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

==== 7.4.2.1 Planck Response ====

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The Planck response represents the additional thermal or longwave (LW) emission to space arising from vertically uniform warming of the surface and the atmosphere. The Planck response α P , often called the Planck feedback, plays a fundamental stabilizing role in Earth’s climate and has a value that is strongly negative: a warmer planet radiates more energy to space. A crude estimate of α P can be made using the normalized greenhouse effect g̃ , defined as the ratio between the greenhouse effect ''G'' and the upwelling LW flux at the surface ( [[#Raval--1989|Raval and Ramanathan, 1989]] ). Current estimates ( [[#7.2|Section 7.2]] , Figure 7.2) give ''G'' = 159 W m <sup>–2</sup> and g̃ ≈ 0.4. Assuming g̃ is constant, one obtains for a surface temperature ''T'' s = 288 K, α P = ( g – 1) 4 σ ''T'' <sup>3</sup> s ≈ –3.3 W m <sup>–2</sup> °C <sup>–1</sup> , where σ is the Stefan–Boltzmann constant. This parameter α P is estimated more accurately using kernels obtained from meteorological reanalysis or climate simulations ( [[#Soden--2006|Soden and Held, 2006]] ; [[#Dessler--2013|Dessler, 2013]] ; [[#Vial--2013|Vial et al., 2013]] ; [[#Caldwell--2016|Caldwell et al., 2016]] ; [[#Colman--2017|Colman and Hanson, 2017]] ; [[#Zelinka--2020|Zelinka et al., 2020]] ). Discrepancies among estimates primarily arise because differences in cloud distributions make the radiative kernels differ ( [[#Kramer--2019|Kramer et al., 2019]] ). Using six different kernels, [[#Zelinka--2020|Zelinka et al. (2020)]] obtained a spread of ±0.1 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation). Discrepancies among estimates secondarily arise from differences in the pattern of equilibrium surface temperature changes among ESMs. For the CMIP5 and CMIP6 models this introduces a spread of ±0.04 W m <sup>–2</sup> °C <sup>–1</sup> (one standard deviation). The multi-kernel and multi-model mean of α P is equal to –3.20 W m <sup>–2</sup> °C <sup>–1</sup> for the CMIP5 and –3.22 W m <sup>–2</sup> °C <sup>–1</sup> for the CMIP6 models (Supplementary Material, Table 7.SM.5). Overall, there is ''high confidence'' in the estimate of the Planck response, which is assessed to be α P = –3.22 W m <sup>–2</sup> °C <sup>–1</sup> with a ''very likely'' range of –3.4 to –3.0 W m <sup>–2</sup> °C <sup>–1</sup> and a ''likely range'' of –3.3 to –3.1 W m <sup>–2</sup> °C <sup>–1</sup> .

The Planck temperature response Δ ''T'' P is the equilibrium temperature change in response to a forcing Δ ''F'' when the net feedback parameter is equal to the Planck response parameter: Δ ''T'' P ''= –'' Δ ''F /'' α P .

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