Atmospheric greenhouse gas concentrations

Assessment versions

Published (reviewed and quality assured)

• No published assessments

Rationale

Justification for indicator selection

Greenhouse gases can affect the radiation balance on Earth and are, therefore, 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 various greenhouse gases affect the climate system in different ways. In order to sum their effects on the atmosphere, the so-called greenhouse gas 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 greenhouse gasess 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 greenhouse gases can be found here. 

There are, in general, three ways the greenhouse gas equivalent concentration can be aggregated, all of which are presented here. First, an approach often used is to group together the six main human-made greenhouse gases under the Kyoto Protocol (i.e. CO₂, CH₄, N₂O, HFCs, PFCs and SF₆). A second approach is to group all long-living greenhouse gases (i.e. Kyoto Protocol gases plus the Montreal Protocol gases (i.e. CFCs, HCFCs & CH₃CCl₃)). Velders et al (2007) have shown that reducing the concentration of these Montreal gases has a considerable beneficial effect for the climate. Finally, in a third approach, ozone, water vapour and aerosols can also be included to show the net effect of all greenhouse gases 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).

Considering all greenhouse gases 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. 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 assessment reports (AR) (1750–2011), showing the uncertainties, especially in direct and indirect aerosol forcing (IPCC, 2013), is presented below in W m^-2:

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.
• 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 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 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 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.
• 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 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.
• 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.
• IPCC (2014) IPCC (2014) Climate Change 2014 Mitigation of Climate Change. Working Group 3 Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Technical Summary and Chapter 6 (Assessing Transformation Pathways).
• Klimont, Z., S. J. Smith, et al. (2013) Klimont, Z., S. J. Smith, et al. (2013). "The last decade of global anthropogenic sulfur dioxide: 2000–2011 emissions." Environmental Research Letters 8(1): 014003.

Indicator definition

This indicator shows the observed trends in greenhouse gas concentrations. Greenhouse gases differ in the way they affect the climate system. In order to sum the effects of the individual greenhouse gasses and other forcing agents in the atmosphere, the so-called ‘greenhouse gas equivalent concentration’ has been defined. This is the concentration of CO₂ that would cause the same amount of radiative forcing as a mixture of CO₂ and other forcing agents (greenhouse gases and aerosols). The forcing is expressed here in CO_2-equivalent concentrations, rather than watts per square meter (W/m²), because they are more easily understood by the general public. For a more detailed assessment of radiative forcing expressed in W/m², the reader is referred to IPCC (2013).

Global average annual concentrations are presented here. Although greenhouse gases are mainly emitted in the northern hemisphere, the use of global average values is considered justified, because the atmospheric lifetime of most greenhouse gases is long compared to the timescales of global atmospheric mixing. This leads to a rather uniform mixture around the globe. The exceptions are ozone and aerosols. However, as described earlier, these gases are less relevant, over the long term.

Units

Atmospheric concentration is measured in parts per million CO₂-equivalents (ppm CO₂ eq.).

Policy context and targets

Context description

The overall objective of the United Nations Framework Convention on Climate Change (UNFCCC), is ‘to stabilise atmospheric greenhouse gas concentrations at a level that would prevent dangerous anthropogenic interference with the climate system’ (UNFCCC, 1993). Both at global level (UNFCCC, 2009) and EU levels (October 2008 Environment Council conclusions), this ‘dangerous anthropogenic interference’ has been recognised in an ambition to keep the long-term global average temperature rise below 2 °C, compared to pre-industrial levels. In December 2015, the Paris Agreement strengthened this by stating its objective as ‘holding the increase in the global average temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5 °C above pre-industrial levels’(UNFCC, 2015).

CO₂ equivalent concentration levels can be defined that correspond to these and other temperature increases. Note that these temperature increases represent a specific long-term equilibrium warming. The equilibrium climate sensitivity is defined as the change in global mean temperature, T2x, that results when the climate system, or a climate model, attains a new equilibrium with the forcing change, F2x, resulting from a doubling of the atmospheric CO₂ concentration (IPCC, 2013).

Studies have been done to assess the probability of achieving this objective. These include both the 1.5 °C and 2 °C targets (above pre-industrial levels) in relation to various stabilisation levels of greenhouse gases in the atmosphere (Meinshausen et al., 2009, 2011; Van Vuuren et al., 2011; IPCC, 2014), which would result in the main objective of the COP21 Paris agreement being achieved (see also Table 1).

From estimates on climate sensitivity, it can be derived that the concentration of all greenhouse gases in the atmosphere – including ozone, water vapour and aerosols – would need to remain below approximately 430 ppm CO₂ equivalent to achieve a 50 % probability of keeping the increase in global mean temperature below 1.5 °C. This concentration that is lower than the one currently observed. The atmospheric concentration level of all greenhouse gases that would be consistent with a maximum temperature increase of 2 °C is between 480 ppm and 650 ppm CO₂ equivalent. Respectively, these figures give a 67 % and <10 % probability of staying below these temperature thresholds, see Table 1). 

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 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 °C above today's level). Scenarios consistent with this target show global emissions reductions of 40 % to 70% by 2050, compared to 2010 levels (i.e. 40 % to 60 % for scenarios with negative emissions, and 60 % to 70% for scenarios without negative emissions).^[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 small additional number of 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. Such a new global agreement was concluded at the COP21 meeting in Paris, in November 2015. The sum of the proposed emissions reductions for all countries would still be insufficient to meet the target (UN Gap analysis).

[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
• Paris Agreement
  The Paris Agreement. Report of the Conference of the Parties on its twenty-first session, held in Paris from 30 November to 11 December 2015.

Methodology

Methodology for indicator calculation

The trends in global average concentrations of atmospheric CO₂ are taken directly from NOAA (NOAA, 2015, see also www.esrl.noaa.gov/gmd/ccgg/trends/global.html). Global average concentration values for the other gases are mainly based on AGAGE figures (2014) (see agage.eas.gatech.edu/index.htm). These global figures were derived by averaging the data from four observatory stations across the world, equally distributed over the northern and southern hemispheres.

Radiative forcings are calculated using an approximate equation according to IPCC (2013), based on the observed atmospheric concentrations and using radiative efficiencies for CO₂, CH₄, N₂O, ozone (both stratospheric and tropospheric) and vapour. IPCC (2013) estimates were also used for the radiative forcing of aerosols between 1970 and 2013.

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

 A similar approach was applied for the Montreal Protocol gases (i.e. CFCs and HCFCs):

Overview of used alpha values for chlorine Kyoto and Montreal Gases (also see CDIAC):

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. A factor of -0.154 for correcting the forcing of Montreal Protocol gases, based on IPCC (2013) (= -0.05 W/m² or 10 ppm in 2014), was used. 

To quantify the total concentration of all greenhouse gases, the forcing of tropospheric ozone, water vapour in the atmosphere, cloud interaction, changes in albedo (e.g. due to black carbon) and the direct effects of multiple aerosols were added. Due to uncertainties in the measurements, and the large inter-annual and seasonal variation, forcing for tropospheric ozone, water vapour and land-use change was kept constant over the years at a respective 0.4, 0.07 (IPCC, 2013, p. 696) and 0.15 W/m² (IPCC, 2013, p. 698).

For aerosols, we used figures (i) based on recent the IPCC report. For example, the forcing of black carbon (BC) from fossil fuel and biofuel use has increased as a result of increased emissions, especially in east and southeast Asia; (ii) changed the methodology for the period from 1970 to 2014. In this new methodology radiative forcing figures for for sulphate, black carbon, organic carbon, nitrate and other forcers (= sum of mineral dust, cloud interaction and contrails), based on IPCC (2013, Fig. 8.17, pg. 698), were defined. These figures are provided in the table below. Then the radiative forcing for these five groups back to 1970 was calculated. For sulphate, historical emissions data from Klimont et al. (2013) and IPCC (2013, Figure 8.8, p. 683) were used. For black carbon, organic carbon and nitrate, we used the historical radiative forcing figures as given by the IPCC (Figure 8.8, p. 683). Note that here 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 measurement data from a particular station are missing for a certain year, the global trend is derived from data available from other stations. 

Methodology references

• CDIAC 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   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)
• 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 network 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 (IPCC, 2013).

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 shown in the tables below (given in 10 % and 90 % confidence ranges).

Best estimate values and 10 % and 90 % confidence ranges for total forcing (IPCC, 2013) are presented below:

Another way of showing the effect of uncertainties on the total greenhouse gas concentration is to use alternative values for the forcing of aerosols. New estimates of this forcing have been presented in previous years (IPCC, 2013). The cooling effect of aerosols is now estimated to be 0.35 W/m² smaller (= more warming) than reported in previous years, mainly due to the greater warming potential of black carbon. When using these older values, the total greenhouse gas concentration in 2014 is estimated to be about 34 ppm lower than when using the more recent figures. Given a similar trend in time, this lower concentration level would imply an additional 10 years before the various critical concentration levels (Table 1) would be reached. 

Overall, this shows the effect of a lower climate sensitivity (Rogelj et al., 2014) and higher heat uptake by the oceans (Doney et al., 2014). An important question here is whether this increased heat uptake will prove to be structural, over time. 

Data sets uncertainty

Comparability of direct measurements is good. Although methods for calculating radiative forcing and CO₂ equivalent are expected to be further improved, any update of these methods will be applied to the complete data set 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.