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

==== 7.5.3.1 Estimates of ECS from the Last Glacial Maximum ====

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The LGM (Cross-Chapter Box 2.1) has been used to provide estimates of ECS (see Table 7.11 for estimates since AR5; [[#Sherwood--2020|Sherwood et al., 2020]] ; [[#Tierney--2020b|Tierney et al., 2020b]] ). The major forcings and feedback processes that led to the cold climate at that time (e.g., CO <sub>2</sub> , non-CO <sub>2</sub> greenhouse gases, and ice sheets) are relatively well-known ( [[IPCC:Wg1:Chapter:Chapter-5#5.1|Section 5.1]] ), orbital forcing relative to pre-industrial was negligible, and there are relatively high spatial resolution and well-dated paleoclimate temperature data available for this time period ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1|Section 2.3.1]] ). Uncertainties in deriving global surface temperature from the LGM proxy data arise partly from uncertainties in the calibration from the paleoclimate data to local annual mean surface temperature, and partly from uncertainties in the conversion of the local temperatures to an annual mean global surface temperature. Overall, the global mean LGM cooling relative to pre-industrial is assessed to be ''very likely'' from 5 to 7 °c ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1|Section 2.3.1]] ). The LGM climate is often assumed to be in full equilibrium with the forcing, such that Δ ''N'' in Equation 7.1, Box 7.1, is zero. A calculation of sensitivity using solely CO <sub>2</sub> forcing, and assuming that the LGM ice sheets were in equilibrium with that forcing, would give an Earth System Sensitivity (ESS) rather than an ECS (see Box 7.1). In order to calculate an ECS, which is defined here to include all feedback processes except ice sheets, the approach of [[#Rohling--2012|Rohling et al. (2012)]] can be used. This approach introduces an additional forcing term in Equation 7.1, Box 7.1, that quantifies the resulting forcing associated with the ice-sheet feedback (primarily an estimate of the radiative forcing associated with the change in surface albedo). However, differences between studies as to which processes are considered as forcings (for example, some studies also include vegetation and/or aerosols, such as dust, as forcings), means that published estimates are not always directly comparable. Additional uncertainty arises from the magnitude of the ice-sheet forcing itself ( [[#Stap--2019|Stap et al., 2019]] ; [[#Zhu--2021|Zhu and Poulsen, 2021]] ), which is often estimated using ESMs. Furthermore, the ECS at the LGM may differ from that of today due to state-dependence ( [[#7.4.3|Section 7.4.3]] ). Here, only studies that report values of ECS that have accounted for the long-term feedbacks associated with ice sheets, and therefore most closely estimate ECS as defined in this chapter, are assessed here (Table 7.11).

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