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Indicator Assessment
The concentration of greenhouse gases (GHG) in the atmosphere has increased during the 20th century and first part of the 21st century, extremely likely[1]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) (IPCC, 2013, See also Carbon Budget at Global Carbon Project, www.globalcarbonproject.org/carbonbudget/index.htm). The increase of all GHGs has been particularly rapid since 1950. The first 50 ppm increase above the pre-industrial value of carbon dioxide (CO2), the most important human greenhouse gas,- was reached in the 1970s more than 200 years since pre-industrial times, whereas the second 50 ppm increase occurred after just approximately 30 years.
The various greenhouse gases (Text box 1) each affect the climate system differently (see rationale). To evaluate the GHG concentration in the atmosphere in relation to temperature change, it is important to consider all greenhouse 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, 2007, 2013). Considering these gases, the total CO2-equivalent concentration reached a level of 416 ppm CO2 eq. in 2011[1], again exceeding the 400 ppm (Figure 2)[2]. The annual concentration increase is 2.5 eq.yr-1 in 2011, somewhat smaller than annual increase in earlier years. This due to the economic situation in the world, leading to less emissions.
The contribution of tropospheric ozone to the climate system is considered to be stable in the recent decades when comparing large annual and special variation (IPCC, 2007a, IPCC, 20133). Long-term data on tropospheric ozone are difficult to develop due to the scarcity of representative observing sites with long records and the large spatial heterogeneity (IPCC, 2013). Overall, assuming a concentration threshold of 450 ppm CO2 equivalents will result in a 2oC temperature change (see rationale), means concentrations can only increase by a further about 45 ppm before this threshold value is exceeded. Assuming the 2000-2011 trend of annual increase of total GHG concentrations will also continue in the coming years, the threshold value may be exceeded in less than 25 years. The lower band of the uncertainty range has been exceeded already in 2010, whereas it may take more than 50 years before the upper uncertainty band is exceeded.
Text box 1: Greenhouse gases and their inclusion in international legislation
Greenhouse gases (GHG) can intercept solar radiation and in such a way affect the climate system. In order to control the emissions of such gases, many of them are included within different 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).
- GHG in the Kyoto Protocol are: Carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and three fluorinated gasses (Hydrofluorocarbons (HFC), PerfluorocarbonsPFC, Sulphur Hexafluoride (SF6))
- GHG in the Montreal Protocol are three other groups of fluorinated gases: CFCs, HCFCs and CH3CCl3
- In addition, GHGs exist that are not included in global treaties, here called non-protocol gases (NPG), including stratospheric and tropospheric ozone (O3), aerosols such as black carbon, and water vapour.
Excluding water vapour, ozone and aerosols, the total concentration of the remaining, long-lived GHGs has increased from 278 in pre-industrial times to 468 ppm CO2 equivalents in 2011. This is about 190 ppm higher than pre-industrial levels. That this concentration is higher than when considering all gases is caused by the overall cooling effect of aerosols - although certain aerosols act in an opposite manner by enhancing the warming. Overall, aerosols are compensating for around 45% of the current warming induced by the Kyoto and Montreal GHGs. Aerosols have a relatively short lifetime. The Montreal Protocol gases contributed as a group about 10% to the current warming (Figure 3). The concentrations of these gases has peaked around the millennium change and started to decline due to natural removal processes (IPCC, 2007a).
Six GHGs are included in the Kyoto Protocol. Their concentration in the atmosphere has reached 446 ppm CO2-equivalent in 2011, an increase of nearly 170 ppm compared to pre-industrial times (Figure 1). Changes in atmospheric CO2 contributed by far most of the increase (about 67% of the increase from pre-industrial period). When translating the overall 450 ppm CO2-equivalent limit into a limit just for the Kyoto gases (491 ppm), this means only an additional 45 ppm CO2-equivalent increase is possible (with an uncertainty range of -2 – 97 ppm CO2-equivalent).
The concentrations of the individual GHGs under the Kyoto protocol remain increasing (Figure 4). Overall, the concentrations in the atmosphere of CO2, CH4 and N2O exceeded the range of concentrations recorded in ice cores during the past 800,000 years (IPCC, 2013).
The CO2 concentration reached a level of 391 ppm in 2011, and increased further up 393 ppm in 2012 (Figure 4, Carbon Dioxide). This is an increase of about 115 ppm (+41%) compared to the pre-industrial levels (i.e. before 1750) (NOAA, 2012).
The concentration of methane (CH4) has increased to 1804 parts per billion (ppb) in 2011, which is an increase by a factor of about 2.5 from pre-industrial levels (Figure 4, Methane). The growth of the CH4 concentration in the atmosphere has been variable in recent decades. But since 2007 it is steadily growing again. The exact drivers of this (renewed) growth are still debated (Dlugokencky et al., 2009) IPCC, 2013). Anthropogenic CH4 sources play a dominant role in the increase, while fluctuations in natural sources are most likely causing the global inter-annual variability of CH4 emissions (high confidence), with a smaller contribution from biomass burning emissions during high fire years.
The nitrous oxide (N2O) concentration in 2011 was 324 ppb (Figure 4, Nitrous Oxide), about 20% above the pre-industrial level. This concentration has not been exceeded during at least the past 1 000 years.
The concentrations of the F-gases within the scope of the Kyoto Protocol (i.e. HFCs, PFCs and SF6) have increased by factors 1.2 to 38 (depending on the gas) between 1995 (reference year under Kyoto Protocol) and 2011. These gases are very effective absorbers of radiation and even small amounts can significantly affect the climate system. Their contribution to the total climate forcing is rapidly increasing in the past years.
[1] Defined as >95% probability (IPCC, 2007)[2]More recent data are not available for the annual-average concentration except for CO2, for which data for 2012 are available
[3] Note that the value is higher than reported in 2012. This due to updated values for the radiative forcing of tropospheric ozone (0.4 instead of 0.35). This based on recent IPCC information (IPCC, 2013)
The indicator shows the observed trends of 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
Next to the concentration of individual GHGs, greenhouse gasses are grouped in two ways, 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.
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 1.1 W.m-2; IPCC, 2013, Table 8.6). The probability of staying below the 2oC becomes very low when stabilization at 550 ppm CO2 eq., ranging between 0% and 37%, considering all GHGs (Meinshausen, 2006; den Elzen et al., 2007; Van Vuuren et al., 2008).
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 2oC 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 2oC 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 (December 2009) recognized 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.
For atmospheric CO2, the global average values are directly taken from NOAA (NOAA, 2011, see also www.esrl.noaa.gov/gmd/ccgg/trends/global.html). Global average concentration values for the other gases are mainly based on CDIAC numbers (2013a,b) (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 based on IPCC (2007a), and according toWMO (2002) for other gases. 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 |
|---|---|---|
|
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 Greenhouse Gases also the Montreal Gases (i.e. CFCs & HCFCs) need to be included. 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 Montral Gases
|
Kyoto gases |
Montreal gases |
||
|
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 |
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. These values are 0.35 (result of 0.4 W.m-2 for tropospheric ozone and due to –0.05 W.m-2 for stratospheric ozone) and 0.07 W.m-2 for ozone and water vapor, respectively (IPCC, 2013, pg 8-22 & 8-23). For aerosols, cloud interactions & land-cover changes a constant value of -1.1 W.m2 was used back to 2000 (IPCC, 2013, Table 8.6). 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). This because of higher concentrations of especially SO2 aerosols.
For all three representations of the GHG concentration (i.e. Kyoto gasses only, all long-living GHG and all Greenhouse gases including zone and aerosols), the following approach has been used, adding the different climate forcing:
|
|
Ceq = C0 exp ((SUM change in F) / alpha) Ceq 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 |
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 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.
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.
For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/atmospheric-greenhouse-gas-concentrations-3/assessment or scan the QR code.
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