Editing IPCC:AR6/WGI/TS (section)

=== TS.3.2 Climate Sensitivity and Earth System Feedbacks ===

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==== TS.3.2.1 Equilibrium Climate Sensitivity, Transient Climate Response, and Transient Climate Response to Cumulative Carbon-dioxide Emissions ====

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'''Since AR5, substantial quantitative progress has been made in combining new evidence of Earth’s climate sensitivity with improvements in the understanding and quantification of Earth’s energy imbalance, the instrumental record of global surface temperature change, paleoclimate change from proxy records, climate feedbacks and their dependence on time scale and climate state. A key advance is the broad agreement across these multiple lines of evidence, supporting a best estimate of equilibrium climate sensitivity of 3°C, with a ''very likely'' range of 2°C to 5°C. The ''likely'' range of 2.5°C to 4°C is narrower than the AR5 ''likely'' range of 1.5°C to 4.5°C. Links to chapters 7.4, 7.5'''
Constraints on equilibrium climate sensitivity (ECS) and transient climate response (TCR) (see Glossary) are based on four main lines of evidence: feedback process understanding, climate change and variability seen within the instrumental record, paleoclimate evidence, and so-called ‘emergent constraints’, whereby a relationship between an observable quantity and either ECS or TCR established within an ensemble of models is combined with observations to derive a constraint on ECS or TCR. In reports up to and including the IPCC Third Assessment Report, ECS and TCR derived directly from ESMs were the primary line of evidence. However, since AR4, historical warming and paleoclimates provided useful additional evidence (Figure TS.16a). This Report differs from previous reports in not directly using climate model estimates of ECS and TCR in the assessed ranges of climate sensitivity. Links to chapters 1.5, 7.5

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'''Figure TS.16 |''' '''(a) Evolution of equilibrium climate sensitivity (ECS) assessments from the Charney Report through a succession of IPCC Assessment Reports to AR6, and lines of evidence and combined assessment for (b) ECS and (c) transient climate response (TCR) in AR6.''' ''The intent of this figure is to show the progression in estimates of ECS, including uncertainty and the lines of evidence used for assessment, and to show the lines of assessment used to assess ECS and TCR in AR6.'' In panel (a), the lines of evidence considered are listed below each assessment. Best estimates are marked by horizontal bars, ''likely'' ranges by vertical bars, and ''very likely'' ranges by dotted vertical bars. In panel (b) and (c), assessed ranges are taken from Tables 7.13 and 7.14 for ECS and TCR respectively. Note that for the ECS assessment based on both the instrumental record and paleoclimates, limits (i.e., one-sided distributions) are given, which have twice the probability of being outside the maximum/minimum value at a given end, compared to ranges (i.e., two tailed distributions) which are given for the other lines of evidence. For example, the ''extremely likely'' limit of greater than 95% probability corresponds to one side of the ''very likely'' (5% to 95%) range. Best estimates are given as either a single number or by a range represented by grey box. Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth system model (ESM) values are not directly used as a line of evidence but are presented on the figure for comparison. Links to chapters 1.5, 7.5; Tables 7.13 and 7.14; Figure 7.18

It is now clear that when estimating ECS and TCR, the dependence of feedbacks on time scales and the climate state must be accounted for. Feedback processes are expected to become more positive overall (more amplifying of global surface temperature changes) on multi-decadal time scales as the spatial pattern of surface warming evolves and global surface temperature increases, leading to an ECS that is higher than was inferred in AR5 based on warming over the instrumental record ( ''high confidence'' ). Historical surface temperature change since 1870 has shown relatively little warming in several key regions of positive feedbacks, including the eastern equatorial Pacific Ocean and the Southern Ocean, while showing greater warming in key regions of negative feedbacks, including the western Pacific warm pool. Based on process understanding, climate modelling, and paleoclimate reconstructions of past warm periods, it is expected that future warming will become enhanced over the eastern Pacific Ocean ( ''medium confidence'' ) and Southern Ocean ( ''high confidence'' ) on centennial time scales. This new understanding, along with updated estimates of historical temperature change, ERF, and energy imbalance, reconciles previously disparate ECS estimates. Links to chapters 7.4.4, 7.5.2, 7.5.3

The AR6 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. There is a high level of agreement among the four main lines of evidence listed above (Figure TS.16b), and altogether it is ''virtually certain'' that ECS is larger than 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 with ''medium confidence'' and the other bounds with ''high confidence'' . Links to chapters 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. There is a high level of agreement among the different lines of evidence (Figure TS.16c) ( ''high confidence'' ). Links to chapters 7.5.5

On average, CMIP6 models have higher mean ECS and TCR values than the CMIP5 generation of models and also have higher mean values and wider spreads than the assessed best estimates and ''very likely'' ranges within this Report. These higher mean ECS and TCR values can be traced to a positive net cloud feedback that is larger in CMIP6 by about 20%. 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 future warming range, which is based on multiple lines of evidence (Cross-Section Box TS.1). However, some of the high-sensitivity CMIP6 models (Section TS.1.2.2) are less consistent with observed recent changes in global warming and with paleoclimate proxy records than models with ECS within the ''very likely'' range. Similarly, some of the low-sensitivity models are less consistent with the paleoclimate data. The CMIP6 models with the highest ECS and TCRs values provide insights into low-likelihood, high-impact futures, which cannot be excluded based on currently available evidence (Cross-Section Box TS.1). Links to chapters 4.3.1, 4.3.4, 7.4.2, 7.5.6

Uncertainties regarding the true value of ECS and TCR are the dominant source of uncertainty in global temperature projections over the 21st century under moderate to high GHG concentrations scenarios. For scenarios that reach net zero CO <sub>2</sub> emissions (Section TS.3.3), the uncertainty in the ERF values of aerosol and other SLCFs contribute substantial uncertainty in projected temperature. Global ocean heat uptake is a smaller source of uncertainty in centennial warming. Links to chapters 7.5.7

The transient climate response to cumulative CO <sub>2</sub> emissions (TCRE) is the ratio between globally averaged surface temperature increase and cumulative CO <sub>2</sub> emissions (see Glossary). This Report reaffirms with ''high confidence'' the finding of AR5 that there is a near-linear relationship between cumulative CO <sub>2</sub> emissions and the increase in global average temperature caused by CO <sub>2</sub> over the course of this century for global warming levels up to at least 2°C relative to 1850–1900. The TCRE falls ''likely'' in the 1.0°C–2.3°C per 1000 PgC range, with a best estimate of 1.65°C per 1000 PgC. This is equivalent to a 0.27°C–0.63°C range with a best estimate of 0.45°C when expressed in units per 1000 GtCO <sub>2</sub> . This range is about 15% narrower than the 0.8°–2.5°C per 1000 PgC assessment of AR5 because of a better integration of evidence across chapters, in particular the assessment of TCR. Beyond this century, there is ''low confidence'' that the TCRE alone remains an accurate predictor of temperature changes in scenarios of very low or net negative CO <sub>2</sub> emissions because of uncertain Earth system feedbacks that can result in further changes in temperature or a path dependency of warming as a function of cumulative CO <sub>2</sub> emissions. Links to chapters 4.6.2, 5.4, 5.5.1

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==== 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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