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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, in extreme likeliness [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 GHG gases has been particularly rapid since 1950. The first 50 ppm increase above the pre-industrial value of carbon dioxide (CO2), the most important 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, 2013). Considering these gases, the total CO2-equivalent concentration reached a level of 435 ppm CO2 equivalent. in 2012 [2]; an increase of 3.3 ppm compared to 2011 (Figure 2). The increase is slightly higher than the average increase of the annual concentration over the past decade (i.e. is now 2.9 eq.yr-1).
Overall, assuming a concentration threshold of 450 ppm CO2 equivalent could result in a 2oC temperature change (see rationale), this means concentrations can only increase by about a further 15 ppm before this threshold value is exceeded. Assuming the 2000-2012 trend of annual increase of total GHG concentrations will also continue in the coming years, the threshold value may be exceeded in about 5-10 years. The lower band of the uncertainty range has been exceeded already around the millennium change, whereas it may take 20-25 years before the upper uncertainty band is exceeded.
Note that this total GHG concentration is considerably higher than that reported in previous years. This is mainly caused by new estimates on the role of aerosols (IPCC, 2013). Their cooling effect is now estimated to be 0.4 W/m2 smaller (implying more warming), mainly due to the higher warming potential of black carbon (see Summary table of radiative forcing estimates for the latest two IPCC assessments (for the period 1750–2011) under 'Justification for indicator selection'). When taking these older values, the total GHG concentration is estimated to be about 23 ppm lower in 2012. Given a similar trend in time, this lower concentration would imply an additional eight years before the 450 ppm is reached. Overall, this shows the effect of a lower climate sensitivity (Rogelj et al, 2014) and higher heat uptake of oceans (Doney et al, 2014). An important question here is how structural this increased heat uptake will be over time.
Furthermore, note that long-term data on tropospheric ozone is difficult to develop due to the scarcity of representative observing sites with long records and the large spatial heterogeneity (IPCC, 2007).
Text box 1: Greenhouse gases and their inclusion in international legislation
Greenhouse gases (GHG) can intercept solar radiation and, therefore, 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).
- GHGs in the Kyoto Protocol are: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and three fluorinated gasses (Hydrofluorocarbons (HFC), Perfluorocarbons (PFC) and Sulphur Hexafluoride (SF6))
- GHGs 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 472 ppm CO2 equivalents in 2012. This is about 194 ppm higher than pre-industrial levels. That this concentration is higher than when all gases are considered is caused by the overall cooling effect of aerosols - although certain aerosols act in an opposing manner by enhancing the warming. Overall, aerosols compensate for nearly 20% 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 9% to the current warming (Figure 3). The concentrations of these gases have peaked around the millennium change and are still declining due to natural removal processes (IPCC, 2013).
Six GHGs are included in the Kyoto Protocol. Their total concentration in the atmosphere has reached a level of 449 ppm CO2-equivalent in 2012, an increase of about 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 the pre-industrial period). When translating the overall 450 ppm CO2-equivalent threshold into a limit for the Kyoto gases alone (491 ppm), this means only an additional 42 ppm CO2-equivalent increase is possible (with an uncertainty range of ±50 ppm CO2-equivalent).
This increase of the total Kyoto 'bucket' is due to increasing concentrations of all the individual GHGs under the Kyoto protocol (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 393 ppm in 2012, and increased further to 396 ppm in 2013. This is an increase of more than 118 ppm (+42%) compared to the pre-industrial levels (i.e. before 1750) (NOAA, 2014).
The concentration of methane (CH4) has increased to 1810 and 1814 parts per billion (ppb) in 2012 and 2013 respectively, which is an increase by a factor of about 2.6 from pre-industrial levels. The growth in CH4 concentration in the atmosphere has been variable in recent decades, but since 2006 it was again growing steadily. The exact drivers of this (renewed) growth are still debated (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.
The nitrous oxide (N2O) concentration in 2012 was 325 ppb, and 326 ppb in 2013, 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 (HFCs, PFCs and SF6) have increased by large factors. Their contribution to the current climate forcing is still limited, but has nearly doubled over the past decade (from 0.6% to 1.1%, Figure 3). Due to the long lifetimes of most F gasses, the expectation is that their contribution will considerably increase in near future.
[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 2013 are available
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.
For references, please go to https://www.eea.europa.eu/data-and-maps/indicators/atmospheric-greenhouse-gas-concentrations-4/assessment or scan the QR code.
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