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

==== 7.3.2.1 Carbon Dioxide (CO <sub>2</sub> ) ====

<div id="h3-4-siblings" class="h3-siblings"></div>

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.

<div id="_idContainer023" class="Basic-Text-Frame"></div>

'''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%

|}

<div id="_idContainer024" class="Basic-Text-Frame mt-4"></div>

'''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%

|}

<div id="7.3.2.2" class="h3-container"></div>

<span id="methane-ch-4"></span>