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

==== 5.5.1.4 Combined Assessment of TCRE ====

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Studies differ in how they define TCRE, in the methods they use, and their assumptions, such as the assumed climate sensitivity distribution or the choice of metrics of global temperature change (e.g., GMST or GSAT, see Table 5.7). This makes TCRE estimates from individual studies difficult to compare. The combined assessment of TCRE therefore takes advantage of the well-established decomposition of TCRE in two factors: the TCR and the AF ([[#5.5.1.3|Section 5.5.1.3]]). This provides a TCRE assessment range for CO <sub>2</sub> -induced warming at the time of doubling CO <sub>2</sub> concentrations that builds on the broader Working Group 1 assessment. Expert judgement based on the airborne fraction range found in CMIP6 models ([[#Arora--2020|Arora et al., 2020]] ; [[#Jones--2020|Jones and Friedlingstein, 2020]]) suggest a value of 53% with a 1-sigma range of ±6%, which is double the sigma range based on the spread of CMIP6 models only. Combining this range with the AR6 TCR assessment ([[IPCC:Wg1:Chapter:Chapter-7#7.5|Section 7.5]] , best estimate 1.8°C, 1.4°C–2.2°C ''likely'' and 1.2°C–2.4°C ''very likely'' range) results in a 5–95% range of 1.0–2.3°C per 1000 PgC (0.27°C–0.63°C per 1000 GtCO <sub>2</sub>). Based on expert judgement that accounts for the incomplete coverage of all Earth system components, this results in a consolidated assessment that TCRE would fall ''likely'' in the range of 1.0–2.3°C per 1000 PgC, with a best estimate of 1.65°C per 1000 PgC (0.45°C per 1000 GtCO <sub>2</sub>). Warming here reflects the human-induced GSAT increase and assumes a normal distribution. Some studies using observational constraints support a lognormal shape for the TCRE distribution ([[#Spafford--2020|Spafford and Macdougall, 2020]]), but such a distribution is currently not supported by the combined assessment of TCR and airborne fraction. Finally, this assessed TCRE range needs to be considered in combination with the ZEC ([[IPCC:Wg1:Chapter:Chapter-4#4.7.1.1|Section 4.7.1.1]]) when estimating the CO <sub>2</sub> -induced warming of low-emissions scenarios.

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