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

===== 5.5.2.2.2 Historical warming =====

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Advances in methods to estimate remaining carbon budgets have shown the importance of applying an estimate of historical warming to date that is as accurate as possible ( [[#Millar--2017b|Millar et al., 2017b]] ; [[#Tokarska--2018|Tokarska and Gillett, 2018]] ). This becomes particularly important when assessing remaining carbon budgets for global warming levels that are relatively close to present-day warming, such as a 1.5°C or 2°C levels ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ). Also shown to be important is the definition of global average temperature by which historical warming is estimated (Cross-Chapter Box 2.3; [[#Cowtan--2014|Cowtan and Way, 2014]] ; [[#Allen--2018|Allen et al., 2018]] ; [[#Pfleiderer--2018|Pfleiderer et al., 2018]] ; [[#Richardson--2018|Richardson et al., 2018]] ; [[#Tokarska--2019b|Tokarska et al., 2019b]] ), as is the correct isolation of human-induced global warming ( [[#Haustein--2017|Haustein et al., 2017]] ; [[#Allen--2018|Allen et al., 2018]] ) to remove the effect of internal variability. Based on the assessment in [[IPCC:Wg1:Chapter:Chapter-3#3.3|Section 3.3]] (Table 3.1), here we apply an assessedbest-estimate of historical warming expressed as an increase in GSAT of 1.07°C (0.8–1.3°C, ''likely'' range) between 1850–1900 and 2010–2019. This choice implies global coverage and is consistent with AR5 where carbon budgets were reported in GSAT (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; T.F. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ), SR1.5 where GSAT was the central metric for remaining carbon budgets ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ), and recent studies that highlight how GSAT enables an easy translation with AR5 ( [[#Tokarska--2019b|Tokarska et al., 2019b]] ). The use of other historical reference periods (Cross-Chapter Box 1.2) or temperature metrics and updated data products (Cross-Chapter Box 2.3) can result in a different estimated historical warming and thus a changed remaining carbon budget.

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