Atmospheric greenhouse gas concentrations

Assessment versions

Published (reviewed and quality assured)

• No published assessments

Rationale

Justification for indicator selection

Greenhouse gases (GHG) have the ability to affect the radiation balance on Earth and are crucial for the global climate. Without these gases, the global average temperature would be about 32^oC lower than it is now (i.e. -18^oC instead of the current +14^oC global mean temperature average).

The different GHGs all affect the climate system in different ways. In order to sum the effects of different GHGs on the atmosphere, the so-called GHG equivalent concentration has been defined. This is the concentration of CO₂ that would cause the same amount of radiative forcing as the mixture of CO₂ and other GHGs over a 100-year time horizon. CO₂-equivalent concentrations, rather than radiative forcings, are presented here, because they are more easily understood by the general public. For an overview on radiative forcing, the reader is referred to IPCC (2013). A summary of radiative forcing of different GHGs is found here. 

There are, in general, three ways the GHG equivalent concentration can be aggregated, all of which are presented here. First, an approach often used is to group together the six Kyoto Protocol GHGs (i.e. CO₂, CH₄, N₂O, HFCs, PFCs & SF₆), as they are the main human made GHGs. A second approach is to group all long-living GHGs (i.e. Kyoto Protocol gases plus the Montreal Protocol Gases CFCs, HCFCs & CH₃CCl₃). Velders et al (2007) has shown that reducing the concentration of these Montreal gases has a considerable beneficial effect also for the climate. Finally, in a third approach, ozone, water vapour and aerosols can also be included to show the net effect of all GHGs on the Earth’s radiation balance, and as such, the climate system. Unfortunately the data record of this third approach is shorter due to the lack of long-term data series for ozone. Furthermore, the forcing of many of the aerosol species is highly uncertain (e.g. they have also changed between IPCC, 2007 and IPCC, 2013, see table below). The consideration of all GHGs together can provide a lower concentration level compared to the other two approaches due to the net cooling effect of aerosols. For the current period (e.g. in 2012), the three approaches lead to quite different concentration levels (see assessment), but for the long-term the three approaches could converge as a result of the decrease in Montreal Protocol gases that is starting to occur (Montzka et al, 2011). Global sulphur and aerosols emissions are also likely to be reduced due to non-climate related policies. 

A summary table of radiative forcing estimates for the latest two IPCC assessments (for the period 1750–2011), showing the uncertainties, especially in direct and indirect aerosol forcing (IPCC, 2013), is presented below:

Although the emissions of GHGs mainly occur in the northern hemisphere, the use of global average values is considered justified because the atmospheric lifetime of GHGs is long compared with the timescales of global atmospheric mixing. This leads to a rather uniform mixing around the globe. The exceptions are ozone, sulphur and aerosols. However, as described earlier, these gases are less relevant over the long-term.

Scientific references

• UNFCCC (1993) UNFCCC (1993) The United Framework Convention on Climate Change. United Nations.
• Butler (2009) Butler, J. (2009). The NOAA annual greenhouse gas index (AGGI). NOAA Earth System Research Laboratory, Boulder. (Update September 2009)
• Meinshausen et al. (2009) Meinshausen, M., Meinshausen, N., Hare, W., Raper, S. C. B., Frieler, K., Knutti, R., Frame, D. J. & Allen, M. (2009) Greenhouse gas emission targets for limiting global warming to 2°C. Nature, 458: 1158-1163.
• UNFCCC (2009) UNFCCC (2009). Report of the Conference of the Parties on its fifteenth session, held in Copenhagen from 7 to 19 December 2009; Part Two: Decisions adopted by the Conference of the Parties
• Den Elzen et al. (2007) Den Elzen, M.G.J. , M. Meinshausen & D.P. van Vuuren (2007), ‘Multi-gas emission envelopes to meet greenhouse gas targets: costs versus certainty of limiting temperature increase’, Global Environmental Change 17: 260-280.
• Van Vuuren et al. (2008) Van Vuuren, D.P. et al. (2008), Temperature increase of 21st century mitigation scenarios’, PNAS 105 (40): 15258-15262.
• Montzka et al. (2011) Montzka, S. A.;  E J Dlugokencky, J. H. Butler (2011). Non-CO2 greenhouse gases and climate change Nature 476, 43-50.
• WMO (2002) Scientific Assessment of Ozone Depletion: 2002, Global Ozone Research and Monitoring Project–Report No. 47, World Meteorological Organization,Geneva,Switzerland.
• Velders et al. (2007) Velders, G. J.M.; S.O. Andersen, J. S. Daniel, D. W. Fahey & M. McFarland (2007)The Importance of the Montreal Protocol in Protecting Climate, PNAS 104:4814 – 4819
• IPCC (2013) Climate Change 2013: The Physical Science Basis. Working Group 1 Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Technical Summary, Chapter 2 (Changes in Atmospheric Constituents and in Radiative Forcing), Chapter 8 (Anthropogenic and Natural Radiative Forcing).
• CDIAC (2013a) http://agage.eas.gatech.edu/data_archive/agage/gc-ms-medusa/monthly/ for CFC-113 HCFC-22, HCFC-141b, HCFC-142b HFC-125, HFC-134a, HFC-152a, HFC-365mfc, HFC-23 Halon-1211, Halon-1301 CH3Cl, CH2Cl2, CHCl3, CH3Br CH3CCl3, CHClCCl2, CCl2CCl2 SF6, SO2F2 PFC-14, PFC-116, PFC-218 HFC-227ea HFC-236fa HFC-245fa
• CDIAC (2013b) http://agage.eas.gatech.edu/data_archive/agage/gc-md/monthly/ for CH4, N2O, CO, H2, CFC-11, CFC-12, CH3CCl3, CCl4, CFC-113, and CHCl3. 
• AGAGE (2014) Advanced Global Atmospheric Gases Experiment
• Doney et all. (2014) Doney, S.C., L. Bopp, and M.C. Long. Historical and future trends in ocean climate and biogeochemistry. Oceanography 27(1):108–119.
• Fry et all. (2012) Fry, M.M., V. Naik, J.J. West, M.D. Schwarzkopf, A.M. Fiore, W.J. Collins, F.J. Dentener, D.T. Shindell, C. Atherton, D. Bergmann, B.N. Duncan, P. Hess, I.A. MacKenzie, E. Marmer, M.G. Schultz, S. Szopa, O. Wild, and G. Zeng, 2012 . The influence of ozone precursor emissions from four world regions on tropospheric composition and radiative climate forcing. Journal of Geophysical Research: Atmospheres 117:D07306.
• Meinshausen et all. (2011) Meinshausen, M. , S. J. Smith, K. V. Calvin, J. S. Daniel, M. Kainuma, J.-F. Lamarque, K. Matsumoto, S. A. Montzka, S. C. B. Raper, K. Riahi, A. M. Thomson, G. J. M. Velders and D. van Vuuren. The RCP Greenhouse Gas Concentrations and their Extension from 1765 to 2300. Climate Change 109:213-241.
• NOAA (2014) NOAA (2014). National Oceanic & Atmospheric Administration, ESRL/GMD FTP Data Finder.
• Rogelj et all. (2014) Rogelj, J., Meinshausen, M., Sedláček, J., Knutti, R. Implications of potentially lower climate sensitivity on climate projections and policy. Environmental Research Letters 9:031003.
• Rosenfeld et all. (2013) Rosenfeld D., M. O. Andreae, A. Asmi, M. Chin, G. de Leeuw, D. P. Donovan, R. Kahn, S. Kinne, N. Kivekäs, M. Kulmala, W. Lau, S. Schmidt, T. Suni, T. Wagner, M. Wild, J. Quaas. Global observations of aerosol-cloud-precipitation-climate interactions. Submitted to Reviews Of Geophysics :2013RG000441.
• Van Vuuren et all. (2011) Van Vuuren, D., Edmonds, J., Kainuma, M., Riahi, K., Thomson, A., Hibbard, K., Hurtt, G., Kram, T., Krey, V., Lamarque, J-F., Masui, T., Meinshausen, M., Nakicenovic, N., Smith, S., Rose, S. The representative concentration pathways: an overview. Climatic Change 109:5-31.

Indicator definition

This indicator shows the observed trends in greenhouse gas concentrations. The various greenhouse gases have been grouped in three different ways:

(1) the six Kyoto Protocol GHGs (CO₂, CH₄, N₂O, HFCs, PFCs & SF₆)

(2) long-living GHGs (Kyoto Protocol gases plus the Montreal Protocol gases CFCs, HCFCs & CH₃CCl₃)

(3) long-living GHGs and ozone, water vapour and aerosols

Greenhouse gasses are grouped in two ways. In addition to the concentration of individual GHGs, they are grouped using the CO₂-equivalent concentration as a way to add the different gasses. The CO₂-equivalent concentration of a gas is the CO₂ concentration that would cause the same amount of radiative forcing as the mixture of all GHGs. Global annual averages are considered, because in general the gases mix quite well in the atmosphere. 

Units

Atmospheric concentration in parts per million in CO₂-equivalent (ppm CO₂ eq.) and parts per billion (ppb) of individual species.

Policy context and targets

Context description

Greenhouse gas concentrations is a key indicator relevant to international climate negotiations, since the overall objective of the United Nations Framework Convention on Climate Change (UNFCCC), is ‘to stabilize atmospheric greenhouse gas concentrations at a level that would prevent dangerous anthropogenic interference with the climate system’ (UNFCCC, 1993). Both at the global (UNFCCC, 2009) and the EU level (October 2008 Environment Council conclusions), this ‘dangerous anthropogenic interference’ has been recognised by formulating the objective of keeping the long-term global average temperature rise below 2°C compared to pre-industrial times. Studies have assessed the probability of keeping the long-term temperature rise below this 2°C target in relation to different stabilisation levels of GHGs in the atmosphere (Meinshausen et al, 2009, 2011; Van Vuuren et al, 2011). These studies showed that to have a 50% probability of limiting the global mean temperature increase to 2°C (above pre-industrial levels), the concentration of all GHGs in the atmosphere would need to be stabilised at about 450 ppm CO₂ equivalent (range 400–500 ppm CO₂-eq.). This includes ozone, water vapour and aerosols. For CO₂ only, the 50% probability concentration threshold is around 400 ppm, and for all Kyoto gases about 480 ppm CO_2-equivalent (range 432–532 ppm CO₂ eq.). The probability of staying below 2°C becomes very low when stabilisation occurs at 550 ppm CO₂-eq., ranging between 0% and 37%, considering all GHGs.

Note that the value for the Kyoto gases only is higher than when considering all GHGs, due to the cooling effect of aerosols. This cooling effect is, however, currently estimated to be considerably lower than previous estimates (currently about 0.75 W.m-2 or about 67 ppm CO₂ eq.) (IPCC, 2013, Fry et al, 2012; Rosenfeld et al, 2014). 

Targets

In 1992, countries adopted the United Nations Framework Convention on Climate Change (UNFCCC) to cooperatively consider options for limiting average global temperature increases and the resulting climate change.^[1] Continuous discussions under the UNFCCC led to the adoption, in 1997, of the Kyoto Protocol ^[2], which legally binds developed countries to achieve greenhouse gas (GHG) emissions reduction targets.

In 2010, the international community agreed on the need to reduce emissions in order to prevent global temperature increases from exceeding 2°C compared to pre-industrial levels^[3] (no more than 1.2°C above today's level). This would require cutting global emissions by 40% to 70% compared to 2010 by 2050.^[4]

More than 90 countries agreed to take on mitigation commitments until 2020, including the major developed and developing nations. The European Union (EU) and a handful of other developed countries made their commitments under the Doha Amendment to the Kyoto Protocol for a second commitment period running from 2013 to 2020.^[5] To secure the chance to stay below 2°C, the international community has decided to work towards an international climate agreement for the period after 2020, which should be applicable to all. The negotiation on this new global agreement is expected to be concluded in 2015 in Paris.

[1] United Nations Framework Convention on Climate Change, United Nations (9 May 1992) New York, accessed 3 July 2014.

[2] Kyoto Protocol, United Nations Framework Convention on Climate Change, accessed 3 July 2014.

[3] Decision 1/CP.16: The Cancun Agreements: Outcome of the work of the Ad Hoc Working Group on Long-term Cooperative Action under the Convention, accessed 3 July 2014.

[4] IPCC (2014): Summary for Policymakers, In: Climate Change 2014, Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Edenhofer, O., R. Pichs-Madruga, Y. Sokona, E. Farahani, S. Kadner, K. Seyboth, A. Adler, I. Baum, S. Brunner, P. Eickemeier, B. Kriemann, J. Savolainen, S. Schlömer, C. von Stechow, T. Zwickel and J.C. Minx (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, accessed 3 July 2014.

[5] Doha Amendment to the Kyoto Protocol, Doha (8 December 2012), accessed 27 August 2014. 

Related policy documents

• Council Decision (2002/358/EC) of 25 April 2002
  Council Decision (2002/358/EC) of 25 April 2002 concerning the approval, on behalf of the European Community, of the Kyoto Protocol to the United Nations Framework Convention on Climate Change and the joint fulfilment of commitments thereunder.
• Greenhouse gas monitoring mechanism Decision
  Decision No 280/2004/EC of the European Parliament and of the Council of 11 February 2004 concerning a mechanism for monitoring Community greenhouse gas emissions and for implementing the Kyoto Protocol

Methodology

Methodology for indicator calculation

For atmospheric CO₂, the global average values are taken directly from NOAA (NOAA, 2014, see also www.esrl.noaa.gov/gmd/ccgg/trends/global.html). Global average concentration values for the other gases are mainly based on AGAGE numbers (2014) (see agage.eas.gatech.edu). Radiative forcings are calculated with approximate equation according to IPCC (2013), based on the observed atmospheric concentrations and using radiative efficiencies for CO₂, CH₄, and N₂O, ozone (both stratospheric and tropospheric) and vapour based on IPCC (2013). IPCC (2013) estimates have also been used for the radiative forcing of aerosols for the period 1970–2012 (see also Summary table of radiative forcing estimates for the period 1750-2011). 

The equations used to compute the contribution of the individual gases are presented below: 

In order to calculate the concentration of all long-living GHGs, the Montreal Gases (i.e. CFCs and HCFCs) need to be included also. A similar approach is applied for these gases:

Overview of used alpha values for chlorine Kyoto and Montreal Gases (also see CDIAC, 2011 a, b)

In calculating the radiative forcing (and accompanying concentration levels) of the Montreal Protocol gases, the effect of ozone depleting substances on the stratospheric ozone layer was also considered. We used a forcing of -0.05 W/m², based on IPCC (2013, 2007) (or about 10 ppm CO₂ eq.). 

To quantify the concentration of all greenhouse gases, important in relation to the 2°C target, the forcing of tropospheric ozone, water vapor in the atmosphere, cloud interaction, changes in albedo (e.g. due to black carbon) and direct effects of multiple aerosols have been added. Due to uncertainties in the measurements and the large inter-annual and seasonal variation, the forcing is kept constant over the years for tropospheric ozone and water vapour. These values are 0.4 and 0.07 W.m^-2 for ozone and water vapor, respectively (IPCC, 2013, pg. 696). For aerosols we have now (i) used updated numbers, based on IPCC. E.g. the forcing of black carbon from fossil fuel and biofuel use has increased as a result of increased emissions in especially east and southeast Asia; (ii) changed the methodology for the period 1970 to 2012. In this new methodology we firstly defined radiative forcing numbers for the year 2011 for sulfate, black carbon, organic carbon, nitrate and other forcers (i.e. the sum of mineral dust, cloud interaction, contrails and land use), based on IPCC (2013, Fig. 8.17, page. 698). The numbers are given in the table below. Then we calculated the radiative forcing for these five groups back to 1970. For sulfate, historic emission data from Klimont et al (2013) and IPCC (2013) was used. Because of a lack of other information, the same trend has been used for the group of other forcers. For black carbon, organic carbon and nitrate we used the historic radiative forcing numbers as given by IPCC (Figure 8.8, pg. 683). We noted here that the forcing of biomass burning was kept to zero, as it is balanced by a positive forcing due to black carbon and negative forcing from organic carbon (see also IPCC, 2013, Table 8.4).

Calculated radiative forcing for multiple aerosols for some years in the past (all in W/m2) is presented below.

Methodology for gap filling

If measurements from a station are missing for a certain year, the global trend is derived from available stations data. 

Methodology references

• CDIAC (2013a) http://agage.eas.gatech.edu/data_archive/agage/gc-ms-medusa/monthly/ for CFC-113 HCFC-22, HCFC-141b, HCFC-142b HFC-125, HFC-134a, HFC-152a, HFC-365mfc, HFC-23 Halon-1211, Halon-1301 CH3Cl, CH2Cl2, CHCl3, CH3Br CH3CCl3, CHClCCl2, CCl2CCl2 SF6, SO2F2 PFC-14, PFC-116, PFC-218 HFC-227ea HFC-236fa HFC-245fa
• CDIAC (2013b)   http://agage.eas.gatech.edu/data_archive/agage/gc-md/monthly/ for CH 4 , N 2 O, CO, H 2 , CFC-11, CFC-12, CH3CCl3, CCl4, CFC-113, and CHCl3. 
• IPCC (2007) Climate Change 2007: The Physical Science Basis.  (eds.) Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt K, Tignor MMB & Miller HL,. Working Group 1 Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Technical Summary and Chapter 10 (Global Climate Projections)
• WMO (2002) Scientific Assessment of Ozone Depletion: 2002, Global Ozone Research and Monitoring Project–Report No. 47, World Meteorological Organization), Geneva, Switzerland
• IPCC (2013) Climate Change 2013: The Physical Science Basis. Working Group 1 Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Technical Summary and Chapter 2.

Data specifications

EEA data references

• No datasets have been specified here.

External data references

• CO2 concentrations
• NCEI Data and Products (NOAA)
• HFC-134a and SF6 concentrations
• Climate Change 2001 - The Scientific Basis (dataset URL is not available)
• CH4 and N2O concentrations (dataset URL is not available)

Data sources in latest figures

Uncertainties

Methodology uncertainty

Global average concentrations since approximately 1980 are determined by averaging measurements from several ground-station networks (SIO, NOAA/CMDL,ALE/GAGE/AGAGE), each consisting of several stations distributed across the globe.

Absolute accuracies of global annual average concentrations are of the order of 1% for CO₂, CH₄ and N₂O, and CFCs; for HFCs, PFCs, and SF₆, absolute accuracies can be up to 10-20%. However, the year-to-year variations are much more accurate.

Radiative forcing is calculated using parameterisations that relate the measured concentrations of greenhouse gases to radiative forcing. The overall uncertainty in radiative forcing calculations is given in the tables below (giving in 5% and 95% ranges).

Best estimate values and 5 and 95% ranges for total forcing (IPCC, 2013) is presented below:

Data sets uncertainty

Direct measurements have good comparability. Although methods for calculating radiative forcing and CO₂-equivalent are expected to improve further, any update of these methods will be applied to the complete dataset covering all years, so this will not affect the comparability of the indicator over time.

Rationale uncertainty

Atmospheric concentrations of greenhouse gases are a well-established indicator of changes in atmospheric composition, which causes changes in the global climate system. Here we only present observed trends, as they have lower uncertainties than model projections.