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

==== 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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