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

==== 5.5.2.1 Framework and Earlier Approaches ====

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The AR6 Glossary (Annex VII) defines remaining carbon budgets as the maximum amount of cumulative net global anthropogenic CO <sub>2</sub> emissions expressed from a recent specified date that would result in limiting global warming to a given level with a given probability, taking into account the effect of other anthropogenic climate forcers, consistent with the definition used inSR1.5 ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ). Studies, however, apply a variety of definitions that result in published remaining carbon budget estimates informing to cumulative emissions at the time when global-mean temperature increase would reach, exceed, avoid, or peak at a given warming level with a given probability (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; T.F. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ; [[#Clarke--2014|Clarke et al., 2014]] ; [[#Friedlingstein--2014a|Friedlingstein et al., 2014a]] ; [[#IPCC--2014|IPCC, 2014]] ; [[#Rogelj--2016|Rogelj et al., 2016]] ; [[#Millar--2017b|Millar et al., 2017b]] ).

This section provides an assessment of remaining carbon budgets consistent with the AR6 Glossary definition (Annex VII). Given that some feedbacks are time dependent, the values in this section apply to limiting warming over the 21st century, consistent with recent studies highlighting the usefulness of time-limited carbon budgets ( [[#Sanderson--2020|Sanderson, 2020]] ). Irrespective of the exact definition of the remaining carbon budget, the finding that higher cumulative CO <sub>2</sub> emissions lead to higher temperatures implies that annual net CO <sub>2</sub> emissions have to decline to close to zero in order to halt global warming, whether at 1.5°C, 2°C or another level ( [[#Allen--2018|Allen et al., 2018]] ).

Two approaches were used in AR5 to determine carbon budgets (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; T.F. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ; [[#Clarke--2014|Clarke et al., 2014]] ; [[#IPCC--2014|IPCC, 2014]] ; [[#Rogelj--2016|Rogelj et al., 2016]] ). Working Group I (WGI) reported threshold exceedance budgets (TEB) that correspond to the amount of cumulative CO <sub>2</sub> emissions at the time a specific temperature threshold is exceeded, with a given probability in a particular greenhouse-gas and aerosol (pre-cursor) emissions scenario (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; [[#IPCC--2013b|IPCC, 2013b]] ; T.F. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ). WGI also reported TEBs for the hypothetical case that only CO <sub>2</sub> would be emitted by human activities (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; [[#IPCC--2013b|IPCC, 2013b]] ; T.F. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ). The AR5 Working Group III used threshold avoidance budgets (TAB) that correspond to the cumulative CO <sub>2</sub> emissions over a given time period of a subset of greenhouse-gas and aerosol (precursor) emissions scenarios in which global-mean temperature increase stays below a specific temperature threshold with at least a given probability ( [[#Clarke--2014|Clarke et al., 2014]] ). The AR5 synthesis report used TABs defined until the time of peak warming over the 21st century ( [[#IPCC--2014|IPCC, 2014]] ). Drawbacks have been identified for TEBs and TABs ( [[#Rogelj--2016|Rogelj et al., 2016]] ). TABs provide an estimate of the cumulative CO <sub>2</sub> emissions under pathways that have as a common characteristic the fact that they do not exceed a specific global warming threshold. However, the actual level of maximum warming can vary between pathways, leading to an unnecessary and poorly constrained spread in TAB estimates ( [[#Rogelj--2016|Rogelj et al., 2016]] ). Therefore, the TAB approach typically does not result in accurate projections of the remaining carbon budget. One drawback of TEBs is that they provide an estimate of the cumulative CO <sub>2</sub> emissions at the time global warming crosses a given threshold of interest in a specific emissions scenario – for example, most of the standard scenarios used in climate change research, such as the Representative Concentration Pathways (RCPs) or Shared Socio-economic Pathways (SSPs), exceed global warming of 1.5°C or 2°C (see Cross-Chapter Box 1.5) (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; T.F. [[#Stocker--2013|]] [[#Stocker--2013|Stocker et al., 2013]] ; [[#Friedlingstein--2014a|Friedlingstein et al., 2014a]] ; [[#Millar--2017b|Millar et al., 2017b]] ). Because of potential variations in non-CO <sub>2</sub> warming at that point in time, or potential lags of about a decade in CO <sub>2</sub> warming ( [[#Joos--2013|Joos et al., 2013]] ; [[#Ricke--2014|Ricke and Caldeira, 2014]] ; [[#Rogelj--2015a|Rogelj et al., 2015a]] , 2016, 2018b; [[#Zickfeld--2015|Zickfeld and Herrington, 2015]] ) TEBs also do not provide a precise estimate of the remaining carbon budget for limiting warming to a specific level.

Since the publication of AR5 (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ), several new approaches have been proposed that provide a solution to the identified limitations of TABs and TEBs. Most of these approaches indirectly rely on the concept of TCRE ( [[#5.5.1|Section 5.5.1]] ), for example, because they estimate modelled cumulative CO <sub>2</sub> emissions until a temperature threshold is crossed and use this budget to infer insights for pathways that attempt to limit warming to below this threshold and thus need to follow a different path ( [[#Friedlingstein--2014a|Friedlingstein et al., 2014a]] ; [[#Matthews--2017|Matthews et al., 2017]] ; [[#Millar--2017b|Millar et al., 2017b]] ; [[#Goodwin--2018|Goodwin et al., 2018]] ; [[#Tokarska--2018|Tokarska and Gillett, 2018]] ). In this report, the assessment framework of SR1.5 for remaining carbon budgets is applied ( [[#Rogelj--2018b|Rogelj et al., 2018b]] , 2019). This framework allows integration of multiple lines of evidence to assess the contributions of five components that together result in a consolidated assessment of the remaining carbon budget (TCRE, historical human-induced warming, non-CO <sub>2</sub> warming, the ZEC, and adjustments due to additional Earth system feedbacks, see [[#5.5.2.2|Section 5.5.2.2]] ). It builds on the advances in estimating remaining carbon budgets or related quantities that have been published since AR5 ( [[#Rogelj--2015a|Rogelj et al., 2015a]] ; [[#Haustein--2017|Haustein et al., 2017]] ; [[#Matthews--2017|Matthews et al., 2017]] , 2021; [[#Millar--2017b|Millar et al., 2017b]] ; [[#Gasser--2018|Gasser et al., 2018]] ; [[#Lowe--2018|Lowe and Bernie, 2018]] ; [[#Tokarska--2018|Tokarska et al., 2018]] ; [[#Nicholls--2020|Nicholls et al., 2020]] ).

Recent studies suggest further changes to this framework by including non-linear adjustments to the TCRE contribution ( [[#Nicholls--2020|Nicholls et al., 2020]] ), or including non-CO <sub>2</sub> forcers in different ways by accounting for their different forcing effects ( [[#Matthews--2021|Matthews et al., 2021]] ). Figure 5.31 provides a conceptual schematic of how the various individually assessed contributions are combined into a consolidated assessment of the remaining carbon budget. Together with estimates of historical CO <sub>2</sub> emissions to date ( [[#5.2.1|Section 5.2.1]] ), these remaining carbon budgets provide the overall amount of cumulative CO <sub>2</sub> emissions consistent with limiting global warming to specific levels. A comparison with the approach applied in AR5 (M. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; [[#Clarke--2014|Clarke et al., 2014]] ) is available in SR1.5 [[IPCC:Wg1:Chapter:Chapter-2#2.2.2|Section 2.2.2]] ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ) as well as Box 5.2.

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[[File:fd897abc34bb54789178645dac08d60e IPCC_AR6_WGI_Figure_5_31.png]]

'''Figure 5.31 |''' '''Illustration of relationship between cumulative emissions of carbon dioxide (CO''' <sub>2</sub> ''') and global mean surface air temperature (GSAT) increase (left) and conceptual schematic of the assessment of the remaining carbon budget from its constituting components (right).''' Carbon budgets consistent with various levels of additional warming are provided in Table 5.8 and should not be read from the illustrations in either panel. Left-hand panel: Historical data (thin black line data) shows historical CO <sub>2</sub> emissions as reported in [[#Friedlingstein--2020|Friedlingstein et al. (2020)]] together with the assessed global mean surface air temperature increase from 1850–1900 as assessed in [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] (Box 2.3, GSAT). The orange-brown range with its central line shows the estimated human-induced share of historical warming ( [[#Haustein--2017|Haustein et al., 2017]] ). The vertical orange-brown line shows the assessed range of historical human-induced warming for the 2010–2019 period relative to 1850–1900 (Chapter 3). The grey cone shows the assessed range for the transient climate response to cumulative CO <sub>2</sub> emissions (TCRE) assessed to fall ''likely'' in the 1.0°C–2.3°C per 1000 PgC range ( [[#5.5.1.4|Section 5.5.1.4]] ), starting from 2015. Thin coloured lines show CMIP6 simulations for the five scenarios of the AR6 core set (SSP1-1.9, sky blue; SSP1-2.6, dark blue; SSP2-4.5, yellow; SSP3-7.0, red; SSP5-8.5, maroon), starting from 2015. Diagnosed carbon emissions ( [[#Arora--2020|Arora et al., 2020]] ) are complemented with estimated land-use change emissions for each respective scenario ( [[#Gidden--2019|Gidden et al., 2019]] ). Coloured areas show the [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] assessed ''very likely'' range of GSAT projections and thick coloured central lines the median estimate, for each respective scenario. These projections are expressed relative to the cumulative CO <sub>2</sub> emissions that are available for emissions-driven CMIP6 ScenarioMIP experiments for each respective scenario ( [[#Riahi--2017|Riahi et al., 2017]] ; [[#Rogelj--2018a|Rogelj et al., 2018a]] ; [[#Gidden--2019|Gidden et al., 2019]] ). Right-hand panel: schematic illustration of assessment of remaining carbon budget based on multiple lines of evidence. The remaining allowable warming is estimated by combining the global warming limit of interest with the assessed historical human induced warming ( [[#5.5.2.2.2|Section 5.5.2.2.2]] ), the assessed future potential non-CO <sub>2</sub> warming contribution ( [[#5.5.2.2.3|Section 5.5.2.2.3]] ) and the zero emissions commitment ( [[#5.5.2.2.4|Section 5.5.2.2.4]] ). Note that contributions in the right-hand panel are illustrative and contributions are not to scale. For example, the central ZEC estimate was assessed to be zero. The remaining allowable warming (vertical blue bar) is subsequently combined with the assessed TCRE (Sections 5.5.1.4 and 5.5.2.2.1) and contribution of unrepresented Earth system feedbacks in models used to estimate ZEC and TCRE ( [[#5.5.2.2.5|Section 5.5.2.2.5]] ) to provide an assessed estimate of the remaining carbon budget (horizontal blue bar; see Table 5.8). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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