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Indicator Assessment
Greenhouse gases in the atmosphere have the ability to affect the radiation balance on Earth and are an important determinant of the global climate. Without these gases, the global average temperature would be about 32 ºC lower than it is today (i.e. -18 ºC instead of the current +14 ºC).
Greenhouse gas concentrations in the atmosphere have increased during the 20th century and the first part of the 21st century, mainly as a result of human activities related to the use of fossil fuels (e.g. for power generation), agricultural activities and land-use change (mainly deforestation) (IPCC, 2013, see also Carbon Budget at Global Carbon Project, www.globalcarbonproject.org/carbonbudget/index.htm). The increase has been particularly rapid since 1950. In 1970, the carbon dioxide (CO2) concentration was 50 ppm above the pre-industrial level of more than 200 years ago. The second increase of 50 ppm took only around 30 years, the increase in the last 10 years was more than 20 ppm.
The various greenhouse gases (Text box 1) affect the climate system differently. To evaluate greenhouse gas concentrations in the atmosphere in relation to temperature change, it is important to consider all gases, i.e. the long-living GHGs under the Kyoto Protocol, those under the Montreal Protocol (direct and indirect), as well as ozone, water vapour and aerosols (IPCC, 2013).
Text box 1: Greenhouse gases and their inclusion in international legislation
Greenhouse gases can intercept solar radiation and, thus, affect the climate system. In order to control the emissions of such gases, many of them are included within multiple international agreements, including the UNEP Montreal Protocol on Substances that deplete the ozone layer (1987) and the Kyoto Protocol to the UNFCCC, which aims to limit global warming (1997).
Total greenhouse gas concentration and its relevance
Considering all greenhouse gases, the total CO2 equivalent concentration reached a level of 445 ppm CO2eq in 2015, which was an increase of 3.7 ppm compared with 2014, and 32 ppm compared with 2004 (Figure 1).
The CO2 equivalent concentration levels can be assessed in the light of the 2015 Climate Agreement in Paris. The objective of this agreement is 'to hold 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' (UNFCCC, 2015). The CO2 equivalent concentration level can be defined that corresponds to a specific equilibrium warming (i.e. the long-term temperature increase). Table 1 shows that the current atmospheric total greenhouse gas concentration is already close to a level that would keep long-term warming below the 'temperature threshold' of +1.5 oC, assuming a 50 % likelihood of exceedance. A concentration level of 480 ppm CO2eq is 'likely' (= 67 % probability) to limit the long-term global temperature increase to less than 2 oC (Table 1). This level is only about 35 ppm higher than the current concentration. Assuming that the 2006–2015 trend of annual increases in total greenhouse gas concentration (i.e. 3.5 ppm per year) will continue in the coming years, this 480 ppm 'threshold' could be exceeded within the next 10 years (Table 1). A 50 % likelihood ('about as likely as not') of keeping the increase in the global average temperature below 2 °C corresponds with a concentration level of 500 ppm CO2eq (range 480-530 ppm) (IPCC, 2013). Under a continuation of current trends, this level will be reached in about 16 years.
Table 1: Long-term concentrations of greenhouse gases in the atmosphere, consistent with keeping the global average temperature increase below 1.5 and 2 °C, for various probability levels. The number of years within which these concentration levels could be exceeded are provided in brackets, given the trend of the past 10 years (source, IPCC, 2013; Meinshausen et al., 2009)
|
Probability of staying below target |
1.5 °C |
2.0 °C |
|
67 % (likely) |
Not calculated |
480 ppm (<10 years) |
|
50 % (about as likely as not) |
450 ppm (430-480 ppm) (exceeded in 2012) |
500 ppm (480-530 ppm) (16 years) |
|
<33 % (unlikely) |
>480 ppm (>10 years) |
>530 ppm (>25 years) |
|
|
|
|
Long-term data sets on tropospheric ozone concentrations are difficult to develop, due to the scarcity of representative observation sites with long records and to the large spatial heterogeneity (IPCC, 2013).
Furthermore, note that for the total equivalent greenhouse gas concentration values presented, new estimates on the climate forcing of aerosols have been used (IPCC, 2013). Overall, aerosols have a cooling effect, although certain aerosols (e.g. black carbon) have a warming effect. The cooling effect is now estimated to be 0.8 W/m2, which is 0.4 W/m2 smaller (i.e. less cooling) than previously thought, mainly due to the greater warming potential of black carbon (see also the section on uncertainty). The forcing trend of these aersols is decreasing slightly (=less cooling)
Relevance of individual gases
An assessment of the contribution by the various groups of greenhouse gases has shown that the non-protocol gases (NPGs) are compensating for nearly 20 % of the current forcing induced by the Kyoto Protocol Gasses (KPGs) and Montreal Protocol (MPGs). The MPGs, as a group, contribute about 9 % to current forcing (Figure 2). Concentrations of these gases peaked around 2000 and have been declining ever since, due to natural removal processes (IPCC, 2013). Most of the forcing is caused by KPGs, especially CO2.
The average annual CO2 concentration reached 400 and 403 ppm in 2015 and 2016, respectively (Figure 3). This is an increase of more than 119 ppm (+43 %), compared with pre-industrial levels (i.e. before 1800) (NOAA, 2015). Overall, CO2 concentrations in the atmosphere exceed the range of concentrations as recorded in ice cores over the past 800 000 years (IPCC, 2013).
The average annual concentration of methane (CH4) — the second most important greenhouse gas — reached a level of 1 834 parts per billion (ppb) in 2015, an increase of a factor of 2.4, compared with pre-industrial levels (Figure 4). CH4 concentration levels in the atmosphere were relatively stable over the 2000–2006 period, but have been steadily increasing again since 2007, increasing by 6 ppb per year over the last decade. The exact drivers of this increase are still debated (IPCC, 2013).
The nitrous oxide (N2O) concentration level was 328 ppb in 2015, 20 % above the pre-industrial level (Figure 5). The rate of change has increased slightly over the past 20 years, from 0.6 ppb per year to 0.8 ppm per year.
The group of fluorinated gases (F gases) within the scope of the Kyoto Protocol can be grouped into HFCs, PFCs and SF6. The HFCs group is very broad. The concentrations of these F gases have increased substantially over the past decades. Their contribution to the current climate forcing is currently still limited, although steadily increasing (from 0.5 % in 1990, to 0.8 % in 2004 and to 1.4 % in 2015, see Figure 2). Their contribution is expected to continuously increase in the near future, due to the long lifetimes of most F gases (in some cases more than 1 000 years) and to the increase in the emissions of new HFCs, such as HFC-134a (a CFC substitute, as they do not have an ozone depleting impact in the stratosphere).
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 CO2-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 with 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 in the long term.
Atmospheric concentration is measured in parts per million CO2 equivalents (ppm CO2e).
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 with 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 concentrations 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, PBL (2017), 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 the range of 430-480 (average 450) ppm CO2e to achieve a 50 % probability of keeping the increase in global mean temperature below 1.5 °C. This concentration is close to the one currently observed. The atmospheric concentration of all greenhouse gases that would be consistent with a maximum temperature increase of 2 °C is between 480 ppm and 530 ppm CO2e. Respectively, these figures give a 67 % and <33 % probability of staying below these temperature thresholds (see Table 1).
In 1992, countries adopted the 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 achieving greenhouse gas emission 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 with pre-industrial levels[3] (no more than 1 °C above today's level). Scenarios consistent with this target show global emission reductions of 40-70 % by 2050, compared with 2010 levels (i.e. 40-60 % for scenarios with negative emissions, and 60-70 % 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 emission 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.
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:
|
Trace gas |
Parameterisation, radiative forcing, change in forcing (F) (Wm-2) |
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 depend on molecules (see below), taken from WMO, 2002. |
A similar approach was applied for the Montreal Protocol gases (i.e. CFCs and HCFCs):
|
CFCs and 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. |
Overview of used alpha values for chlorine Kyoto and Montreal Gases (also see CDIAC):
|
Kyoto gases |
Alpha values |
Montreal gases |
Alpha values |
|---|---|---|---|
|
HFC-23 |
0.16 |
CFC-11 |
0.25 |
|
HFC-134a |
0.159 |
CFC-12 |
0.32 |
|
CF4 |
0.116 |
CFC-13 |
0.25 |
|
C2F6 |
0.26 |
CFC-113 |
0.3 |
|
SF6 |
0.52 |
CFC-114 |
0.31 |
|
HFC-23 |
0.16 |
CFC-115 |
0.18 |
|
|
|
HCFC-22 |
0.2 |
|
|
|
HCFC-141 |
0.14 |
|
|
|
HCFC-142 |
0.163 |
|
|
|
CCl |
0.13 |
|
|
|
CH4CC |
0.01 |
|
|
|
CH4CCl3 |
0.06 |
|
|
|
CH3Br |
0.05 |
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 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 emission 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.
|
Gas (group) |
2014 |
2000 |
1990 |
1970 |
|---|---|---|---|---|
|
Sulphate |
-0.38 |
-0.42 |
-0.44 |
-0.36 |
|
BC |
0.64 |
0.61 |
0.58 |
0.45 |
|
OC (incl. sec.) |
-0.32 |
-0.31 |
-0.30 |
-0.27 |
|
Nitrate |
-0.11 |
-0.11 |
-0.11 |
-0.05 |
|
Other gases |
-0.60 |
-0.61 |
-0.61 |
-0.61 |
|
Total |
-0.77 |
-0.84 |
-0.87 |
-0.84 |
If measurement data from a particular station are missing for a certain year, the global trend is derived from data available from other stations.
Global average concentrations since approximately 1980 are determined by averaging measurements from several ground-station networks (SIO, NOAA, CMDL, ALE, GAGE, AGAGE), where each network consists 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:
|
Group |
Forcers |
Best estimate |
10 % |
90 % |
|---|---|---|---|---|
|
Kyoto |
CO2 |
1.82 |
1.63 |
2.01 |
|
|
CH4 |
0.49 |
0.44 |
0.54 |
|
|
N2O |
0.18 |
0.15 |
0.21 |
|
|
HFC, PFCs, SF6 |
0.03 |
0.027 |
0.033 |
|
Montreal |
Montreal F-gases |
0.27 |
0.11 |
0.41 |
|
Non-protocol |
Tropospheric O3 |
0.4 |
0.2 |
0.6 |
|
|
Sulfate aerosols |
-0.40 |
-0.60 |
-0.20 |
|
|
Nitrate aerosols |
-0.11 |
-0.19 |
0.03 |
|
|
Black carbon |
0.64 |
0.22 |
1.02 |
|
|
Organic carbon |
-0.32 |
-0.42 |
-0.22 |
|
|
Cloud Interaction |
-0.55 |
-1.1 |
0.00 |
|
|
Land use |
-0.15 |
-0.25 |
-0.05 |
|
|
Vapour |
0.07 |
0.02 |
0.12 |
|
|
Mineral dust |
-0.10 |
-0.30 |
0.10 |
|
|
Air contrails |
0.05 |
0.00 |
0.10 |
|
|
Total |
2.33 |
-0.06 |
4.70 |
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 because of 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.
Comparability of direct measurements is good. Although methods for calculating radiative forcing and CO2e 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.
Atmospheric concentrations of greenhouse gases are a well-established indicator of changes in atmospheric composition, which causes changes in the global climate system.
For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/atmospheric-greenhouse-gas-concentrations-10/assessment or scan the QR code.
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