Editing IPCC:AR6/WGI/TS (section)

==== TS.3.2.2 Earth System Feedbacks ====

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The combined effect of all climate feedback processes is to amplify the climate response to forcing ( ''virtually certain'' ). While major advances in the understanding of cloud processes have increased the level of confidence and decreased the uncertainty range for the cloud feedback by about 50% compared to AR5, clouds remain the largest contribution to overall uncertainty in climate feedbacks ( ''high confidence'' ). Uncertainties in the ECS and other climate sensitivity metrics, such as the TCR and TCRE, are the dominant source of uncertainty in global temperature projections over the 21st century under moderate to high GHG emissions scenarios. CMIP6 models have higher mean values and wider spreads in ECS and TCR than the assessed best estimates and ''very likely'' ranges within this Report, leading the models to project a range of future warming that is wider than the assessed future warming range (Section TS.2.2). Links to chapters 7.1, 7.4.2, 7.5

Earth system feedbacks can be categorized into three broad groups: physical feedbacks, biogeophysical and biogeochemical feedbacks, and feedbacks associated with ice sheets. In previous assessments, the ECS has been associated with a distinct set of physical feedbacks (Planck response, water vapour, lapse rate, surface albedo, and cloud feedbacks). In this assessment, a more general definition of ECS is adopted whereby all biogeophysical and biogeochemical feedbacks that do not affect the atmospheric concentration of CO <sub>2</sub> are included. These include changes in natural CH <sub>4</sub> emissions, natural aerosol emissions, N <sub>2</sub> O, ozone, and vegetation, which all act on time scales of years to decades and are therefore relevant for temperature change over the 21st century. Because the total biogeophysical and non-CO <sub>2</sub> biogeochemical feedback is assessed to have a central value that is near zero ( ''low confidence'' ), including it does not affect the assessed ECS but does contribute to the net feedback uncertainty. The biogeochemical feedbacks that affect the atmospheric concentration of CO <sub>2</sub> are not included because ECS is defined as the response to a sustained doubling of CO <sub>2</sub> . Moreover, the long-term feedbacks associated with ice sheets are not included in the ECS owing to their long time scales of adjustment. Links to chapters 5.4, 6.4, 7.4, 7.5, Box 7.1

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% (Figure TS.17a). An assessment of the low-altitude cloud feedback over the subtropical ocean, 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 is assessed to be +0.42 [–0.10 to 0.94] W m <sup>–2</sup> °C <sup>–</sup> <sup>1</sup> . A net negative cloud feedback is ''very unlikely'' . The CMIP5 and CMIP6 ranges of cloud feedback are similar to this assessed range, with CMIP6 having a slightly more positive median cloud feedback ( ''high confidence'' ). The surface albedo feedback and combined water vapour-lapse rate feedback are positive (Figure TS.17a), with ''high confidence'' in the estimated value of each based on multiple lines of evidence, including observations, models and theory (Box TS.6). Links to chapters 7.4.2, Figure 7.14, Table 7.10

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'''Figure TS.17 |''' '''An overview of physical and biogeochemical feedbacks in the climate system.''' ''The intent of this figure is to summarize assessed estimates of physical, biogeophysical and biogeochemical feedbacks on global temperature based on Chapters 5, 6 and 7.'' '''(a)''' Synthesis of physical, biogeophysical and non-carbon dioxide (CO <sub>2</sub> ) biogeochemical feedbacks that are included in the definition of equilibrium climate sensitivity (ECS) assessed in this Technical Summary. These feedbacks have been assessed using multiple lines of evidence including observations, models and theory. The net feedback is the sum of the Planck response, water vapour and lapse rate, surface albedo, cloud, and biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks. Bars denote the mean feedback values, and uncertainties represent ''very likely'' ranges; '''(b)''' Estimated values of individual biogeophysical and non-CO <sub>2</sub> biogeochemical feedbacks. The atmospheric methane (CH <sub>4</sub> ) lifetime and other non-CO <sub>2</sub> biogeochemical feedbacks have been calculated using global Earth system model simulations from AerChemMIP, while the CH <sub>4</sub> and nitrous oxide (N <sub>2</sub> O) source responses to climate have been assessed for the year 2100 using a range of modelling approaches using simplified radiative forcing equations. The estimates represent the mean and 5–95% range. The level of confidence in these estimates is ''low'' owing to the large model spread. '''(c)''' Carbon-cycle feedbacks as simulated by models participating in the C4MIP of the Coupled Model Intercomparison Project Phase 6 (CMIP6). An independent estimate of the additional positive carbon-cycle climate feedbacks from permafrost thaw, which is not considered in most C4MIP models, is added. The estimates represent the mean and 5–95% range. Note that these feedbacks act through modifying the atmospheric concentration of CO <sub>2</sub> and thus are not included in the definition of ECS, which assumes a doubling of CO <sub>2</sub> <sub>, 4</sub> but are included in the definition and assessed range of the transient climate response to cumulative CO <sub>2</sub> emissions (TCRE). Links to chapters 5.4.7, 5.4.8, Box 5.1, Figure 5.29, 6.4.5, Table 6.9, 7.4.2, Table 7.10

Natural sources and sinks of non-CO <sub>2</sub> greenhouse gases such as methane (CH <sub>4</sub> ) and nitrous oxide (N <sub>2</sub> O) respond both directly and indirectly to atmospheric CO <sub>2</sub> concentration and climate change, and thereby give rise to additional biogeochemical feedbacks in the climate system. Many of these feedbacks are only partially understood and are not yet fully included in ESMs. There is ''medium confidence'' that the net response of natural ocean and land CH <sub>4</sub> and N <sub>2</sub> O sources to future warming will be increased emissions, but the magnitude and timing of the responses of each individual process is known with ''low confidence'' . Links to chapters 5.4.7

Non-CO <sub>2</sub> biogeochemical feedbacks induced from changes in emissions, abundances or lifetimes of SLCFs mediated by natural processes or atmospheric chemistry are assessed to decrease ECS (Figure TS.17b). These non-CO <sub>2</sub> biogeochemical feedbacks are estimated from ESMs, which since AR5 have advanced to include a consistent representation of biogeochemical cycles and atmospheric chemistry. However, process-level understanding of many biogeochemical feedbacks involving SLCFs, particularly natural emissions, is still emerging, resulting in ''low confidence'' in the magnitude and sign of the feedbacks. The central estimate of the total biogeophysical and non-CO <sub>2</sub> biogeochemical feedback is assessed to be −0.01 [–0.27 to +0.25] W m <sup>–2</sup> °C <sup>–1</sup> (Figure TS.17a). Links to chapters 5.4.7, 5.4.8, 6.2.2, 6.4.5, 7.4, 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 (in the absence of feedbacks), also known as the Planck temperature response <sup>[[#footnote-001|20]]</sup> ( ''virtually certain'' ) '''.''' Combining these feedbacks with the Planck response, the net climate feedback parameter 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'' ) Links to chapters 7.4.2, 7.4.3, Figure 7.14, Table 7.10

The carbon cycle provides for additional feedbacks on climate owing to the sensitivity of land–atmosphere and ocean–atmosphere carbon fluxes and storage to changes in climate and in atmospheric CO <sub>2</sub> (Figure TS.17c). Because of the time scales associated with land and ocean carbon uptake, these feedbacks are known to be scenario dependent. Feedback estimates deviate from linearity in scenarios of stabilizing or reducing concentrations. With ''high confidence'' , increased atmospheric CO <sub>2</sub> will lead to increased land and ocean carbon uptake, acting as a negative feedback on climate change. It is ''likely'' that a warmer climate will lead to reduced land and ocean carbon uptake, acting as a positive feedback (Box TS.5). Links to chapters 4.3.2, 5.4.1–5

Thawing terrestrial permafrost will lead to carbon release ( ''high confidence'' ), but there is ''low confidence'' in the timing, magnitude and the relative roles of CO <sub>2</sub> versus CH <sub>4</sub> as feedback processes. An ensemble of models projects CO <sub>2</sub> release from permafrost to be 3–41 PgC per 1ºC of global warming by 2100, leading to warming strong enough that it must be included in estimates of the remaining carbon budget but weaker than the warming from fossil fuel burning. However, the incomplete representation of important processes, such as abrupt thaw, combined with weak observational constraints, only allow ''low confidence'' in both the magnitude of these estimates and in how linearly proportional this feedback is to the amount of global warming. There is emerging evidence that permafrost thaw and thermokarst give rise to increased CH <sub>4</sub> and N <sub>2</sub> O emissions, which leads to the combined radiative forcing from permafrost thaw being larger than from CO <sub>2</sub> emissions only. However, the quantitative understanding of these additional feedbacks is low, particularly for N <sub>2</sub> O. These feedbacks, as well as potential additional carbon losses due to climate-induced fire feedback are not routinely included in Earth system models. Links to chapters Box 5.1, 5.4.3, 5.4.7, 5.4.8

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