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).
Methodology for indicator calculation
The trend 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 (HFCs, PFCs, 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, land-use related), 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, while more historic data are derived from Etheridge et al. 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 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 . Pre-observational data for F-gases are the result of modeling . Data for the non-protocol agents are taken from IPCC and Myhre et al.
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 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:
Trace gas | Parameterisation, radiative forcing (F), in Wm | Constants(IPCC, 2013; Butler, 2009) |
|---|---|---|
| CO2 | change in F = alpha ln (C/C0) C and C0 are the current and pre-industrial concentrations (ppm) of CO2, respectively | alpha = 5.35 |
| CH4 | change in F = alpha (sq. root of M - sq. root of M0 ) - (f (M,N0) - f (M0,N0)) where f (M,N)= 0.47 ln [1+2.01*10-5 (MN)0.75 + 5.31*10-15 M(MN)1.52] M and M0 are the current and pre-industrial concentrations (ppb) of CH4, respectively; N and N0 are the current and pre-industrial concentrations (ppb) of N2O, respectively. | alpha = 0.036 |
| N2O | change in F = alpha (sq. root of N - sq. root of N0 ) - (f (M0,N) - f (M0,N0)) where f (M,N)= 0.47 ln [1+2.01*10-5 (MN)0.75 + 5.31*10-15 M(MN)1.52] M and M0 are the current and pre-industrial concentrations of CH4, respectively; N and N0 are the current and pre-industrial concentrations of N2O, respectively. | alpha = 0.12 |
| HFC, PFC & SF6 | change in F = alpha (X-X0) X and X0 are the current and pre-industrial concentrations (ppb) of gas X, respectively. | Values for alpha, depending on molecule (see below), taken from WMO, 2002. |
A similar approach was applied for the Montreal Protocol gases (i.e. CFCs & HCFCs):
| CFCs & HCFCs | change in F = alpha (X-X0) X and X0 are the current and pre-industrial concentrations (ppb) of gas X, respectively. | Values for alpha, depending on molecule (see below), taken from WMO, 2002. |
Table A1 Overview of alpha values used for Kyoto and Montreal Protocol Gases (see also CDIAC, 2011a, 2011b)
Kyoto Protocol gases | Montreal Protocol gases | ||
HFC-23 | 0.191 | CFC-11 | 0.259 |
HFC-134a | 0.167 | CFC-12 | 0.320 |
HFC-32 | 0.111 | CFC-13 | 0.278 |
HFC-125 | 0.234 | CFC-113 | 0.301 |
HFC-143a | 0.168 | CFC-114 | 0.314 |
HFC152a | 0.102 | CFC-115 | 0.246 |
HFC-227ea | 0.273 | HCFC-22 | 0.214 |
HFC-136fa | 0.251 | HCFC-124 | 0.207 |
HFC-245fa | 0.251 | HCFC-141 | 0.161 |
HFC365mf | 0.228 | HCFC-142 | 0.193 |
CF4 | 0.099 | CCl4 | 0.166 |
C2F6 | 0.261 | CH3Cl | 0.005 |
SF6 | 0.57 | CHCl3 | 0.074 |
SF5CF3 | 0.567 | CHCCl3 | 0.065 |
NF3 | 0.204 | CH3Br | 0.004 |
PCF-318 | 0.314 | H1211 | 0.300 |
PCF-218 | 0.314 | H1301 | 0.299 |
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| H2402 | 0.312 |
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.
Also new data and information has been used to compute the forcing of aerosols and other agents (based on figures in the 2021 IPCC report and underlying literature like Meinshausen et al., 2020) (see Table A2). Firstly, the direct radiative (RFari) and indirect forcing of aerosols thought the cloud interaction (RFaci) and tropospheric ozone forcing has been derived from 1850 to 2019 (see Annex III, IPCC 2021). Then we summed the forcing of remaining gasses (e.g. stratospheric water vapour, aircraft contrails) and processes (esp. land use) to define the total of non-protocol gasses. Note that this forcing is stronger (more negative) than reported in earlier literature (e.g. IPCC, 2013) mainly due to the stronger signal of the indirect cloud effect.
Table A2 Calculated direct radiative forcing for multiple aerosols and tropospheric ozone for a number of past years (W/m2)
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| Source: Based on IPCC, 2021 Annex III |
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.
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’ . Both at the global level 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’ . The agreements made at the COP in Glasgow (2021) and Sharm el-Sheikh (2022) even stated to “drive efforts to limit the temperature increase to 1.5 °C".
Targets
No targets have been specified
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% .
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), based on ranges in IPCC, 2021.
Table A3: Best estimate ERF values from 1750 to 2020 (and 5% and 95% confidence ranges)
Group | Forcers | Best estimate | 5% | 95% |
Kyoto Protocol | CO2 | 2.11 | 1.86 | 2.35 |
| CH4 | 0.64 | 0.51 | 0.77 |
| N2O | 0.20 | 0.17 | 0.23 |
| HFC, PFCs, SF6 | 0.06 | 0.05 | 0.07 |
Montreal Protocol | Montreal F gases | 0.33 | 0.26 | 0.39 |
Non-Protocol | Tropospheric O3 | 0.47 | 0.24 | 0.71 |
| Aerosols direct radiation effect | -0.22 | -0.46 | 0.04 |
| Aerosols indirect cloud interaction | -0.83 | -1.43 | -0.25 |
| Stratospheric Vapour | 0.05 | 0.00 | 0.10 |
| Land use | -0.2 | -0.30 | -0.10 |
| BC on snow | 0.08 | 0.00 | 0.18 |
| Air contrails | 0.06 | 0.02 | 0.10 |
| Total | 2.75 | 1.98 | 3.51 |
(source: IPCC, 2013)
Data sets uncertainty
Uncertainty in relation to peak concentration values
The IPCC 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 (67%, 50% and 37% staying below). This 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 stronger decrease later in the century. Risking such a temperature overshoot would put an additional burden on strong and even n egative emissions reductions. 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 .
Probability of staying | Temperature | 1.5°C target |
| 2.0°C target |
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| Peak | Concentration | Peak | Concentration |
>67 % (likely) | No | 465 (445-485) | 411 (390-430) | 505 (470-540) | 480 (460-500) |
| Yes | 485 (465-505) | 405 (385-425) | 520 (505-540) | 465 (440-480) |
50 % (33 %-67 %) | No | 485 (465-500) | 440 (430-445) | 540 (510-560) | 520 (500-535) |
| Yes | 525 (520-540) | 425 (410-435) | 560 (540-575) | 505 (480-530) |
<33 % (unlikely) | No | 500 (480-530) | 470 (445-480) | 580 (550-615) | 570 (540-600) |
| Yes | 535 (510-550) | 450 (425-460) | 600 (560-630) | 540 (505-580) |
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Rationale uncertainty
No uncertainty has been specified.
