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

==== 7.4.3.2 State-dependence of Feedbacks in the Paleoclimate Proxy Record ====

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Several studies have estimated ECS from observations of the glacial–interglacial cycles of the last approximately 2 million years, and found a state-dependence, with more-negative α (i.e., lower sensitivity to forcing) during colder periods of the cycles and less-negative α during warmer periods ( [[#von%20der%20Heydt--2014|von der Heydt et al., 2014]] ; [[#Köhler--2015|Köhler et al., 2015]] , 2017; [[#Friedrich--2016|Friedrich et al., 2016]] ; [[#Royer--2016|Royer, 2016]] ; [[#Snyder--2019|Snyder, 2019]] ); see summaries in [[#Skinner--2012|Skinner (2012)]] and [[#von%20der%20Heydt--2016|von der Heydt et al. (2016)]] . However, the nature of the state-dependence derived from these observations is dependent on the assumed ice-sheet forcing ( [[#Köhler--2015|Köhler et al., 2015]] ; [[#Stap--2019|Stap et al., 2019]] ), which is not well known, due to a relative lack of proxy indicators of ice-sheet extent and distribution prior to the LGM (Cross-Chapter Box 2.1). Furthermore, many of these glacial–interglacial studies estimate a very strong temperature-dependence of α (Figure 7.11) that is hard to reconcile with the other lines of evidence, including proxy estimates from warmer paleoclimates. However, if the analysis excludes time periods when the temperature and CO <sub>2</sub> data are not well correlated, which occurs in general at times when sea level is falling and obliquity is decreasing, the state-dependence reduces ( [[#Köhler--2018|Köhler et al., 2018]] ). Despite these uncertainties, due to the agreement in the sign of the temperature-dependence from all these studies, there is ''medium confidence'' from the paleoclimate proxy record that the net feedback parameter, α , was less negative in the warm periods than in the cold periods of the glacial–interglacial cycles.

Paleoclimate proxy evidence from past high-CO <sub>2</sub> time periods much warmer than present (the early Eocene and Paleocene–Eocene Thermal Maximum, PETM; Cross-Chapter Box 2.1) show that the feedback parameter increases as temperature increases ( [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020; [[#Shaffer--2016|Shaffer et al., 2016]] ). However, such temperature-dependence of feedbacks was not found in the warm Pliocene relative to the cooler Pleistocene ( [[#Martínez-Botí--2015|Martínez-Botí et al., 2015]] ), although the temperature changes are relatively small at this time, making temperature-dependence challenging to detect given the uncertainties in reconstructing global mean temperature and forcing. Overall, the paleoclimate proxy record provides ''medium confidence'' that the net feedback parameter, α , was less negative in these past warm periods than in the present day.

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[[File:cede88dd139299f5d25e764a5f855c21 IPCC_AR6_WGI_Figure_7_11.png]]

'''Figure 7.11''' '''|''' '''Feedback parameter,''' α '''(W m''' –2 '''°C''' –1 '''), as a function of global mean surface air temperature anomaly relative to pre-industrial, for ESM simulations (red circles and lines)''' ( [[#Caballero--2013|Caballero and Huber, 2013]] ; [[#Jonko--2013|Jonko et al., 2013]] ; [[#Meraner--2013|Meraner et al., 2013]] ; [[#Good--2015|Good et al., 2015]] ; [[#Duan--2019|Duan et al., 2019]] ; [[#Mauritsen--2019|Mauritsen et al., 2019]] ; [[#Stolpe--2019|Stolpe et al., 2019]] ; [[#Zhu--2019a|Zhu et al., 2019a]] ), '''and derived from paleoclimate proxies (grey squares and lines)''' ( [[#von%20der%20Heydt--2014|von der Heydt et al., 2014]] ; [[#Anagnostou--2016|Anagnostou et al., 2016]] , 2020; [[#Friedrich--2016|Friedrich et al., 2016]] ; [[#Royer--2016|Royer, 2016]] ; [[#Shaffer--2016|Shaffer et al., 2016]] ; [[#Köhler--2017|Köhler et al., 2017]] ; [[#Snyder--2019|Snyder, 2019]] ; [[#Stap--2019|Stap et al., 2019]] ). For the ESM simulations, the value on The x -axis refers to the average of the temperature before and after the system has equilibrated to a forcing (in most cases a CO <sub>2</sub> doubling), and is expressed as an anomaly relative to an associated pre-industrial global mean temperature from that model. The light blue shaded square extends across the assessed range of '''α''' (Table 7.10) on The y -axis, and on The x -axis extends across the approximate temperature range over which the assessment of α is based (taken as from zero to the assessed central value of ECS; see Table 7.13). Further details on data sources and processing are available in the chapter data table (Table 7.SM.14).

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