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

==== 7.4.2.2 Water-vapour and Temperature Lapse-rate Feedbacks ====

<div id="h3-25-siblings" class="h3-siblings"></div>

Two decompositions are generally used to analyse the feedbacks associated with a change in the water-vapour and temperature lapse-rate in the troposphere. As in any system, many feedback decompositions are possible, each of them highlighting a particular property or aspect of the system ( [[#Ingram--2010|Ingram, 2010]] ; [[#Held--2012|Held and Shell, 2012]] ; [[#Dufresne--2016|Dufresne and Saint-Lu, 2016]] ). The first decomposition considers separately the changes (and therefore feedbacks) in the lapse rate (LR) and specific humidity (WV). The second decomposition considers changes in the lapse rate assuming constant relative humidity (LR*) separately from changes in relative humidity (RH).

The specific humidity (WV) feedback, also known as the water-vapour feedback, quantifies the change in radiative flux at the TOA due to changes in atmospheric water vapour concentration associated with a change in global mean surface air temperature. According to theory, observations and models, the water vapour increase approximately follows the Clausius–Clapeyron relationship at the global scale with regional differences dominated by dynamical processes ( [[IPCC:Wg1:Chapter:Chapter-8#8.2.1|Section 8.2.1]] ; [[#Sherwood--2010a|Sherwood et al., 2010a]] ; [[#Chung--2014|Chung et al., 2014]] ; [[#Romps--2014|Romps, 2014]] ; R. [[#Liu--2018|]] [[#Liu--2018|Liu et al., 2018]] ; [[#Schröder--2019|Schröder et al., 2019]] ). Greater atmospheric water vapour content, particularly in the upper troposphere, results in enhanced absorption of LW and SW radiation and reduced outgoing radiation. This is a positive feedback. Atmospheric moistening has been detected in satellite records ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.3.3|Section 2.3.1.3.3]] ), it is simulated by climate models ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.2.2|Section 3.3.2.2]] ), and the estimates agree within model and observational uncertainty ( [[#Soden--2005|Soden et al., 2005]] ; [[#Dessler--2013|Dessler, 2013]] ; [[#Gordon--2013|Gordon et al., 2013]] ; [[#Chung--2014|Chung et al., 2014]] ). The estimate of this feedback inferred from satellite observations is α WV <sub></sub> = 1.85 ± 0.32 W m <sup>–2</sup> °C <sup>–1</sup> (R. [[#Liu--2018|]] [[#Liu--2018|Liu et al., 2018]] ). This is consistent with the value α WV <sub></sub> = 1.77 ± 0.20 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation) obtained with CMIP5 and CMIP6 models ( [[#Zelinka--2020|Zelinka et al., 2020]] ).

The lapse-rate (LR) feedback quantifies the change in radiative flux at the TOA due to a nonuniform change in the vertical temperature profile. In the tropics, the vertical temperature profile is mainly driven by moist convection and is close to a moist adiabat. The warming is larger in the upper troposphere than in the lower troposphere ( [[#Manabe--1975|Manabe and Wetherald, 1975]] ; [[#Santer--2005|Santer et al., 2005]] ; [[#Bony--2006|Bony et al., 2006]] ), leading to a larger radiative emission to space and therefore a negative feedback. This larger warming in the upper troposphere than at the surface has been observed over the last 20 years thanks to the availability of sufficiently accurate observations ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.2.2|Section 2.3.1.2.2]] ). In the extratropics, the vertical temperature profile is mainly driven by a balance between radiation, meridional heat transport and ocean heat uptake ( [[#Rose--2014|Rose et al., 2014]] ). Strong winter temperature inversions lead to warming that is larger in the lower troposphere ( [[#Payne--2015|Payne et al., 2015]] ; [[#Feldl--2017a|Feldl et al., 2017a]] ) and a positive LR feedback in polar regions ( [[#7.4.4.1|Section 7.4.4.1]] ; [[#Manabe--1975|Manabe and Wetherald, 1975]] ; [[#Bintanja--2012|Bintanja et al., 2012]] ; [[#Pithan--2014|Pithan and Mauritsen, 2014]] ). However, the tropical contribution dominates, leading to a negative global mean LR feedback ( [[#Soden--2006|Soden and Held, 2006]] ; [[#Dessler--2013|Dessler, 2013]] ; [[#Vial--2013|Vial et al., 2013]] ; [[#Caldwell--2016|Caldwell et al., 2016]] ). The LR feedback has been estimated at interannual time scales using meteorological reanalysis and satellite measurements of TOA fluxes ( [[#Dessler--2013|Dessler, 2013]] ). These estimates from climate variability are consistent between observations and ESMs ( [[#Dessler--2013|Dessler, 2013]] ; [[#Colman--2017|Colman and Hanson, 2017]] ). The mean and standard deviation of this feedback under global warming based on the cited studies are α LR <sub></sub> = –0.50 ± 0.20 W m <sup>–2</sup> °C <sup>–1</sup> ( [[#Dessler--2013|Dessler, 2013]] ; [[#Caldwell--2016|Caldwell et al., 2016]] ; [[#Colman--2017|Colman and Hanson, 2017]] ; [[#Zelinka--2020|Zelinka et al., 2020]] ).

The second decomposition was proposed by [[#Held--2012|Held and Shell (2012)]] to separate the response that would occur under the assumption that relative humidity remains constant from that due to the change in relative humidity. The feedback is decomposed into three: (i) change in water vapour due to an identical temperature increase at the surface and throughout the troposphere assuming constant relative humidity, which will be called the Clausius–Clapeyron (CC) feedback here; (ii) change in LR assuming constant relative humidity (LR*); (iii) change in relative humidity (RH). Since AR5 it has been clarified that by construction, the sum of the temperature lapse rate and specific humidity (LR + WV) feedbacks is equal to the sum of the Clausius–Clapeyron feedback, the lapse rate feedback assuming constant relative humidity, and the feedback from changes in relative humidity (that is, CC + LR* + RH). Therefore, each of these two sums may simply be referred to as the ‘water-vapour plus lapse-rate’ feedback.

The CC feedback has a large positive value due to well understood thermodynamic and radiative processes: α CC <sub></sub> = 1.36 ± 0.04 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation; [[#Held--2012|Held and Shell, 2012]] ; [[#Zelinka--2020|Zelinka et al., 2020]] ). The lapse-rate feedback assuming a constant relative humidity (LR*) in CMIP6 models has small absolute values ( α LR <sub>*</sub> = –0.10 ± 0.07 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation)), as expected from theoretical arguments ( [[#Ingram--2010|Ingram, 2010]] , 2013). It includes the pattern effect of surface warming that modulates the lapse rate and associated specific humidity changes ( [[#Po-Chedley--2018b|Po-Chedley et al., 2018b]] ). The relative humidity feedback is close to zero ( α RH = 0.00 ± 0.06 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation)) and the spread among models is confined to the tropics ( [[#Sherwood--2010b|Sherwood et al., 2010b]] ; [[#Vial--2013|Vial et al., 2013]] ; [[#Takahashi--2016|Takahashi et al., 2016]] ; [[#Po-Chedley--2018b|Po-Chedley et al., 2018b]] ). The change in upper tropospheric RH is closely related to model representation of current climate ( [[#Sherwood--2010b|Sherwood et al., 2010b]] ; [[#Po-Chedley--2019|Po-Chedley et al., 2019]] ), and a reduction in model RH biases is expected to reduce the uncertainty of the RH feedback. At interannual time scales, it has been shown that the change in RH in the tropics is related to the change of the spatial organization of deep convection ( [[#Holloway--2017|Holloway et al., 2017]] ; [[#Bony--2020|Bony et al., 2020]] ).

Both decompositions allow estimates of the sum of the lapse-rate and specific humidity feedbacks α LR+WV . The multi-kernel and multi-model mean of α LR+WV <sub></sub> is equal to 1.24 and 1.26 W m <sup>–2</sup> °C <sup>–1</sup> respectively for CMIP5 and CMIP6 models, with a standard deviation of 0.10 W m <sup>–2</sup> °C <sup>–1</sup> ( [[#Zelinka--2020|Zelinka et al., 2020]] ). These values are larger than the recently assessed value of 1.15 W m <sup>–2</sup> °C <sup>–1</sup> by [[#Sherwood--2020|Sherwood et al. (2020)]] as a larger set of kernels, including those obtained from meteorological reanalysis, are used here.

Since AR5, the effect of the water vapour increase in the stratosphere as a result of global warming has been investigated by different studies. This increase produces a positive feedback between 0.1 and 0.3 W m <sup>–2</sup> °C <sup>–1</sup> if the stratospheric radiative response is computed assuming temperatures that are adjusted with fixed dynamical heating ( [[#Dessler--2013|Dessler et al., 2013]] ; [[#Banerjee--2019|Banerjee et al., 2019]] ). However, various feedbacks reduce this temperature adjustment and the overall physical (water vapour, temperature and dynamical) stratospheric feedback becomes much smaller (0.0 to 0.1 W m <sup>–2</sup> °C <sup>–1</sup> ; [[#Huang--2016|Huang et al., 2016]] , 2020; [[#Li--2020|Li and Newman, 2020]] ), with uncertainty arising from limitations of current ESMs in simulating stratospheric processes. The total stratospheric feedback is assessed at 0.05 ± 0.1 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation).

The combined ‘water-vapour plus lapse-rate’ feedback is positive. The main physical processes that drive this feedback are well understood and supported by multiple lines of evidence including models, theory and observations. The combined ‘water-vapour plus lapse-rate’ feedback parameter is assessed to be α LR+WV <sub></sub> = 1.30 W m <sup>–2</sup> °C <sup>–1</sup> , with a ''very likely'' range of 1.1 to 1.5 W m <sup>–2</sup> °C <sup>–1</sup> and a ''likely'' range of 1.2 to 1.4 W m <sup>–2</sup> °C <sup>–1</sup> with ''high confidence.''

<div id="7.4.2.3" class="h3-container"></div>

<span id="surface-albedo-feedback"></span>