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

=== 7.5.5 Combined Assessment of ECS and TCR ===

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Substantial quantitative progress has been made in interpreting evidence of Earth’s climate sensitivity since AR5, through innovation, scrutiny, theoretical advances and a rapidly evolving data base from current, recent and paleo climates. It should be noted that, unlike AR5 and earlier reports, our assessment of ECS is not directly informed by ESM simulations ( [[#7.5.6|Section 7.5.6]] ). The assessments of ECS and TCR are focussed on the following lines of evidence: process-understanding; the instrumental record of warming; paleoclimate evidence; and emergent constraints. ESMs remain essential tools for establishing these lines of evidence, for instance, in estimating part of the feedback parameters and radiative forcings, and emergent constraints rely on substantial model spread in ECS and TCr ( [[#7.5.6|Section 7.5.6]] ).

A key advance over the AR5 assessment is the broad agreement across multiple lines of evidence. These support a central estimate of ECS close to, or at least not inconsistent with, 3°C. This advance is foremost following improvements in the understanding and quantification of Earth’s energy imbalance, the instrumental record of global temperature change, and the strength of anthropogenic radiative forcing. Further advances include increased understanding of how the pattern effect influences ECS inferred from historical global warming (Sections 7.4.4 and 7.5.3), improved quantification of paleo climatechange from proxy evidence and a deepened understanding of how feedback mechanisms increase ECS in warmer climate states (Sections 7.4.3, 7.4.4 and 7.5.4), and also an improved quantification of individual cloud feedbacks (Sections 7.4.2 and 7.5.4.2). The assessment findings for ECS and TCR are summarized in Table 7.13 and Table 7.14, respectively, and also visualized in Figure 7.18.

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'''Table 7.12''' '''|''' '''Emergent constraint studies used in the assessment of equilibrium climate sensitivity (ECS).''' These are studies that rely on global or near-global temperature change as the observable.

{| class="wikitable"
|-
| Study

| Emergent Constraint Description

| Published Best Estimate and Uncertainty (°C)

| Uncertainty Estimate

|-
| [[#Bender--2010|Bender et al. (2010)]]

| Pinatubo integrated forcing normalized by CMIP3 models’ own forcing versus temperature change regressed against ECS

| 2.4 [1.7 to 4.1]

| 5–95%

|-
| [[#Dessler--2018|Dessler and Forster (2018)]]

| Emergent constraint on TOA radiation variations linked to mid-tropospheric temperature in CMIP5 models

| 3.3 [2.4 to 4.5]

| 17–83%

|-
| [[#Hargreaves--2012|Hargreaves et al. (2012)]]

| Last Glacial Maximum tropical SSTs in PMIP2 models

| 2.5 [1.3 to 4.2]

| 5–95%

|-
| [[#Hargreaves--2016|Hargreaves and Annan (2016)]]

| Pliocene tropical SSTs in PlioMIP models

| [1.9 to 3.7]

| 5–95%

|-
| [[#Jiménez-de-la-Cuesta--2019|Jiménez-de-la-Cuesta and Mauritsen (2019)]]

| Post-1970s global warming, 1995–2005 relative to 1970–1989, CMIP5 models

| 2.83 [1.72 to 4.12]

| 5–95%

|-
| [[#Nijsse--2020|Nijsse et al. (2020)]]

| Post-1970s global warming, 2009–2019 relative to 1975–1985, CMIP6 models

| 2.6 [1.5 to 4.0]

| 5–95%

|-
| [[#Renoult--2020|Renoult et al. (2020)]]

| Combined Last Glacial Maximum and Pliocene tropical SSTs in PMIP2, PMIP3, PMIP4, PlioMIP and PlioMIP2 models

| 2.5 [0.8 to 4.0]

| 5–95%

|}

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'''Table 7.13''' '''|''' '''Summary of equilibrium climate sensitivity (ECS) assessment.'''

{| class="wikitable"
|-
| Equilibrium Climate Sensitivity (ECS)

| Central Value

| Likely

| Very likely

| Extremely likely

|-
| Process understanding ( [[#7.5.1|Section 7.5.1]] )

| 3.4°C

| 2.5°C to 5.1°C

| 2.1°C to 7.7°C

| –

|-
| Warming over instrumental record ( [[#7.5.2|Section 7.5.2]] )

| 2.5°C to 3.5°C

| &gt;2.2°C

| &gt;1.8°C

| &gt;1.6°C

|-
| Paleoclimates ( [[#7.5.3|Section 7.5.3]] )

| 3.3°C to 3.4°C

| &lt;4.5°C

| &gt;1.5°C

| &lt;8°C

|-
| Emergent constraints ( [[#7.5.4|Section 7.5.4]] )

| 2.4°C to 3.3°C

| –

| 1.5°C to 5.0°C

| –

|-
| Combined assessment

| 3°C

| 2.5°C to 4.0°C

| 2.0°C to 5.0°C

| –

|}

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'''Table 7.14''' '''|''' '''Summary of TCR assessment.'''

{| class="wikitable"
|-
| Transient Climate Response (TCR)

| Central Value

| Likely Range

| Very likely Range

|-
| Process understanding ( [[#7.5.1|Section 7.5.1]] )

| 2.0°C

| 1.6°C to 2.7°C

| 1.3°C to 3.1°C

|-
| Warming over instrumental record ( [[#7.5.2|Section 7.5.2]] )

| 1.9°C

| 1.5°C to 2.3°C

| 1.3°C to 2.7°C

|-
| Emergent constraints ( [[#7.5.4|Section 7.5.4]] )

| 1.7°C

| –

| 1.1°C to 2.3°C

|-
| Combined assessment

| 1.8°C

| 1.4°C to 2.2°C

| 1.2°C to 2.4°C

|}

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[[File:79d3d92af99cc0b748591882f981bbc9 IPCC_AR6_WGI_Figure_7_18.png]]

'''Figure 7.18''' '''|''' '''Summary of the equilibrium climate sensitivity (ECS panel (a)) and transient climate response (TCR panel (b)) assessments using different lines of evidence.''' 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'' (5–95%) range. Best estimates are given as either a single number or by a range represented by a grey box. CMIP6 model values are not directly used as a line of evidence but presented on the Figure for comparison. ECS values are taken from [[#Schlund--2020|Schlund et al. (2020)]] and TCR values from [[#Meehl--2020|Meehl et al. (2020)]] ; see Supplementary Material 7.SM.4. Further details on data sources and processing are available in the chapter data table (Table 7.SM.14).

The AR5 assessed ECS to have a ''likely'' range from 1.5 to 4.5 °C (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ) based on the majority of studies and evidence available at the time. The broader evidence base presented in this Report and the general agreement among different lines of evidence means that they can be combined to yield a narrower range of ECS values. This can be done formally using Bayesian statistics, though such a process is complex and involves formulating likelihoods and priors ( [[#Annan--2006|Annan and Hargreaves, 2006]] ; [[#Stevens--2016|Stevens et al., 2016]] ; [[#Sherwood--2020|Sherwood et al., 2020]] ). However, it can be understood that if two lines of independent evidence each give a low probability of an outcome being true, for example, that ECS is less than 2.0°C, then the combined probability that ECS is less than 2.0°C is lower than that of either line of evidence. On the contrary, if one line of evidence is unable to rule out an outcome, but another is able to assign a low probability, then there is a low probability that the outcome is true ( [[#Stevens--2016|Stevens et al., 2016]] ). This general principle applies even when there is some dependency between the lines of evidence ( [[#Sherwood--2020|Sherwood et al., 2020]] ), for instance between historical energy budget constraints ( [[#7.5.2.1|Section 7.5.2.1]] ) and those emergent constraints that use the historically observed global warming ( [[#7.5.4.1|Section 7.5.4.1]] ). Even in this case the combined constraint will be closer to the narrowest range associated with the individual lines of evidence.

In the process of providing a combined and self-consistent ECS assessment across all lines of evidence, the above principles were all considered. As in earlier reports, a 0.5°C precision is used. Starting with the ''very likely'' lower bound, there is broad support for a value of 2.0°C, including process understanding and the instrumental record (Table 7.13). For the ''very likely'' upper bound, emergent constraints give a value of 5.0°C whereas the three other lines of evidence are individually less tightly constrained. Nevertheless, emergent constraints are a relatively recent field of research, in part taken into account by adding uncertainty to the upper bound ( [[#7.5.4.3|Section 7.5.4.3]] ), and the underlying studies use, to a varying extent, information that is also used in the other three lines of evidence, causing statistical dependencies. However, omitting emergent constraints and statistically combining the remaining lines of evidence likewise yields 95th percentiles close to 5.0°C ( [[#Sherwood--2020|Sherwood et al., 2020]] ). Information for the ''likely'' range is partly missing or one-sided, however it must necessarily reside inside the ''very likely'' range and is therefore supported by evidence pertaining to both the ''likely'' and ''very likely'' ranges. Hence, the upper ''likely'' bound is assessed to be about halfway between the best estimate and the upper ''very likely'' bound while the lower ''likely'' bound is assessed to be about halfway between the best estimate and the lower ''very likely'' bound. In summary, based on multiple lines of evidence the best estimate of ECS is 3°C, it is ''likely'' within the range 2.5 to 4 °C and ''very likely'' within the range 2 to 5 °C. It is ''virtually certain'' that ECS is larger than 1.5°C. Whereas there is ''high confidence'' based on mounting evidence that supports the best estimate, ''likely'' range and ''very likely'' lower end, a higher ECS than 5°C cannot be ruled out, hence there is ''medium confidence'' in the upper end of the ''very likely'' range. Note that the best estimate of ECS made here corresponds to a feedback parameter of –1.3 W m <sup>–2</sup> °C <sup>–1</sup> which is slightly more negative than the feedback parameter from process-based evidence alone that is assessed in ( [[#7.4.2.7|Section 7.4.2.7]] .

There has long been a consensus ( [[#Charney--1979|Charney et al., 1979]] ) supporting an ECS estimate of 1.5°C–4.5°C. In this regard it is worth remembering the many debates challenging an ECS of this magnitude. These started as early as [[#Ångström--1900|Ångström (1900)]] criticizing the results of [[#Arrhenius--1896|Arrhenius (1896)]] arguing that the atmosphere was already saturated in infrared absorption such that adding more CO <sub>2</sub> would not lead to warming. The assertion of Ångström was understood half a century later to be incorrect. History has seen a multitude of studies (e.g., [[#Svensmark--1998|Svensmark, 1998]] ; [[#Lindzen--2001|Lindzen et al., 2001]] ; [[#Schwartz--2007|Schwartz, 2007]] ) mostly implying lower ECS than the range assessed as ''very likely'' here. However, there are also examples of the opposite, such as very large ECS estimates based on the Pleistocene records ( [[#Snyder--2016|Snyder, 2016]] ), which have been shown to be overestimated due to a lack of accounting for orbital forcing and long-term ice-sheet feedbacks ( [[#Schmidt--2017b|Schmidt et al., 2017b]] ), or suggestions that global climate instabilities may occur in the future ( [[#Steffen--2018|Steffen et al., 2018]] ; [[#Schneider--2019|Schneider et al., 2019]] ). There is, however, no evidence for unforced instabilities of such magnitude occurring in the paleo-record temperatures of the past 65 million years ( [[#Westerhold--2020|Westerhold et al., 2020]] ), possibly short of the Paleocene–Eocene Thermal Maximum (PETM) excursion ( [[IPCC:Wg1:Chapter:Chapter-5#5.3.1.1|Section 5.3.1.1]] ) that occurred at more than 10°C above present-day levels ( [[#Anagnostou--2020|Anagnostou et al., 2020]] ). Looking back, the resulting debates have led to a deeper understanding, strengthened the consensus, and have been scientifically valuable.

In the climate sciences, there are often good reasons to consider representing deep uncertainty, or what are sometimes referred to as ‘unknown unknowns’. This is natural in a field that considers a system that is both complex and at the same time challenging to observe. For instance, since emergent constraints represent a relatively new line of evidence, important feedback mechanisms may be biased in process-level understanding; pattern effects and aerosol cooling may be large; and paleo evidence inherently builds on indirect and incomplete evidence of past climate states, there certainly can be valid reasons to add uncertainty to the ranges assessed on individual lines of evidence. This has indeed been addressed throughout Sections 7.5.1–7.5.4. Since it is neither probable that all lines of evidence assessed here are collectively biased nor is the assessment sensitive to single lines of evidence, deep uncertainty is not considered as necessary to frame the combined assessment of ECS.

The evidence for TCR is less abundant than for ECS, and focuses on the instrumental temperature record (Sections 7.5.2 and 7.5.6), emergent constraints ( [[#7.5.4.3|Section 7.5.4.3]] ) and process understanding ( [[#7.5.1|Section 7.5.1]] ). The AR5 assessed a ''likely'' range for TCR of 1.0 to 2.5 °C. TCR and ECS are related, though, and in any case TCR is less than ECS (see the introduction to ( [[#7.5|Section 7.5]] ). Furthermore, estimates of TCR from the historical record are not as strongly influenced by externally forced surface temperature pattern effects as estimates of ECS are since both historical transient warming and TCR are affected by this phenomenon ( [[#7.4.4|Section 7.4.4]] ). A slightly higher weight is given to instrumental record warming and emergent constraints since these are based on observed transient warming, whereas the process-understanding estimate relies on pattern effects and ocean heat uptake efficiency from ESMs to represent the transient dampening effects of the ocean. If these effects are underestimated by ESMs then the resulting TCR would be lower. Given the interdependencies of the other two lines of evidence, a conservative approach to combining them as reflected in the assessment is adopted. Since uncertainty is substantially lower than in AR5 a 0.1°C precision is therefore used here. Otherwise the same methodology for combining the lines of evidence as applied to ECS is used for TCR. Based on process understanding, warming over the instrumental record and emergent constraints, the best estimate TCR is 1.8°C, it is ''likely'' 1.4 to 2.2 °C and ''very likely'' 1.2 to 2.4 °C. The assessed ranges are all assigned ''high confidence'' due to the high level of agreement among the lines of evidence.

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