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Indicator Assessment

Atmospheric greenhouse gas concentrations

Indicator Assessment
Prod-ID: IND-2-en
  Also known as: CSI 013 , CLIM 052
Published 10 Dec 2020 Last modified 11 May 2021
1 min read

The total concentration of all greenhouse gases and other forcing agents, including cooling aerosols, reached 457 parts per million CO2 equivalents in 2018. If this concentration continues to increase at the present decadal rate, concentrations could, in the next few years, exceed the peak level that the Intergovernmental Panel on Climate Change states should not be exceeded if — with a 67 % likelihood — the global temperature increase is to be limited to 1.5 oC above pre-industrial levels by the end of the century. The peak concentrations corresponding to a temperature increase of 2 oC could be exceeded before 2034.

This indicator assesses the global atmospheric concentration of greenhouse gases and checks how the status and trend of that concentration relate to scientific knowledge and policy ambitions for limiting a global temperature increase at the end of the century. The objective of the 2015 Paris Climate 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). It is important to consider all gases and other forcing agents using the so-called ‘CO2 equivalent’ (CO2e); that is an equivalent amount to 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 amount of forcing is expressed here in CO2e and, in some cases such as for sulphate aerosols, this can be a negative forcing (i.e. have a cooling effect) (IPCC, 2013).

Considering all greenhouse gases and other forcing agents (including aerosols), total CO2e reached 457 ppm in 2018, which is an increase of nearly 4 ppm from 2017, and is 30 ppm more than in 2008 (Figure 1). Assessing the contribution of the various groups of greenhouse gases has shown that by far the most forcing is caused by gases covered by the Kyoto Protocol (KPGs), especially CO2, the average annual concentration of which reached 408 and 410 ppm in 2018 and 2019, respectively, or more than 125 ppm (+145 %), above pre-industrial levels (NOAA, 2020). As a group, the gases covered by the Montreal Protocol (MPGs), contributed about 25 ppm to the climate signal in 2018. Concentrations of these gases peaked around 2000 and have been slowly declining ever since, as a result of natural removal processes (IPCC, 2013). The contribution of non-protocol gases (NPGs) has a net cooling effect overall. In 2018, this effect amounted to about 39 ppm CO2e, and, as such compensated for about 18 % of the forcing induced by other greenhouse gases. The forcing trend (cooling) has been relatively stable over the past 5 years.

Peak and 2100 concentrations of total greenhouse gases in the atmosphere consistent with a 67 % probability of keeping the average global temperature increase below 1.5 °C (left) and 2 °C (right)

Note: Peak and 2100 concentrations of total greenhouse gases in the atmosphere consistent with a 67 % probability of keeping the average global temperature increase below 1.5 °C (left) and 2 °C (right) are shown. The periods within which peak concentrations could be exceeded are shown by purple arrows, based on the trend of the past 10 years in total greenhouse gas concentrations and without allowing for a temperature overshoot (based on IPCC, 2018)

Data source:

Pathways developed by the Intergovernmental Panel on Climate Change (IPCC, 2018) show concentrations of atmospheric greenhouse gases in relation to the chances of staying below specific increases in temperature compared with pre-industrial levels. These pathways show (1) peak concentrations that should not be exceeded to ensure that (2) CO2e concentrations in the year 2100 remain compatible with limiting the temperature increase to 1.5 or 2 °C above pre-industrial levels. According to the IPCC’s (2018) most conservative peak and 2100 concentration levels — those corresponding to a 67 % chance of staying below target values — the global greenhouse gas concentrations must not exceed 465 (range 445-485) ppm and should have returned to 411 (390-430) ppm by 2100 to limit the increase to 1.5 °C; for the 2 °C limit, the corresponding values are 505 (470-540) and 480 (460-500) ppm, respectively. Therefore, at the present decadal growth rate of 3.0 ppm per year, the peak concentration for limiting the increase to 1.5 °C will be exceeded around 2021. In the case of the 2 °C limit, the peak concentration will be reached around 2034. Taking into account uncertainty ranges (see supporting information), peak concentrations will be reached within 0-9 years (1.5 °C) or between 4-27 years (2 °C) (compared to 2018).

Supporting information

Indicator definition

The indicator shows the observed trends in greenhouse gas concentration levels. Greenhouse gases differ in the way they affect the climate system. In order to sum the effects of the individual greenhouse gases 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).

Units

Atmospheric concentration in parts per million in CO2 equivalents (ppm CO2e)


 

Policy context and targets

Context description

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 level (UNFCCC, 2009) and the EU level (October 2008 Environment Council conclusions), this ‘dangerous anthropogenic interference’ has been recognised by formulating an ambition of keeping the long-term global average temperature rise below 2 °C, compared to 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).

Targets

No targets have been specified

Related policy documents

No related policy documents have been specified

 

Methodology

Methodology for indicator calculation

The trend in this indicator is based on combining data for numerous gases that affect the radiation balance on earth:

  • Greenhouse gases included in the Kyoto Climate Protocol: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and three groups of fluorinated gases (HFC’s, PFC’s, SF6)
  • Gases under the Montreal Protocol on ‘Substances that deplete the ozone layer’ CFCs, HCFCs, CH3Cl’s, CH3Br’s and halons
  • Other forcing agents and greenhouse gases that are not included in global treaties are dealt with at a regional level (e.g. UNECE Convention on Long Range Transboundary Air Pollution). This includes aerosols (sulphate, black carbon, organic carbon, nitrate, mineral dust), tropospheric and stratospheric ozone (O3), stratospheric water vapour and aircraft contrails. Both the direct forcing effect of these agents and the indirect effect through aerosol-cloud interaction are included.

The trends in global average concentration levels of atmospheric CO2 for the period from 1950 are based on data available from the NOAA observatory (NOAA, 2020). Trend data for CH4, N2O, the fluorinated gases and other compounds under the Montreal Protocol were derived from station data that are available in the AGAGE (2020) data set. The global figures were derived by averaging the data from different observatory stations across the world, equally distributed over the northern and southern hemisphere. Pre-observational data for CO2, CH4 and N2O are based on ice core data (Etheridge et al, 1998, 2002, Machida, et al, 1995). Pre-observational data for F-gases are the result of modeling (Meinshausen et al, 2017). Data for the non-protocol agents are taken from IPCC (2013) and Myhre et al (2017).

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, and N2O, O3 (both stratospheric and tropospheric) and vapour based on IPCC (2013). IPCC (2013) and Myhre et al (2017) estimates were used for the radiative forcing of non-protocol related compounds.

The equations used to compute contributions by the individual gases are presented below:

 

Trace gas

Parameterisation, radiative forcing (F), in Wm

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, depending on molecule (see below), taken from WMO, 2002.

A similar approach was applied for the Montreal Protocol gases (i.e. CFCs & HCFCs)

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 alpha values used for Kyoto and Montreal Protocol Gases (see also CDIAC, 2011a, 2011b)

 

Kyoto Protocol gases

Montreal Protocol gases

HFC-23

0.18

CFC-11

0.26

HFC-134a

0.16

CFC-12

0.32

HFC-32

0.11

CFC-13

0.25

HFC-125

0.23

CFC-113

0.3

HFC-143a

0.16

CFC-114

0.31

HFC152a

0.1

CFC-115

0.2

HFC-227ea

0.26

HCFC-22

0.21

HFC-136fa

0.24

HCFC-124

0.2

HFC-245fa

0.24

HCFC-141

0.16

HFC365mf

0.22

HCFC-142

0.19

CF4

0.09

CCl4

0.17

C2F6

0.25

CH3Cl

0.01

SF6

0.57

CHCl3

0.08

SF5CF3

0.59

CHCCl3

0.07

NF3

0.2

CH3Br

0.004

PCF-318

0.32

H1211

0.29

PCF-218

0.28

H1301

0.3

 

 

H2402

0.31

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 factor of 0.154 (-) for correcting forcing of Montreal Protocol gases, based on IPCC (2013) (= -0.05 W/m2 or 10 ppm in 2015).

To quantify the total concentration of all greenhouse gases, the direct and indirect effect of multiple aerosols (sulphate, black carbon, organic carbon, nitrate, and mineral dust), and the forcing of tropospheric ozone, stratospheric water vapour, changes in albedo (e.g. due to black carbon) were added. Data on forcing have been based on IPCC (2013), Klimont et al, (2013), Wang et al. (2016), Nazarenko et al., (2017) and Myhre et al, (2017). 

For the aerosols, we used figures, based on the 2013 IPCC report and subsequent literature. First, the direct radiative forcing (RFari) from 1850 to 2010 has been defined for the individual aerosols and tropospheric ozone, using IPCC (2013, Figures 8.7 and 8.8, Figure TS7, Table 8.6). Then, updates from Wang et al. (2014), Wang et al. (2016), Nazarenko et al., (2017) and Myhre et al (2017) have been used for the period 1990 to 2015. Finally, recent numbers of emissions have been used to extrapolate the time series until 2018. See table below for historic forcing of some of the aerosols and ozone. The historical indirect forcing of aerosols through cloud interaction (RFaci) has been defined by using the total forcing as provided by the IPCC (2013, annex II) and the direct forcing. This is done for the period up to 2011. Due to the lack of more recent data, the indirect forcing has been kept constant after 2011. Note that the net effect 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).

The climate forcing due to stratospheric vapour, aircraft contrails and land use are taken from IPCC (2013, annex II). The numbers have been kept constant at their 2011 value for the period 2011-2018, due to lack of more recent data.

Calculated direct radiative forcing for multiple aerosols and tropospheric ozone for a number of past years (W/m2)

Gas (group)

2018

2010

1990

1950

1900

1850

Sulphate

-0.34

-0.35

-0.39

-0.16

-0.06

-0.02

Black carbon (including snow albedo)

0.45

0.46

0.41

0.30

0.17

0.05

Organic carbon (including secondary organic carbon.)

-0.12

-0.12

-0.10

-0.06

-0.04

-0.02

Nitrate

-0.17

-0.16

-0.11

-0.03

-0.02

-0.02

Tropospheric ozone

0.41

0.40

0.37

0.16

0.09

0.04

 

Methodology for gap filling

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

Methodology references

No methodology references available.

 

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), with each network consisting of several stations distributed across the globe.

Absolute accuracies of global average annual concentrations are around 1% for CO2, CH4, N2O and CFCs; for HFCs, PFCs, and SF6, absolute accuracies are between 10 % and 20 %. The largest uncertainties have been determined for the concentration of different aerosols such as sulphur, and black and organic carbon. The uncertainty in the trend of these aerosols could be 50-60 % (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 RF values from 1750 to 2011(and 10 % and 90 % confidence ranges) (source: IPCC, 2013)

Group

Forcers

Best estimate

10 %

90 %

Kyoto Protocol

CO2

1.82

1.62

2.02

 

CH4

0.49

0.44

0.54

 

N2O

0.18

0.15

0.21

 

HFC, PFCs, SF6

0.03

0.03

0.04

Montreal Protocol

Montreal F-gases

0.28

0.14

0.44

Non-Protocol

Tropospheric O3

0.4

0.2

0.6

 

Sulphate aerosols

-0.39

-0.59

-0.19

 

Nitrate aerosols

-0.13

-0.18

0.08

 

Black carbon

0.42

0.02

0.82

 

Organic carbon

-0.12

-0.32

0.08

 

Land use

-0.15

-0.25

-0.05

 

Vapour

0.07

0.04

0.10

 

Mineral dust

-0.10

-0.30

0.10

 

Air contrails

0.05

0.01

0,10

 

Indirect effect

(cloud interaction)

-0.55

-1.1

0.10

 

 

 

 

 

 

Total

2.32

1.2

3.4

Data sets uncertainty

Uncertainty in relation to peak concentration values

The IPCC (2014, 2018) has modelled concentration levels of all greenhouse gases in the atmosphere, which are consistent with keeping the global average temperature increase below 1.5 and 2 °C, for various probability levels. The indicator assessment uses those peak concentrations and 2100 concentration values of greenhouse gases in the atmosphere that, according to the IPCC, give a likely (67 %) probability of staying below a 1.5 °C and a 2 °C temperature increase by the end of the century. This means that the concentration could peak somewhere between 2020 and 2100, and then become (strongly) reduced again (e.g. through zero emissions and even negative emissions though active CO2 removal). The IPCC also introduced the temperature overshoot feature, which means that temperature targets may become temporarily exceeded, followed by a decrease later in the century. Risking such a temperature overshoot would put an additional burden on strong and even negative emissions reductions and, hence, this has not been considered in the messaging of this indicator. These values and other probability values are presented below to indicate the uncertainty and variation in peak and 2100 concentration values.

Peak and 2100 concentration levels of total greenhouse gases in the atmosphere consistent with keeping the global average temperature increase below 1.5 °C and 2 °C, for various probability levels. The ranges are given in brackets, (all based on IPCC, 2014, 2018)

Probability of staying
below target

Temperature
overshoot

1.5 °C

 

2.0 °C

 

 

 

Peak 
concentration

Concentration
2100

Peak
concentration

Concentration
2100

>67 % (likely)

No

465

(445-485)

411

(390-430)

505

(470-540)

480

(460-500)

 

Yes

485

(465-505)

405

(385-425)

520

(505-540)

465

(440-480)

50 %
(about as likely as not)

(33 %-67 %)

No

485

(465-500)

440

(430-445)

540

(510-560)

520

(500-535)

 

Yes

525

(520-540)

425

(410-435)

560

(540-575)

505

(480-530)

<33 % (unlikely)

No

500

(480-530)

470

(445-480)

580

(550-615)

570

(540-600)

 

Yes

535

(510-550)

450

(425-460)

600

(560-630)

540

(505-580)

 

 

 

 

 

 


Rationale uncertainty

No uncertainty has been specified

Data sources

Other info

DPSIR: State
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
Indicator codes
  • CSI 013
  • CLIM 052
Frequency of updates
Updates are scheduled once per year
EEA Contact Info info@eea.europa.eu
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