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

===== 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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