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

==== 7.4.3.1 State-dependence of Feedbacks in Models ====

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There are several modelling studies since AR5 in which ESMs of varying complexity have been used to explore temperature dependence of feedbacks, either under modern ( [[#Hansen--2013|Hansen et al., 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]] ; [[#Rohrschneider--2019|Rohrschneider et al., 2019]] ; [[#Stolpe--2019|Stolpe et al., 2019]] ; [[#Bloch-Johnson--2020|Bloch-Johnson et al., 2020]] ; [[#Rugenstein--2020|Rugenstein et al., 2020]] ) or paleo ( [[#Caballero--2013|Caballero and Huber, 2013]] ; [[#Zhu--2019a|Zhu et al., 2019a]] ) climate conditions, typically by carrying out multiple simulations across successive CO <sub>2</sub> doublings. A non-linear temperature response to these successive doublings may be partly due to forcing that increases more (or less) than expected from a purely logarithmic dependence ( [[#7.3.2|Section 7.3.2]] ; [[#Etminan--2016|Etminan et al., 2016]] ), and partly due to state-dependence in feedbacks; however, not all modelling studies have partitioned the non-linearities in temperature response between these two effects. Nonetheless, there is general agreement among ESMs that the net feedback parameter, α , increases (i.e., becomes less negative) as temperature increases from pre-industrial levels (i.e., sensitivity to forcing increases as temperature increases; e.g., [[#Meraner--2013|Meraner et al., 2013]] ; see Figure 7.11). The associated increase in sensitivity to forcing is, in most models, due to the water vapour ( [[#7.4.2.2|Section 7.4.2.2]] ) and cloud ( [[#7.4.2.4|Section 7.4.2.4]] ) feedback parameters increasing with warming ( [[#Caballero--2013|Caballero and Huber, 2013]] ; [[#Meraner--2013|Meraner et al., 2013]] ; [[#Zhu--2019a|Zhu et al., 2019a]] ; [[#Rugenstein--2020|Rugenstein et al., 2020]] ; [[#Sherwood--2020|Sherwood et al., 2020]] ). These changes are offset partially by the surface-albedo feedback parameter decreasing ( [[#Jonko--2013|Jonko et al., 2013]] ; [[#Meraner--2013|Meraner et al., 2013]] ; [[#Rugenstein--2020|Rugenstein et al., 2020]] ), as a consequence of a reduced amount of snow and sea ice cover in a much warmer climate. At the same time, there is little change in the Planck response ( [[#7.4.2.1|Section 7.4.2.1]] ), which has been shown in one model to be due to competing effects from increasing Planck emission at warmer temperatures and decreasing planetary emissivity due to increased CO <sub>2</sub> and water vapour ( [[#Mauritsen--2019|Mauritsen et al., 2019]] ). Analysis of the spatial patterns of the non-linearities in temperature response ( [[#Good--2015|Good et al., 2015]] ) suggests that these patterns are linked to a reduced weakening of the AMOC, and changes to evapotranspiration. The temperature dependence of α is also found in model simulations of high-CO <sub>2</sub> paleoclimates ( [[#Caballero--2013|Caballero and Huber, 2013]] ; [[#Zhu--2019a|Zhu et al., 2019a]] ). The temperature dependence is not only evident at very high CO <sub>2</sub> concentrations in excess of 4×CO <sub>2</sub> , but also apparent in the difference in temperature response to a 2×CO <sub>2</sub> forcing compared with to a 4×CO <sub>2</sub> forcing ( [[#Mauritsen--2019|Mauritsen et al., 2019]] ; [[#Rugenstein--2020|Rugenstein et al., 2020]] ), and as such is relevant for interpreting century-scale climate projections.

Despite the general agreement that α increases as temperature increases from pre-industrial levels (Figure 7.11), other modelling studies have found the opposite ( [[#Duan--2019|Duan et al., 2019]] ; [[#Stolpe--2019|Stolpe et al., 2019]] ). Modelling studies exploring state-dependence in climates colder than today, including in cold paleoclimates such as the LGM, provide conflicting evidence of either decreased ( [[#Yoshimori--2011|Yoshimori et al., 2011]] ) or increased ( [[#Kutzbach--2013|Kutzbach et al., 2013]] ; [[#Stolpe--2019|Stolpe et al., 2019]] ) temperature response per unit forcing during cold climates compared to the modern era.

In contrast to most ESMs, the majority of Earth system models of intermediate complexity (EMICs) do not exhibit state-dependence, or have a net feedback parameter that decreases with increasing temperature ( [[#Pfister--2017|Pfister and Stocker, 2017]] ). This is unsurprising since EMICs usually do not include process-based representations of water-vapour and cloud feedbacks. Although this shows that care must be taken when interpreting results from current generation EMICs, [[#Pfister--2017|Pfister and Stocker (2017)]] also suggest that non-linearities in feedbacks can take a long time to emerge in model simulations due to slow adjustment time scales associated with the ocean; longer simulations also allow better estimates of equilibrium warming ( [[#Bloch-Johnson--2020|Bloch-Johnson et al., 2020]] ). This implies that multi-century simulations ( [[#Rugenstein--2020|Rugenstein et al., 2020]] ) could increase confidence in ESM studies examining state-dependence.

The possibility of more substantial changes in climate feedbacks, sometimes accompanied by hysteresis and/or irreversibility, has been suggested from some theoretical and modelling studies. It has been postulated that such changes could occur on a global scaleand across relatively narrow temperature changes ( [[#Popp--2016|Popp et al., 2016]] ; [[#von%20der%20Heydt--2016|von der Heydt and Ashwin, 2016]] ; [[#Steffen--2018|Steffen et al., 2018]] ; [[#Schneider--2019|Schneider et al., 2019]] ; [[#Ashwin--2020|Ashwin and von der Heydt, 2020]] ; [[#Bjordal--2020|Bjordal et al., 2020]] ). However, the associated mechanisms are highly uncertain, and as such there is ''low confidence'' as to whether such behaviour exists at all, and in the temperature thresholds at which it might occur.

Overall, the modelling evidence indicates that there is ''medium confidence'' that the net feedback parameter, α , increases (i.e., becomes less negative) with increasing temperature (i.e., that sensitivity to forcing increases with increasing temperature), under global surface background temperatures at least up to 40°C ( [[#Meraner--2013|Meraner et al., 2013]] ; [[#Seeley--2021|Seeley and Jeevanjee, 2021]] ), and ''medium confidence'' that this temperature dependence primarily derives from increases in the water-vapour and shortwave cloud feedbacks. This assessment is further supported by recent analysis of CMIP6 model simulations ( [[#Bloch-Johnson--2020|Bloch-Johnson et al., 2020]] ) in the framework of nonlinMIP ( [[#Good--2016|Good et al., 2016]] ), which showed that out of 10 CMIP6 models, seven of them showed an increase of the net feedback parameter with temperature, primarily due to the water-vapour feedback.

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