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Indicator Specification
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 32oC lower than it is now (i.e. -18oC instead of the current +14oC 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:
Radiative forcing estimates |
AR4 (1970-2005) |
AR5 (1970-2011) |
Comments |
---|---|---|---|
Well-mixed greenhouse gases |
2.63 |
2.83 |
Change due to increase in concentrations |
Tropospheric ozone |
0.35 |
0.4 |
Slightly modified estimate |
Stratospheric ozone |
-0.05 |
-0.05 |
Estimate unchanged |
Stratospheric water vapour from CH4 |
0.07 |
0.07 |
Estimate unchanged |
Aerosol–radiation interactions |
-0.5 |
-0.35 |
Re-evaluated to be smaller in magnitude, mainly due to more warming by black carbon |
Aerosol–cloud interactions |
-0.7 |
-0.45 |
Re-evaluated to be smaller in magnitude |
Surface albedo (land use) |
-0.2 |
0.15 |
Re-evaluated to be smaller in magnitude |
Surface albedo (black carbon aerosol on snow and ice) |
0.1 |
0.04 |
Re-evaluated to be weaker |
Contrails |
0.01 |
0.01 |
Unchanged |
Combined contrails and contrail induced cirrus |
Not estimated |
0.05 |
- |
Total anthropogenic |
1.6 |
2.3 |
Stronger positive due to changes in various forcing agents |
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.
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.
Atmospheric concentration in parts per million in CO2-equivalent (ppm CO2 eq.) and parts per billion (ppb) of individual species.
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 CO2-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).
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.
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:
Trace gas |
Parameterisation, Radiative forcing, change in 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 molecule (see below), taken from WMO, 2002. |
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:
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. |
Overview of used alpha values for chlorine Kyoto and Montreal Gases (also see CDIAC, 2011 a, b)
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 |
|
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. 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.
Gas (group) |
2012 |
2000 |
1990 |
1970 |
---|---|---|---|---|
Sulphate |
-0.39 |
-0.42 |
-0.46 |
-0.37 |
BC |
0.64 |
0.61 |
0.58 |
0.45 |
OC |
-0.29 |
-0.28 |
-0.27 |
-0.25 |
Nitrate |
-0.11 |
-0.11 |
-0.11 |
-0.05 |
Other gases |
-0.75 |
-0.82 |
-0.82 |
-0.82 |
Total |
-0.90 |
-1.04 |
-1.08 |
-1.03 |
If measurements from a station are missing for a certain year, the global trend is derived from available stations data.
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:
Forcers |
Best estimate |
5% |
95% |
---|---|---|---|
Well-mixed greenhouse gases |
2.83 |
2.26 |
3.4 |
Tropospheric and stratospheric Ozone |
0.35 |
0.14 |
0.56 |
Vapour |
0.07 |
0.02 |
0.12 |
Land use |
-0.15 |
-0.25 |
-0.05 |
Black carbon on snow |
0.04 |
0.02 |
0.09 |
Contrails |
0.05 |
0.02 |
0.15 |
Aerosols |
-0.9 |
-1.9 |
-0.1 |
Total |
2.3 |
1.1 |
3.3 |
Group |
Forcers |
Best estimate |
5% |
95% |
---|---|---|---|---|
Kyoto |
CO2 |
1.82 |
1.47 |
2.17 |
|
CH4 |
0.51 |
0.28 |
0.74 |
|
N2O |
0.18 |
0.14 |
0.22 |
|
HFC, PFCs, SF6 |
0.03 |
0.02 |
0.04 |
Montreal |
Montreal gases |
0.26 |
0.09 |
0.43 |
Non-Protocol |
Tropospheric O3 |
0.35 |
0.15 |
0.55 |
|
Sulfate aerosols |
-0.40 |
-0.60 |
-0.20 |
|
Nitrate aerosols |
-0.07 |
-0.17 |
0.03 |
|
Black carbon |
0.64 |
0.24 |
1.04 |
|
Organic carbon |
-0.29 |
-0.49 |
-0.09 |
|
Cloud Interaction |
-0.45 |
-0.90 |
0.00 |
|
Land use |
-0.15 |
-0.25 |
-0.05 |
|
Vapour |
0.07 |
0.02 |
0.12 |
|
Other |
-0.15 |
-0.35 |
0.05 |
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.
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.
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For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/atmospheric-greenhouse-gas-concentrations-4 or scan the QR code.
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