Climate Change May Be Twice Previous Estimates |publisher=Nasa.gov |date=30 November 2007 |accessdate=2010-10-16}}</ref>

When ranked by their direct contribution to the greenhouse effect, the most important are:<ref name="kiehl197"/>

{| class="wikitable" style="text-align:center"
|-
! Compound  
! style="text-align:left;"| Formula  
! Contribution (%)
|-
| Water vapor and clouds || {{chem|H|2|O}} || 36 – 72%  
|-
| Carbon dioxide || {{CO2}} || 9 – 26%
|-
|Methane || {{chem|CH|4}} || 4 – 9%  
|-
|Ozone || {{chem|O|3}} || 3 – 7%  
|}

In addition to the main greenhouse gases listed above, other greenhouse gases include [[sulfur hexafluoride]], [[hydrofluorocarbon]]s and [[perfluorocarbon]]s (see [[IPCC list of greenhouse gases]]). Some greenhouse gases are not often listed. For example, [[nitrogen trifluoride]] has a high [[global warming potential]] (GWP) but is only present in very small quantities.<ref name="NF3">{{cite journal |last= Prather |first= Michael J. |coauthors= J Hsu |title= {{chem|NF|3}}, the greenhouse gas missing from Kyoto |journal= [[Geophysical Research Letters]] |volume=35 |pages=L12810 |year=2008 |doi=10.1029/2008GL034542 |bibcode=2008GeoRL..3512810P |issue= 12}}</ref>

===Proportion of direct effects at a given moment===
It is not possible to state that a certain gas causes an exact percentage of the greenhouse effect. This is because some of the gases absorb and emit radiation at the same frequencies as others, so that the total greenhouse effect is not simply the sum of the influence of each gas. The higher ends of the ranges quoted are for each gas alone; the lower ends account for overlaps with the other gases.<ref name="kiehl197"/><ref name="realclimate.org"/> In addition, some gases such as methane are known to have large indirect effects that are still being quantified.<ref>{{cite journal|last=Isaksen|first=Ivar S. A.|coauthors=Michael Gauss, Gunnar Myhre, Katey M. Walter Anthony, and Carolyn Ruppel|title=Strong atmospheric chemistry feedback to climate warming from Arctic methane emissions|journal=Global Biogeochemical Cycles|date=20 April 2011|volume=25|doi=10.1029/2010GB003845|url=http://www.atmos.washington.edu/academics/classes/2011Q2/558/IsaksenGB2011.pdf|accessdate=29 July 2011|bibcode = 2011GBioC..25B2002I|issue=2 }}</ref>

===Atmospheric lifetime===

Aside from [[water vapor]], which has a residence time of about nine days,<ref>{{cite web|url=http://www.eso.org/gen-fac/pubs/astclim/espas/pwv/mockler.html |title=AGU Water Vapor in the Climate System |publisher=Eso.org |date=27 April 1995 |accessdate=2011-09-11}}</ref> major greenhouse gases are well-mixed, and take many years to leave the atmosphere.<ref name='betts'>{{cite book |url= http://www.grida.no/publications/other/ipcc%5Ftar/?src=/climate/ipcc_tar/wg1/218.htm
|contribution=6.3 Well-mixed Greenhouse Gases
|title=Chapter 6 Radiative Forcing of Climate Change
|series=Working Group I: The Scientific Basis IPCC Third Assessment Report — Climate Change 2001
|publisher=UNEP/GRID-Arendal — Publications
|year=2001
|author=Betts
|accessdate=2010-10-16 |display-authors= 1}}</ref> Although it is not easy to know with precision how long it takes greenhouse gases to leave the atmosphere, there are estimates for the principal greenhouse gases.
Jacob (1999)<ref name=JacobDJ1999/> defines the lifetime <math>\tau</math> of an atmospheric [[chemical species|species]] X in a [[Climate model#Box models|one-box model]] as the average time that a molecule of X remains in the box. Mathematically <math>\tau</math> can
be defined as the ratio of the mass <math>m</math> (in kg) of X in the box to its removal rate, which is the sum of the flow of X out of the box
(<math>F_{out}</math>),
chemical loss of X
(<math>L</math>),
and [[deposition (chemistry)|deposition]] of X
(<math>D</math>)
(all in kg/s):
<math>\tau = \frac{m}{F_{out}+L+D}</math>.<ref name=JacobDJ1999>{{cite book
|last = Jacob
|first = Daniel
|title = Introduction to atmospheric chemistry
|publisher = [[Princeton University Press]]
|year = 1999
|url = http://www-as.harvard.edu/people/faculty/djj/book/
|isbn = 0-691-00185-5
|pages = 25–26
}}</ref>
If one stopped pouring any of this gas into the box, then after a time <math>\tau</math>, its concentration would be about halved.

The atmospheric lifetime of a species therefore measures the time required to restore equilibrium following a sudden increase or decrease in its concentration in the atmosphere. Individual atoms or molecules may be lost or deposited to sinks such as the soil, the oceans and other waters, or vegetation and other biological systems, reducing the excess to background concentrations. The average time taken to achieve this is the [[mean lifetime]].

[[Carbon dioxide]] has a variable atmospheric lifetime, and cannot be specified precisely.<ref>{{cite web|url=http://www.realclimate.org/index.php/archives/2005/03/how-long-will-global-warming-last |title=How long will global warming last? |publisher=RealClimate |date= |accessdate=2012-06-12}}</ref> The atmospheric lifetime of {{CO2}} is estimated of the order of 30–95 years.<ref name=JacobsonMZ2005>{{cite news
|last = Jacobson
|first = MZ
|title = Correction to "Control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming."
|journal = J. Geophys. Res.
|volume = 110
|pages = D14105
|year = 2005
|doi = 10.1029/2005JD005888
|url = http://www.agu.org/pubs/crossref/2005/2005JD005888.shtml
}}</ref>
This figure accounts for {{CO2}} molecules being removed from the atmosphere by mixing into the ocean, photosynthesis, and other processes. However, this excludes the balancing fluxes of {{CO2}} into the atmosphere from the geological reservoirs, which have slower characteristic rates.<ref name=ArcherD2009>{{cite news
|last = Archer
|first = David
|title = Atmospheric lifetime of fossil fuel carbon dioxide
|journal = Annual Review of Earth and Planetary Sciences
|volume = 37
|pages = 117–134
|year = 2009
|doi = 10.1146/annurev.earth.031208.100206
|url=http://www.annualreviews.org/doi/abs/10.1146/annurev.earth.031208.100206
}}</ref> While more than half of the {{CO2}} emitted is removed from the atmosphere within a century, some fraction (about 20%) of emitted {{CO2}} remains in the atmosphere for many thousands of years.<ref>
{{cite book
| year=2007
| contribution=Frequently Asked Question 10.3: If emissions of greenhouse gases are reduced, how quickly do their concentrations in the atmosphere decrease?
| title=Chapter 10: Global Climate Projections
| series=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
| editor=S. Solomon, ''et al.'', (eds.)
| publisher=Print version:CUP. This version: IPCC website
| location=Cambridge University Press (CUP), Cambridge, United Kingdom and New York, NY, USA.
| author=Meehl, G.A.
| url=http://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-10-3.html
| accessdate=2011-06-01
| display-authors=1}}
</ref><ref name="carbon_lifetime1">See also: {{Cite journal|url=http://geosci.uchicago.edu/~archer/reprints/archer.2005.fate_co2.pdf |format=PDF| first = David|last = Archer|title = Fate of fossil fuel {{CO2}} in geologic time|journal = [[Journal of Geophysical Research]]|volume = 110|issue=C9|pages=C09S05.1–6|year = 2005|
doi=10.1029/2004JC002625|accessdate=27 July 2007|bibcode=2005JGRC..11009S05A}}</ref><ref name="carbon_lifetime2">See also: {{Cite journal|first1=Ken|last1=Caldeira|first2=Michael E.|last2= Wickett|url=http://www.ipsl.jussieu.fr/~jomce/acidification/paper/Caldeira_Wickett_2005_JGR.pdf |format=PDF| title = Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean|journal = [[Journal of Geophysical Research]]|volume = 110|issue=C9|pages=C09S04.1–12| year = 2005|doi=10.1029/2004JC002671|accessdate=27 July 2007 |archiveurl = http://web.archive.org/web/20070810202611/http://www.ipsl.jussieu.fr/~jomce/acidification/paper/Caldeira_Wickett_2005_JGR.pdf |archivedate = 10 August 2007 |bibcode=2005JGRC..11009S04C}}</ref> Similar issues apply to other greenhouse gases, many of which have longer mean lifetimes than {{CO2}}. E.g., [[#N2O|N2O]] has a mean atmospheric lifetime of 114 years.<ref name="TableOfWarmingPotentials"/>

===Radiative forcing===

The Earth absorbs some of the radiant energy received from the sun, reflects some of it as light and reflects or re-radiates the rest back to space as [[thermal radiation|heat]].<ref name="epa radiative forcing">
Edited quote from public-domain source: {{cite web
| year=2010
| author=US Environmental Protection Agency (EPA)
| title= Climate Change Indicators in the United States
| at= Greenhouse Gases: Figure 1. The Annual Greenhouse Gas Index, 1979–2008: Background
| publisher=EPA
| url=http://www.epa.gov/climatechange/science/indicators/ghg/climate-forcing.html
}}. This publication is also available as a [http://www.epa.gov/climatechange/science/indicators/download.html PDF (page 18).]
</ref> The Earth's surface temperature depends on this balance between incoming and outgoing energy.<ref name="epa radiative forcing"/> If this [[First law of thermodynamics|energy balance]] is shifted, the Earth's surface could become warmer or cooler, leading to a variety of changes in global climate.<ref name="epa radiative forcing"/>

A number of natural and man-made mechanisms can affect the global energy balance and force changes in the Earth's climate.<ref name="epa radiative forcing"/> Greenhouse gases are one such mechanism.<ref name="epa radiative forcing"/> Greenhouse gases in the atmosphere absorb and re-emit some of the outgoing energy radiated from the Earth's surface, causing that heat to be retained in the lower atmosphere.<ref name="epa radiative forcing"/> As [[greenhouse gas#Atmospheric lifetime|explained above]], some greenhouse gases remain in the atmosphere for decades or even centuries, and therefore can affect the Earth's energy balance over a long time period.<ref name="epa radiative forcing"/> Factors that influence Earth's energy balance can be quantified in terms of "[[radiative forcing|radiative climate forcing]]."<ref name="epa radiative forcing"/> Positive radiative forcing indicates warming (for example, by increasing incoming energy or decreasing the amount of energy that escapes to space), while negative forcing is associated with cooling.<ref name="epa radiative forcing"/>

===Global warming potential===

The [[global warming potential]] (GWP) depends on both the efficiency of the molecule as a greenhouse gas and its atmospheric lifetime. GWP is measured relative to the same '''mass''' of {{CO2}} and evaluated for a specific timescale. Thus, if a gas has a high (positive) [[radiative forcing]] but also a short lifetime, it will have a large GWP on a 20-year scale but a small one on a 100-year scale. Conversely, if a molecule has a longer atmospheric lifetime than {{CO2}} its GWP will increase with the timescale considered. Carbon dioxide is defined to have a GWP of 1 over all time periods.

[[Methane]] has an atmospheric lifetime of 12 ± 3 years and a GWP of 72 over 20 years, 25 over 100 years and 7.6 over 500 years. The decrease in GWP at longer times is because [[Atmospheric methane#Removal processes|methane]] is degraded to water and {{CO2}} through chemical reactions in the atmosphere.

Examples of the atmospheric lifetime and [[global warming potential|GWP]] relative to {{CO2}} for several greenhouse gases are given in the following table:<ref name="TableOfWarmingPotentials">[[Intergovernmental Panel on Climate Change|IPCC]] Fourth Assessment Report, [http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter2.pdf Table 2.14, Chap. 2, p. 212]</ref>

{| class="wikitable" style="text-align: right"
|+ Atmospheric lifetime and [[global warming potential|GWP]] relative to {{CO2}} at different time horizon for various greenhouse gases.
|-
! rowspan="2" style="text-align:left;" | Gas name
! rowspan="2" | Chemical formula !! rowspan="2" | Lifetime (years)
! colspan="3" | Global warming potential (GWP) for given time horizon
|-
! 20-yr !! 100-yr !! 500-yr
|-
| style="text-align:left;"| '''[[Carbon dioxide]]'''
| style="text-align:center;"| {{CO2}}|| See above|| 1 || 1 || 1
|-
| style="text-align:left;"| '''[[Methane]]'''
| style="text-align:center;"| {{chem|CH|4}}|| 12 || 72 || 25 || 7.6
|- id="N2O"
| style="text-align:left;"| '''[[Nitrous oxide]]'''
| style="text-align:center;"| {{chem|N|2|O}} || 114 || 289 || 298 || 153
|-
| style="text-align:left;"| '''[[CFC-12]]'''
| style="text-align:center;"| {{chem|CCl|2|F|2}}|| 100 || 11 000 || 10 900 || 5 200
|-
| style="text-align:left;"| '''[[HCFC-22]]'''
| style="text-align:center;"| {{chem|CHClF|2}}|| 12 || 5 160 || 1 810 || 549
|-
| style="text-align:left;"| '''[[Tetrafluoromethane]]'''
| style="text-align:center;"| {{chem|CF|4}}|| 50 000 || 5 210 || 7 390 || 11 200
|-
| style="text-align:left;"| '''[[Hexafluoroethane]]'''
| style="text-align:center;"| {{chem|C|2|F|6}}|| 10 000 || 8 630 || 12 200 || 18 200
|-
| style="text-align:left;"| '''[[Sulfur hexafluoride]]'''
| style="text-align:center;"| {{chem|SF|6}}|| 3 200 || 16 300 || 22 800 || 32 600
|-
| style="text-align:left;"| '''[[Nitrogen trifluoride]]'''
| style="text-align:center;"| {{chem|NF|3}}|| 740 || 12 300 || 17 200 || 20 700
|}

The use of [[CFC-12]] (except some essential uses) has been phased out due to its [[Ozone depletion|ozone depleting]] properties.<ref>[http://www.norden.org/pub/ebook/2003-516.pdf Use of ozone depleting substances in laboratories]. TemaNord 2003:516</ref> The phasing-out of less active [[Haloalkane|HCFC-compounds]] will be completed in 2030.<ref>[[Montreal Protocol]]</ref>

==Natural and anthropogenic sources==

[[File:Carbon History and Flux Rev.png|thumb|alt=refer to caption and article text|right|Top: Increasing atmospheric [[carbon dioxide]] levels as measured in the atmosphere and reflected in [[ice core]]s. Bottom: The amount of net carbon increase in the atmosphere, compared to carbon emissions from burning [[fossil fuel]].]]
[[File:Diagram showing a simplified representation of the Earth's annual carbon cycle (US DOE).png|thumb|alt=refer to caption and image description|right|This diagram shows a simplified representation of the contemporary global [[carbon cycle]]. Changes are measured in [[gigaton]]s of carbon per year (GtC/y). Canadell ''et al.'' (2007) estimated the growth rate of global average atmospheric {{CO2}} for 2000-2006 as 1.93 parts-per-million per year (4.1 [[petagram]]s of carbon per year).<ref>
{{citation
| author=Canadell, J.G., ''et al.''
| title=Contributions to Accelerating Atmospheric CO2 Growth from Economic Activity, Carbon Intensity, and Efficiency of Natural Sinks (Results and Discussion: Growth in Atmospheric CO2)
| url=http://www.pnas.org/content/104/47/18866.full
| date=20 November 2007
| journal=Proceedings of the National Academy of Sciences of the United States of America
| volume=104
| issue=47
| pages=18866–18870
| doi=10.1073/pnas.0702737104
| pmid=17962418
| pmc=2141868
|bibcode = 2007PNAS..10418866C }}
</ref> Image credit: [[U.S. Department of Energy]] Genomic Science program<ref>
{{citation
| author=US DOE, Prepared by the Biological and Enviornmental Research Information System, Oak Ridge National Laboratory, genomicscience.energy.gov/ and genomics.energy.gov/
| url=https://public.ornl.gov/site/gallery/detail.cfm?id=445&topic=&citation=&general=carbon&restsection=BERPublic
| title=Global Carbon Cycle Components
}}. Used in: {{citation
| date=December 2008
| author=US DOE
| url=http://genomicscience.energy.gov/carboncycle/report/index.shtml
| title=Carbon Cycling and Biosequestration: Integrating Biology and Climate Through Systems Science; Report from the March 2008 Workshop, DOE/SC-108
| publisher=U.S. Department of Energy Office of Science (genomicscience.energy.gov/carboncycle/)
}}
</ref>]]

Aside from purely human-produced synthetic halocarbons, most greenhouse gases have both natural and human-caused sources. During the pre-industrial [[Holocene]], concentrations of existing gases were roughly constant. In the industrial era, human activities have added greenhouse gases to the atmosphere, mainly through the burning of fossil fuels and clearing of forests.<ref>{{cite web|url= http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter1.pdf|format=PDF|title= Chapter 1 Historical Overview of Climate Change Science — FAQ 1.3 Figure 1 description page 116|work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change|accessdate=25 April 2008|date=5 February 2007|publisher=[[Intergovernmental Panel on Climate Change]]}}</ref><ref>{{cite web|url=http://www.grida.no/Climate/ipcc/emission/049.htm |title=Chapter 3, IPCC Special Report on Emissions Scenarios, 2000 |publisher=Intergovernmental Panel on Climate Change |year=2000 |accessdate=2010-10-16}}</ref>

The 2007 [[IPCC Fourth Assessment Report|Fourth Assessment Report]] compiled by the IPCC (AR4) noted that "changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system", and concluded that "increases in anthropogenic greenhouse gas concentrations is very likely to have caused most of the increases in global average temperatures since the mid-20th century".<ref>{{cite web|url=http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf |title=AR4 SYR SPM page 5|format=PDF |date= |accessdate=2010-10-16}}</ref> In AR4, "most of" is defined as more than 50%.

''Abbreviations used in the two tables below: ppm = [[parts-per notation|parts-per-million]]; ppb = parts-per-billion; ppt = parts-per-trillion; W/m2 = [[watt]]s per [[square metre]]''

{| class="wikitable"
|+ Current greenhouse gas concentrations<ref name="blasing ghg concentrations">
From non-copyrighted source: {{citation
| title=Current Greenhouse Gas Concentrations
| author=Blasing, T.J.
| doi=10.3334/CDIAC/atg.032
| date=February 2013
| url=http://cdiac.ornl.gov/pns/current_ghg.html
}}, on {{harvnb|CDIAC|2013}}. Details on copyright status: {{citation
| title=Frequently Asked Global Change Questions
| at=[http://cdiac.ornl.gov/faq.html#Q34 Q34: I would like to use a diagram, image, graph, table, or other materials from the CDIAC Web site. How can I obtain permission? Are there copyright restrictions?]
| url=http://cdiac.ornl.gov/faq.html
| accessdate=2012-09-26
}}, on {{harvnb|CDIAC|2013}}. "All of the reports, graphics, data, and other information on the CDIAC Web site are freely and publicly available without copyright restrictions. However as a professional courtesy, we ask that the original data source be acknowledged."
</ref>
{| summary=Atmospheric GHG concentrations have increased above the [[Pre-industrial society|pre-industrial]] level (1750)
! Gas !! Pre-1750 [[tropospheric]] concentration<ref name="blasing pre 1750 ghg conc">
{{citation
| at=[http://www.grida.no/climate/ipcc_tar/wg1/130.htm Table 4.1]
| chapter=Ch 4. Atmospheric Chemistry and Greenhouse Gases
| author=Ehhalt, D., ''et al.''
}}, in {{harvnb|IPCC TAR WG1|2001|pp=244–245}}. Referred to by: {{citation
| title=Current Greenhouse Gas Concentrations
| author=Blasing, T.J.
| doi=10.3334/CDIAC/atg.032
| date=February 2013
| url=http://cdiac.ornl.gov/pns/current_ghg.html
}}, on {{harvnb|CDIAC|2013}}. Based on Blasing ''et al.'' (2013): Pre-1750 concentrations of CH4,N2O and current concentrations of O3, are taken from Table 4.1 (a) of the IPCC Intergovernmental Panel on Climate Change), 2001. Following the convention of IPCC (2001), inferred global-scale trace-gas concentrations from prior to 1750 are assumed to be practically uninfluenced by human activities such as increasingly specialized [[agriculture]], [[land clearing]], and combustion of fossil fuels. Preindustrial concentrations of industrially manufactured compounds are given as zero. The short atmospheric lifetime of ozone (hours-days) together with the spatial variability of its sources precludes a globally or vertically homogeneous distribution, so that a fractional unit such as parts per billion would not apply over a range of altitudes or geographical locations. Therefore a different unit is used to integrate the varying concentrations of ozone in the vertical dimension over a unit area, and the results can then be averaged globally. This unit is called a [[Dobson Unit]] (D.U.), after G. M. B. Dobson, one of the first investigators of atmospheric ozone. A Dobson unit is the amount of ozone in a column which, unmixed with the rest of the atmosphere, would be 10 micrometers thick at standard temperature and pressure.
</ref>!! Recent [[tropospheric]] concentration<ref>Because atmospheric concentrations of most gases tend to vary systematically over the course of a year, figures given represent averages over a 12-month period for all gases except ozone (O3), for which a current global value has been estimated (IPCC, 2001, Table 4.1a). {{CO2}} averages for year 2012 are taken from the National Oceanic and Atmospheric Administration, Earth System Research Laboratory, web site: www.esrl.noaa.gov/gmd/ccgg/trends maintained by Dr. Pieter Tans. For other chemical species, the values given are averages for 2011. These data are found on the CDIAC AGAGE web site: http://cdiac.ornl.gov/ndps/alegage.html or the AGAGE home page: http://agage.eas.gatech.edu.
</ref>!! Absolute increase since 1750 !! Percentage increase since 1750 !! Increased radiative forcing (W/m2)<ref name="forster table of ghg conc">
{{citation
| at=[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-3.html Table 2.1]
| chapter=Ch 2: Changes in Atmospheric Constituents and in Radiative Forcing
| author=Forster, P., ''et al.''
}}, in {{harvnb|IPCC AR4 WG1|2007|p=141}}. Referred to by: {{citation
| title=Current Greenhouse Gas Concentrations
| author=Blasing, T.J.
| doi=10.3334/CDIAC/atg.032
| date=February 2013
| url=http://cdiac.ornl.gov/pns/current_ghg.html
}}, on {{harvnb|CDIAC|2013}}. For the latest updates, see the NOAA Annual Greenhouse Gas Index at: [http://www.esrl.noaa.gov/gmd/aggi].
</ref>
|-
| [[Carbon dioxide in Earth's atmosphere|Carbon dioxide]] ({{CO2}}) || 280 [[parts-per notation|ppm]]<ref>
{{citation
| at=[http://www.grida.no/climate/ipcc_tar/wg1/096.htm Executive summary]
| chapter=Ch 3. The Carbon Cycle and Atmospheric Carbon Dioxide
| author=Prentice, I.C., ''et al.''
}}, in {{harvnb|IPCC TAR WG1|2001|p=185}}. Referred to by: {{citation
| title=Current Greenhouse Gas Concentrations
| author=Blasing, T.J.
| doi=10.3334/CDIAC/atg.032
| date=February 2013
| url=http://cdiac.ornl.gov/pns/current_ghg.html
}}, on {{harvnb|CDIAC|2013}}
</ref> || 392.6 ppm<ref>Recent {{CO2}} concentration (392.6 ppm) is the 2012 average taken from globally averaged marine surface data given by the National Oceanic and Atmospheric Administration Earth System Research Laboratory, website: http://www.esrl.noaa.gov/gmd/ccgg/trends/index.html#global. Please read the material on that web page and reference Dr. Pieter Tans when citing this average (Dr. Pieter Tans, NOAA/ESRL http://www.esrl.noaa.gov/gmd/ccgg/trends). The oft-cited Mauna Loa average for 2012 is 393.8 ppm, which is a good approximation although typically about 1 ppm higher than the spatial average given above. Refer to http://www.esrl.noaa.gov/gmd/ccgg/trends for records back to the late 1950s.
</ref> || 112.6 ppm || 40.2% || 1.85
|-
| [[Methane]] ({{chem|CH|4}}) || 700 ppb<ref>ppb = parts-per-billion</ref> || 1874 ppb /<ref name="blasing ghg conc notes">The first value in a cell represents Mace Head, [[Ireland]], a mid-latitude Northern-Hemisphere site, and the second value represents [[Cape Grim]], [[Tasmania]], a mid-latitude Southern-Hemisphere site. "Current" values given for these gases are annual arithmetic averages based on monthly background concentrations for year 2011. The {{chem|SF|6}} values are from the AGAGE gas chromatography - mass spectrometer (gc-ms) Medusa measuring system. Source: Advanced Global Atmospheric Gases Experiment (AGAGE) data posted on CDIAC web site at: http://cdiac.ornl.gov/ftp/ale_gage_Agage/. These data are compiled from data on finer time scales in the ALE/GAGE/AGAGE database [http://cdiac.ornl.gov/ndps/alegage.html] (Prinn et al., 2000). These data represent the work of several investigators at various institutions; guidelines on citing the various parts of the AGAGE database are found within the ALE/GAGE/AGAGE database, see: [http://cdiac.ornl.gov/ftp/ale_gage_Agage/].
</ref> 1758 ppb<ref name="blasing ghg conc notes"/> || 1174 ppb / 1058 ppb || 167.7% / 151.1% || 0.51
|-
| [[Nitrous oxide]] ({{chem|N|2|O}}) || 270 ppb<ref name="forster table of ghg conc"/><ref>The pre-1750 value for {{chem|N|2|O}} is consistent with ice-core records from 10,000 B.C.E. through 1750 C.E.: {{citation
| at=[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-human-and.html Figure SPM.1]
| chapter=Summary for policymakers
| author=IPCC
}}, in {{harvnb|IPCC AR4 WG1|2007|p=3}}. Referred to by: {{citation
| title=Current Greenhouse Gas Concentrations
| author=Blasing, T.J.
| doi=10.3334/CDIAC/atg.032
| date=February 2013
| url=http://cdiac.ornl.gov/pns/current_ghg.html
}}, on {{harvnb|CDIAC|2013}}
</ref> || 324 ppb /<ref name="blasing ghg conc notes"/> 323 ppb<ref name="blasing ghg conc notes"/> || 54 ppb / 53 ppb || 20.0% / 19.6% || 0.18
|-
| [[Tropospheric]] [[ozone]] ({{chem|O|3}}) || 25 ppb<ref name="blasing pre 1750 ghg conc"/> || 34 ppb<ref name="blasing pre 1750 ghg conc"/> || 9 ppb || 36% || 0.35<ref>Changes in [[stratospheric]] ozone have resulted in a decrease in radiative forcing of 0.05 W/m2: {{citation
| at=[http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-9-3.html Table 2.12]
| chapter=Ch 2: Changes in Atmospheric Constituents and in Radiative Forcing
| author=Forster, P., ''et al.''
}}, in {{harvnb|IPCC AR4 WG1|2007|p=204}}. Referred to by: {{citation
| title=Current Greenhouse Gas Concentrations
| author=Blasing, T.J.
| doi=10.3334/CDIAC/atg.032
| date=February 2013
| url=http://cdiac.ornl.gov/pns/current_ghg.html
}}, on {{harvnb|CDIAC|2013}}
</ref>
|}

{| class="wikitable"
|+ Relevant to radiative forcing and/or [[ozone depletion]]; all of the following have no natural sources and hence zero amounts pre-industrial<ref name="blasing ghg concentrations"/>
{| summary=Atmospheric concentrations of these gases are very low, but these gases are extremely effective at trapping heat
! Gas !! Recent [[tropospheric]] concentration !! Increased radiative forcing (W/m2)
|-
| [[CFC-11]] (trichlorofluoromethane) ({{chem|CCl|3|F}}) || 238 ppt / 236 ppt || 0.060
|-
| [[CFC-12]] ({{chem|CCl|2|F|2}}) || 531 ppt / 529 ppt || 0.17
|-
| [[CFC-113]] ({{chem|Cl|2|FC-CClF|2}}) || 75 ppt / 75 ppt || 0.024
|-
| [[HCFC-22]] ({{chem|CHClF|2}}) || 226 ppt / 203 ppt || 0.041
|-
| [[HCFC-141b]] ({{chem|CH|3|CCl|2|F}}) || 23 ppt / 20 ppt || 0.0025
|-
| [[HCFC-142b]] ({{chem|CH|3|CClF|2}}) || 23 ppt / 21 ppt || 0.0031
|-
| [[Halon 1211]] ({{chem|CBrClF|2}}) || 4.2 ppt / 4.0 ppt || 0.001
|-
| [[Halon 1301]] ({{chem|CBrClF|3}}) || 3.3 ppt / 3.2 ppt || 0.001
|-
| [[HFC-134a]] ({{chem|CH|2|FCF|3}}) || 68 ppt / 58 ppt || 0.0055
|-
| [[Carbon tetrachloride]] ({{chem|CCl|4}}) || 86 ppt / 84 ppt || 0.012
|-
| [[Sulfur hexafluoride]] ({{chem|SF|6}}) || 7.47 ppt /<ref name="cdiac sf6">For {{chem|SF|6}} data from January 2004 onward see: [http://cdiac.ornl.gov/ftp/ale_gage_Agage/AGAGE/gc-ms-medusa/monthly]. For data from 1995 through 2004, see the National Oceanic and Atmospheric Administration (NOAA), Halogenated and other Atmospheric Trace Species (HATS) site at: [http://www.esrl.noaa.gov/gmd/hats/airborne/index.html]. Concentrations of {{chem|SF|6}} from 1970 through 1999, obtained from Antarctic [[firn]] (consolidated deep snow) air samples, can be found in [http://cdiac.ornl.gov/trends/otheratg/sturges/sturges.html W. T. Sturges et al.]
</ref> 7.09 ppt<ref name="cdiac sf6"/> || 0.0029
|-
| Other [[halocarbon]]s || Varies by substance || collectively 0.021
|-
| Halocarbons in total || || 0.323
|}
[[File:Vostok Petit data.svg|thumb|alt=refer to caption and article text|right|400,000 years of ice core data.]]
[[Ice core]]s provide evidence for greenhouse gas concentration variations over the past 800,000 years (see the [[greenhouse gas#Ice cores|following section]]). Both {{CO2}} and {{chem|CH|4}} vary between glacial and interglacial phases, and concentrations of these gases correlate strongly with temperature. Direct data does not exist for periods earlier than those represented in the ice core record, a record that indicates {{CO2}} [[mole fraction]]s stayed within a range of 180 ppm to 280 ppm throughout the last 800,000 years, until the increase of the last 250 years. However, various proxies and modeling suggests larger variations in past epochs; 500 million years ago {{CO2}} levels were likely 10 times higher than now.<ref>[[:Image:Phanerozoic Carbon Dioxide.png]]</ref> Indeed higher {{CO2}} concentrations are thought to have prevailed throughout most of the [[Phanerozoic]] eon, with concentrations four to six times current concentrations during the Mesozoic era, and ten to fifteen times current concentrations during the early Palaeozoic era until the middle of the [[Devonian]] period, about 400 [[annum|Ma]].<ref name=Berner1994>{{cite journal
|last=Berner |first=Robert A. |date=January 1994|title=GEOCARB II: a revised model of atmospheric {{CO2}} over Phanerozoic time |url=http://www.neotrucks.com/pdf/01.1994.02berner.pdf |journal=American Journal of Science |volume=294 |issue=1 |pages=56–91 |doi=10.2475/ajs.294.1.56 }}</ref><ref name=Royeretal2001>{{cite journal |last=Royer |first=DL |coauthors=RA Berner and DJ Beerling |year=2001 |title= Phanerozoic atmospheric {{CO2}} change: evaluating geochemical and paleobiological approaches |journal=Earth-Science Reviews |volume=54 |pages=349–92 |doi=10.1016/S0012-8252(00)00042-8 |bibcode=2001ESRv...54..349R |issue=4}}</ref><ref name=Berner&Kothavala2001>{{cite journal |last=Berner |first=Robert A. |coauthors=Kothavala, Zavareth |year=2001 |title=GEOCARB III: a revised model of atmospheric {{CO2}} over Phanerozoic time |url=http://www.geology.yale.edu/~ajs/2001/Feb/qn020100182.pdf |journal=American Journal of Science |volume=301 |issue=2 |pages=182–204 |doi=10.2475/ajs.301.2.182}}</ref> The spread of land plants is thought to have reduced {{CO2}} concentrations during the late Devonian, and plant activities as both sources and sinks of {{CO2}} have since been important in providing stabilising feedbacks.<ref name=Beerling2005>{{cite journal |last=Beerling |first=DJ |coauthors=Berner, RA |year=2005 |title=Feedbacks and the co-evolution of plants and atmospheric {{CO2}} |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=102 |pages=1302–5 |doi=10.1073/pnas.0408724102 |pmid=15668402 |issue=5 |pmc=547859|bibcode = 2005PNAS..102.1302B }}</ref>
Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator ([[Snowball Earth]]) appears to have been ended suddenly, about 550 Ma, by a colossal volcanic outgassing that raised the {{CO2}} concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as [[limestone]] at the rate of about 1 mm per day.<ref name=Hoffmannetal1998>{{cite journal |last=Hoffmann |first=PF |coauthors=AJ Kaufman, GP Halverson, DP Schrag|year=1998 |title=A neoproterozoic snowball earth |url=http://www.sciencemag.org/cgi/content/full/281/5381/1342 |journal=Science |volume=281| issue=5381 |pages=1342–6 |doi=10.1126/science.281.5381.1342 |pmid=9721097 |bibcode=1998Sci...281.1342H}}</ref> This episode marked the close of the Precambrian eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic carbon dioxide emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are only about 1% of emissions from human sources.<ref name=Hoffmannetal1998 /><ref name=gerlach1991>{{cite journal |last=Gerlach |first=TM |year=1991 |title=Present-day {{CO2}} emissions from volcanoes |journal=Transactions of the American Geophysical Union |volume=72 |pages=249–55 |doi=10.1029/90EO10192 |bibcode=1991EOSTr..72..249. |issue=23}}</ref><ref>See also: {{cite web|url=http://www.usgs.gov/newsroom/article.asp?ID=2827&from=rss_home#.UHvSclFA92M
|title=U.S. Geological Survey
|date=14 June 2011
|accessdate=15 October 2012
}}</ref>

===Ice cores===

[[Carbon dioxide in the Earth's atmosphere#Past variation|Measurements from Antarctic ice cores]]
show that before industrial emissions started atmospheric {{CO2}} mole fractions were about 280 [[parts per million]] (ppm), and stayed between 260 and 280 during the preceding ten thousand years.<ref>{{cite journal|doi=10.1029/2001GB001417|title=High-resolution Holocene {{chem|N|2|O}} ice core record and its relationship with {{chem|CH|4}} and {{CO2}}|year=2002|last1=Flückiger|first1=Jacqueline|journal=Global Biogeochemical Cycles|volume=16|page=1010|bibcode=2002GBioC..16a..10F}}</ref> Carbon dioxide mole fractions in the atmosphere have gone up by approximately 35 percent since the 1900s, rising from 280 parts per million by volume to 387 parts per million in 2009. One study using evidence from [[stomata]] of fossilized leaves suggests greater variability, with carbon dioxide mole fractions above 300 ppm during the period seven to ten thousand years ago,<ref>{{cite journal |author=Friederike Wagner, Bent Aaby and Henk Visscher |title=Rapid atmospheric {{CO2}} changes associated with the 8,200-years-B.P. cooling event |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=99 |issue=19 |year=2002 |pages=12011–4 |doi=10.1073/pnas.182420699 |pmid=12202744 |pmc=129389|bibcode = 2002PNAS...9912011W }}</ref> though others have argued that these findings more likely reflect calibration or contamination problems rather than actual {{CO2}} variability.<ref>{{cite journal |author=Andreas Indermühle, Bernhard Stauffer, Thomas F. Stocker |title=Early Holocene Atmospheric {{CO2}} Concentrations |journal=Science |volume=286 |issue=5446 |year=1999 |page=1815 |doi=10.1126/science.286.5446.1815a}} {{cite web|title=Early Holocene atmospheric {{CO2}} concentrations|work=Science|url=http://www.sciencemag.org/cgi/content/full/286/5446/1815a|accessdate = 26 May 2005}}</ref><ref>{{cite journal|author=H.J. Smith, M Wahlen and D. Mastroianni| title=The {{CO2}} concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition| journal=Geophysical Research Letters| volume=24| issue=1| year=1997| pages=1–4| doi=10.1029/96GL03700| bibcode=1997GeoRL..24....1S}}</ref> Because of the way air is trapped in ice (pores in the ice close off slowly to form bubbles deep within the firn) and the time period represented in each ice sample analyzed, these figures represent averages of atmospheric concentrations of up to a few centuries rather than annual or decadal levels.

===Changes since the Industrial Revolution===

[[File:CO2 increase rate.png|thumb|left|alt=Refer to caption|Recent year-to-year increase of atmospheric {{CO2}}.]]
[[File:Major greenhouse gas trends.png|alt=Refer to caption|thumb|right|Major greenhouse gas trends.]]

Since the beginning of the [[Industrial Revolution]], the concentrations of most of the greenhouse gases have increased. For example, the mole fraction of carbon dioxide has increased from 280 ppm by about 36% to 380 ppm, or 100 ppm over modern pre-industrial levels. The first 50 ppm increase took place in about 200 years, from the start of the Industrial Revolution to around 1973.{{citation needed|date=May 2013|reason=next cite only covers most recent 50 years}}; however the next 50 ppm increase took place in about 33 years, from 1973 to 2006.<ref>{{cite web|url=http://cdiac.ornl.gov/trends/co2/graphics/mlo145e_thrudc04.pdf|title=Monthly Average Carbon Dioxide Concentration, Mauna Loa Observatory|year=2005|publisher=Carbon Dioxide Information Analysis Center|format=PDF|accessdate=14 December 2008}}</ref>

Recent data also shows that the concentration is increasing at a higher rate. In the 1960s, the average annual increase was only 37% of what it was in 2000 through 2007.<ref>Dr. Pieter Tans (3 May 2008) [ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/co2_gr_mlo.txt "Annual {{CO2}} mole fraction increase (ppm)" for 1959–2007] [[National Oceanic and Atmospheric Administration]] Earth System Research Laboratory, Global Monitoring Division ([http://www.esrl.noaa.gov/gmd/ccgg/trends/ additional details]; see also {{cite journal |author=K.A. Masarie, P.P. Tans |year=1995 |title=Extension and integration of atmospheric carbon dioxide data into a globally consistent measurement record|journal=J. Geophys. Research |volume=100 |pages=11593–610 |doi=10.1029/95JD00859 |bibcode=1995JGR...10011593M}}</ref>

Today, the stock of carbon in the atmosphere increases by more than 3 million tonnes per annum (0.04%) compared with the existing stock.{{clarify|date=April 2011|reason=in what year was this measurement made?}} This increase is the result of human activities by burning fossil fuels, deforestation and forest degradation in tropical and boreal regions.<ref name="Tarziu2011">
{{cite journal
|author= Dumitru-Romulus Târziu, Victor-Dan Păcurar
|title= Pădurea, climatul și energia
|language= {{ro icon}}
|journal= [[Revista pădurilor|Rev. pădur.]]
|issn= 1583-7890
|volume= 126
|issue= 1
|pages= 34–39
|year= 2011
|month= Jan
|url= http://www.revistapadurilor.ro/(16720)
|id= 16720
|accessdate= 2012-06-11 }}(webpage has a translation button)
</ref>

The other greenhouse gases produced from human activity show similar increases in both amount and rate of increase. Many observations are available online in a variety of [[Atmospheric Chemistry Observational Databases]].
{{-}}

==Anthropogenic greenhouse gases==
{{Multiple image
|direction=vertical
| align=right
| image1=NOAA Annual Greenhouse Gas Index 2012.png
| image2=Global greenhouse gas emissions by sector, 1990-2005, in carbon dioxide equivalents (EPA, 2010).png
| image3= Global Carbon Emissions.svg
| width=250
| caption1=This graph shows changes in the annual greenhouse gas index (AGGI) between 1979 and 2011.<ref name="annual greenhouse gas index">
{{cite web
| year=2012
| author=NOAA
| title= Climate Change Indicators in the United States
| at= Figure 4. The Annual Greenhouse Gas Index, 1979–2011
| publisher=NOAA
| url=http://www.esrl.noaa.gov/gmd/aggi/
}}
</ref> The AGGI measures the levels of greenhouse gases in the atmosphere based on their ability to cause changes in the Earth's climate.<ref name="annual greenhouse gas index"/>
| caption2=This bar graph shows global greenhouse gas emissions by sector from 1990 to 2005, measured in [[carbon dioxide equivalent]]s.<ref>
{{cite web
| year=2010
| author=US Environmental Protection Agency (EPA)
| title= Climate Change Indicators in the United States
| at= Figure 2. Global Greenhouse Gas Emissions by Sector, 1990–2005
| publisher=EPA
| url= http://www.epa.gov/climatechange/science/indicators/ghg/global-ghg-emissions.html
}}
</ref>
| caption3=Modern global anthropogenic [[carbon]] emissions.
}}

Since about 1750 human activity has increased the concentration of carbon dioxide and other greenhouse gases. Measured atmospheric concentrations of carbon dioxide are currently 100 ppm higher than pre-industrial levels.<ref>{{cite web| url=http://www.grida.no/climate/ipcc_tar/wg1/fig6-6.htm|title=Climate Change 2001: Working Group I: The Scientific Basis: figure 6-6| accessdate=1 May 2006}}</ref> Natural sources of carbon dioxide are more than 20 times greater than sources due to human activity,<ref>{{cite web|url=http://www.grida.no/climate/vital/13.htm |title=The present carbon cycle — Climate Change |publisher=Grida.no |date= |accessdate=2010-10-16}}</ref> but over periods longer than a few years natural sources are closely balanced by natural sinks, mainly photosynthesis of carbon compounds by plants and marine plankton. As a result of this balance, the atmospheric mole fraction of carbon dioxide remained between 260 and 280 parts per million for the 10,000 years between the end of the last glacial maximum and the start of the industrial era.<ref name="IPCC AR4 WG1 Chapter 7" />

It is likely that anthropogenic (i.e., human-induced) warming, such as that due to elevated greenhouse gas levels, has had a discernible influence on many physical and biological systems.<ref>
{{cite book
|year=2007d
|contribution=6.1 Observed changes in climate and their effects, and their causes
|title=6 Robust findings, key uncertainties
|series=Climate Change 2007: Synthesis Report. A Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC)
|publisher=IPCC
|location= Geneva, Switzerland
|author=IPCC
|url=http://www.ipcc.ch/publications_and_data/ar4/syr/en/mains6-1.html
}}
</ref> Future warming is projected to have a range of [[effects of global warming|impacts]], including [[sea level rise]],<ref name="projected impacts of climate change">
{{cite book
|year=2007d
|contribution=6.2 Drivers and projections of future climate changes and their impacts
|title=6 Robust findings, key uncertainties
|series=Climate Change 2007: Synthesis Report. A Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC)
|publisher=IPCC
|location= Geneva, Switzerland
|author=IPCC
|url=http://www.ipcc.ch/publications_and_data/ar4/syr/en/mains6-2.html
}}
</ref> increased frequencies and severities of some [[extreme weather]] events,<ref name="projected impacts of climate change"/> loss of [[climate change and ecosystems|biodiversity]],<ref name="projected impacts of climate change by sector">
{{cite book
|year=2007d
|contribution=3.3.1 Impacts on systems and sectors
|title=3 Climate change and its impacts in the near and long term under different scenarios
|series=Climate Change 2007: Synthesis Report. A Contribution of Working Groups I, II, and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC)
|publisher=IPCC
|location= Geneva, Switzerland
|author=IPCC
|url=http://www.ipcc.ch/publications_and_data/ar4/syr/en/mains3-3-1.html
}}
</ref> and regional changes in [[climate change and agriculture|agricultural productivity]].<ref name="projected impacts of climate change by sector"/>

The main sources of greenhouse gases due to human activity are:
*burning of [[fossil fuel]]s and [[deforestation]] leading to higher carbon dioxide concentrations in the air. Land use change (mainly deforestation in the tropics) account for up to one third of total anthropogenic {{CO2}} emissions.<ref name="IPCC AR4 WG1 Chapter 7">{{Cite book
| author=IPCC
| authorlink=Intergovernmental Panel on Climate Change
| coauthors=Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)
| title=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
| year=2007
| publisher=[[Cambridge University Press]]
| location= [[Cambridge]], United Kingdom and [[New York City|New York, NY]], USA
| isbn=978-0-521-88009-1
| chapter=Chapter 7. Couplings Between Changes in the Climate System and Biogeochemistry
| chapter-url=http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter7.pdf
| accessdate=13 May 2008
}}</ref>
*livestock [[enteric fermentation]] and manure management,<ref name=livestock>H. Steinfeld, P. Gerber, T. Wassenaar, V. Castel, M. Rosales, C. de Haan (2006) [http://www.fao.org/docrep/010/a0701e/a0701e00.htm Livestock’s long shadow. Environmental issues and options.] FAO Livestock, Environment and Development (LEAD) Initiative.</ref> paddy [[rice]] farming, land use and wetland changes, pipeline losses, and covered vented landfill emissions leading to higher methane atmospheric concentrations. Many of the newer style fully vented septic systems that enhance and target the fermentation process also are sources of [[atmospheric methane]].
*use of chlorofluorocarbons (CFCs) in [[refrigeration]] systems, and use of CFCs and [[Halomethane#Fire extinguishing|halons]] in [[fire extinguisher|fire suppression]] systems and manufacturing processes.
*agricultural activities, including the use of fertilizers, that lead to higher nitrous oxide ({{chem|N|2|O}}) concentrations.

The seven sources of {{CO2}} from fossil fuel combustion are (with percentage contributions for 2000–2004):<ref name=Raupach/>
{| class="wikitable" style="text-align:center"
|-
! Seven main fossil fuel combustion sources !! Contribution (%)
|-
| style="text-align:left;"| Liquid fuels (e.g., [[gasoline]], [[fuel oil]]) || 36%
|-
| style="text-align:left;"| Solid fuels (e.g., [[coal]]) || 35%
|-
| style="text-align:left;"| Gaseous fuels (e.g., [[natural gas]]) || 20%
|-
| style="text-align:left;"| [[Cement]] production ||  3 %
|-
| style="text-align:left;"| [[Gas flare|Flaring]] gas industrially and at wells || < 1%  
|-
| style="text-align:left;"| Non-fuel hydrocarbons || < 1%  
|-
| style="text-align:left;"| "International [[bunker fuel]]s" of transport not included in national inventories<ref name="Schrooten2">{{cite journal
| last = Schrooten
| first = L
| title = Inventory and forecasting of maritime emissions in the Belgian sea territory, an activity based emission model
| journal = Atmospheric Environment - 42(4)667-676(2008)
| volume = 42
| issue = 4
| pages = 667–676
| year = 2008
| url =
| doi =
| pmid =
| month =
| last2 = De Vlieger
| first2 = Ina
| last3 = Int Panis
| first3 = Luc
| last4 = Styns, R. Torfs
| first4 = K
| last5 = Torfs
| first5 = R}}</ref> ||  4 %
|}

[[Carbon dioxide]], [[methane]], [[nitrous oxide]] ({{chem|N|2|O}}) and three groups of [[fluorinated gases]] ([[sulfur hexafluoride]] ({{chem|SF|6}}), [[hydrofluorocarbons]] (HFCs), and [[perfluorocarbons]] (PFCs)) are the major anthropogenic greenhouse gases,<ref name="grubb kyoto protocol">
{{cite journal
|title=The economics of the Kyoto protocol
|author=Grubb, M.
|date=July–September 2003
|journal=World Economics
|volume=4
|issue=3
|url=http://www.econ.cam.ac.uk/rstaff/grubb/publications/J36.pdf
|format=PDF
}}
</ref>{{Rp|147}}<ref name="Environmental issues: essential primary sources.">
{{cite web|last = Lerner & K. Lee Lerner|first = Brenda Wilmoth|year = 2006
|url = http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=1&DB=local&CMD=010a+2006000857&CNT=10+records+per+page|title = Environmental issues: essential primary sources
|publisher = Thomson Gale
|accessdate = 11 September 2006
}}
</ref> and are regulated under the [[Kyoto Protocol]] international [[treaty]], which came into force in 2005.<ref name="unfccc kyoto protocol">
{{cite web
| title=Kyoto Protocol
| url=http://unfccc.int/kyoto_protocol/items/2830.php
| publisher=United Nations Framework Convention on Climate Change
| at=Home > Kyoto Protocol
}}
</ref> Emissions limitations specified in the Kyoto Protocol expire in 2012.<ref name="unfccc kyoto protocol"/> The [[2010 United Nations Climate Change Conference|Cancún agreement]], agreed in 2010, includes voluntary pledges made by 76 countries to control emissions.<ref name="cancun agreement">
{{citation
| date=July 2011
| author=King, D., ''et al.''
| chapter=Copenhagen and Cancun
| title=International climate change negotiations: Key lessons and next steps
| publisher=Smith School of Enterprise and the Environment, University of Oxford
| location=Oxford, UK
| page=12
| doi=10.4210/ssee.pbs.2011.0003
| url=http://edition2a.intellimag.com/?id=ssee-july2011
}} PDF version is also [http://www.smithschool.ox.ac.uk/wp-content/uploads/2011/03/Climate-Negotiations-report_Final.pdf available]
</ref> At the time of the agreement, these 76 countries were collectively responsible for 85% of annual global emissions.<ref name="cancun agreement"/>

Although [[CFCs]] are greenhouse gases, they are regulated by the [[Montreal Protocol]], which was motivated by CFCs' contribution to [[ozone depletion]] rather than by their contribution to global warming. Note that ozone depletion has only a minor role in greenhouse warming though the two processes often are confused in the media.

===Sectors===
{{Expand section|1=Information on emissions from other sectors|date=July 2013}}
; Tourism

According to [[UNEP]] global [[tourism]] is closely linked to [[climate change]]. Tourism is a significant contributor to the increasing concentrations of greenhouse gases in the atmosphere. Tourism accounts for about 50% of traffic movements. Rapidly expanding air traffic contributes about 2.5% of the production of {{CO2}}. The number of international travelers is expected to increase from 594 million in 1996 to 1.6 billion by 2020, adding greatly to the problem unless steps are taken to reduce emissions.<ref>[http://www.unep.org/resourceefficiency/Business/SectoralActivities/Tourism/TheTourismandEnvironmentProgramme/FactsandFiguresaboutTourism/ImpactsofTourism/EnvironmentalImpacts/EnvironmentalImpactsofTourism-GlobalLevel/tabid/78777/Default.aspx Environmental Impacts of Tourism - Global Level] UNEP</ref>

==Role of water vapor==
[[File:BAMS climate assess boulder water vapor 2002.png|thumb|350px|Increasing water vapor in the stratosphere at Boulder, Colorado.]]

[[Water vapor]] accounts for the largest percentage of the greenhouse effect, between 36% and 66% for clear sky conditions and between 66% and 85% when including clouds.<ref name="realclimate.org"/> Water vapor concentrations fluctuate regionally, but human activity does not significantly affect water vapor concentrations except at local scales, such as near irrigated fields. The atmospheric concentration of vapor is highly variable and depends largely on temperature, from less than 0.01% in extremely cold regions up to 3% by mass at in saturated air at about 32 °C.(see [[Relative humidity#other important facts]]) <ref>{{cite book |chapter=The greenhouse effect and climate change |author=Evans, Kimberly Masters |title=The environment: a revolution in attitudes |publisher=Thomson Gale |location=Detroit |year=2005 |isbn=0-7876-9082-1 }}</ref>

The average residence time of a water molecule in the atmosphere is only about nine days, compared to years or centuries for other greenhouse gases such as {{chem|CH|4}} and {{CO2}}.<ref>{{cite web | url=http://www.epa.gov/climatechange/Downloads/ghgemissions/US-GHG-Inventory-2012-Main-Text.pdf | title=INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS AND SINKS: 1990–2010 | date=15 April 2012 | publisher=U.S. Environmental Protection Agency | accessdate=2 June 2012 | page=1.4}}</ref> Thus, water vapor responds to and amplifies effects of the other greenhouse gases. The [[Clausius-Clapeyron relation]] establishes that more water vapor will be present per unit volume at elevated temperatures. This and other basic principles indicate that warming associated with increased concentrations of the other greenhouse gases also will increase the concentration of water vapor (assuming that the [[relative humidity]] remains approximately constant; modeling and observational studies find that this is indeed so). Because water vapor is a greenhouse gas, this results in further warming and so is a "[[positive feedback]]" that amplifies the original warming. Eventually other earth processes offset these positive feedbacks, stabilizing the global temperature at a new equilibrium and preventing the loss of Earth's water through a Venus-like [[runaway greenhouse effect]].<ref>{{cite doi | 10.1146/annurev.energy.25.1.441}}</ref>

==Direct greenhouse gas emissions==

Between the period 1970 to 2004, GHG emissions (measured in [[CO2 equivalent|{{CO2}}-equivalent]])<ref>Includes the [[Kyoto Protocol|Kyoto "basket"]] of GHGs</ref> increased at an average rate of 1.6% per year, with {{CO2}} emissions from the use of fossil fuels growing at a rate of 1.9% per year.<ref name="rogner summary of emission trends">
{{cite book
|year=2007
|author=Rogner, H.-H., D. Zhou, R. Bradley. P. Crabbé, [[Ottmar Edenhofer|O. Edenhofer]], B.Hare, L. Kuijpers, M. Yamaguchi
|contribution=Executive Summary |title= Introduction
|series=Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
|editor= B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer
|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch1s1-es.html
|publisher=Cambridge University Press
|isbn=978-0-521-88011-4
}}</ref><ref>
{{cite book
|year=2007
|author=Rogner, H.-H., D. Zhou, R. Bradley. P. Crabbé, O. Edenhofer, B.Hare, L. Kuijpers, M. Yamaguchi
|contribution=1.3.1 Review of the last three decades |title= Introduction
|series=Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
|editor= B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer
|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch1s1-3.html#1-3-1
|publisher=Cambridge University Press
|isbn=978-0-521-88011-4
}} This citation clarifies the time period (1970-2004) for the observed emissions trends</ref> Total anthropogenic emissions at the end of 2009 were estimated at 49.5 [[giga-|gigatonnes]] {{CO2}}-equivalent.<ref name="Bridging the Emissions Gap">
{{citation
| date=November 2011
| author=UNEP
| title=Bridging the Emissions Gap: A UNEP Synthesis Report
| url=http://www.unep.org/pdf/UNEP_bridging_gap.pdf
| format=PDF
| publisher=United Nations Environment Programme (UNEP)
| location=[[Nairobi]], [[Kenya]]
| isbn=978-92-807-3229-0
}} UNEP Stock Number: DEW/1470/NA</ref>{{Rp|15}} These emissions include {{CO2}} from fossil fuel use and from land use, as well as emissions of methane, nitrous oxide and other GHGs covered by the [[Kyoto Protocol]].

At present, the two primary sources of {{CO2}} emissions are from burning [[coal]] used for electricity generation and [[petroleum]] used for motor transport.{{Citation needed|date=February 2012}}

===Regional and national attribution of emissions===
{{See also|Kyoto Protocol and government action}}
[[File:Greenhouse Gas by Sector.png|thumb|This figure shows the relative fraction of man-made greenhouse gases coming from each of eight categories of sources, as estimated by the Emission Database for Global Atmospheric Research version 3.2, fast track 2000 project [1]. These values are intended to provide a snapshot of global annual greenhouse gas emissions in the year 2000. The top panel shows the sum over all man-made greenhouse gases, weighted by their global warming potential over the next 100 years. This consists of 72% carbon dioxide, 18% methane, 8% nitrous oxide and 1% other gases. Lower panels show the comparable information for each of these three primary greenhouse gases, with the same coloring of sectors as used in the top chart. Segments with less than 1% fraction are not labeled.<ref>https://commons.wikimedia.org/wiki/File:Greenhouse_Gas_by_Sector.png</ref>]]
There are several different ways of measuring GHG emissions, for example, see World Bank (2010)<ref name="world bank emissions data">
{{cite book
|year=2010
|author=World Bank
|at=Tables A1 and A2
|chapter=Selected Development Indicators
|chapter-url=http://siteresources.worldbank.org/INTWDRS/Resources/477365-1327504426766/8389626-1327510418796/Statistical-Annex.pdf
|format=PDF
|title=World Development Report 2010: Development and Climate Change
|publisher=The International Bank for Reconstruction and Development / The World Bank
|location=Washington DC, USA
|url=http://go.worldbank.org/CHZJNP7X30
|isbn=9780821379875
|doi=10.1596/978-0-8213-7987-5
}}
</ref>{{Rp|362}} for tables of national emissions data. Some variables that have been reported<ref name="sapiens.revues.org">{{cite web|url=http://sapiens.revues.org/index854.html |author=Bader, N. and Bleichwitz, R. |year= 2009 |title=Measuring urban greenhouse gas emissions: The challenge of comparability. ''S.A.P.I.EN.S.'' '''2''' (3) |publisher=Sapiens.revues.org |accessdate=2011-09-11}}</ref> include:

* Definition of measurement boundaries: Emissions can be attributed geographically, to the area where they were emitted (the territory principle) or by the activity principle to the territory produced the emissions. These two principles result in different totals when measuring, for example, electricity importation from one country to another, or emissions at an international airport.
* Time horizon of different GHGs: Contribution of a given GHG is reported as a {{CO2}} equivalent. The calculation to determine this takes into account how long that gas remains in the atmosphere. This is not always known accurately and calculations must be regularly updated to reflect new information.
* What sectors are included in the calculation (e.g., energy industries, industrial processes, agriculture etc.): There is often a conflict between transparency and availability of data.
* The measurement protocol itself: This may be via direct measurement or estimation. The four main methods are the emission factor-based method, mass balance method, predictive emissions monitoring systems, and continuous emissions monitoring systems. These methods differ in accuracy, cost, and usability.

These different measures are sometimes used by different countries to assert various policy/ethical positions on climate change (Banuri ''et al.'', 1996, p. 94).<ref name=banuri>{{cite book
|year=1996
|author=Banuri, T.
|title=Equity and social considerations. In: Climate change 1995: Economic and social dimensions of climate change. Contribution of Working Group III to the Second Assessment Report of the Intergovernmental Panel on Climate Change (J.P. Bruce ''et al.'' Eds.)
|publisher=This version: Printed by Cambridge University Press, Cambridge, UK, and New York, NY, USA. PDF version: IPCC website
|format=PDF
|url=http://www.ipcc.ch/ipccreports/sar/wg_III/ipcc_sar_wg_III_full_report.pdf
|isbn=978-0-521-56854-8
|doi=10.2277/0521568544
|display-authors=1}}</ref>
This use of different measures leads to a lack of comparability, which is problematic when monitoring progress towards targets. There are arguments for the adoption of a common measurement tool, or at least the development of communication between different tools.<ref name="sapiens.revues.org"/>

Emissions may be measured over long time periods. This measurement type is called historical or cumulative emissions. Cumulative emissions give some indication of who is responsible for the build-up in the atmospheric concentration of GHGs (IEA, 2007, p. 199).<ref name=iea>{{cite book
|year=2007
|author=IEA
|title=World energy outlook 2007 edition – China and India insights
|url=http://www.iea.org/publications/free_new_Desc.asp?PUBS_ID=1927
|page=600
|publisher=International Energy Agency (IEA), Head of Communication and Information Office, 9 rue de la Fédération, 75739 Paris Cedex 15, France
|isbn=978-92-64-02730-5
|accessdate=2010-05-04}}</ref>

The national accounts balance would be positively related to carbon emissions. The national accounts balance shows the difference between exports and imports. For many richer nations, such as the United States, the accounts balance is negative because more goods are imported than they are exported. This is mostly due to the fact that it is cheaper to produce goods outside of developed countries, leading the economies of developed countries to become increasingly dependent on services and not goods. We believed that a positive accounts balance would means that more production was occurring in a country, so more factories working would increase carbon emission levels.(Holtz-Eakin, 1995, pp.;85;101).<ref name=holtz-eakin>{{cite journal
|title=Stoking the fires? {{CO2}} emissions and economic growth
|author=Holtz-Eakin, D.
|year=1995
|journal=Journal of Public Economics
|volume=57
|issue=1
|pages=85–101
|url=http://www.nber.org
|accessdate=2011-04-20
|doi=10.1016/0047-2727(94)01449-X}}</ref>

Emissions may also be measured across shorter time periods. Emissions changes may, for example, be measured against a base year of 1990. 1990 was used in the [[United Nations Framework Convention on Climate Change]] (UNFCCC) as the base year for emissions, and is also used in the [[Kyoto Protocol]] (some gases are also measured from the year 1995).<ref name="grubb kyoto protocol"/>{{Rp|146,149}} A country's emissions may also be reported as a proportion of global emissions for a particular year.

Another measurement is of per capita emissions. This divides a country's total annual emissions by its mid-year population.<ref name="world bank emissions data"/>{{Rp|370}} Per capita emissions may be based on historical or annual emissions (Banuri ''et al.'', 1996, pp. 106–107).<ref name=banuri/>

===Greenhouse gas intensity and land-use change===

{{Multiple image
|direction=vertical
| align=right
| image1=GHG intensity 2000.svg
| image2=Cumulative energy-related carbon dioxide emissions between 1850-2005 for low-income, middle-income, high-income, the EU-15, and OECD countries.png
| image3=Cumulative energy-related carbon dioxide emissions between 1850-2005 for different countries.png
| image4=CO2 responsibility 1950-2000.svg
| image5=Yearly trends in annual regional carbon dioxide emissions from fuel combustion between 1971 and 2009.png
| image6=Regional trends in annual per capita carbon dioxide emissions from fuel combustion between 1971 and 2009.png
| width=180
| caption1=Greenhouse gas intensity in the year 2000, including land-use change.
| caption2=Cumulative energy-related {{CO2}} emissions between the years 1850–2005 grouped into low-income, middle-income, high-income, the [[EU-15]], and the [[OECD]] countries.
| caption3=Cumulative energy-related {{CO2}} emissions between the years 1850–2005 for individual countries.
| caption4=Map of cumulative per capita anthropogenic atmospheric {{CO2}} emissions by country. Cumulative emissions include land use change, and are measured between the years 1950 and 2000.
| caption5=Regional trends in annual {{CO2}} emissions from fuel combustion between 1971 and 2009.
| caption6=Regional trends in annual per capita {{CO2}} emissions from fuel combustion between 1971 and 2009.
}}

The first figure shown opposite is based on data from the [[World Resources Institute]], and shows a measurement of GHG emissions for the year 2000 according to [[carbon intensity|greenhouse gas intensity]] and [[land use|land-use]] change. Herzog ''et al.'' (2006, p. 3) defined greenhouse gas intensity as GHG emissions divided by economic output.<ref>{{cite book
|date=November 2006|author=Herzog, T.
|editor=Yamashita, M.B.
|title=Target: intensity — an analysis of greenhouse gas intensity targets
|url=http://pdf.wri.org/target_intensity.pdf
|format=PDF
|publisher=World Resources Institute
|isbn=1-56973-638-3
|accessdate=2011-04-11
|display-authors=1}}</ref> GHG intensities are subject to uncertainty over whether they are calculated using [[Gross domestic product#Cross-border comparison|market exchange rate]]s (MER) or [[purchasing power parity]] (PPP) (Banuri ''et al.'', 1996, p. 96).<ref name=banuri/> Calculations based on MER suggest large differences in intensities between developed and developing countries, whereas calculations based on PPP show smaller differences.

Land-use change, e.g., the clearing of forests for agricultural use, can affect the concentration of GHGs in the atmosphere by altering how much carbon flows out of the atmosphere into [[carbon sink]]s.<ref>{{cite book
|year=2007
|author=IPCC
|contribution=Annex I: Glossary J-P
|editor=B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer
|title=Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/annex1sglossary-j-p.html
|publisher=Print version: Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. This version: IPCC website
|isbn=978-0-521-88011-4
|accessdate=2011-04-11}}</ref> Accounting for land-use change can be understood as an attempt to measure “net” emissions, i.e., gross emissions from all GHG sources minus the removal of emissions from the atmosphere by carbon sinks (Banuri ''et al.'', 1996, pp. 92–93).<ref name=banuri/>

There are substantial uncertainties in the measurement of net carbon emissions.<ref>{{cite book
|year=2001
|contribution=7.3.5 Cost Implications of Alternative GHG Emission Reduction Options and Carbon Sinks
|title=Costing Methodologies
|series=Climate Change 2001: Mitigation. Contribution of Working Group III to the Third Assessment Report of the Intergovernmental Panel on Climate Change
|editor=B. Metz ''et al.''
|publisher=Print version: Cambridge University Press, Cambridge, U.K., and New York, N.Y., U.S.A.. This version: GRID-Arendal website
|isbn=978-0-521-01502-8
|doi=10.2277/0521015022
|author=Markandya, A.
|url=http://www.grida.no/climate/ipcc_tar/wg3/293.htm
|accessdate=2011-04-11
|display-authors=1}}</ref> Additionally, there is controversy over how carbon sinks should be allocated between different regions and over time (Banuri ''et al.'', 1996, p. 93).<ref name=banuri/> For instance, concentrating on more recent changes in carbon sinks is likely to favour those regions that have deforested earlier, e.g., Europe.

===Cumulative and historical emissions===

Cumulative anthropogenic (i.e., human-emitted) emissions of {{CO2}} from fossil fuel use are a major cause of [[global warming]],<ref>
{{cite journal
| year=2008
| author=Botzen, W.J.W., ''et al.''
| title=Cumulative {{CO2}} emissions: shifting international responsibilities for climate debt
| journal=Climate Policy
| volume=8
| doi=10.3763/cpol.2008.0539
| page=570
}}
</ref> and give some indication of which countries have contributed most to human-induced climate change.<ref name="hohne 2010 regional contribution to global warming">
{{cite journal
| date=24 September 2010
| author=Höhne, N., ''et al.''
| title=Contributions of individual countries’ emissions to climate change and their uncertainty
| journal=Climatic Change
| doi=10.1007/s10584-010-9930-6
| publisher=Springer Science+Business Media B.V.
| url=http://www.gcca.eu/usr/documents/Contributions_Individual_countries_201011229410.pdf
| format=PDF
}}
</ref>{{Rp|15}}

{| class="wikitable" | style="text-align:center"
|+ Top-5 historic {{CO2}} contributors by region over the years 1800 to 1988 (in %)
! Region
! Industrial {{CO2}}
! Total {{CO2}}
|-
| style="text-align:left;"| OECD North America || 33.2|| 29.7
|-
| style="text-align:left;"| OECD Europe || 26.1|| 16.6
|-
| style="text-align:left;"| Former USSR || 14.1|| 12.5
|-
| style="text-align:left;"| China ||   5.5||   6.0
|-
| style="text-align:left;"| Eastern Europe ||   5.5||   4.8
|}

The table above to the left is based on Banuri ''et al.'' (1996, p. 94).<ref name=banuri/> Overall, developed countries accounted for 83.8% of industrial {{CO2}} emissions over this time period, and 67.8% of total {{CO2}} emissions. Developing countries accounted for industrial {{CO2}} emissions of 16.2% over this time period, and 32.2% of total {{CO2}} emissions. The estimate of total {{CO2}} emissions includes [[Biota (ecology)|biotic]] carbon emissions, mainly from deforestation. Banuri ''et al.'' (1996, p. 94)<ref name=banuri/> calculated per capita cumulative emissions based on then-current population. The ratio in per capita emissions between industrialized countries and developing countries was estimated at more than 10 to 1.

Including biotic emissions brings about the same controversy mentioned earlier regarding carbon sinks and land-use change (Banuri ''et al.'', 1996, pp. 93–94).<ref name=banuri/> The actual calculation of net emissions is very complex, and is affected by how carbon sinks are allocated between regions and the dynamics of the climate system.

Non-[[OECD]] countries accounted for 42% of cumulative energy-related {{CO2}} emissions between 1890–2007.<ref name="world energy outlook 2009">
{{citation
|year=2009
|title=World Energy Outlook 2009
|publisher=IEA
|location=Paris, France
|author=International Energy Agency (IEA)
|format=PDF
|pages=179–180
|isbn=978-92-64-06130-9
|url=http://www.iea.org/textbase/nppdf/free/2009/weo2009.pdf
}}
</ref>{{Rp|179–180}} Over this time period, the US accounted for 28% of emissions; the EU, 23%; Russia, 11%; China, 9%; other OECD countries, 5%; Japan, 4%; India, 3%; and the rest of the world, 18%.<ref name="world energy outlook 2009"/>{{Rp|179–180}}

===Changes since a particular base year===

{{See also|Kyoto Protocol#Government action and emissions}}

Between 1970–2004, global growth in annual {{CO2}} emissions was driven by North America, Asia, and the Middle East.<ref>
{{citation
|year=2007
|author=Rogner, H.-H., D. Zhou, R. Bradley. P. Crabbé, O. Edenhofer, B.Hare, L. Kuijpers, M. Yamaguchi
|contribution=1.3.1 Review of the last three decades
|title= Introduction
|series=Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
|editor= B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer
|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch1s1-3.html#1-3-1
|publisher=Cambridge University Press
|isbn=978-0-521-88011-4
}}
</ref> The sharp acceleration in {{CO2}} emissions since 2000 to more than a 3% increase per year (more than 2 ppm per year) from 1.1% per year during the 1990s is attributable to the lapse of formerly declining trends in [[carbon intensity]] of both developing and developed nations. China was responsible for most of global growth in emissions during this period. Localised plummeting emissions associated with the collapse of the [[Soviet Union]] have been followed by slow emissions growth in this region due to more [[efficient energy use]], made necessary by the increasing proportion of it that is exported.<ref name=Raupach>{{cite journal
|author=Raupach, M.R.
|year=2007
|url=http://www.pnas.org/cgi/reprint/0700609104v1.pdf
|title=Global and regional drivers of accelerating {{CO2}} emissions
|journal=Proc. Natl. Acad. Sci. U.S.A.
|volume=104
|issue=24
|pages=10288–93
|doi=10.1073/pnas.0700609104
|pmid=17519334
|pmc=1876160
|bibcode=2007PNAS..10410288R
|display-authors=1
|last2=Marland
|first2=G.
|last3=Ciais
|first3=P.
|last4=Le Quere
|first4=C.
|last5=Canadell
|first5=J. G.
|last6=Klepper
|first6=G.
|last7=Field
|first7=C. B.}}</ref> In comparison, methane has not increased appreciably, and {{chem|N|2|O}} by 0.25% y−1.

Using different base years for measuring emissions has an effect on estimates of national contributions to global warming.<ref name="hohne 2010 regional contribution to global warming"/>{{Rp|17–18}}<ref>The cited paper uses the term "start date" instead of "base year."</ref> This can be calculated by dividing a country's highest contribution to global warming starting from a particular base year, by that country's minimum contribution to global warming starting from a particular base year. Choosing between different base years of 1750, 1900, 1950, and 1990 has a significant effect for most countries.<ref name="hohne 2010 regional contribution to global warming"/>{{Rp|17–18}} Within the [[G8]] group of countries, it is most significant for the UK, France and Germany. These countries have a long history of {{CO2}} emissions (see the section on [[Greenhouse gas#Cumulative and historical emissions|Cumulative and historical emissions]]).

===Annual emissions===

[[File:GHG per capita 2000.svg|thumb|180px|Per capita anthropogenic greenhouse gas emissions by country for the year 2000 including land-use change.]]

Annual per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries.<ref name="grubb kyoto protocol"/>{{Rp|144}} Due to China's fast economic development, its annual per capita emissions are quickly approaching the levels of those in the [[Kyoto Protocol#2012 emission targets and "flexible mechanisms"|Annex I group]] of the Kyoto Protocol (i.e., the developed countries excluding the USA).<ref name="pbl annual emissions in 2008">
{{cite web
|date=25 June 2009
|author=PBL
|title=Global {{CO2}} emissions: annual increase halves in 2008
|url=http://www.pbl.nl/en/publications/2009/Global-CO2-emissions-annual-increase-halves-in-2008.html
|publisher=Netherlands Environmental Assessment Agency (PBL) website
|accessdate=2010-05-05}}</ref> Other countries with fast growing emissions are [[South Korea]], Iran, and Australia. On the other hand, annual per capita emissions of the EU-15 and the USA are gradually decreasing over time.<ref name="pbl annual emissions in 2008"/> Emissions in [[Russia]] and the [[Ukraine]] have decreased fastest since 1990 due to economic restructuring in these countries.<ref>{{cite web
|date=March 2009
|title=Global Carbon Mechanisms: Emerging lessons and implications (CTC748)
|author=Carbon Trust
|url=http://www.carbontrust.com/resources/reports/advice/global-carbon-mechanisms
|publisher=Carbon Trust website
|page=24
|accessdate=2010-03-31}}</ref>

Energy statistics for fast growing economies are less accurate than those for the industrialized countries. For China's annual emissions in 2008, the [[Netherlands Environmental Assessment Agency]] estimated an uncertainty range of about 10%.<ref name="pbl annual emissions in 2008"/>

The [[GHG footprint]], or greenhouse gas footprint, refers to the amount of GHG that are emitted during the creation of products or services. It is more comprehensive than the commonly used carbon footprint, which measures only carbon dioxide, one of many greenhouse gases.

===Top emitters===

{{Multiple image
| direction=vertical
| align=right
| image1=Annual per capita carbon dioxide emissions from fuel combustion in 2009 for 140 countries.png
| image2=Cumulative energy-related per capita carbon dioxide emissions between 1850-2008 for 185 countries.png
| width=75
| caption1=Bar graph of annual per capita {{CO2}} emissions from fuel combustion for 140 countries in 2009.
| caption2=Bar graph of cumulative energy-related per capita {{CO2}} emissions between 1850–2008 for 185 countries.
}}
{{See also|List of countries by carbon dioxide emissions|List of countries by carbon dioxide emissions per capita|List of countries by greenhouse gas emissions|List of countries by greenhouse gas emissions per capita}}

====Annual====

In 2009, the annual top ten emitting countries accounted for about two-thirds of the world's annual energy-related {{CO2}} emissions.<ref>
{{citation
| year=2011
| author=International Energy Agency (IEA)
| title={{CO2}} Emissions From Fuel Combustion: Highlights (2011 edition)
| url=http://www.iea.org/publications/free_new_Desc.asp?PUBS_ID=2450
| page=9
| publisher=IEA
| location=Paris, France
}}
</ref>

{| class="wikitable" | style="text-align:center"
|+Top-10 annual energy-related {{CO2}} emitters for the year 2009<ref>
{{citation
| year=2011
| author=International Energy Agency (IEA)
| title={{CO2}} Emissions From Fuel Combustion: Highlights (2011 edition): Excel spreadsheet
| url=http://www.iea.org/co2highlights/CO2highlights.xls
| format=XLS
| publisher=IEA
| location=Paris, France
}}
</ref>
! Country
! % of global total annual emissions
! Tonnes of GHG per capita
|-
| style="text-align:left;"| [[China|People's Rep. of China]] || 23.6 || 5.13
|-
| style="text-align:left;"| United States || 17.9 || 16.9
|-
| style="text-align:left;"| India || 5.5 || 1.37
|-
| style="text-align:left;"| [[Russian Federation]] || 5.3 || 10.8
|-
| style="text-align:left;"| Japan || 3.8 || 8.6
|-
| style="text-align:left;"| Germany || 2.6 || 9.2
|-
| style="text-align:left;"| [[Iran|Islamic Rep. of Iran]] || 1.8 || 7.3
|-
| style="text-align:left;"| [[Canada]] || 1.8 || 15.4
|-
| style="text-align:left;"| [[Korea]] || 1.8 || 10.6
|-
| style="text-align:left;"| United Kingdom || 1.6 || 7.5
|}

====Cumulative====

{| class="wikitable" | style="text-align:center"
|+Top-10 cumulative energy-related {{CO2}} emitters between 1850–2008<ref>
{{citation
| year=2011
| author=World Resources Institute (WRI)
| title=Climate Analysis Indicators Tool (CAIT): Indicators: GHG Emissions: Cumulative Emissions (free registration required)
| url=http://cait.wri.org/cait.php
| publisher=WRI
| location=Washington DC, USA
}}
</ref>
! Country
! % of world total
! Metric tonnes {{CO2}} per person
|-
| style="text-align:left;"| United States || 28.5 || 1,132.7
|-
| style="text-align:left;"| China || 9.36 || 85.4
|-
| style="text-align:left;"| [[Russian Federation]] || 7.95 || 677.2
|-
| style="text-align:left;"| Germany || 6.78 || 998.9
|-
| style="text-align:left;"| United Kingdom || 5.73 || 1,127.8
|-
| style="text-align:left;"| Japan || 3.88 || 367
|-
| style="text-align:left;"| France || 2.73 || 514.9
|-
| style="text-align:left;"| India || 2.52 || 26.7
|-
| style="text-align:left;"| [[Canada]] || 2.17 || 789.2
|-
| style="text-align:left;"| [[Ukraine]] || 2.13 || 556.4
|}

===Embedded emissions===

One way of attributing greenhouse gas (GHG) emissions is to measure the [[embedded emissions]] (also referred to as "embodied emissions") of goods that are being consumed. Emissions are usually measured according to production, rather than consumption.<ref>
{{cite book
|date=10 December 2007
|author=Helm, D., ''et al.''
|title=Too Good To Be True? The UK's Climate Change Record
|url=http://www.dieterhelm.co.uk/sites/default/files/Carbon_record_2007_0.pdf
|format=PDF
|publisher=Website of Dieter Helm, a professor at the University of Oxford and a Fellow of New College, Oxford
|page=3
}}
</ref> For example, in the main international [[treaty]] on climate change (the [[UNFCCC]]), countries report on emissions produced within their borders, e.g., the emissions produced from burning fossil fuels.<ref name="world energy outlook 2009"/>{{Rp|179}}<ref name="davis consumption emissions"/>{{Rp|1}} Under a production-based accounting of emissions, embedded emissions on imported goods are attributed to the exporting, rather than the importing, country. Under a consumption-based accounting of emissions, embedded emissions on imported goods are attributed to the importing country, rather than the exporting, country.

Davis and Caldeira (2010)<ref name="davis consumption emissions">
{{cite journal
|url=http://www.pnas.org/content/early/2010/02/23/0906974107.full.pdf+html
|format=PDF
|author=Davis, S.J. and K. Caldeira
|title=Consumption-based Accounting of {{CO2}} Emissions
|date=8 March 2010
|journal=Proceedings of the National Academy of Sciences of the United States of America
|doi=10.1073/pnas.0906974107
|accessdate=2011-04-18|bibcode = 2010PNAS..107.5687D
|volume=107
|issue=12
}}
</ref>{{Rp|4}} found that a substantial proportion of {{CO2}} emissions are traded internationally. The net effect of trade was to export emissions from China and other emerging markets to consumers in the US, Japan, and Western Europe. Based on annual emissions data from the year 2004, and on a per-capita consumption basis, the top-5 emitting countries were found to be (in t{{CO2}} per person, per year): Luxembourg (34.7), the US (22.0), Singapore (20.2), Australia (16.7), and Canada (16.6).<ref name="davis consumption emissions"/>{{Rp|5}} Carbon Trust research revealed that approximately 25% of all CO2 emissions from human activities 'flow' (i.e. are imported or exported) from one country to another. Major developed economies were found to be typically net importers of embodied carbon emissions — with UK consumption emissions 34% higher than production emissions, and Germany (29%), Japan (19%) and the USA (13%) also significant net importers of embodied emissions.<ref>{{cite web|url=http://www.carbontrust.com/resources/reports/advice/international-carbon-flows|title=International Carbon Flows| publisher =Carbon Trust|date=May 2011|accessdate=12 November 2012}}</ref>

===Effect of policy===

{{See also|Kyoto Protocol and government action}}

Governments have taken action to reduce GHG emissions ([[climate change mitigation]]). Assessments of policy effectiveness have included work by the [[Intergovernmental Panel on Climate Change]],<ref>e.g., Gupta ''et al.'' (2007) assessed the scientific literature on climate change mitigation policy: {{cite book
|year=2007
|author=Gupta, S., ''et al.''
|title=Chapter 13: Policies, instruments, and co-operative arrangements. In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds))
|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch13.html
|publisher=Cambridge University Press
|isbn=9780521880114
}}
</ref> [[International Energy Agency]],<ref>
{{cite web
| title=International Energy Agency — Energy Policy
| url=http://iea.org/subjectqueries/keyresult.asp?KEYWORD_ID=4151
| year=2012
| author=International Energy Agency (IEA)
| publisher=IEA
| location=Paris, France
}}
</ref><ref>
{{cite web
| title=IEA Publications Bookshop: IEA Publications on 'Energy Policy'
| url=http://iea.org/w/bookshop/b.aspx?Subject=Energy%20Policy
| year=2012
| author=Organization for Economic Co-operation and Development (OECD) / International Energy Agency (IEA)
| publisher=OECD/IEA
| location=Paris, France
}}
</ref> and [[United Nations Environment Programme]].<ref>
{{citation
| date=November 2011
| author=United Nations Environment Programme (UNEP)
| title=Bridging the Emissions Gap: A UNEP Synthesis Report
| url=http://www.unep.org/pdf/UNEP_bridging_gap.pdf
| format=PDF
| publisher= UNEP
| location=[[Nairobi]], [[Kenya]]
| isbn=978-92-807-3229-0
}} UNEP Stock Number: DEW/1470/NA
</ref> Policies implemented by governments have included<ref>
{{cite book
|year=2010
|author=World Bank
|at=p 192, Box 4.2: Efficient and clean energy can be good for development
|chapter=4. Energizing development without compromising the climate
|chapter-url=http://siteresources.worldbank.org/INTWDRS/Resources/477365-1327504426766/8389626-1327510418796/Chapter-4.pdf
|format=PDF
|title=World Development Report 2010: Development and Climate Change
|publisher=The International Bank for Reconstruction and Development / The World Bank
|location=Washington DC, USA
|url=http://go.worldbank.org/CHZJNP7X30
|isbn=9780821379875
|doi=10.1596/978-0-8213-7987-5
}}
</ref><ref>
{{cite book
| year=2005
| author=[[United Nations Framework Convention on Climate Change]] (UNFCCC)
| title=Sixth compilation and synthesis of initial national communications from Parties not included in Annex I to the Convention. Note by the secretariat. Executive summary.
| url=http://unfccc.int/resource/docs/2005/sbi/eng/18.pdf
| format=PDF
| pages=10–12
| publisher=United Nations Office
| location=Geneva, Switzerland
}}
</ref><ref name="2011 unfccc synthesis of annex I communications">
{{cite book
| year=2011
| author=[[United Nations Framework Convention on Climate Change]] (UNFCCC)
| title=Compilation and synthesis of fifth national communications. Executive summary. Note by the secretariat.
| url=http://unfccc.int/resource/docs/2011/sbi/eng/inf01.pdf
| format=PDF
| pages=9–10
| publisher=United Nations Office at Geneva
| location=Geneva (Switzerland)
}}
</ref> national and regional targets to reduce emissions, promoting [[Efficient energy use|energy efficiency]], and support for [[renewable energy]].

Countries and regions listed in Annex I of the [[United Nations Framework Convention on Climate Change]] (UNFCCC) (i.e., the OECD and former planned economies of the [[Soviet Union]]) are required to submit periodic assessments to the UNFCCC of actions they are taking to address climate change.<ref name="2011 unfccc synthesis of annex I communications"/>{{Rp|3}} Analysis by the UNFCCC (2011)<ref name="2011 unfccc synthesis of annex I communications"/>{{Rp|8}} suggested that policies and measures undertaken by Annex I Parties may have produced emission savings of 1.5 thousand [[Orders of magnitude (mass)|Tg]] [[carbon dioxide equivalent|{{CO2}}-eq]] in the year 2010, with most savings made in the [[energy sector]]. The projected emissions saving of 1.5 thousand Tg {{CO2}}-eq is measured against a hypothetical "[[economics of climate change mitigation#Baselines|baseline]]" of Annex I emissions, i.e., projected Annex I emissions in the absence of policies and measures. The total projected Annex I saving of 1.5 thousand {{CO2}}-eq does not include emissions savings in seven of the Annex I Parties.<ref name="2011 unfccc synthesis of annex I communications"/>{{Rp|8}}

===Projections===

{{Further|climate change scenario#Quantitative emissions projections}}
{{See also|Global climate model#Projections of future climate change}}

A wide range of projections of future GHG emissions have been produced.<ref>
{{cite book
|year=2007
|author=Fisher, B., ''et al.''
|contribution=3.1 Emissions scenarios |title= Chapter 3: Issues related to mitigation in the long-term context
|series=Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
|editor= B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer
|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch3s3-1.html
|publisher=Cambridge University Press
|isbn=978-0-521-88011-4
}}
</ref> Rogner ''et al.'' (2007)<ref>
{{cite book
|year=2007
|author=Rogner, H.-H., D. Zhou, R. Bradley. P. Crabbé, [[Ottmar Edenhofer|O. Edenhofer]], B.Hare, L. Kuijpers, M. Yamaguchi
|contribution=1.3.2 Future outlook |title= Introduction
|series=Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
|editor= B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer
|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch1s1-3-2.html
|publisher=Cambridge University Press
|isbn=978-0-521-88011-4
}}</ref> assessed the scientific literature on GHG projections. Rogner ''et al.'' (2007)<ref name="rogner summary of emission trends"/> concluded that unless energy policies changed substantially, the world would continue to depend on fossil fuels until 2025–2030. Projections suggest that more than 80% of the world's energy will come from fossil fuels. This conclusion was based on "much evidence" and "high agreement" in the literature.<ref name="rogner summary of emission trends"/> Projected annual energy-related {{CO2}} emissions in 2030 were 40–110% higher than in 2000, with two-thirds of the increase originating in developing countries.<ref name="rogner summary of emission trends"/> Projected annual per capita emissions in developed country regions remained substantially lower (2.8–5.1 [[tonne]]s {{CO2}}) than those in developed country regions (9.6–15.1 tonnes {{CO2}}).<ref>
{{cite book
|year=2007
|author=Rogner, H.-H., D. Zhou, R. Bradley. P. Crabbé, [[Ottmar Edenhofer|O. Edenhofer]], B.Hare, L. Kuijpers, M. Yamaguchi
|contribution=1.3.2.4 Total GHG emissions |title= Introduction
|series=Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change
|editor= B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer
|url=http://www.ipcc.ch/publications_and_data/ar4/wg3/en/ch1s1-3-2-4.html
|publisher=Cambridge University Press
|isbn=978-0-521-88011-4
}}
</ref> Projections consistently showed increase in annual world GHG emissions (the [[Kyoto Protocol|"Kyoto"]] gases,<ref>carbon dioxide, methane, nitrous oxide, sulfur hexafluoride</ref> measured in [[carbon dioxide equivalent|{{CO2}}-equivalent]]) of 25–90% by 2030, compared to 2000.<ref name="rogner summary of emission trends"/>

===Relative {{CO2}} emission from various fuels===
One liter of gasoline, when used as a fuel, produces {{nowrap|2.32 kg}} (about 1300 liters or 1.3 cubic meters) of carbon dioxide, a greenhouse gas. One US gallon produces 19.4 lb (1,291.5 gallons or 172.65 cubic feet)<ref>{{cite web|url=http://www.epa.gov/otaq/climate/documents/420f11041.pdf |title=Greenhouse Gas Emissions from a Typical Passenger Vehicle, US Environment Protection Agency |publisher=Epa.gov |date= |accessdate=2011-09-11}}</ref><ref>{{cite web|last=Engber |first=Daniel |url=http://www.slate.com/id/2152685/ |title=How gasoline becomes {{CO2}}, Slate Magazine |publisher=Slate.com |date=1 November 2006 |accessdate=2011-09-11}}</ref><ref>{{cite web|url=http://www.icbe.com/carbondatabase/CO2volumecalculation.asp |title=Volume calculation for carbon dioxide |publisher=Icbe.com |date= |accessdate=2011-09-11}}</ref>

{| class="wikitable sortable" border="1" | style="text-align:center"
|+ Mass of [[carbon dioxide]] emitted per quantity of energy for various fuels<ref>{{cite web| url=http://www.eia.doe.gov/oiaf/1605/coefficients.html| accessdate=21 August 2009| title=Voluntary Reporting of Greenhouse Gases Program| publisher=[[Energy Information Administration]]}}</ref>
! Fuel name
! {{CO2}} emitted (lbs/106 Btu)
! {{CO2}} emitted (g/MJ)
|-
| style="text-align:left;"| [[Natural gas]]
| 117
| 50.30
|-
| style="text-align:left;"| [[Liquefied petroleum gas]]
| 139
| 59.76
|-
| style="text-align:left;"| [[Propane]]
| 139
| 59.76
|-
| style="text-align:left;"| [[Avgas|Aviation gasoline]]
| 153
| 65.78
|-
| style="text-align:left;"| Automobile [[gasoline]]
| 156
| 67.07
|-
| style="text-align:left;"| [[Kerosene]]
| 159
| 68.36
|-
| style="text-align:left;"| [[Fuel oil]]
| 161
| 69.22
|-
| style="text-align:left;"| [[Tire]]s/[[tire derived fuel]]
| 189
| 81.26
|-
| style="text-align:left;"| [[Wood]] and wood waste
| 195
| 83.83
|-
| style="text-align:left;"| [[Bituminous coal|Coal (bituminous)]]
| 205
| 88.13
|-
| style="text-align:left;"| [[Sub-bituminous coal|Coal (sub-bituminous)]]
| 213
| 91.57
|-
| style="text-align:left;"| [[Lignite|Coal (lignite)]]
| 215
| 92.43
|-
| style="text-align:left;"| [[Petroleum coke]]
| 225
| 96.73
|-
| style="text-align:left;"| [[Oil sands|Tar-sand Bitumen]]
| {{citation needed|date=August 2013}}
| {{citation needed|date=August 2013}}
|-
| style="text-align:left;"| [[Anthracite|Coal (anthracite)]]
| 227
| 97.59
|}

==Life-cycle greenhouse-gas emissions of energy sources==
A literature review of numerous energy sources {{CO2}} emissions by the [[IPCC]] in 2011, found that, the {{CO2}} emission value, that fell within the 50th [[percentile]] of all total life cycle emissions studies conducted, was as follows.<ref>{{cite journal | url = http://srren.ipcc-wg3.de/report/IPCC_SRREN_Annex_II.pdf | page = 10 | author = Moomaw, W., P. Burgherr, G. Heath, M. Lenzen, J. Nyboer, A. Verbruggen | year = 2011 | title = Annex II: Methodology | work = IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation}}</ref>
{|class="wikitable sortable"
|+ '''Lifecycle greenhouse gas emissions by electricity source.
|-
! Technology !! Description !! 50th percentile (g {{CO2}}/kWhe)
|-
| [[Hydroelectricity|Hydroelectric]] || reservoir || 4
|-
| [[Wind]] || [[List of onshore wind farms|onshore]] || 12
|-
| [[Nuclear power|Nuclear]] || various [[generation II reactor]] types || 16
|-
| [[Biomass]] || various || 18
|-
| [[Concentrating solar power|Solar thermal]] || [[parabolic trough]] || 22
|-
| [[Geothermal]] || [[hot dry rock]] || 45
|-
| [[Solar PV]] || [[Polycrystalline silicon photovoltaics|Polycrystaline silicon]] || 46
|-
| [[Natural gas]] || various combined cycle turbines without scrubbing || 469
|-
| [[Coal]] || various generator types without scrubbing || 1001
|-
|}

==Removal from the atmosphere ("sinks")==

===Natural processes===
Greenhouse gases can be removed from the atmosphere by various processes, as a consequence of:
* a physical change (condensation and precipitation remove water vapor from the atmosphere).

* a chemical reaction within the atmosphere. For example, methane is [[oxidized]] by reaction with naturally occurring [[hydroxyl]] [[free radical|radical]], OH'''·''' and degraded to {{CO2}} and water vapor ({{CO2}} from the oxidation of methane is not included in the methane [[Global warming potential]]). Other chemical reactions include solution and solid phase chemistry occurring in atmospheric aerosols.
* a physical exchange between the atmosphere and the other compartments of the planet. An example is the mixing of atmospheric gases into the oceans.
* a chemical change at the interface between the atmosphere and the other compartments of the planet. This is the case for {{CO2}}, which is reduced by [[photosynthesis]] of plants, and which, after dissolving in the oceans, reacts to form [[carbonic acid]] and [[bicarbonate]] and [[carbonate]] ions (see [[ocean acidification]]).
* a [[Photochemistry|photochemical change]]. [[Halocarbons]] are dissociated by [[Ultraviolet|UV]] light releasing Cl'''·''' and F'''·''' as [[free radical]]s in the [[stratosphere]] with harmful effects on [[ozone]] (halocarbons are generally too stable to disappear by chemical reaction in the atmosphere).

===Negative emissions===
{{see also|Bio-energy with carbon capture and storage|Carbon dioxide air capture|Geoengineering|Greenhouse gas remediation}}

A number of technologies remove greenhouse gases emissions from the atmosphere. Most widely analysed are those that remove carbon dioxide from the atmosphere, either to geologic formations such as [[bio-energy with carbon capture and storage]]<ref>{{Cite journal |author=Obersteiner M |title=Managing climate risk |journal=Science |volume=294 |issue=5543 |pages=786–7 |date=October 2001|pmid=11681318 |doi=10.1126/science.294.5543.786b |author-separator=, |author2=Azar C |author3=Kauppi P |display-authors=3 |last4=Möllersten |first4=K |last5=Moreira |first5=J |last6=Nilsson |first6=S |last7=Read |first7=P |last8=Riahi |first8=K |last9=Schlamadinger |first9=B }}</ref><ref>{{cite journal |author=Azar, C., Lindgren, K., Larson, E.D. and Möllersten, K. |title=Carbon capture and storage from fossil fuels and biomass – Costs and potential role in stabilising the atmosphere |journal=Climatic Change |volume=74 |pages=47–79 |year=2006 |url=http://www.environmental-expert.com/Files%5C6063%5Carticles%5C6220%5Cw30h4274h130580u.pdf |doi=10.1007/s10584-005-3484-7}}</ref><ref name=RoyalSociety>{{cite web | work=The Royal Society |year=2009 |url=http://royalsociety.org/displaypagedoc.asp?id=35151 | title = Geoengineering the climate: science, governance and uncertainty | accessdate = 12 September 2009 }} {{Dead link|date=October 2010|bot=H3llBot}}</ref> and [[carbon dioxide air capture]],<ref name="RoyalSociety"/> or to the soil as in the case with [[biochar]].<ref name="RoyalSociety"/> The IPCC has pointed out that many long-term climate scenario models require large scale manmade negative emissions to avoid serious climate change.<ref name=IPCC2007>Fischer, B.S., N. Nakicenovic, K. Alfsen, J. Corfee Morlot, F. de la Chesnaye, J.-Ch. Hourcade, K. Jiang, M. Kainuma, E. La Rovere, A. Matysek, A. Rana, K. Riahi, R. Richels, S. Rose, D. van Vuuren, R. Warren, (2007)[http://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter3.pdf "Issues related to mitigation in the long term context", In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Inter-governmental Panel on Climate Change] [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge.</ref>

==History of scientific research==
Late 19th century scientists experimentally discovered that {{chem|N|2}} and {{chem|O|2}} do not absorb infrared radiation (called, at that time, "dark radiation") while, on the contrary, water, both as true vapor and condensed in the form of microscopic droplets suspended in clouds, as well as {{CO2}} and other poly-atomic gaseous molecules, do absorb infrared radiation. It was recognized in the early 20th century that greenhouse gases in the atmosphere made the Earth's overall temperature higher than it would be without them. During the late 20th century, a [[Scientific opinion on climate change|scientific consensus]] evolved that increasing concentrations of greenhouse gases in the atmosphere are causing a substantial rise in global temperatures and changes to other parts of the climate system,<ref>{{cite doi|10.1088/1748-9326/8/2/024024}}</ref> with [[Effects of global warming|consequences]] for the [[Physical impacts of climate change|environment]] and for [[Effects of global warming on human health|human health]].

==See also==
{{Columns-list|2|
*[[Attribution of recent climate change]]
*[[Carbon credit]]
*[[Carbon Disclosure Project]]
*[[Carbon emissions reporting]]
*[[Carbon neutrality]]
*[[Carbon offset]]
*[[Clean Air Act (United States)|Clean Air Act]]
*[[Cool Earth 50]]
*[[Infrared window]] Atmospheric window
*[[Integrated Carbon Observation System]]
*[[Eddy covariance]] (also known as eddy correlation and eddy flux)
*[[Effects of global warming]]
*[[Emission standard]]
*[[Environmental impact of aviation]]
*[[European Climate Change Programme]]
*[[Externality]]
*[[Global Atmosphere Watch]]
*[[Greenhouse debt]]
*[[Hydrogen economy]]
*[[List of countries by electricity production from renewable sources]]
*[[List of international environmental agreements]]
*[[Low-carbon economy]]
*[[Low-carbon fuel standard]]
*[[Mobile source air pollution]]
*[[North American Carbon Program]]
*[[Perfluorotributylamine]]
*[[Physical properties of greenhouse gases]]
*[[Regional Greenhouse Gas Initiative]]
*[[Regulation of greenhouse gases under the Clean Air Act]]
*[[Renewable energy commercialization]]
*[[Sustainability measurement]]
*[[World energy consumption]]
*[[Zero-emissions vehicle]]
}}

== Notes and references==
{{Reflist|2}}

==Bibliography==

* {{citation
| title=Carbon Dioxide Information Analysis Center (CDIAC)
| author=CDIAC
| year=2012
| url=http://cdiac.ornl.gov/
| publisher=CDIAC
| location=Oak Ridge, Tennessee, USA
}}
* {{Citation
|year = 2001
|author = IPCC TAR WG1
|author-link = IPCC
|title = Climate Change 2001: The Scientific Basis
|series = Contribution of Working Group I (WG1) to the [[IPCC Third Assessment Report|Third Assessment Report]] (TAR) of the Intergovernmental Panel on Climate Change (IPCC)
|editor = Houghton, J.T.; Ding, Y.; Griggs, D.J.; Noguer, M.; van der Linden, P.J.; Dai, X.; Maskell, K.; and Johnson, C.A.
|publisher = Cambridge University Press
|url = http://www.grida.no/publications/other/ipcc%5Ftar/?src=/climate/ipcc_tar/wg1/index.htm
|isbn = 0-521-80767-0
}} (pb: {{ISBNT|0-521-01495-6}})
* {{Citation
|year = 2007
|author = IPCC AR4 WG1
|author-link = IPCC
|title = Climate Change 2007: The Physical Science Basis
|series = Contribution of Working Group I (WG1) to the [[IPCC Fourth Assessment Report|Fourth Assessment Report]] (AR4) of the Intergovernmental Panel on Climate Change (IPCC)
|editor = Solomon, S.; Qin, D.; Manning, M.; Chen, Z.; Marquis, M.; Averyt, K.B.; Tignor, M.; and Miller, H.L.
|publisher = Cambridge University Press
|url = http://www.ipcc.ch/publications_and_data/ar4/wg1/en/contents.html
|isbn = 978-0-521-88009-1
}} (pb: {{ISBNT|978-0-521-70596-7}})
*{{cite journal |author=Van Dijk, P.; Zhang, J.; Jun, W.; Kuenzer, C.; WOLF, K.H. |title=Assessment of the contribution of in-situ combustion of coal to greenhouse gas emission; based on a comparison of Chinese mining information to previous remote sensing estimates |journal=International Journal of Coal Geology |volume=86 |issue=1 Special Issue RS/GIS |pages=108–119 |year=2011 |doi=10.1016/j.coal.2011.01.009 }}

* Zhou, Yiqin (2011). ''Compar[ison of] Fresh or Ensiled Fodders (e.g., Grass, Legume, Corn) on the Production of Greenhouse Gases Following Enteric Fermentation in Beef Cattle''. Rouyn-Noranda, Qué.: Université du Québec en Abitibi-Témiscamingue. ''N.B''.: Research report.

==External links==
*{{dmoz|Science/Environment/Global_Change/}}
*[http://www.cmdl.noaa.gov/aggi/ Annual Greenhouse Gas Index (AGGI)] from [[NOAA]]
*[http://www.spectralcalc.com/ Atmospheric spectra of GHGs and other trace gases]
*[http://tonto.eia.doe.gov/energy_in_brief/greenhouse_gas.cfm How Much Greenhouse Gas Does the United States Emit?]
*[http://gristmill.grist.org/story/2009/1/11/192838/298 Grist article on convenient summary from various sources incl IPCC of greenhouse gas emissions] * *
*[http://spreadsheets.google.com/ccc?key=pzrff2j0rl2wNrQfxOKkYYQ Summary of Greenhouse gas emissions] at Google Docs
*[http://www.lakescientist.com/learn-about-lakes/lakes-climate-change/lakes-and-greenhouse-gases.html Greenhouse Gases]
*[http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html Greenhouse Gases] Sources, Levels, Study results — University of Michigan; eia.doe.gov findings
*[http://www.eastmain1.org/en/greenhouse-gas-emission.html EM-1 | Greenhouse gas emissions research project]

;Carbon dioxide emissions
* [http://image.guardian.co.uk/sys-files/Guardian/documents/2011/02/10/CarbonWeb.pdf Carbon Emissions World Map in 2009] Mark McCormick and Paul Scruton, [[The Guardian]] February 2011
*[http://www.eia.doe.gov/emeu/iea/res.html International Energy Annual: Reserves]
*[http://www.eia.doe.gov/emeu/iea/carbon.html International Energy Annual 2003: Carbon Dioxide Emissions]
*[http://www.eia.gov/emeu/iea/Notes%20for%20Table%20H_1co2.html International Energy Annual 2003: Notes and Sources for Table H.1co2] (Metric tons of carbon dioxide can be converted to metric tons of carbon equivalent by multiplying by 12/44)
*[http://www.licor.com/env/applications/eddy_covariance/book.jsp Textbook on Eddy Covariance Measurements of Gas Emissions]
*[http://www.cmdl.noaa.gov/ccgg/trends/ Trends in Atmospheric Carbon Dioxide] at NOAA
*[http://www.ngdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok.html NOAA Paleoclimatology Program — Vostok Ice Core]
*[http://www.cmdl.noaa.gov/ccgg/iadv/ NOAA CMDL CCGG — Interactive Atmospheric Data Visualization] NOAA {{CO2}} data
*[http://cdiac.ornl.gov/ Carbon Dioxide Information Analysis Center] (CDIAC)
**[http://cdiac.esd.ornl.gov/pns/faq.html Carbon Dioxide Information Analysis Centre FAQ] Includes links to Carbon Dioxide statistics
*[http://siteresources.worldbank.org/INTDATASTA/64199955-1178226923002/21322619/LGDB2007.pdf Little Green Data Book 2007], World Bank. Lists {{CO2}} statistics by country, including per capita and by country income class.
*[http://www.carma.org/ Database of carbon emissions of power plants]
*[http://oco.jpl.nasa.gov/ NASA's Orbiting Carbon Observatory]
*[http://zeroemissionproject.com/blog/article/53/my-carbon-bag The Carbon Bag: the carbon dioxide emission of a typical British home]

;Methane emissions
*[http://news.bbc.co.uk/2/hi/science/nature/5321046.stm BBC News — Thawing Siberian bogs are releasing more methane]
*[http://www.licor.com/env/applications/eddy_covariance/book.jsp Textbook on Eddy Covariance Measurements of Gas Emissions]

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