Indicator Assessment

Atmospheric greenhouse gas concentrations

Indicator Assessment

Prod-ID: IND-2-en

Also known as: CSI 013

Published 02 Apr 2008 Last modified 11 May 2021

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Topics: Climate change mitigation

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The global average concentrations of various greenhouse gasses in the atmosphere reached their highest levels ever recorded, and continue increasing. The combustion of fossil fuels from human activities and land-use changes are largely responsible for this increase.
The concentration in 2006 of the six greenhouse gases (GHG) included in the Kyoto Protocol has reached 433 ppm CO2 equivalent, which is an increase of 155 ppm compared to the pre-industrial level. Considering all GHGs (incl. ozone and various cooling aerosols), the concentration is 393 ppm CO2 equivalents, which is 115 ppm higher than in pre-industrial times. The concentration of CO2 - the most important greenhouse gas - has reached in 2006 a level of 381 ppm, showing an increase of 103 ppm compared to the pre-industrial level.
Under the IPCC scenarios the overall concentration of the six Kyoto gasses is projected to increase up to 638-1360 ppm CO2 -equivalent by 2100, whereas the concentration of all GHGs may increase up to 608-1535 ppm CO2 -equivalent. The global atmospheric GHG concentration of 450 ppm CO2-equivalent may be exceeded between 2015 and 2030.

Observed and projected changed in the overall Kyoto gasses (Fig 1a) and all greenhouse gasses, expressed in CO2-equivalents (IPCC, 2007a, partly based on IPCC, 2001).

Note: There is no historical trend in figure 1a due to the unavailability of long-term data for aerosols and ozone

Data source:

IPCC, 2007a

Downloads and more info

The concentration of greenhouse gases (GHG) in the atmosphere has increased during the 20th century, extremely likely, defined as >95% probability (IPCC, 2007), caused mainly by human activities related to the use of fossil fuels (e.g. for electric power generation), agricultural activities and land-use change (mainly deforestation). The increase of all GHG gasses has been particularly rapid since 1950. The first 50 ppm CO2 eq. increase above the pre-industrial value of carbon dioxide (CO2) for example, was reached in the 1970s after more than 200 years, whereas the second 50 ppm was achieved in about 30 years. In the 10 years the highest average growth rate has been recorded for any decade since atmospheric CO2 measurements began (IPCC, 2007a).
Compared with the pre-industrial era (before 1750), concentrations up to 2006 of CO2, methane (CH4) and nitrous oxide (N2O) have increased by 37%, 153%, and 19%, respectively. The CO2 increase is nearly entirely caused by human activities (of which about 2/3 caused by fossil fuel use, 1/3 due to land-use change), whereas humans are directly responsible for two third (mainly fossil fuel exploitation, rice agriculture, biomass burning, landfills) and one third (as fuel combustion, biomass burning, fertilizer use and some industrial processes) of the increase in CH4 and N2O, respectively. The present concentrations of CO2 (381 parts per million, ppm) and CH4 (1775 part per billion, ppb) have not been exceeded during the past 420 000 years (for CO2 probably not even during the past 20 million years); the present N2O concentration (320 ppb) has not been exceeded during at least the past 1 000 years. The fluorine-containing Kyoto Protocol gases (HFCs, PFCs and SF6) are very effective absorbers of radiation and as such even small amounts can affect significantly the climate system. Their concentrations have increased by large factors (between 1.3 and 4.3, depending on the gas) between 1998 and 2005. As such their role in the total climate forcing is rapidly increasing in the past years. The Montreal Protocol gases (CFCs, HCFCs, and CH3CCl3) have peaked in 2003 and are now beginning to decline due to natural removal processes (IPCC, 2007a) and policy measures. For example, governments have recently agreed to freeze production of HCFCs in developing countries in 2013 and bring forward the final phase-out date of these chemicals by ten years in both developed and developing countries (Montreal/Nairobi, 22 September 2007). See core set indicator 'production and consumption of ozone depleting substances'. Finally, the contribution of stratospheric ozone to the climate system is decreasing in the recent decades (although it is unclear whether this is indicative for a recovery of the global ozone layer), whereas assessments of long-term trends in tropospheric ozone are difficult due to the scarcity of representative observing sites with long records and the large spatial heterogeneity (IPCC, 2007a).
The overall concentration of the six Kyoto GHGs (i.e. CO2, CH4, N2O, HFC, PFC, SF6) has increased from 278 ppm CO2 eq. pre-industrial to 433 ppm CO2-equivalents in 2006, thus an increase of 155 ppm. Considering all long living greenhouse gasses (i.e. the Kyoto Gasses plus the CFCs & HCFCs, that are included in the Montreal Protocol), a level of 460 ppm CO2-equivalents has been reached for 2006. Including also ozone and various aerosols, the GHG concentration has reached a level of 393 ppm CO2 equivalents in 2006. Thus, aerosols are important for the global climate, since they have in general a strong cooling affect - although some aerosols enhance the warming. In total aerosols are compensating for about 70% of the climate forcing by CO2. Note that these aerosols have a relative short lifetime, the emissions will be reduced due to non-climate related policy measures and as such their importance for the future climate will diminish. Likewise, the Montreal Protocol gases (CFCs, HCFCs, and CH3CCl3) as a group contributed significantly (about 18%) to the current warming. Also their contribution is likely to decrease in the near-term future due to policy measures (IPCC, 2007a) such as the phase out of these substances - see core set indicator 'consumption and production of ozone depleting substances'.
The IPCC (2001, 2007a) showed various projected future greenhouse gas concentrations for the 21st century, varying due to a range of scenarios of socio-economic, technological and demographic developments (Figure 1, Table 1). These scenarios assume no implementation of specific climate-driven policy measures. Under these scenarios, the overall concentration of the six Kyoto gasses is projected to increase up to 638-1360 ppm CO2-equivalent by 2100, whereas the concentration of all GHGs (incl. aerosols) may increase up to 608-1535 ppm CO2-equivalent by 2100 (Fig. 1). Note the importance of the non-Kyoto gasses (especially aerosols) is projected to strongly decrease, resulting in decreasing differences between only-Kyoto and all-GHG projections, with the exception of the A1FI scenario (where especially Montreal gasses and ozone remain high).
The IPCC projections show that a global atmospheric GHG concentration of 450 ppm may be exceeded between 2010-2015 (in case of Kyoto gasses only) or between 2020-2030 (all GHGs). A level of 550 ppm CO2-equivalent may become exceeded a decade later (Figure 1). Substantial global emission reductions are therefore needed to remain below these targets or return back to these levels after an overshoot. The probability to stay below the 2^o C global temperature increase above pre-industrial levels (EU target) is about 50% at a total greenhouse gas concentration of 450 ppm CO2 equivalent - see also core set indicator 'global and European temperature'.

Table 1: Projected changes in atmospheric GHG concentration (considering either Kyoto gasses only or all GHGs) in ppm CO2 equivalent

		A1B	A1T	A1FI	A2	B1	B2
Kyoto only	2020	489	478	484	484	475	470
	2050	645	613	707	653	571	575
	2100	877	722	1360	1196	638	800

all GHGs	2020	416	442	417	407	416	432
	2050	605	622	686	575	515	555
	2100	861	717	1535	1256	608	808

(source: IPCC, 2001, 2007a)

Supporting information

Indicator definition

The indicator shows the observed trends of greenhouse gas concentrations. The various greenhouse gases have been grouped in three different ways (see rationale). Except for the concentration of individual GHGs, the effect on the enhanced greenhouse effect is presented as CO₂-equivalent concentrations, which is the CO₂concentration 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.

Units

Atmospheric concentration in parts per million in CO₂-equivalent (ppm CO₂-eq.).

Rationale

Justification for indicator selection

Various greenhouse gases (GHG) exist. They are characterized by 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 32^oC lower than it is now (i.e. -18^oC instead of the current +14^oC global mean temperature average).

The GHGs differently affect the climate system. 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 (Figure 5). 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 (2007a).

There are, in general, three ways the GHG equivalent concentration can be aggregated, all presented here. Firstly, 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 antropgenic 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 are also be included to show the net effect of all GHGs on the earth’s radiation balance, and as such, the climate system. The data record of this third approach is shorter due to the lack of long-term data series for ozone. The consideration of all GHGs together can provide a lower concentration level compared to the other two approaches due to the cooling effect of aerosols. For the current period (e.g. in 2009), 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.

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.

Scientific references

IPCC (2001) Climate Change 2001 IPCC (2001). Climate Change 2001: The Physical Science Basis. (eds.) J. T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P. J. van der Linden and D. Xiaosu. Working Group 1 Contribution to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Appendix II.3.11 on GHG forcing)
IPCC (2007a) Climate Change 2007 IPCC (2007a). 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)
IPCC (2007b) Climate Change 2007 IPCC (2007b). Climate Change 2007: Mitigation. Eds. B. Metz, O. R. Davidson, P. R. Bosch, R. Dave, L. A. Meyer, Working Group III contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), Technical Summary, chapters 3 (Issues related to mitigation in the long term context) and 11 (Mitigation from a cross sectoral perspective)
Meinshausen M (2006) Meinshausen, M., (2006). What Does a 2°C Target Mean for Greenhouse Gas Concentrations. In: Avoiding Dangerous Climate Change (eds. Schellnhuber HJ, Cramer W, Nakicenovic N, Wigley T & Yohe G), pp. 265-280. Cambridge University Press, Exeter.
UNFCCC (1993) UNFCCC (1993). The United Framework Convention on Climate Change. United Nations.
WMO (2010) WMO (2010) Greenhouse Gas Bulletin: The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2009.
Butler (2009) Butler, J. (2009). The NOAA annual greenhouse gas index (AGGI). NOAA Earth System Research Laboratory, Boulder. (Update September 2009)
Dlugokencky et al. (2009). Dlugokencky, E., J., and Bruhwiler, L. (2009). Observational constraints on recent increases in the atmospheric CH4 burden, Geophys. Res. Lett., 36 (18), 5p
Mascarelli (2009) Mascarelli, A., L. (2009). A sleeping giant? Nature Reports. Vol 3: 46-49
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;
Moss et al. (2010) Moss, R.,H., Edmonds, J.,A., Hibbard, K.,A., Manning, M.,R., Rose, S.,K., van Vuuren, D.,P.,, Carter, T.,R., Emori, S., Kainuma, M., Kram, T., Meehl, G.,A., Mitchell, J.,F., Nakicenovic, N., Riahi, K., Smith, S.,J., Stouffer, R.,J., Thomson, A.,M., Weyant, J.,P., Wilbanks, T.,J. (2010). The next generation of scenarios for climate change research and assessment. Nature. 463(7282):747-56.
NOAA (2009) NOAA (2009). The NOAA Annual Greenhouse Gas Index (AGGI). The National Oceanic and Atmospheric Administration
Shakhova et al. (2010) Shakhova N, Semiletov I, Salyuk A, Yusupov V, Kosmach D, and Gustafsson O. (2010). Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf. Science Vol 327: 1246-1250.
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
Bollen et al. (2009) Bollen, J.C, C.J. Brink, H.C. Eerens & A.J.G. Manders (2009) Co-benefits of climate change mitigation policies: literature review and new results. Report 500116005 PBL Netherlands Environmental Assessment Agency, 72 pg.
CDIAC (2011a) 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 (2011b) 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.
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.
Van Vuuren et al. (2008) Van Vuuren, D.P. et al. (2008), Temperature increase of 21st century mitigation scenarios’, PNAS 105 (40): 15258-15262.
Montzka et al. (2011) Montzka, S. A.; E J Dlugokencky, J. H. Butler (2011) Non-CO2 greenhouse gases and climate change Nature 476, 43-50
NOAA (2008) http://www.noaanews.noaa.gov/stories2008/20080423_methane.html
NOAA (2011) http://www.esrl.noaa.gov/gmd/ccgg/trends/global.html (used for CO2 data)
WMO (2002) WMO(2002) Scientific Assessment of Ozone Depletion: 2002, Global Ozone Research and Monitoring Project–Report No. 47, World Meteorological Organization),Geneva,Switzerland.
WMO (2011a) Scientific Assessment of Ozone Depletion: 2010, Global Ozone Research and Monitoring Project–Report 52, World Meteorological Organization),Geneva, Switzerland
WMO (2011b) Greenhouse Gas Bulletin: The State of Greenhouse Gases in the Atmosphere Based on Global Observations through 2010
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

Policy context and targets

Context description

GHG concentrations is a key indicator relevant to international climate negotiations as 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 stabilization levels of GHGs in the atmosphere (Meinshausen, 2006; den Elzen et al., 2007; Van Vuuren et al., 2008). 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 below 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. ). Note that the value for the Kyoto gases only, is higher than when considering all GHGs, due to the cooling affect of aerosols (currently about 1.2 W.m-2 or about 70 ppm CO₂ eq.). According to the scientific literature the probability of staying below the 2^oC becomes very low when stabilization at 550 ppm CO2 eq, ranging between 0% and 37%, considering all GHGs.

Targets

The ultimate objective of the United Nations Framework Convention on Climate Change (UNFCCC) is to achieve 'stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner'.

To reach the UNFCCC objective, the EU has specified more quantitative targets in its 6th environmental action programme (6th EAP) which mentions a long-term EU climate change objective of limiting global temperature rise to a maximum of 2^oC compared with pre-industrial levels. This target was confirmed by the Environment Councils of 20 December 2004 and 22-23 March 2005. Scientific insight shows that in order to have a high chance of meeting the EU policy target of limiting global temperature rise to 2^oC above pre-industrial levels, global GHG concentrations may need to be stabilised at much lower levels, e.g. 450 ppm CO₂-equivalent. Stabilisation of concentrations at well below 550 ppm CO₂-equivalent may be needed and global GHG emissions would have to peak within two decades, followed by substantial reductions by 2050 compared with 1990 levels.

The EU Environment Council (October 2008) adopted the conclusion that to achieve stabilisation in an equitable manner, developed countries should reduce emissions by about 15-30% by 2020 and 80-95% by 2050, below the base year levels (1990).

The Copenhagen Accord (Dec. 2009) recognised the objective of keeping the maximum global average temperature rise below 2 °C, although without specifying the base year or period, and the need for a review in 2015 to consider a possible goal of limiting temperature rise to 1.5 °C using new scientific insights.

Methodology

Methodology for indicator calculation

For atmospheric CO₂, the global average values are directly taken from NOAA (2011).

Global average concentration values for the other gasses are mainly based on CDIAC (2011). Radiative forcings are calculated with approximate equation according to (IPCC, 2001; IPCC, 2007a), based on the observed atmospheric concentrations and using radiative efficiencies for CO₂, CH₄, and N₂O based on IPCC (2007a), and according to WMO (2002) for the other gases. Slightly updated values became available for some substances (WMO, 2011a). However, too late for this assessment. Updates will be included in a next version. The equations used to compute the contribution of the individual gasses are presented below:

Trace gas	Parameterisation, Radiative forcing, change in F (Wm^-2)	Constants
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+2.0110^-5(MN)^0.75+ 5.3110^-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+2.0110^-5(MN)^0.75+ 5.3110^-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, and are taken from WMO, 2002.

In order to calculate the concentration of all long-living Greenhouse Gasses also the Montreal Gasses (i.e. CFCs & HCFCs) need to be included. A similar approach is applied for these gasses

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 Montral Gasses

Kyoto gasses		Montreal gasses
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. Velders et al (2007) estimated that the observed changes in stratospheric ozone between 2000 and 2010 contributed a forcing of -0.06 W.m2 (or about 10 ppm ppm CO2 eq.). To quantify the concentration of all greenhouse gases, important in relation to the 2oC target, the forcing of ozone, water vapor in the atmosphere and 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 ozone and water vapour (IPCC, 2007a). These values are 0.35 and 0.07 W.m-2 for ozone and water vapor, respectively (IPCC, 2007a, pg 204). For aerosols a constant value of -1.2 W.m2 was used back to 2000. Between 1990 and 2000 2% higher values were assumed, and between 1970 and 1990 10% higher values (back to 1.35 W.m2 in 1970) (Bollen et al, 2009).

For all three representations of the GHG concentration (i.e. Kyoto gasses only, all long-living GHG and all Greenhouse gasses including zone and aerosols), the following approach has been used, adding the different climate forcing:

C_eq = C₀ exp ((SUM change in F) / alpha)

C_eq is the current CO₂-equivalent concentration; C is the pre-industrial CO₂ concentration. Summation is over radiative forcings of all greenhouse gases considered.

alpha = 5.35

Methodology for gap filling

If measurements from a station are missing for a certain year, the global trend is derived from available stations data.

Methodology references

NOAA (2011) http://www.esrl.noaa.gov/gmd/ccgg/trends/global.html (used for CO2 data)
CDIAC (2011a) 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 (2011b) 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 (2001) Climate Change 2001: The Physical Science Basis. (eds.) J. T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P. J. van der Linden and D. Xiaosu. Working Group 1 Contribution to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Appendix II.3.11 on GHG forcing)
IPCC (2007a) 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
WMO (2011a) Scientific Assessment of Ozone Depletion: 2010, Global Ozone Research and Monitoring Project–Report 52, World Meteorological Organization),Geneva, Switzerland
Butler (2009) The NOAA annual greenhouse gas index (AGGI). Update September 2009. www.esrl.noaa.gov/gmd/aggi
Velders et al. (2009) 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
Bollen et al. (2009) Bollen, J.C, C.J. Brink, H.C. Eerens & A.J.G. Manders (2009) Co-benefits of climate change mitigation policies: literature review and new results. Report 500116005 PBL Netherlands Environmental Assessment Agency, 72 pg.
WMO (2011b) Greenhouse Gas Bulletin: The State ofGreenhouseGases in the Atmosphere Based on Global Observations through 2010

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 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 calculations have an absolute accuracy of 10% (IPCC, 2001); trends in radiative forcing are much more accurate.

The dominant sources of error for radiative forcing are the uncertainties in modelling radiative transfer in the Earths atmosphere and in the spectroscopic parameters of the molecules involved. Radiative forcing is calculated using parameterisations that relate the measured concentrations of greenhouse gases to radiative forcing. The overall uncertainty in radiative forcing calculations (all species together) is estimated to be 10 % (IPCC, 2001). Radiative forcing is also expressed as CO2-equivalent concentration; both have the same uncertainty. The uncertainty in the trend in radiative forcing/CO2-equivalent concentration is determined by the precision of the method rather than the absolute uncertainty discussed above. The uncertainty in the trend is therefore much less than 10 %, and is determined by the precision of concentration measurements (0.1 %).

It is important to note that global warming potentials are not used to calculate radiative forcing. They are used only to compare the time-integrated climate effects of emissions of different greenhouse gases.

Data sets uncertainty

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.

Rationale uncertainty

Atmospheric concentrations of greenhouse gases are a well-established indicator of changes in atmospheric composition, which causes changes of the global climate system. Here we only present observed trends, having lower uncertainties than model projections.

Data sources

CO2 concentrations
provided by Scripps Institution of Oceanography (SIO)
Climate Change 2001 - The Scientific Basis (dataset URL is not available)
provided by Intergovernmental Panel on Climate Change (IPCC)
HFC-134a and SF6 concentrations
provided by National Oceanic and Atmospheric Administration (NOAA)
CH4 and N2O concentrations (dataset URL is not available)
provided by Advanced Global Atmospheric Gases Experiment (AGAGE)
NCEI Data and Products (NOAA)
provided by National Oceanic and Atmospheric Administration (NOAA)