Atmospheric greenhouse gas concentrations

Assessment versions

Published (reviewed and quality assured)

• 10 Dec 2020, 05:15 PM
  - Atmospheric greenhouse gas concentrations

Rationale

Justification for indicator selection

The rationale for this indicator has not been described

Scientific references

• Butler, J., 2009, The NOAA annual greenhouse gas index (AGGI). NOAA Earth System Research Laboratory, Boulder, (Update September 2009)
• Etheridge, D. M., et al., 1998, Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability, Journal of Geophysical Research. 103: 15979-15993
• Etheridge, D.M., et al., 2002, Historical CH4 Records Since About 1000 A.D. From Ice Core Data, In Trends: A Compendium of Data on Global Change, CDIAC, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A
• 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 8 (Anthropogenic and Natural Radiative Forcing)
• 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)
• IPCC, 2018, Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty.
• Klimont, Z., et al., 2013, The last decade of global anthropogenic sulfur dioxide: 2000–2011 emissions, Environmental Research Letters 8(1): 014003
• Machida, T., et al., 1995, Increase in the atmospheric nitrous oxide concentration during the last 250 years. Geophysical Research Letters 22:2921-2924.
• Meinshausen, et al., 2017, Historical greenhouse gas concentrations for climate modelling (CMIP6) Geoscientific Model Development 10, 2057-2116
• Myhre, G., et al., 2017, Multi-model simulations of aerosol and ozone radiative forcing due to anthropogenic emission changes during the period 1990–2015 Atmospheric. Chemistry and Physics. 17, 2709-2720
• Nazarenko, L., et al., 2017, Interactive nature of climate change and aerosol forcing, Journal of Geophysical Research Atmospheres 122(6), 3457-3480. doi:10.1002/2016JD025809
• NOAA, 2020, National Oceanic & Atmospheric Administration, ESRL/GMD. Internet: www.esrl.noaa.gov/gmd/ccgg/trends/global.html
• UNFCCC, 1993, The United Framework Convention on Climate Change. United Nations
• 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
• UNFCCC, 2015, 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 
• Wang, R., et al., 2016, Estimation of global black carbon direct radiative forcing and its uncertainty constrained by observations. 121(10), Journal of Geophysical Research 5948-5971. doi:doi:10.1002/2015JD024326
• WMO, 2002, Scientific Assessment of Ozone Depletion: 2002 Global Ozone Research and Monitoring Project–Report No. 47 (World Meteorological Organization), Geneva, Switzerland

Indicator definition

The indicator shows the observed trends in greenhouse gas concentration levels. Greenhouse gases differ in the way they affect the climate system. In order to sum the effects of the individual greenhouse gases 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).

Units

Atmospheric concentration in parts per million in CO₂ equivalents (ppm CO₂e)

Policy context and targets

Context description

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 level (UNFCCC, 2009) and the EU level (October 2008 Environment Council conclusions), this ‘dangerous anthropogenic interference’ has been recognised by formulating an ambition of keeping 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).

Targets

No targets have been specified

Related policy documents

No related policy documents have been specified

Methodology

Methodology for indicator calculation

The trend in this indicator is based on combining data for numerous gases that affect the radiation balance on earth:

• Greenhouse gases included in the Kyoto Climate Protocol: carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and three groups of fluorinated gases (HFC’s, PFC’s, SF₆)
• Gases under the Montreal Protocol on ‘Substances that deplete the ozone layer’ CFCs, HCFCs, CH₃Cl’s, CH₃Br’s and halons
• Other forcing agents and greenhouse gases that are not included in global treaties are dealt with at a regional level (e.g. UNECE Convention on Long Range Transboundary Air Pollution). This includes aerosols (sulphate, black carbon, organic carbon, nitrate, mineral dust), tropospheric and stratospheric ozone (O₃), stratospheric water vapour and aircraft contrails. Both the direct forcing effect of these agents and the indirect effect through aerosol-cloud interaction are included.

The trends in global average concentration levels of atmospheric CO₂ for the period from 1950 are based on data available from the NOAA observatory (NOAA, 2020). Trend data for CH₄, N₂O, the fluorinated gases and other compounds under the Montreal Protocol were derived from station data that are available in the AGAGE (2020) data set. The global figures were derived by averaging the data from different observatory stations across the world, equally distributed over the northern and southern hemisphere. Pre-observational data for CO₂, CH₄ and N₂O are based on ice core data (Etheridge et al, 1998, 2002, Machida, et al, 1995). Pre-observational data for F-gases are the result of modeling (Meinshausen et al, 2017). Data for the non-protocol agents are taken from IPCC (2013) and Myhre et al (2017).

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, and N₂O, O₃ (both stratospheric and tropospheric) and vapour based on IPCC (2013). IPCC (2013) and Myhre et al (2017) estimates were used for the radiative forcing of non-protocol related compounds.

The equations used to compute contributions by the individual gases are presented below:

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

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

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 factor of 0.154 (-) for correcting forcing of Montreal Protocol gases, based on IPCC (2013) (= -0.05 W/m² or 10 ppm in 2015).

To quantify the total concentration of all greenhouse gases, the direct and indirect effect of multiple aerosols (sulphate, black carbon, organic carbon, nitrate, and mineral dust), and the forcing of tropospheric ozone, stratospheric water vapour, changes in albedo (e.g. due to black carbon) were added. Data on forcing have been based on IPCC (2013), Klimont et al, (2013), Wang et al. (2016), Nazarenko et al., (2017) and Myhre et al, (2017). 

For the aerosols, we used figures, based on the 2013 IPCC report and subsequent literature. First, the direct radiative forcing (RFari) from 1850 to 2010 has been defined for the individual aerosols and tropospheric ozone, using IPCC (2013, Figures 8.7 and 8.8, Figure TS7, Table 8.6). Then, updates from Wang et al. (2014), Wang et al. (2016), Nazarenko et al., (2017) and Myhre et al (2017) have been used for the period 1990 to 2015. Finally, recent numbers of emissions have been used to extrapolate the time series until 2018. See table below for historic forcing of some of the aerosols and ozone. The historical indirect forcing of aerosols through cloud interaction (RFaci) has been defined by using the total forcing as provided by the IPCC (2013, annex II) and the direct forcing. This is done for the period up to 2011. Due to the lack of more recent data, the indirect forcing has been kept constant after 2011. Note that the net effect 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).

The climate forcing due to stratospheric vapour, aircraft contrails and land use are taken from IPCC (2013, annex II). The numbers have been kept constant at their 2011 value for the period 2011-2018, due to lack of more recent data.

Calculated direct radiative forcing for multiple aerosols and tropospheric ozone for a number of past years (W/m²)

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

No methodology references available.

Data specifications

EEA data references

• No datasets have been specified here.

External data references

• Peak and 2100 concentration levels of total greenhouse gasses in the atmosphere
• AGAGE Data & Figures
• Greenhouse Gases (NOAA)

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), with each network consisting of several stations distributed across the globe.

Absolute accuracies of global average annual concentrations are around 1% for CO₂, CH₄, N₂O and CFCs; for HFCs, PFCs, and SF6, absolute accuracies are between 10 % and 20 %. The largest uncertainties have been determined for the concentration of different aerosols such as sulphur, and black and organic carbon. The uncertainty in the trend of these aerosols could be 50-60 % (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 RF values from 1750 to 2011(and 10 % and 90 % confidence ranges) (source: IPCC, 2013)

Data sets uncertainty

Uncertainty in relation to peak concentration values

The IPCC (2014, 2018) has modelled concentration levels of all greenhouse gases in the atmosphere, which are consistent with keeping the global average temperature increase below 1.5 and 2 °C, for various probability levels. The indicator assessment uses those peak concentrations and 2100 concentration values of greenhouse gases in the atmosphere that, according to the IPCC, give a likely (67 %) probability of staying below a 1.5 °C and a 2 °C temperature increase by the end of the century. This means that the concentration could peak somewhere between 2020 and 2100, and then become (strongly) reduced again (e.g. through zero emissions and even negative emissions though active CO₂ removal). The IPCC also introduced the temperature overshoot feature, which means that temperature targets may become temporarily exceeded, followed by a decrease later in the century. Risking such a temperature overshoot would put an additional burden on strong and even negative emissions reductions and, hence, this has not been considered in the messaging of this indicator. These values and other probability values are presented below to indicate the uncertainty and variation in peak and 2100 concentration values.

Peak and 2100 concentration levels of total greenhouse gases in the atmosphere consistent with keeping the global average temperature increase below 1.5 °C and 2 °C, for various probability levels. The ranges are given in brackets, (all based on IPCC, 2014, 2018)

Rationale uncertainty

No uncertainty has been specified