Editing IPCC:AR6/WGI/TS (section)

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

<div id="h3-7-siblings" class="h3-siblings"></div>

'''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

<div id="_idContainer110" class="_idGenObjectLayout-1 _idGenObjectStyleOverride-1 mb-3"></div>

[[File:39bc7e4f4acf60be8e07e65e21f3aecc IPCC_AR6_WGI_TS_Figure_16.png]]
<div id="_idContainer109" class="_idGenObjectStyleOverride-2"></div>

'''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

<div id="TS.3.2.2" class="h3-container"></div>

<span id="ts.3.2.2-earth-system-feedbacks"></span>