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

=== Climate Feedbacks and Sensitivity ===

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'''The net effect of changes in clouds in response to global warming is to amplify human-induced warming, that is, the net cloud feedback is positive''' ( ''high confidence'' '''). Compared to AR5, major advances in the understanding of cloud processes have increased the level of confidence and decreased the uncertainty range in the cloud feedback by about 50%.''' An assessment of the low-altitude cloud feedback over the subtropical oceans, which was previously the major source of uncertainty in the net cloud feedback, is improved owing to a combined use of climate model simulations, satellite observations, and explicit simulations of clouds, altogether leading to strong evidence that this type of cloud amplifies global warming. The net cloud feedback, obtained by summing the cloud feedbacks assessed for individual regimes, is 0.42 [–0.10 to +0.94] W m <sup>–2</sup> °C <sup>–1</sup> . A net negative cloud feedback is ''very unlikely'' ( ''high confidence'' ). {7.4.2, Figure 7.10, Table 7.10}

'''The combined effect of all known radiative feedbacks (physical, biogeophysical, and non-CO''' <sub>2</sub> '''biogeochemical) is to amplify the base climate response, also known as the Planck temperature response''' ( ''virtually certain'' ''').''' Combining these feedbacks with the base climate response, the net feedback parameter based on process understanding is assessed to be –1.16 [–1.81 to –0.51] W m <sup>–2</sup> °C <sup>–1</sup> , which is slightly less negative than that inferred from the overall ECS assessment. The combined water-vapour and lapse-rate feedback makes the largest single contribution to global warming, whereas the cloud feedback remains the largest contribution to overall uncertainty. Due to the state-dependence of feedbacks, as evidenced from paleoclimate observations and from models, the net feedback parameter will increase (become less negative) as global temperature increases. Furthermore, on long time scales the ice-sheet feedback parameter is ''very likely'' positive, promoting additional warming on millennial time scales as ice sheets come into equilibrium with the forcing ( ''high confidence'' ). {7.4.2, 7.4.3, 7.5.7}

'''Radiative feedbacks, particularly from clouds, are expected to become less negative (more amplifying) on multi-decadal time scales as the''' ''spatial pattern'' '''of surface warming evolves, leading to an ECS that is higher than was inferred in AR5 based on warming over the instrumental record. This new understanding, along with updated estimates of historical temperature change, ERF, and Earth’s energy imbalance, reconciles previously disparate ECS estimates''' ( ''high confidence'' ''').''' However, there is currently insufficient evidence to quantify a ''likely'' range of the magnitude of future changes to current climate feedbacks. Warming over the instrumental record provides robust constraints on the lower end of the ECS range ( ''high confidence'' ), but owing to the possibility of future feedback changes it does not, on its own, constrain the upper end of the range, in contrast to what was reported in AR5. {7.4.4, 7.5.2, 7.5.3}

'''Based on multiple lines of evidence the best estimate of ECS is 3°C, the''' ''likely'' '''range is 2.5°C to 4°C, and the''' ''very likely'' '''range is 2°C to 5°C. It is''' ''virtually certain'' '''that ECS is larger than 1.5°C.''' Substantial advances since AR5 have been made in quantifying ECS based on feedback process understanding, the instrumental record, paleoclimates and emergent constraints. There is a high level of agreement among the different lines of evidence. All lines of evidence help rule out ECS values below 1.5°C, but currently it is not possible to rule out ECS values above 5°C. Therefore, the 5°C upper end of the ''very likely'' range is assessed to have ''medium confidence'' and the other bounds have ''high confidence'' . {7.5.5}

'''Based on process understanding, warming over the instrumental record, and emergent constraints, the best estimate of TCR is 1.8°C, the''' ''likely'' '''range is 1.4°C to 2.2°C and the''' ''very likely'' '''range is 1.2°C to 2.4°C''' ( ''high confidence'' ''').''' {7.5.5}

'''On average, Coupled Model Intercomparison Project Phase 6 (CMIP6) models have higher mean ECS and TCR values than the Phase 5 (CMIP5) generation of models. They also have higher mean values and wider spreads than the assessed best estimates and''' ''very likely'' '''ranges within this Report.''' These higher ECS and TCR values can, in some models, be traced to changes in extra-tropical cloud feedbacks that have emerged from efforts to reduce biases in these clouds compared to satellite observations ( ''medium confidence'' ). The broader ECS and TCR ranges from CMIP6 also lead the models to project a range of future warming that is wider than the assessed warming range, which is based on multiple lines of evidence. However, some of the high-sensitivity CMIP6 models are less consistent with observed recent changes in global warming and with paleoclimate proxy data than models with ECS within the ''very likely'' range. Similarly, some of the low-sensitivity models are less consistent with the paleoclimate data. The CMIP models with the highest ECS and TCR values provide insights into low-likelihood, high-impact outcomes, which cannot be excluded based on currently available evidence ( ''high confidence'' ). {4.3.1, 4.3.4, 7.4.2, 7.5.6}

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