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

=== 5.5.2 Remaining Carbon Budget Assessment ===

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Estimates of remaining carbon budgets consistent with holding global warming below a specific temperature threshold depend on a range of factors which are increasingly being studied and quantified. These factors include: (i) well-understood methodological and definitional choices (Sections 5.5.2.1 and 5.5.2.2; [[#Friedlingstein--2014a|Friedlingstein et al., 2014a]] ; [[#Rogelj--2016|Rogelj et al., 2016]] , 2018b); and (ii) a set of contributing factors such as historical warming, the TCRE and its limitations, the ZEC (the amount of warming projected to occur following a complete cessation of emissions; see [[IPCC:Wg1:Chapter:Chapter-4#4.7.1.1|Section 4.7.1.1]] ), as well as contributions of non-CO <sub>2</sub> climate forcers ( [[#5.5.2.2|Section 5.5.2.2]] ; [[#MacDougall--2015|MacDougall and Friedlingstein, 2015]] ; [[#Rogelj--2015a|Rogelj et al., 2015a]] , b; [[#MacDougall--2016|MacDougall, 2016]] ; [[#Simmons--2016|Simmons and Matthews, 2016]] ; [[#Ehlert--2017|Ehlert et al., 2017]] ; [[#Matthews--2017|Matthews et al., 2017]] , 2021; [[#Millar--2017b|Millar et al., 2017b]] ; [[#Goodwin--2018|Goodwin et al., 2018]] ; [[#Mengis--2018|Mengis et al., 2018]] ; [[#Pfleiderer--2018|Pfleiderer et al., 2018]] ; [[#Tokarska--2018|Tokarska et al., 2018]] ; [[#Cain--2019|Cain et al., 2019]] ). These contributing factors are integrated in an overarching assessment of remaining carbon budgets for limiting global average warming to levels ranging from 1.5°C to 2.5°C relative to pre-industrial levels provided in [[#5.5.2.3|Section 5.5.2.3]] . Box 5.2 provides an overview of the methodological advances since AR5 (W.J. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ).

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==== 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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==== 5.5.2.2 Assessment of Individual Components ====

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Remaining carbon budgets are assessed through the combination of five separate components ( [[#Forster--2018|Forster et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ). Each component is discussed and assessed separately in the sections below, based on all available lines of evidence. Box 5.1 details the differences compared to AR5 and SR1.5 estimates(W.J. [[#Collins--2013|]] [[#Collins--2013|Collins et al., 2013]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ).

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===== 5.5.2.2.1 TCRE =====

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The first and central component for estimating remaining carbon budgets is the TCRE. Based on the assessment in [[#5.5.1.4|Section 5.5.1.4]] , an assessed ''likely'' range for TCRE of 1.0°C–2.3°C per 1000 PgC with a normal distribution is used.

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===== 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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===== 5.5.2.2.3 Non-CO <sub>2</sub> warming contribution =====

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Non-CO <sub>2</sub> emissions contribute either cumulatively (N <sub>2</sub> O, and other long-lived climate forcers) or in proportion to their annual emissions (CH <sub>4</sub> and other short-lived climate forcers) to global warming, and thus also affect estimates of remaining carbon budgets by reducing the amount of warming that could still result from CO <sub>2</sub> emissions ( [[#Meinshausen--2009|Meinshausen et al., 2009]] ; [[#Friedlingstein--2014a|Friedlingstein et al., 2014a]] ; [[#Knutti--2015|Knutti and Rogelj, 2015]] ; [[#Rogelj--2015a|Rogelj et al., 2015a]] , 2016; R.G. [[#Williams--2016|Williams et al., 2016]] , 2017b; [[#Matthews--2017|Matthews et al., 2017]] ; [[#Collins--2018|Collins et al., 2018]] ; [[#Mengis--2018|Mengis et al., 2018]] ; [[#Tokarska--2018|Tokarska et al., 2018]] ; [[#Zickfeld--2021|Zickfeld et al., 2021]] ). The size of this contribution has been estimated both implicitly ( [[#Meinshausen--2009|Meinshausen et al., 2009]] ; [[#Friedlingstein--2014a|Friedlingstein et al., 2014a]] ; [[#Rogelj--2016|Rogelj et al., 2016]] ; [[#Matthews--2017|Matthews et al., 2017]] ; [[#Mengis--2018|Mengis et al., 2018]] ; [[#Tokarska--2018|Tokarska et al., 2018]] ) and explicitly ( [[#Rogelj--2015a|Rogelj et al., 2015a]] , 2018b; [[#Collins--2018|Collins et al., 2018]] ; [[#Matthews--2021|Matthews et al., 2021]] ) by varying the assumptions of non-CO <sub>2</sub> emissions and associated warming. Internally consistent evolutions of future CO <sub>2</sub> and non-CO <sub>2</sub> emissions allow for derivation of non-CO <sub>2</sub> warming contributions consistent with global CO <sub>2</sub> emissions reaching net zero levels, and therewith capping maximum future CO <sub>2</sub> emissions ( [[#Smith--2013|Smith and Mizrahi, 2013]] ; [[#Clarke--2014|Clarke et al., 2014]] ; [[#Huppmann--2018|Huppmann et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ; [[#Matthews--2021|Matthews et al., 2021]] ). Pathways that reflect such development typically show a stabilization or decline in non-CO <sub>2</sub> radiative forcing and warming at, and after the time of, global CO <sub>2</sub> emissions reaching net zero levels, as illustrated in the scenario database underlying SR1.5 ( [[#Huppmann--2018|Huppmann et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ).

The impact of non-CO <sub>2</sub> emissions on remaining carbon budgets is assessed with emulators ( [[#Meinshausen--2009|Meinshausen et al., 2009]] ; [[#Millar--2017b|Millar et al., 2017b]] ; [[#Gasser--2018|Gasser et al., 2018]] ; [[#Goodwin--2018|Goodwin et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ; C.J. [[#Smith--2018|]] [[#Smith--2018|Smith et al., 2018]] ; [[#Matthews--2021|Matthews et al., 2021]] ) that incorporate synthesized climate and carbon-cycle knowledge (Cross-Chapter Box 7.1). The estimated implied non-CO <sub>2</sub> warming can subsequently be applied to reduce the remaining allowable warming for estimating the remaining carbon budget (Figure 5.31; [[#Rogelj--2018b|Rogelj et al., 2018b]] , 2019). Alternative methods estimate the non-CO <sub>2</sub> fraction of total anthropogenic forcing ( [[#Matthews--2021|Matthews et al., 2021]] ), or do not correct for non-CO <sub>2</sub> warming directly. The latter methods instead consider CO <sub>2</sub> and non-CO <sub>2</sub> warming together to define a CO <sub>2</sub> -forcing equivalent carbon budget from which eventual non-CO <sub>2</sub> contributions expressed in CO <sub>2</sub> -forcing-equivalent emissions have to be subtracted to obtain a remaining carbon budget ( [[#Jenkins--2018|Jenkins et al., 2018]] ; [[#Matthews--2020|Matthews et al., 2020]] ). These studies also use emulators to invert a specified evolution of non-CO <sub>2</sub> forcing to a corresponding amount of equivalent CO <sub>2</sub> emissions ( [[#Matthews--2020|Matthews et al., 2020]] ), or alternatively use empirical relationships linking changes in non-CO <sub>2</sub> greenhouse gas emissions to warming ( [[#Cain--2019|Cain et al., 2019]] ). Methods to express non-CO <sub>2</sub> emissions in CO <sub>2</sub> equivalence are assessed in [[IPCC:Wg1:Chapter:Chapter-7#7.6|Section 7.6]] , yet their applicability and related uncertainties for remaining carbon budgets have not yet been covered in-depth in the literature.

Application of the SR1.5 method ( [[#Forster--2018|Forster et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ) with AR6-calibrated emulators (Box 7.1) suggests a median additional non-CO <sub>2</sub> warming contribution at the time global CO <sub>2</sub> emissions reach net zero levels of about 0.1°C–0.2°C relative to 2010–2019. Uncertainty surrounding this range due to geophysical uncertainties such as non-CO <sub>2</sub> -forcing uncertainties and TCR is of the order of ±0.1°C. Differences in the choices of mitigation strategies considered in low-emissions scenarios ( [[#Huppmann--2018|Huppmann et al., 2018]] ) result in a potential additional variation around the central range of at least ±0.1°C (spread across scenarios, referred to as non-CO <sub>2</sub> scenario uncertainty in Table 5.8).

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'''Table 5.8 |''' '''The assessed remaining carbon budget and corresponding uncertainties''' . Assessed estimates are provided for additional human-induced warming expressed as global average surface air temperature since the recent past (2010–2019), which ''likely'' amounted to 0.8 to 1.3 with a best estimate of 1.07°C relative to 1850–1900 (Table 3.1 in Chapter 3).

{| class="wikitable"
|-
! Additional Warming Since 2010–2019 <sup>a</sup>

! Warming Since 1850–1900 <sup>a</sup>

! colspan="5"| Remaining Carbon Budget <sup>b</sup> starting from 1 January 2020 and subject to variations and uncertainties quantified in the columns on the right

! Scenario Variation

! colspan="4"| Geophysical Uncertainties

|-
| ''°C''

| ''°C''

| colspan="5"| Percentiles of TCRE <sup>c,d</sup> ''PgC (GtCO'' 2 '')''

| Non-CO <sub>2</sub> scenario variation <sup>e</sup>

| Non-CO <sub>2</sub> forcing and response uncertainty <sup>f</sup>

| Historical temperature uncertainty <sup>a</sup>

| Zero emissions commitment (ZEC)uncertainty <sup>g</sup>

| Recent emissions uncertainty <sup>h</sup>

|-
|
| ''17th''

| ''33rd''

| ''50th''

| ''67th''

| ''83rd''

| ''PgC (GtCO'' 2 '')''

| ''PgC (GtCO'' 2 '')''

| ''PgC (GtCO'' 2 '')''

| ''PgC (GtCO'' 2 '')''

| ''PgC (GtCO'' 2 '')''

|-
| 0.23

| 1.3

| ''100 (400)''

| ''60 (250)''

| ''40 (150)''

| ''30 (100)''

| ''10 (50)''

| rowspan="12"| Values can vary by at least ±60 PgC (±220 GtCO <sub>2</sub> ) due to choices related to non-CO <sub>2</sub> emissions mitigation

| rowspan="12"| Values can vary by at least ±60 PgC (±220 GtCO <sub>2</sub> ) due to uncertainty in the warming reponse to future non-CO <sub>2</sub> emissions

| rowspan="12"| ±150 PgC (±550 GtCO <sub>2</sub> )

| rowspan="12"| ±115 PgC (±420 GtCO <sub>2</sub> )

| rowspan="12"| ±6 PgC (±20 GtCO <sub>2</sub> )

|-
| 0.33

| 1.4

| ''180 (650)''

| ''120 (450)''

| ''90 (350)''

| ''70 (250)''

| ''50 (200)''

|-
| 0.43

| 1.5

| ''250 (900)''

| ''180 (650)''

| ''140 (500)''

| ''110 (400)''

| ''80 (300)''

|-
| 0.53

| 1.6

| ''330 (1200)''

| ''230 (850)''

| ''180 (650)''

| ''150 (550)''

| ''110 (400)''

|-
| 0.63

| 1.7

| ''400 (1450)''

| ''290 ('' ''1050'' '')''

| ''230 (850)''

| ''190 (700)''

| ''150 (550)''

|-
| 0.73

| 1.8

| ''470 (1750)''

| ''350 (1250)''

| ''280 (1000)''

| ''230 (850)''

| ''180 (650)''

|-
| 0.83

| 1.9

| ''550 (2000)''

| ''400 (1450)''

| ''320 ('' ''1200'' '')''

| ''270 (1000)''

| ''210 (800)''

|-
| 0.93

| 2

| ''620 (2300)''

| ''460 (1700)''

| ''370 ('' ''1350'' '')''

| ''310 (1150)''

| ''250 (900)''

|-
| 1.03

| 2.1

| ''700 (2550)''

| ''510 (1900)''

| ''420 ('' ''1500'' '')''

| ''350 (1250)''

| ''280 ('' ''1050'' '')''

|-
| 1.13

| 2.2

| ''770 (2850)''

| ''570 (2100)''

| ''460 ('' ''1700'' '')''

| ''390 (1400)''

| ''310 (1150)''

|-
| 1.23

| 2.3

| ''850 (3100)''

| ''630 (2300)''

| ''510 ('' ''1850'' '')''

| ''430 (1550)''

| ''350 ('' ''1250'' '')''

|-
| 1.33

| 2.4

| ''920 (3350)''

| ''680 (2500)''

| ''550 (2050)''

| ''470 (1700)''

| ''380 ('' ''1400'' '')''

|}

<sup>a</sup> Human-induced global surface air temperature increase between 1850–1900 and 2010–2019 is assessed at 0.8–1.3°C ( ''likely'' range; Chapter 3) with a best estimate of 1.07°C. Warming reflects changes in GSAT, as TCRE and other estimates are GSAT-based. Combined with a central estimate of TCRE (1.65°C EgC <sup>–1</sup> ) the uncertainty in historical human-induced GSAT warming results in a potential variation of remaining carbon budgets of ±150 PgC or ±550 GtCO <sub>2</sub> .

<sup>b</sup> Historical CO <sub>2</sub> emissions between 1850 and 2019 have been estimated at about 655 ± 65 PgC ( ''likely'' range, or 2390 ± 240 GtCO <sub>2</sub> , see Table 5.1). Note that 57 PgC (210 GtCO <sub>2</sub> ) have been emitted from the middle of the 2010–2019 reference period (2015) until the end of 2019 ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ).

<sup>c</sup> TCRE: transient climate response to cumulative CO <sub>2</sub> emissions, assessed to fall ''likely'' between 1.0–2.3°C EgC <sup>–1</sup> with a normal distribution. PgC values are rounded to the nearest 10; GtCO <sub>2</sub> values to the nearest 50. For comparison, assuming a lognormal distribution with a 1.0–2.3°C EgC <sup>–1</sup> central 66% range instead of a normal distribution would increase remaining carbon budgets at the 17th, 33rd, 50th, 67th, and 83rd percentile with 3%, 10%, 12%, 9%, 2%, respectively. Future non-CO <sub>2</sub> contributions in these remaining carbon budget estimates are based on the scenarios assessed in the SR1.5 report and estimated as the median quantile regression of non-CO <sub>2</sub> warming since 2010–2019 relative to total additional warming since 2010–2019 at the time scenarios reach net-zero CO <sub>2</sub> emissions ( [[#Forster--2018|Forster et al., 2018]] ; [[#Huppmann--2018|Huppmann et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ).

<sup>d</sup> Additional Earth system feedbacks are included in the remaining carbon budget estimates as discussed in [[#5.5.2.2.5|Section 5.5.2.2.5]] . The tropospheric ozone and methane lifetime contributions are included through the non-CO <sub>2</sub> warming projections by the AR6-calibrated Model for the Assessment of Greenhouse Gas Induced Climate Change (MAGICC) emulator, while the remaining feedbacks are assessed totalling a combined feedback of magnitude 7 ± 27 PgC K <sup>–1</sup> (1-sigma range, or 26 ± 97 GtCO <sub>2</sub> °C <sup>–1</sup> ).

<sup>e</sup> Variations due to different scenario assumptions related to the future evolution of non-CO <sub>2</sub> emissions in mitigation scenarios reaching net zero CO <sub>2</sub> emissions ( [[#Huppmann--2018|Huppmann et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ) of at least ±0.1°C (spread across scenarios). Combined with a central estimate of TCRE (1.65°C EgC <sup>–1</sup> ) this results in at least ±60 PgC or ±220 GtCO <sub>2</sub> . This spread reflects the variation in the underlying scenario ensemble but is not a formal likelihood. WGIII will re-assess the potential for non-CO <sub>2</sub> mitigation based on literature since SR1.5.

<sup>f</sup> Remaining carbon budget variation due to geophysical uncertainty in forcing and temperature response of non-CO <sub>2</sub> emissions of the order of ±0.1°C, ''very'' ''likely'' range (5–95%) of non-CO <sub>2</sub> response ( [[#5.5.2.2.3|Section 5.5.2.2.3]] ). Combined with a central estimate of TCRE (1.65°C EgC <sup>–1</sup> ) this results in at least ±60 PgC or ±220 GtCO <sub>2</sub> .

<sup>g</sup> The variation due to the ZEC is estimated for a central TCRE value of 1.65°C EgC <sup>–1</sup> and a 1-sigma ZEC range of 0.19°C. In real-world pathways, the magnitude of this effect will depend on the pace of CO <sub>2</sub> emissions reductions to net zero.

<sup>h</sup> Historical emissions uncertainty reflects the ±10% uncertainty in the historical emissions estimate since 1 January 2015.

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===== 5.5.2.2.4 Adjustments due to the zero emissions commitment =====

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Use of TCRE for estimating remaining carbon budgets needs to consider the zero emissions commitment (ZEC), the potential additional warming after a complete cessation of net CO <sub>2</sub> emissions. Based on the ZEC assessment presented in [[IPCC:Wg1:Chapter:Chapter-4#4.7.1.1|Section 4.7.1.1]] , the ZEC’s central value is taken to be zero with a ''likely'' range of ±0.19°C, noting that it might either increase or decrease after half a century. ZEC uncertainty is assessed for a time frame of half a century, as this most appropriately reflects the time between stringent mitigation pathways reaching net zero CO <sub>2</sub> emissions and the end of the century. For shorter time horizons, a similar central zero value applies, but with a smaller range ( [[#MacDougall--2020|MacDougall et al., 2020]] ). Experiments that ramped up and down emissions following a bell-shaped trajectory ( [[#MacDougall--2016a|MacDougall and Knutti, 2016a]] ) show that when annual CO <sub>2</sub> emissions decline to zero at a pace consistent with those currently assumed in mitigation scenarios ( [[#Huppmann--2018|Huppmann et al., 2018]] ; [[#Rogelj--2018b|Rogelj et al., 2018b]] ), the ZEC will already be realized to a large degree at the time of reaching net zero CO <sub>2</sub> emissions ( [[#MacDougall--2020|MacDougall et al., 2020]] ).

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===== 5.5.2.2.5 Adjustments for additional Earth system feedbacks =====

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( [[#5.5.1.2|Section 5.5.1.2]] highlighted recent literature describing potential impacts of Earth system feedbacks that have typically not been included in standard ESMs ( [[#MacDougall--2015|MacDougall and Friedlingstein, 2015]] ; [[#Schneider%20von%20Deimling--2015|Schneider von Deimling et al., 2015]] ; [[#Schädel--2016|Schädel et al., 2016]] ; [[#Burke--2017b|Burke et al., 2017b]] ; [[#Mahowald--2017|Mahowald et al., 2017]] ; [[#Comyn-Platt--2018|Comyn-Platt et al., 2018]] ; [[#Gasser--2018|Gasser et al., 2018]] ; [[#Lowe--2018|Lowe and Bernie, 2018]] ), the most important of which is carbon release from thawing permafrost. The SR1.5 estimated unrepresented Earth system processes to result in a reduction of remaining carbon budgets of up to 100 GtCO <sub>2</sub> over the course of this century, and more thereafter ( [[#Rogelj--2018b|Rogelj et al., 2018b]] ). Here this assessment is updated based on the Earth system feedback assessment of ( [[#5.4.8|Section 5.4.8]] and synthesized in Figure 5.29 by applying the reverse method by [[#Gregory--2009|Gregory et al. (2009)]] .

The assessment in [[#5.4|Section 5.4]] and Box 5.1 highlights the different nature, magnitude and uncertainties surrounding additional Earth system feedback. The remaining carbon budgets reported in Table 5.8 account for these feedbacks, including corrections due to permafrost CO <sub>2</sub> and CH <sub>4</sub> feedbacks as well as those due to aerosol and atmospheric chemistry ( [[#5.4.8|Section 5.4.8]] ). Two of these additional feedbacks (tropospheric ozone and methane lifetime feedbacks) are included in the projections of non-CO <sub>2</sub> warming carried out with AR6-calibrated emulators (Box 7.1). The remainder of these independent Earth system feedbacks combine to a feedback of about 7 ± 27 PgC K <sup>–1</sup> (1-sigma range, or 26 ± 97 GtCO <sub>2</sub> °C <sup>–1</sup> ). Overall, [[#5.4.8|Section 5.4.8]] assessed there to be ''low confidence'' in the exact magnitude of these feedbacks and they represent identified additional amplifying factors that scale with additional warming, and mostly increase the challenge of limiting global warming to or below specific temperature levels.

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==== 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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