Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.5.1.3 Estimates of TCRE ====

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The AR5 (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ) assessed that the TCRE is ''likely'' to fall in the range of 0.8°C –2.5°C per 1000 PgC (or per exagrams of carbon, EgC <sup>–1</sup> ) for cumulative emissions up to 2000 PgC, based on multiple lines of evidence. These include estimates based on ESMs of varying complexity ( [[#Matthews--2009|Matthews et al., 2009]] ; [[#Gillett--2013|Gillett et al., 2013]] ; [[#Zickfeld--2013|Zickfeld et al., 2013]] ), simple climate modelling approaches ( [[#Allen--2009|Allen et al., 2009]] ; [[#Rogelj--2012|Rogelj et al., 2012]] ) or observational constraints and attributable warming ( [[#Gillett--2013|Gillett et al., 2013]] ).

Since AR5, new studies have further expanded the evidence base for estimating the value of TCRE. These studies rely on ESMs or EMICs, observational constraints and concepts of attributable warming, or theoretically derived equations (see Table 5.7 for an overview). Several studies have endeavoured to partition the uncertainty in the value of TCRE into constituent sources. For example, TCRE can be decomposed into terms of TCR and the airborne fraction of anthropogenic CO <sub>2</sub> emissions over time ( [[#Allen--2009|Allen et al., 2009]] ; [[#Matthews--2009|Matthews et al., 2009]] ). These two terms are assessed individually (see [[#5.4|Section 5.4]] and Chapter 7, respectively) and allow the integration of evidence assessed elsewhere in the report into the assessment of TCRE ( [[#5.5.1.4|Section 5.5.1.4]] ). Further studies use a variety of methods, including analysing the outputs from CMIP5 (R.G. [[#Williams--2017|Williams et al., 2017]] b) or CMIP6 ( [[#Arora--2020|Arora et al., 2020]] ; [[#Jones--2020|Jones and Friedlingstein, 2020]] ), conducting perturbed parameter experiments with a single model ( [[#MacDougall--2017|MacDougall et al., 2017]] ), Monte-Carlo methods applied to a simple climate model ( [[#Spafford--2020|Spafford and Macdougall, 2020]] ), or observations and estimates of the contribution of CO <sub>2</sub> and non-CO <sub>2</sub> forcers ( [[#Matthews--2021|Matthews et al., 2021]] ). All of the studies agree that uncertainty in climate sensitivity – either equilibrium climate sensitivity (ECS) or transient climate response (TCR) – is among the most important contribution to uncertainty in TCRE, with uncertainty in the strength of the land carbon feedback and ocean heat uptake or ventilation having also been identified as crucial to uncertainty in TCRE (Matthews et al., 2009; [[#Gillett--2013|Gillett et al., 2013]] ; [[#Ehlert--2017|Ehlert et al., 2017]] ; [[#MacDougall--2017|MacDougall et al., 2017]] ; R.G. [[#Williams--2017|Williams et al., 2017]] a, 2020; [[#Katavouta--2019|Katavouta et al., 2019]] ; [[#Arora--2020|Arora et al., 2020]] ; [[#Jones--2020|Jones and Friedlingstein, 2020]] ; [[#Spafford--2020|Spafford and Macdougall, 2020]] ). Finally, internal variability has been shown to affect the maximum accuracy of TCRE estimates by ±0.1°C per 1000 PgC (5–95% range; [[#Tokarska--2020|Tokarska et al., 2020]] ).

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'''Table 5.7 |''' '''Overview of results from studies estimating the transient climate response to cumulative CO''' <sub>2</sub> '''emissions (TCRE)''' . GSAT = Global mean surface air temperature increase, SAT = surface air temperature (e.g., over land only), SST = sea surface temperature, ECS = equilibrium climate sensitivity. Studies that do not isolate the CO <sub>2</sub> -induced warming contribution in their TCRE estimates are not included.

{| class="wikitable"
|-
! Study

! TCRE Range (°C per 1000 PgC)

! Notes

|-
| colspan="3"| '''Studies available at the t''' '''ime of IPCC AR5'''

|-
| [[#Matthews--2009|Matthews et al. (2009)]]

| 1–2.1

| 5–95% range; GSAT; C <sup>4</sup> MIP model range

|-
| [[#Allen--2009|Allen et al. (2009)]]

| 1.4–2.5

| 5–95% range; blended global mean SAT and SSTs (no infilling of coverage gaps); simple model

|-
| [[#Zickfeld--2009|Zickfeld et al. (2009)]]

| 1.5

| Best estimate; GSAT, EMIC

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

| 0.8–1.9

| Range consistent with 2°C to 4.5°C ECS; GSAT

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

| About 1–2

| 5–95% range; historical constraint on GMST increase, but other constraints on GSAT increase MAGICC model calibrated to C <sup>4</sup> MIP model range and 2°C–4.5°C ''likely'' ECS

|-
| [[#Zickfeld--2013|Zickfeld et al. (2013)]]

| 1.4–2.5; mean: 1.9

| Model range; GSAT, EMICs

|-
| [[#Eby--2013|Eby et al. (2013)]]

| 1.1–2.1; mean: 1.6

| Model range; GSAT, EMICs

|-
| [[#Gillett--2013|Gillett et al. (2013)]]

| 0.8–2.4

| Model range; GSAT, CMIP5 ESMs

|-
| [[#Gillett--2013|Gillett et al. (2013)]]

| 0.7–2.0

| 5–95% range; blended global mean SAT and SSTs; observationally constrained estimates of historical warming and emissions

|-
| IPCC AR5 M. [[#Collins--2013|]] [[#Collins--2013|Collins et al. (2013)]]

| 0.8–2.5

| Assessed ''likely'' range; multiple lines of evidence; mixed definition of global average temperature increase

|-
| colspan="3"| '''Studies published''' '''since IPCC AR5'''

|-
| [[#Tachiiri--2015|Tachiiri et al. (2015)]]

| 0.3–2.4

| 5–95% range; blended global mean SAT and SSTs; JUMP-LCM model perturbed physics ensemble (EMIC)

|-
| [[#Tachiiri--2015|Tachiiri et al. (2015)]]

| 1.1–1.7

| 5–95% range; blended global mean SAT and SSTs; observationally constrained JUMP-LCM perturbed physics ensemble

|-
| [[#Goodwin--2015|Goodwin et al. (2015)]]

| 1.1 ± 0.5

| 5–95% range; theoretically derived TCRE equation constrained by surface warming, radiative forcing, and historic ocean and land carbon uptake from IPCC AR5

|-
| [[#Millar--2017a|Millar et al. (2017a)]]

| 1.0–2.5

| 5 to 95% range; blended global mean SAT and SSTs (HadCRUT4); observationally constrained probabilistic setup of simple climate model

|-
| [[#Steinacher--2016|Steinacher and Joos (2016)]]

| 1.0–2.7; median: 1.7

| 5–95% range; GSAT, observationally constrained BERN3D-LPJ EMIC

|-
| [[#MacDougall--2017|MacDougall et al. (2017)]]

| 0.9–2.5; mean: 1.7

| 5–95% range; GSAT, emulation of 23 CMIP5 ESMs

|-
| [[#Ehlert--2017|Ehlert et al. (2017)]]

| 1.2–2.1

| Model range; GSAT, UVIC EMIC with varying ocean mixing parameters

|-
| R.G. [[#Williams--2017b|Williams et al. (2017b)]]

| 1.4–2.1; mean: 1.8

| 1-sigma range; GSAT, diagnosed from 10 CMIP5 ESMs

|-
| [[#Millar--2018|Millar and Friedlingstein (2018)]]

| 0.9–2.6; best estimate: 1.3

| 5–95% range; blended global mean SAT and SSTs ( [[#Cowtan--2014|Cowtan and Way, 2014]] ); detection attribution with observational constraints

|-
| [[#Millar--2018|Millar and Friedlingstein (2018)]]

| Best estimate: 1.5

| Blended global mean SAT and SSTs (Berkeley Earth); detection attribution with observational constraints

|-
| [[#Millar--2018|Millar and Friedlingstein (2018)]]

| Best estimate: 1.2

| Blended global mean SAT and SSTs ( [[#Cowtan--2014|Cowtan and Way, 2014]] ); detection attribution with observational constraints, with updated historical CO <sub>2</sub> emissions ( [[#Le%20Quéré--2018b|Le Quéré et al., 2018b]] )

|-
| C.J. [[#Smith--2018|]] [[#Smith--2018|Smith et al. (2018)]]

| 1.0–2.2

| 5–95% range; blended global mean SAT and SSTs ( [[#Cowtan--2014|Cowtan and Way, 2014]] ); observationally constrained probabilistic setup of simple climate model

|-
| [[#Matthews--2021|Matthews et al. (2021)]]

| 1.0–2.2; median: 1.5

| 5–95% range; blended global mean SAT and SSTs; human-induced warming ( [[#Haustein--2017|Haustein et al., 2017]] ) based on an average of three full coverage datasets; observationally constrained estimate using the current non-CO <sub>2</sub> fraction of total anthropogenic forcing

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

| 1.3–2.4; mean: 1.8; median: 1.65

| Model range; GSAT, diagnosed CO <sub>2</sub> emissions in CMIP6 ESMs

|-
| R.G. [[#Williams--2020|]] [[#Williams--2020|Williams et al. (2020)]]

| 1.2–2.1; mean: 1.6

| 1-sigma range; GSAT, diagnosed CO <sub>2</sub> emissions in 9 CMIP6 ESMs

|-
| [[#Jones--2020|Jones and Friedlingstein (2020)]]

| 1.2–2.7; median: 1.8

| 5–95% range; GSAT; estimate based on decomposition presented in ( [[#Jones--2020|Jones and Friedlingstein, 2020]] ) with ranges of carbon cycle feedback parameters from CMIP6 ( [[#Arora--2020|Arora et al., 2020]] ), see [[#5.4|Section 5.4]] .

|-
| [[#Spafford--2020|Spafford and Macdougall (2020)]]

| 1.1–2.9; mean: 1.9; median: 1.8

| 5–95% range; ratio of land SAT and SST; probabilistic assessment of with a zero-dimensional ocean diffusive model

|-
| colspan="3"| '''Cross-AR6 li''' '''nes of evidence'''

|-
| Transient Climate Response (TCR) and Airborne Fraction (AF)

| 1.0–2.3; median: 1.6

| 5–95% range; GSAT; TCR–AF decomposition-based estimate using the assessed range of TCR ( [[IPCC:Wg1:Chapter:Chapter-7#7.5|Section 7.5]] , 1.8°C median with 0.4°C 1-sigma range) and an airborne fraction of 53 ± 6% (1-sigma range)

|-
| colspan="3"| '''Ove''' '''rall assessment'''

|-
| IPCC AR6

| 1.0–2.3; best estimate: 1.65

| ''Likely'' range; GSAT; based on combination of cross-AR6 lines of evidence ( [[#5.5.1.4|Section 5.5.1.4]] ); normally distributed

|}

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