Editing IPCC:AR6/WGI/Chapter-7 (section)

=== 7.3.2 Greenhouse Gases ===

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High spectral resolution radiative transfer models provide the most accurate calculations of radiative perturbations due to greenhouse gases (GHGs), with errors in the instantaneous radiative forcing (IRF) of less than 1% ( [[#Mlynczak--2016|Mlynczak et al., 2016]] ; [[#Pincus--2020|Pincus et al., 2020]] ). They can calculate IRFs with no adjustments, or SARFs by accounting for the adjustment of stratospheric temperatures using a fixed dynamical heating. It is not possible with offline radiation models to account for other adjustments. The high-resolution model calculations of SARF for carbon dioxide, methane and nitrous oxide have been updated since AR5, which were based on [[#Myhre--1998|Myhre et al. (1998)]] . The new calculations include the shortwave forcing from methane and updates to the water vapour continuum (increasing the total SARF of methane by 25%) and account for the absorption band overlaps between carbon dioxide and nitrous oxide ( [[#Etminan--2016|Etminan et al., 2016]] ). The associated simplified expressions, from a re-fitting of the [[#Etminan--2016|Etminan et al. (2016)]] results by [[#Meinshausen--2020|Meinshausen et al. (2020)]] , are given in Supplementary Material, Table 7.SM.1. The shortwave contribution to the IRF of methane has been confirmed independently ( [[#Collins--2018|Collins et al., 2018]] ). Since they incorporate known missing effects we assess the new calculations as being a more appropriate representation than [[#Myhre--1998|Myhre et al. (1998)]] .

As described in ( [[#7.3.1|Section 7.3.1]] , ERFs can be estimated using ESMs, however the radiation schemes in climate models are approximations to high spectral resolution radiative transfer models with variations and biases in results between the schemes ( [[#Pincus--2015|Pincus et al., 2015]] ). Hence ESMs alone are not sufficient to establish ERF best estimates for the well-mixed GHGs (WMGHGs). This assessment therefore estimates ERFs from a combined approach that uses the SARF from radiative transfer models and adds the tropospheric adjustments derived from ESMs.

In AR5, the main information used to assess components of ERFs beyond SARF was from [[#Vial--2013|Vial et al. (2013)]] who found a near-zero non-stratospheric adjustment (without correcting for near-surface temperature changes over land) in 4×CO <sub>2</sub> CMIP5 model experiments, with an uncertainty of ±10% of the total CO <sub>2</sub> ERF. No calculations were available for other WMGHGs, so ERF was therefore assessed to be approximately equal to SARF (within 10%) for all WMGHGs.

The effect of WMGHGs in ESMs can extend beyond their direct radiative effects to include effects on ozone and aerosol chemistry and natural emissions of ozone and aerosol precursors, and in the case of CO <sub>2</sub> to vegetation cover through physiological effects. In some cases these can have significant effects on the overall radiative budget changes from perturbing WMGHGs within ESMs ( [[#Myhre--2013b|Myhre et al., 2013b]] ; [[#Zarakas--2020|Zarakas et al., 2020]] ; [[#O’Connor--2021|O’Connor et al., 2021]] ; [[#Thornhill--2021a|Thornhill et al., 2021a]] ). These composition adjustments are further discussed in ( [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4.2).

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<span id="carbon-dioxide-co-2"></span>
==== 7.3.2.1 Carbon Dioxide (CO <sub>2</sub> ) ====

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The SARF for carbon dioxide (CO <sub>2</sub> ) has been slightly revised due to updates to spectroscopic data and inclusion of the absorption band overlaps between N <sub>2</sub> O and CO <sub>2</sub> ( [[#Etminan--2016|Etminan et al., 2016]] ). The formulae fitting to the [[#Etminan--2016|Etminan et al. (2016)]] results in [[#Meinshausen--2020|Meinshausen et al. (2020)]] are used. This increases the SARF due to doubling CO <sub>2</sub> slightly from 3.71 W m <sup>–2</sup> in AR5 to 3.75 W m <sup>–2</sup> . Tropospheric responses to CO <sub>2</sub> in fSST experiments have been found to lead to an approximate balance in their radiative effects between an increased radiative forcing due to water vapour, cloud and surface-albedo adjustments and a decrease due to increased tropospheric temperature and land surface temperature response (Table 7.3; [[#Vial--2013|Vial et al., 2013]] ; [[#Zhang--2014|Zhang and Huang, 2014]] ; [[#Smith--2018b|Smith et al., 2018b]] , 2020b). The Δ ''F'' <sub>fsst</sub> includes any effects represented within the ESMs on tropospheric adjustments due to changes in evapotranspiration or leaf area (mainly affecting surface and boundary-layer temperature, low-cloud amount, and albedo) from the CO <sub>2</sub> -physiological effects (Doutriaux- [[#Boucher--2009|Boucher et al., 2009]] ; [[#Cao--2010|Cao et al., 2010]] ; T.B. [[#Richardson--2018|]] [[#Richardson--2018|Richardson et al., 2018]] ). The effect on surface temperature (negative longwave response) is consistent with the expected physiological responses and needs to be removed for consistency with the ERF definition. The split between surface and tropospheric temperature responses was not reported in [[#Vial--2013|Vial et al. (2013)]] or [[#Zhang--2014|Zhang and Huang (2014)]] but the total of surface and tropospheric temperature response agrees with Smith et al. (2018b, 2020b), giving ''medium confidence'' in this decomposition. Doutriaux- [[#Boucher--2009|Boucher et al. (2009)]] and [[#Andrews--2021|Andrews et al. (2021)]] (using the same land surface model) find a 13% and 10% increase respectively in ERF due to the physiological responses to CO <sub>2</sub> . The physiological adjustments are therefore assessed to make a substantial contribution to the overall tropospheric adjustment for CO <sub>2</sub> ( ''high confidence'' ), but there is insufficient evidence to provide a quantification of the split between physiological and thermodynamic adjustments. These forcing adjustments due to the effects of CO <sub>2</sub> on plant physiology differ from the biogeophysical feedbacks due to the effects of temperature changes on vegetation discussed in ( [[#7.4.2.5|Section 7.4.2.5]] . The adjustment is assumed to scale with the SARF in the absence of evidence for non-linearity. The tropospheric adjustment is assessed from Table 7.3 to be +5% of the SARF with an uncertainty of 5%, which is added to the [[#Meinshausen--2020|Meinshausen et al. (2020)]] formula for SARF. Due to the agreement between the studies and the understanding of the physical mechanisms there is ''medium confidence'' in the mechanisms underpinning the tropospheric adjustment, but ''low confidence'' in its magnitude ''.''

The ERF from doubling CO <sub>2</sub> (2×CO <sub>2</sub> ) from the 1750 level (278 ppm; [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.3|Section 2.2.3.3]] ) is assessed to be 3.93 ± 0.47 W m <sup>–2</sup> ( ''high confidence'' ). Its assessed components are given in Table 7.4. The combined spectroscopic and radiative transfer modelling uncertainties give an uncertainty in the CO <sub>2</sub> SARF of around 10% or less ( [[#Etminan--2016|Etminan et al., 2016]] ; [[#Mlynczak--2016|Mlynczak et al., 2016]] ). The overall uncertainty in CO <sub>2</sub> ERF is assessed as ±12%, as the more uncertain adjustments only account for a small fraction of the ERF (Table 7.3). The 2×CO <sub>2</sub> ERF estimate is 0.2 W m <sup>–2</sup> larger than using the AR5 formula ( [[#Myhre--2013b|Myhre et al., 2013b]] ) due to the combined effects of tropospheric adjustments which were assumed to be zero in AR5. CO <sub>2</sub> concentrations have increased from 278 ppm in 1750 to 410 ppm in 2019 [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.3|Section 2.2.3.3]] ). The historical ERF estimate from CO <sub>2</sub> is revised upwards from the AR5 value of 1.82 ± 0.38 W m <sup>–2</sup> (1750–2011) to 2.16 ± 0.26 W m <sup>–2</sup> (1750–2019) in this assessment, from a combination of the revisions described above (0.06 W m <sup>–2</sup> ) and the 19 ppm rise in atmospheric concentrations between 2011 and 2019 (0.27 W m <sup>–2</sup> ). The ESM estimates of 2×CO <sub>2</sub> ERF (Table 7.2) lie within ±12% of the assessed value (apart from CESM2). The definition of ERF can also include further physiological effects – for instance on dust, natural fires and biogenic emissions from the land and ocean – but these are not typically included in the modelling setup for 2×CO <sub>2</sub> ERF.

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'''Tabl''' '''e 7.3 |''' '''Adjustments to the top-of-atmosphere (TOA) carbon dioxide forcing due to changes in stratospheric temperature, surface and tropospheric temperatures, water vapour, clouds, and surface albedo, as a fraction of the stratospheric-temperature-adjusted radiative forcing (SARF).''' Effective radiative forcing (ERF) is defined in this Report as excluding the surface temperature response.

{| class="wikitable"
|-
| Percentage of SARF (source study)

| Surface Temperature

| Tropospheric Temperature

| Stratospheric

Temperature

| Surface Albedo

| Water Vapour

| Clouds

| Troposphere

(Including Surface)

| Troposphere

(Excluding Surface)

|-
| [[#Vial--2013|Vial et al. (2013)]]

| colspan="2"| –20% combined

| N/A

| 2%

| 6%

| 11%

| –1%

| N/A

|-
| [[#Zhang--2014|Zhang and Huang (2014)]]

| colspan="2"| –23% combined

| 26%

| N/A

| 6%

| 16%

| –1%

| N/A

|-
| [[#Smith--2018b|Smith et al. (2018b)]]

| –6%

| –16%

| 30%

| 3%

| 6%

| 12%

| –1%

| +5%

|-
| [[#Smith--2020b|Smith et al. (2020b)]]

| –6%

| –15%

| 35%

| 3%

| 6%

| 15%

| +3%

| +9%

|}

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'''Table 7.''' '''4 |''' '''Assessed effective radiative forcing (ERF), stratospheric-temperature-adjusted radiative forcing (SARF) and tropospheric adjustments to 2×CO''' <sub>2</sub> '''change since pre-industrial times compared to the AR5 assessed range ( [[#Myhre--2013b|Myhre et al., 2013b]] ).''' Adjustments are due to changes in tropospheric temperatures, water vapour, clouds and surface albedo and land cover and are taken from [[#Smith--2018b|Smith et al. (2018b)]] and assessed as a percentage of SARF (Table 7.3). Uncertainties are based on multi-model spread in [[#Smith--2018b|Smith et al. (2018b)]] . Note some of the uncertainties are anticorrelated, which means that they do not sum linearly.

{| class="wikitable"
|-
| 2×CO <sub>2</sub> Forcing

| AR5

SARF/ERF (W m <sup>–2</sup> )

| SARF

(W m <sup>–2</sup> )

| Tropospheric Temperature Adjustment

(W m <sup>–2</sup> )

| Water Vapour Adjustment

(W m <sup>–2</sup> )

| Cloud Adjustment (W m <sup>–2</sup> )

| Surface Albedo and Land-cover Adjustment (W m <sup>–2</sup> )

| Total Tropospheric Adjustment (W m <sup>–2</sup> )

| ERF

(W m <sup>–2</sup> )

|-
| 2×CO <sub>2</sub> ERF components

| 3.71

| 3.75

| –0.60

| 0.22

| 0.45

| 0.11

| 0.18

| 3.93

|-
| 5–95% uncertainty ranges as percentage of ERF

| 10% (SARF)

20% (ERF)

| &lt;10%

| ±6%

| ±4%

| ±7%

| ±2%

| ±7%

| ±12%

|}

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<span id="methane-ch-4"></span>
==== 7.3.2.2 Methane (CH <sub>4</sub> ) ====

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The SARF for methane (CH <sub>4</sub> ) has been substantially increased due to updates to spectroscopic data and inclusion of shortwave absorption ( [[#Etminan--2016|Etminan et al., 2016]] ). Adjustments have been calculated in nine climate models by [[#Smith--2018b|Smith et al. (2018b)]] . Since CH <sub>4</sub> is found to absorb in the shortwave near infrared, only adjustments from those models including this absorption are taken into account. For these models the adjustments act to reduce the ERF because the shortwave absorption leads to tropospheric heating and reductions in upper tropospheric cloud amounts. The adjustment is –14% ± 15%, which counteracts much of the increase in SARF identified by [[#Etminan--2016|Etminan et al. (2016)]] . [[#Modak--2018|Modak et al. (2018)]] also found negative forcing adjustments from a methane perturbation including shortwave absorption in the NCAR CAM5 model, in agreement with the above assessment. The uncertainty in the shortwave component leads to a higher radiative modelling uncertainty (14%) than for CO <sub>2</sub> ( [[#Etminan--2016|Etminan et al., 2016]] ). When combined with the uncertainty in the adjustment, this gives an overall uncertainty of ±20%. There is ''high confidence'' in the spectroscopic revision but only ''medium confidence'' in the adjustment modification. CH <sub>4</sub> concentrations have increased from 729 ppb in 1750 to 1866 ppb in 2019 [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.3|Section 2.2.3.3]] ). The historical ERF estimate from AR5 of 0.48 ± 0.10 W m <sup>–2</sup> (1750–2011) is revised to 0.54 ± 0.11 W m <sup>–2</sup> (1750 to 2019) in this assessment from a combination of spectroscopic radiative efficiency revisions (+0.12 W m <sup>–2</sup> ), adjustments (–0.08 W m <sup>–2</sup> ) and the 63 ppb rise in atmospheric CH <sub>4</sub> concentrations between 2011 and 2019 (+0.03 W m <sup>–2</sup> ). As the adjustments are assessed to be small, there is ''high confidence'' in the overall assessment of ERF from methane. Increased methane leads to tropospheric ozone production and increased stratospheric water vapour, so that an attribution of forcing to methane emissions gives a larger effect than that directly from the methane concentration itself. This is discussed in detail in ( [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4.2) and shown in Figure 6.12.

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==== 7.3.2.3 Nitrous oxide (N <sub>2</sub> O) ====

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The tropospheric adjustments to nitrous oxide (N <sub>2</sub> O) have been calculated from 5 ESMs as 7% ± 13% of the SARF ( [[#Hodnebrog--2020b|Hodnebrog et al., 2020b]] ). This value is therefore taken as the assessed adjustment, but with ''low confidence'' . The radiative modelling uncertainty is ±10% ( [[#Etminan--2016|Etminan et al., 2016]] ), giving an overall uncertainty of ±16%. Nitrous oxide concentrations have increased from 270 ppb in 1750 to 332 ppb in 2019 [[IPCC:Wg1:Chapter:Chapter-2#2.2.3.3|Section 2.2.3.3]] ). The historical ERF estimate from N <sub>2</sub> O is revised upwards from 0.17 ± 0.06 W m <sup>–2</sup> (1750–2011) in AR5 to 0.21 ± 0.03 W m <sup>–2</sup> (1750–2019) in this assessment, of which 0.02 W m <sup>–2</sup> is due to the 7 ppb increase in concentrations, and 0.02 W m <sup>–2</sup> to the tropospheric adjustment. As the adjustments are assessed to be small there remains ''high confidence'' in the overall assessment.

Increased nitrous oxide leads to ozone depletion in the upper stratosphere which will make a positive contribution to the direct ERF here (Section 6.4.2 and Figure 6.12) when considering emissions-based estimates of ERF.

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==== 7.3.2.4 Halogenated Species ====

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The stratospheric-temperature-adjusted radiative efficiencies (SARF per ppb increase in concentration) for halogenated compounds are reviewed extensively in [[#Hodnebrog--2020a|Hodnebrog et al. (2020a)]] , an update to those used in AR5. Many halogenated compounds have lifetimes short enough that they can be considered short-lived climate forcers (SLCFs; Table 6.1). As such, they are not completely ‘well-mixed’ and their vertical distributions are taken into account when determining their radiative efficiencies. The World Meteorological Organization ( [[#WMO--2018|WMO, 2018]] ) updated the lifetimes of many halogenated compounds and these were used in [[#Hodnebrog--2020a|Hodnebrog et al. (2020a)]] .

The tropospheric adjustments to chlorofluorocarbons (CFCs), specifically CFC-11 and CFC-12, have been quantified as 13% ± 10% and 12% ± 14% of the SARF, respectively ( [[#Hodnebrog--2020b|Hodnebrog et al., 2020b]] ). The assessed adjustment to CFCs is therefore 12% ± 13% with ''low confidence'' due to the lack of corroborating studies. There have been no calculations for other halogenated species so for these the tropospheric adjustments are therefore assumed to be 0 ± 13% with ''low confidence.'' The radiative modelling uncertainties are 14% and 24% for compounds with lifetimes greater than and less than five years, respectively ( [[#Hodnebrog--2020a|Hodnebrog et al., 2020a]] ). The overall uncertainty in the ERFs of halogenated compounds is therefore assessed to be 19% and 26% depending on the lifetime. The ERF from CFCs is slowly decreasing, but this is compensated for by the increased forcing from the replacement species (HCFCs and HFCs). The ERF from HFCs has increased by 0.028 ± 0.05 W m <sup>–2</sup> . Thus, the concentration changes mean that the total ERF from halogenated compounds has increased since AR5 from 0.360 ± 0.036 W m <sup>–2</sup> to 0.408 ± 0.078 W m <sup>–2</sup> (Table 7.5). Of this, 0.034 W m <sup>–2</sup> is due to increased radiative efficiencies and tropospheric adjustments, and 0.014 W m <sup>–2</sup> is due to increases in concentrations. As the adjustments are assessed to be small there remains ''high confidence'' in the overall assessment.

Halogenated compounds containing chlorine and bromine lead to ozone depletion in the stratosphere which will reduce the associated ERF ( [[#Morgenstern--2020|Morgenstern et al., 2020]] ). [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] (Section 6.4 and Figure 6.12) assesses the ERF contributions due to the chemical effects of reactive gases.

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==== 7.3.2.5 Ozone ====

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Estimates of the pre-industrial to present-day tropospheric ozone radiative forcing are based entirely on models. The lack of pre-industrial ozone measurements prevents an observational determination. There have been limited studies of ozone ERFs ( [[#MacIntosh--2016|MacIntosh et al., 2016]] ; [[#Xie--2016|Xie et al., 2016]] ; [[#Skeie--2020|Skeie et al., 2020]] ). [[#Skeie--2020|Skeie et al. (2020)]] found little net contribution to the ERF from tropospheric adjustment terms for 1850–2000 change in ozone (tropospheric and stratospheric ozone combined), although [[#MacIntosh--2016|MacIntosh et al. (2016)]] suggested that increases in stratospheric or upper tropospheric ozone reduces high-cloud and increases low-cloud, whereas an increase in lower tropospheric ozone reduces low-cloud. Further studies suggest that changes in circulation due to decreases in stratospheric ozone affect Southern Hemisphere clouds and the atmospheric levels of sea salt aerosol that would contribute additional adjustments, possibly of comparable magnitude to the SARF from stratospheric ozone depletion ( [[#Grise--2013|Grise et al., 2013]] , 2014; [[#Xia--2016|Xia et al., 2016]] , 2020). ESM responses to changes in ozone-depleting substances (ODS) in CMIP6 show a much more negative ERF than would be expected from offline calculations of SARF ( [[#Morgenstern--2020|Morgenstern et al., 2020]] ; [[#Thornhill--2021b|Thornhill et al., 2021b]] ) again suggesting a negative contribution from adjustments. However there is insufficient evidence available to quantify this effect.

Without sufficient information to assess whether the ERFs differ from SARF, this assessment relies on offline radiative transfer calculations of SARF for both tropospheric and stratospheric ozone. [[#Checa-Garcia--2018|Checa-Garcia et al. (2018)]] found SARF of 0.30 W m <sup>–2</sup> for changes in ozone (1850–1860 to 2009–2014). These were based on precursor emissions and ODS concentrations from the Coupled Chemistry Model Initiative (CCMI) project ( [[#Morgenstern--2017|Morgenstern et al., 2017]] ). [[#Skeie--2020|Skeie et al. (2020)]] calculated an ozone SARF of 0.41 ± 0.12 W m <sup>–2</sup> (1850–2010; from five climate models and one chemistry transport model) using CMIP6 precursor emissions and ODS concentrations (excluding models without fully interactive ozone chemistry and one model with excessive ozone depletion). The ozone precursor emissions are higher in CMIP6 than in CCMI, which explains much of the increase compared to [[#Checa-Garcia--2018|Checa-Garcia et al. (2018)]] ''.''

Previous assessments have split the ozone forcing into tropospheric and stratospheric components. This does not correspond to the division between ozone production and ozone depletion and is sensitive to the choice of tropopause ( ''high confidence'' ) ( [[#Myhre--2013b|Myhre et al., 2013b]] ). The contributions to total SARF in CMIP6 ( [[#Skeie--2020|Skeie et al., 2020]] ) are 0.39 ± 0.07 and 0.02 ± 0.07 W m <sup>–2</sup> for troposphere and stratosphere respectively (using a 150 ppb ozone tropopause definition). This small positive (but with uncertainty encompassing negative values) stratospheric ozone SARF is due to contributions from ozone precursors to lower stratospheric ozone and some of the CMIP6 models showing ozone depletion in the upper stratosphere, where depletion contributes a positive radiative forcing ( ''medium confidence'' ).

As there is insufficient evidence to quantify adjustments, for total ozone the assessed central estimate for ERF is assumed to be equal to SARF ( ''low confidence'' ) and follows [[#Skeie--2020|Skeie et al. (2020)]] , since that study uses the most recent emissions data. The dataset is extended over the entire historical period following [[#Skeie--2020|Skeie et al. (2020)]] , with a SARF for 1750–1850 of 0.03 W m <sup>–2</sup> and for 2010–2018 of 0.03 W m <sup>–2</sup> , <sup></sup> to give 0.47 [0.24 to 0.70] W m <sup>–2</sup> for 1750–2019. This maintains the 50% uncertainty (5–95% range) from AR5 which is largely due to the uncertainty in pre-industrial emissions ( [[#Rowlinson--2020|Rowlinson et al., 2020]] ). There is also ''high confidence'' that this range includes uncertainty due to the adjustments. The CMIP6 SARF is more positive than the AR5 value of 0.31 W m <sup>–2</sup> for the period 1850–2011 ( [[#Myhre--2013b|Myhre et al., 2013b]] ) which was based on the Atmospheric Chemistry and Climate Intercomparison Project (ACCMIP; [[#Shindell--2013|Shindell et al., 2013]] ) ''.'' The assessment is sensitive to the assumptions on precursor emissions used to drive the models, which are larger in CMIP6 than ACCMIP.

In summary, although there is insufficient evidence to quantify adjustments, there is ''high confidence'' in the assessed range of ERF for ozone changes over the 1750–2019 period, giving an assessed ERF of 0.47 [0.24 to 0.70] W m <sup>–2</sup> .

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<span id="stratospheric-water-vapour"></span>
==== 7.3.2.6 Stratospheric Water Vapour ====

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This section considers direct anthropogenic effects on stratospheric water vapour by oxidation of methane. Since AR5 the SARF from methane-induced stratospheric water vapour changes has been calculated in [[#Winterstein--2019|Winterstein et al., 2019]] , corresponding to 0.09 W m <sup>–2</sup> when scaling to 1850 to 2014 methane changes. This is marginally larger than the AR5 assessed value of 0.07 ± 0.05 W m <sup>–2</sup> ( [[#Myhre--2013b|Myhre et al., 2013b]] ). [[#Wang--2020|Wang and Huang (2020)]] quantified the adjustment terms to a stratospheric water vapour change equivalent to a forcing from a 2×CO <sub>2</sub> warming (which has a different vertical profile). They found that the ERF was less than 50% of the SARF due to high-cloud decrease and upper tropospheric warming. The assessed ERF is therefore 0.05 ± 0.05 W m <sup>–2</sup> with a lower limit reduced to zero and the central value and upper limit reduced to allow for adjustment terms. This still encompasses the two recent SARF studies. There is ''medium confidence'' in the SARF from agreement with the recent studies and AR5. There is ''low confidence'' in the adjustment terms.

Stratospheric water vapour may also change as an adjustment to species that warm or cool the upper troposphere–lower stratosphere region ( [[#Forster--2005|Forster and Joshi, 2005]] ; [[#Stuber--2005|Stuber et al., 2005]] ), in which case it should be included as part of the ERF for that compound. Changes in GSAT are also associated with changes in stratospheric water vapour as part of the water-vapour–climate feedback ( [[#7.4.2.2|Section 7.4.2.2]] ).

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<span id="synthesis"></span>
==== 7.3.2.7 Synthesis ====

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The ERF of GHGs (excluding ozone and stratospheric water vapour) over 1750–2019 is assessed to be 3.32 ± 0.29 W m <sup>–2</sup> . It has increased by 0.49 W m <sup>–2</sup> compared to AR5 (reference year 2011) ( ''high confidence'' ) ''.'' Most of this has been due to an increase in CO <sub>2</sub> concentration since 2011 [0.27 ± 0.03] W m <sup>–2</sup> , with concentration increases in CH <sub>4</sub> , N <sub>2</sub> O and halogenated compounds adding 0.02, 0.02 and 0.01 W m <sup>–2</sup> respectively (Table 7.5). Changes in the radiative efficiencies (including adjustments) of CO <sub>2</sub> , CH <sub>4</sub> , N <sub>2</sub> O and halogenated compounds have increased the ERF by an additional 0.15 W m <sup>–2</sup> compared to the AR5 values ( ''high confidence'' ). Note that the ERFs in this section do not include chemical effects of GHGs on production or destruction of ozone or aerosol formation (Section 6.2.2). The ERF for ozone is considerably increased compared to AR5 due to an increase in the assumed ozone precursor emissions in CMIP6 compared to CMIP5, and better accounting for the effects of both ozone precursors and ODSs in the stratosphere. The ERF for stratospheric water vapour is slightly reduced. The combined ERF from ozone and stratospheric water vapour has increased since AR5 by 0.10 ± 0.50 W m <sup>–2</sup> ( ''high confidence'' ), although the uncertainty ranges still include the AR5 values.

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'''Table 7.5''' '''|''' '''Present-day mole fractions in parts per trillion (pmol mol''' –1 '''), except where specified, and effective radiative forcing (ERF, in W m''' –2 ''') for the well-mixed greenhouse gases (WMGHGs).''' Data taken from ( [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] [[IPCC:Wg1:Chapter:Chapter-2#2.2.3|Section 2.2.3]] ). The data for 2011 (the time of the AR5 estimates) are also shown. Some of the concentrations vary slightly from those reported in AR5 owing to averaging different data sources. Individual species are reported where 1750–2019 ERF is at least 0.001 W m <sup>–2</sup> . Radiative efficiencies for the minor gases are given in Supplementary Material, Table 7.SM.7. Uncertainties in the ERF for all gases are dominated by the uncertainties in the radiative efficiencies. Tabulated global mixing ratios of all WMGHGs and ERFs from 1750 to 2019 are provided in Annex III.

{| class="wikitable"
|-
|
| colspan="4"| Concentration

| colspan="2"| ERF with Respect to 1850

| colspan="2"| ERF with Respect to 1750

|-
|
| 2019

| 2011

| 1850

| 1750

| 2019

| 2011

| 2019

| 2011

|-
| CO <sub>2</sub> (ppm)

| 409.9

| 390.5

| 285.5

| 278.3

| 2.012 ± 0.241

| 1.738

| 2.156 ± 0.259

| 1.882

|-
| CH <sub>4</sub> (ppb)

| 1866.3

| 1803.3

| 807.6

| 729.2

| 0.496 ± 0.099

| 0.473

| 0.544 ± 0.109

| 0.521

|-
| N <sub>2</sub> O (ppb)

| 332.1

| 324.4

| 272.1

| 270.1

| 0.201 ± 0.030

| 0.177

| 0.208 ± 0.031

| 0.184

|-
| HFC-134a

| 107.6

| 62.7

| 0.0

| 0.0

| 0.018

| 0.010

| 0.018

| 0.010

|-
| HFC-23

| 32.4

| 24.1

| 0.0

| 0.0

| 0.006

| 0.005

| 0.006

| 0.005

|-
| HFC-32

| 20.0

| 4.7

| 0.0

| 0.0

| 0.002

| 0.001

| 0.002

| 0.001

|-
| HFC-125

| 29.4

| 10.3

| 0.0

| 0.0

| 0.007

| 0.002

| 0.007

| 0.002

|-
| HFC-143a

| 24.0

| 12.0

| 0.0

| 0.0

| 0.004

| 0.002

| 0.004

| 0.002

|-
| SF <sub>6</sub>

| 10.0

| 7.3

| 0.0

| 0.0

| 0.006

| 0.004

| 0.006

| 0.004

|-
| CF <sub>4</sub>

| 85.5

| 79.0

| 34.0

| 34.0

| 0.005

| 0.004

| 0.005

| 0.004

|-
| C <sub>2</sub> f <sub>6</sub>

| 4.8

| 4.2

| 0.0

| 0.0

| 0.001

| 0.001

| 0.001

| 0.001

|-
| CFC-11

| 226.2

| 237.3

| 0.0

| 0.0

| 0.066

| 0.070

| 0.066

| 0.070

|-
| CFC-12

| 503.1

| 528.6

| 0.0

| 0.0

| 0.180

| 0.189

| 0.180

| 0.189

|-
| CFC-113

| 69.8

| 74.6

| 0.0

| 0.0

| 0.021

| 0.022

| 0.021

| 0.022

|-
| CFC-114

| 16.0

| 16.3

| 0.0

| 0.0

| 0.005

| 0.005

| 0.005

| 0.005

|-
| CFC-115

| 8.7

| 8.4

| 0.0

| 0.0

| 0.002

| 0.002

| 0.002

| 0.002

|-
| HCFC-22

| 246.8

| 213.2

| 0.0

| 0.0

| 0.053

| 0.046

| 0.053

| 0.046

|-
| HCFC-141b

| 24.4

| 21.4

| 0.0

| 0.0

| 0.004

| 0.003

| 0.004

| 0.003

|-
| HCFC-142b

| 22.3

| 21.2

| 0.0

| 0.0

| 0.004

| 0.004

| 0.004

| 0.004

|-
| CCl <sub>4</sub>

| 77.9

| 86.1

| 0.0

| 0.0

| 0.013

| 0.014

| 0.013

| 0.014

|-
| Sum of HFCs (HFC-134a equivalent)

| 237.1

| 128.6

| 0.0

| 0.0

| 0.040

| 0.022

| 0.040

| 0.022

|-
| Sum of CFCs+HCFCs+other ozone depleting gases covered by the Montreal Protocol (CFC-12 equivalent)

| 1031.9

| 1050.1

| 0.0

| 0.0

| 0.354

| 0.362

| 0.354

| 0.362

|-
| Sum of PFCs (CF <sub>4</sub> equivalent)

| 109.4

| 98.9

| 34.0

| 34.0

| 0.007

| 0.006

| 0.007

| 0.006

|-
| Sum of Halogenated species

|
| 0.408 ±0.078

| 0.394

| 0.408 ±0.078

| 0.394

|-
| Total

|
| 3.118 ±0.258

| 2.782

| 3.317 ±0.278

| 2.981

|}

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