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

==== 5.5.2.3 Remaining Carbon Budget ====

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The combination of the five components assessed in Sections 5.5.2.2.1–5.5.2.2.5 allows for an overall assessment of the remaining carbon budget in line with different levels of global average warming, as documented in SR1.5 ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ). The overall assessment of remaining carbon budgets (Table 5.8) reflects the uncertainty in TCRE quantification and provides estimates of the uncertainties surrounding the contributions of each of the respective further components. A formal combination of all uncertainties is not possible because they are not all independent, or because they represent choices rather than probabilistic uncertainties ( [[#Matthews--2021|Matthews et al., 2021]] ). In light of all uncertainties related to TCRE, non-CO <sub>2</sub> forcing and response, the level of non-CO <sub>2</sub> mitigation, and historical warming, there is a small probability that the remaining carbon budget for limiting warming to 1.5°C since the 1850–1900 period is effectively zero. However, applying best estimate values for all but uncertainties in Earth system feedbacks and TCRE, the remaining carbon budgets in line with the Paris Agreement are generally small yet not zero (see Table 5.8).

There is ''robust evidence'' supporting the concept of TCRE as well as ''high confidence'' in the range of historical human-induced warming. Combined with the assessed uncertainties in the Earth system’s response to non-CO <sub>2</sub> emissions and less well-established quantification of some of the effect of non-linear Earth system feedbacks, this leads to ''medium confidence'' being assigned to the assessed remaining carbon budget estimates while noting the identified and assessed uncertainties and potential variations. The reported values are applicable to warming and cumulative emissions over the 21st century. For climate stabilization beyond the 21st century, this confidence would decline to ''very low confidence'' due to uncertainties in Earth system feedbacks and the ZEC. For estimates of total carbon budgets in line with limiting global warming to a specific level, an estimate of historical CO <sub>2</sub> emissions should be added to the remaining carbon budget values reported in Table 5.8. Historical CO <sub>2</sub> emissions between 1850 and 2019 have been estimated at about 655 ± 65 PgC (1-sigma range, or 2390 ± 240 GtCO <sub>2</sub> , see Table 5.1), while since 1 January 2015, an additional 57 PgC (210 GtCO <sub>2</sub> ) has been emitted until the end of 2019 ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ).

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'''Box 5.2 | Implications of Methodological Advancements in Estimating the Remaining Carbon Budget since the IPCC’s Fifth Assessme''' '''nt Report (AR5)'''

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Methodological advancements since AR5 (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; [[#IPCC--2013b|IPCC, 2013b]] , 2014; T.F. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ; [[#Clarke--2014|Clarke et al., 2014]] ) result in an updated and strengthened assessment of remaining carbon budgets. Methods and approaches at the time of AR5 are described in [[#5.5.2.1|Section 5.5.2.1]] . Since AR5, strengths and weaknesses of various approaches have been more clearly articulated in the literature (e.g., in [[#Rogelj--2016|Rogelj et al., 2016]] ; [[#Millar--2017b|Millar et al., 2017b]] ; [[#Tokarska--2018|Tokarska and Gillett, 2018]] ; [[#Matthews--2020|Matthews et al., 2020]] ), resulting in a new consolidated framework applied in SR1.5 ( [[#Rogelj--2018b|Rogelj et al., 2018b]] , 2019) that is also used in AR6. This framework incorporates five methodological advancements compared to AR5, the implications of which are discussed in this box.

First, publications since AR5 applied methods that limit the effect of uncertainties in historical, diagnosed emissions in coupled Earth system models (ESMs) on estimates of the remaining carbon budget ( [[#Millar--2017b|Millar et al., 2017b]] ; [[#Tokarska--2018|Tokarska and Gillett, 2018]] ). These new methods express remaining carbon budget estimates relative to a recent reference period instead of relative to the pre-industrial period ( [[#Millar--2017b|Millar et al., 2017b]] ; [[#Tokarska--2019b|Tokarska et al., 2019b]] ). Estimates of the full carbon budget since the pre-industrial period can still be obtained by adding estimates of historical CO <sub>2</sub> emissions (Table 5.1) to the estimates in Table 5.8. This methodological update resulted, all other aspects being equal, in median estimates of remaining carbon budgets being about 350–450 GtCO <sub>2</sub> larger compared to AR5 ( [[#IPCC--2014|IPCC, 2014]] ; [[#Millar--2017b|Millar et al., 2017b]] ).

At the time of AR5, Coupled Model Intercomparison Project Phase 5 (CMIP5; [[#Taylor--2012|Taylor et al., 2012]] ) provided global surface air temperature (GSAT) projections for the representative concentration pathways ( [[#Meinshausen--2011c|Meinshausen et al., 2011c]] ), which were used to determine carbon budgets while taking into account the effects of non-CO <sub>2</sub> forcers (T.F. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ). Their use came with two recognized limitations: first, the model spread of the CMIP5 represents an ensemble of opportunity with limited statistical value ( [[#Tebaldi--2007|Tebaldi and Knutti, 2007]] ); and second, the evolution of non-CO <sub>2</sub> emissions as a function of cumulative CO <sub>2</sub> emissions can differ markedly between high and low emissions pathways ( [[#Meinshausen--2011c|Meinshausen et al., 2011c]] ; [[#Friedlingstein--2014a|Friedlingstein et al., 2014a]] ; [[#Rogelj--2016|Rogelj et al., 2016]] ; [[#Matthews--2017|Matthews et al., 2017]] ). Solutions to these two limitations have been published since AR5 and represent the second and third methodological improvement compared to AR5.

The reliance on an ensemble of opportunity (i.e., a serendipitous collection of scenario data from a variety of sources and studies) is avoided by methodologically separating the assessment of future warming contributions of non-CO <sub>2</sub> emissions from the spread in transient climate response to cumulative CO <sub>2</sub> emissions (TCRE; [[#5.5.2|Section 5.5.2]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] , 2019). This facilitates the explicit representation of TCRE uncertainty by a formal distribution, in this case a normal distribution with a 1.0–2.3°C 1000 PgC <sup>–1</sup> 1–sigma range ( [[#5.5.1.4|Section 5.5.1.4]] ). The effect of this methodological advance can be estimated from a direct comparison of the frequency distribution of TCRE in CMIP5 models that were used in AR5, and the formal TCRE distribution used in AR6, but is limited in precision. For estimates of the remaining carbon budget in line with limiting warming to 1.5°C or 2°C relative to pre-industrial levels, this improvement is estimated to lead to a reduction of budgets of the order of about 100 GtCO <sub>2</sub> between AR5 and AR6.

The third methodological improvement is a more direct estimation of the warming contribution of non-CO <sub>2</sub> emissions, consistent with pathways that bring global CO <sub>2</sub> emissions down to net zero. Instead of deriving this contribution implicitly from the CMIP5 ensemble, climate emulators ( [[#Meinshausen--2011b|Meinshausen et al., 2011b]] ; C.J. [[#Smith--2018|]] [[#Smith--2018|Smith et al., 2018]] ; [[#Schwarber--2019|Schwarber et al., 2019]] ) that are calibrated to the combined AR6 assessment (Cross-Chapter Box 7.1) are used to estimate the non-CO <sub>2</sub> contribution across a wide variety of stringent mitigation scenarios ( [[#Huppmann--2018|Huppmann et al., 2018]] ). The specific relative effect of this advance compared to AR5 is not calculable because CMIP5 data does not isolate non-CO <sub>2</sub> from CO <sub>2</sub> -induced warming.

The fourth and fifth methodological advancements are to explicitly account for the zero emissions commitment (ZEC; [[#5.5.2.2.4|Section 5.5.2.2.4]] ) and adjust estimates for Earth system feedbacks that are typically not represented in Earth system models (ESMs; [[#5.5.2.2.5|Section 5.5.2.2.5]] ). The central estimate of the assessed ZEC used in SR1.5 and AR6 is zero ( [[IPCC:Wg1:Chapter:Chapter-4#4.7.1.1|Section 4.7.1.1]] ). ZEC uncertainties are reported separately (Table 5.8), and the additional consideration of ZEC therefore does result in a better understanding but not in a net shift of central estimates of the remaining carbon budget compared to AR5. Furthermore, AR5 did not explicitly account for Earth system feedbacks not represented in ESMs. The SR1.5 assessed that they could reduce the remaining carbon budgets by about 100 GtCO <sub>2</sub> over centennial time scales. This assessment has been updated in AR6, including a wider range of biogeochemical feedbacks and new evidence ( [[#5.5.2.2.5|Section 5.5.2.2.5]] ). Some of these feedbacks are captured in the estimation of non-CO <sub>2</sub> warming (see below), while the combined effect of remaining positive and negative feedbacks is assessed to reduce the remaining carbon budget estimates by 7 ± 27 PgC K <sup>–1</sup> (1-sigma range, or 26 ± 97 GtCO <sub>2</sub> °C <sup>–1</sup> ) compared to AR5.

Between SR1.5 and AR6, each of the five components described in [[#5.5.2.1|Section 5.5.2.1]] and Figure 5.31 have been re-assessed (see Sections 5.5.2.2.1–5.5.2.2.5). Their updated assessments in turn affect the assessment of the remaining carbon budget. The new and narrower assessment of TCRE in AR6 compared to SR1.5 ( ''likely'' range of 1.0°C–2.3°C EgC <sup>–1</sup> compared to 0.8°C–2.5°C EgC <sup>–1</sup> , respectively, with the same central estimate) leads to no change in median estimates and about a 50 and 100 GtCO <sub>2</sub> increase in remaining carbon budgets estimates at the 67th percentile in AR6 compared to SR1.5 for 1.5°C and 2°C of global warming, respectively.

For historical warming, SR1.5 used GSAT increase between 1850–1900 and 2006–2015 of 0.97°C as its main starting point, while also providing values for other temperature metrics. Remaining carbon budgets were expressed starting from 1 January 2018 by accounting for historical emissions emitted from 1 January 2011 until the end of 2017. AR6 uses anthropogenic (human-induced) warming until the 2010–2019 period, which is assessed at the 0.8-1.3°C range, with a best estimate of 1.07°C (Table 3.1), and subsequently accounts for historical emissions from 1 January 2015 until the end of 2019 to express remaining carbon budget estimates from 1 January 2020 onwards. The human-induced warming between the 1850–1900 and 2006–2015 periods used in SR1.5 was assessed by AR6 at 0.97°C (Table 3.1). In a like-with-like comparison, the combined effect of data and methodological updates in historical warming estimates thus results in no shift in estimated remaining carbon budgets between SR1.5 and AR6. However, the emissions of the years passed since SR1.5 reduce the remaining carbon budget by about 85 GtCO <sub>2</sub> . Note that AR6 also updated its GSAT assessment for total warming between the 1850–1900 and 2006–2015 periods, reporting 0.94°C of warming. On a like-with-like basis, this would have resulted in slightly larger remaining carbon budgets compared to SR1.5 (Cross-Chapter Box 2.3).

The non-CO <sub>2</sub> contribution to future warming in emissions scenarios ( [[#Huppmann--2018|Huppmann et al., 2018]] ) is re-assessed with AR6-calibrated emulators, in this case MAGICC7 (Cross-Chapter Box 7.1; [[#Meinshausen--2009|Meinshausen et al., 2009]] , 2011a, 2020). The re-assessment of non-CO <sub>2</sub> warming with MAGICC7 results in a relationship that closely matches the average relationship applied in SR1.5 (shown in Section 2.SM.1.1.2 in [[#Forster--2018|Forster et al., 2018]] ), and therefore does not change estimates of the remaining carbon budget relative to SR1.5. The median ZEC assessment remained the same between SR1.5 and AR6, and therefore does not change the median remaining carbon budget estimates. Finally, as indicated above, AR6 expanded the assessment of Earth system feedbacks compared to SR1.5 and included it in its central remaining carbon budget estimates. Some feedbacks are accounted for through the non-CO <sub>2</sub> warming estimate ( [[#5.5.2.2.5|Section 5.5.2.2.5]] ), while the remainder combines to reduce the median remaining carbon budget estimates for 1.5°C and 2°C of warming by about 10 to 20 GtCO <sub>2</sub> , respectively, compared to SR1.5.

All methodological improvements and new evidence combined result in median and 67th percentile remaining carbon budget estimates for limiting warming to 1.5°C being about 300–350 GtCO <sub>2</sub> larger compared to an assessment that would use the evidence and methods available at the time of the AR5. For limiting warming to 2°C, the difference is about 400–500 GtCO <sub>2</sub> . Since SR1.5, fewer key advancements had to be integrated. In a like-with-like comparison, the combined effects of all AR6 updates result in median remaining carbon budget estimates for limiting warming to 1.5°C and 2°C being the same and about 60 GtCO <sub>2</sub> smaller, respectively, in AR6 compared to SR1.5. At the 67th percentile, remaining carbon budget estimates for limiting warming to 1.5°C and 2°C are about 40 to 60 GtCO <sub>2</sub> larger, respectively, mainly as a result of a narrower assessed TCRE range.

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