Editing IPCC:AR6/WGI/TS (section)

==== TS.3.3.1 Remaining Carbon Budgets and Temperature Stabilization ====

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'''The near-linear relationship between cumulative CO <sub>2</sub> emissions and maximum global surface temperature increase caused by CO <sub>2</sub> implies that stabilizing human-induced global temperature increase at any level requires net anthropogenic CO <sub>2</sub> emissions to become zero. This near-linear relationship further implies that mitigation requirements for limiting warming to specific levels can be quantified in terms of a carbon budget ( ''high confidence'' ). Remaining carbon budget estimates have been updated since AR5 with methodological improvements, resulting in larger estimates that are consistent with SR1.5. Several factors, including estimates of historical warming, future emissions from thawing permafrost, variations in projected non-CO <sub>2</sub> warming, and the global surface temperature change after cessation of CO <sub>2</sub> emissions, affect the exact value of carbon budgets ( ''high confidence'' ). Links to chapters 1.3.5, Box 1.2, 4.7.1, 5.5'''
Limiting further climate change would require substantial and sustained reductions of GHG emissions. Without net zero CO <sub>2</sub> emissions, and a decrease in the net non-CO <sub>2</sub> forcing (or sufficient net negative CO <sub>2</sub> emissions to offset any further warming from net non-CO <sub>2</sub> forcing), the climate system will continue to warm. There is ''high confidence'' that mitigation requirements for limiting warming to specific levels over this century can be estimated using a carbon budget that relates cumulative CO <sub>2</sub> emissions to global mean temperature increase (Figure TS.18, Table TS.3). For the period 1850–2019, a total of 2390 ± 240 GtCO <sub>2</sub> of anthropogenic CO <sub>2</sub> has been emitted. Remaining carbon budgets (starting from 1 January 2020) for limiting warming to 1.5°C, 1.7°C and 2.0°C are estimated at 500 GtCO <sub>2</sub> , 850 GtCO <sub>2</sub> and 1350 GtCO <sub>2</sub> , respectively, based on the 50th percentile of TCRE. For the 67th percentile, the respective values are 400 GtCO <sub>2</sub> , 700 GtCO <sub>2</sub> and 1150 GtCO <sub>2</sub> . The remaining carbon budget estimates for different temperature limits assume that non-CO <sub>2</sub> emissions are mitigated consistent with the median reductions found in scenarios in the literature as assessed in SR1.5, but they may vary by an estimated ±220 GtCO <sub>2</sub> depending on how deeply future non-CO <sub>2</sub> emissions are assumed to be reduced (Table TS.3). Links to chapters 5.5.2, 5.6, Box 5.2, 7.6

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[[File:33f1d196f83ba934a6dfadd271795bdd IPCC_AR6_WGI_TS_Figure_18.png]]
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'''Figure TS.18 |''' '''Illustration of (a) relationship between cumulative emissions of carbon dioxide (CO''' 2 ''') and global mean surface air temperature increase and (b) the assessment of the remaining carbon budget from its constituting components based on multiple lines of evidence.''' ''The intent of this figure is to show (i) the proportionality between cumulative CO'' 2 ''emissions and global surface air temperature in observations and models as well as the assessed range of the transient climate response to cumulative CO'' 2 ''emissions (TCRE), and (ii) how information is combined to derive remaining carbon budgets consistent with limiting warming to a specific level.'' 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. In panel (a) thin black line shows historical CO <sub>2</sub> emissions together with the assessed global surface temperature increase from 1850–1900 as assessed in [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] (Box 2.3). The orange-brown range with its central line shows the estimated human-induced share of historical warming. 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 ''likely'' range for the TCRE ( [[IPCC:Wg1:Chapter:Chapter-5#5.5.1.4|Section 5.5.1.4]] ), starting from 2015. Thin coloured lines show Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations for the five scenarios of the WGI core set (SSP1-1.9, light blue; SSP1-2.6, blue; SSP2-4.5, yellow; SSP3-7.0, red; SSP5-8.5, maroon), starting from 2015 and until 2100. Diagnosed carbon emissions are complemented with estimated land-use change emissions for each respective scenario. Coloured areas show the [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] assessed ''very likely'' range of global surface temperature projections and thick coloured central lines show the median estimate, for each respective scenario. These temperature projections are expressed relative to cumulative CO <sub>2</sub> emissions that are available for emissions-driven CMIP6 ScenarioMIP experiments for each respective scenario. For panel (b), the remaining allowable warming is estimated by combining the global warming limit of interest with the assessed historical human-induced warming ( [[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.2|Section 5.5.2.2.2]] ), the assessed future potential non-CO <sub>2</sub> warming contribution ( [[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.3|Section 5.5.2.2.3]] ) and the zero emissions commitment (ZEC; [[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.4|Section 5.5.2.2.4]] ). 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 ( [[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.5|Section 5.5.2.2.5]] ) to provide an assessed estimate of the remaining carbon budget (horizontal blue bar, Table 5.8). Note that contributions in panel (b) are illustrative and are not to scale. For example, the central ZEC estimate was assessed to be zero. Links to chapters Box 2.3, 5.2.1, 5.2.2, Figure 5.31

There is ''high confidence'' that several factors, including estimates of historical warming, future emissions from thawing permafrost, and variations in projected non-CO <sub>2</sub> warming, affect the value of carbon budgets but do not change the conclusion that global CO <sub>2</sub> emissions would need to decline to net zero to halt global warming. Estimates may vary by ±220 GtCO <sub>2</sub> depending on the level of non-CO <sub>2</sub> emissions at the time global anthropogenic CO <sub>2</sub> emissions reach net zero levels. This variation is referred to as non-CO <sub>2</sub> scenario uncertainty and will be further assessed in the AR6 Working Group III Contribution. Geophysical uncertainties surrounding the climate response to these non-CO <sub>2</sub> emissions result in an additional uncertainty of at least ±220 GtCO <sub>2</sub> , and uncertainties in the level of historical warming result in a ±550 GtCO <sub>2</sub> uncertainty. Links to chapters 5.4, 5.5.2

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'''Table TS.3 |''' '''Estimates of remaining carbon budgets and their uncertainties.''' Assessed estimates are provided for additional human-induced warming, expressed as global surface temperature, since the recent past (2010–2019), ''likely'' amounted to 0.8° to 1.3°C with a best estimate of 1.07°C relative to 1850–1900. Historical CO <sub>2</sub> emissions between 1850 and 2014 have been estimated at about 2180 ± 240 GtCO <sub>2</sub> (1-sigma range), while since 1 January 2015, an additional 210 GtCO <sub>2</sub> has been emitted until the end of 2019. GtCO <sub>2</sub> values to the nearest 50. Links to chapters Table 3.1, 5.5.1, 5.5.2, Box 5.2, Table 5.1, Table 5.7, Table 5.8

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<sup>a</sup> Human-induced global surface temperature increase between 1850–1900 and 2010–2019 is assessed at 0.8–1.3°C ( ''likely'' range; Cross-Section Box TS.1) with a best estimate of 1.07°C. Combined with a central estimate of TCRE (1.65°C per 1000 PgC) this uncertainty in isolation results in a potential variation of remaining carbon budgets of ±550 GtCO <sub>2</sub> , which, however, is not independent of the assessed uncertainty of TCRE and thus not fully additional.

<sup>b</sup> TCRE: transient climate response to cumulative emissions of carbon dioxide, assessed to fall ''likely'' between 1.0–2.3°C per 1000 PgC with a normal distribution, from which the percentiles are taken. Additional Earth system feedbacks are included in the remaining carbon budget estimates as discussed in [[IPCC:Wg1:Chapter:Chapter-5#5.5.2.2.5|Section 5.5.2.2.5]] .

<sup>c</sup> Estimates assume that non-CO <sub>2</sub> emissions are mitigated consistent with the median reductions found in scenarios in the literature as assessed in SR1.5. Non-CO <sub>2</sub> scenario variations indicate how much remaining carbon budget estimates vary due to different scenario assumptions related to the future evolution of non-CO <sub>2</sub> emissions in mitigation scenarios from SR1.5 that reach net zero CO <sub>2</sub> emissions. This variation is additional to the uncertainty in TCRE. The Working Group III Contribution to AR6 will reassess the potential for non-CO <sub>2</sub> mitigation based on literature since SR1.5.

<sup>d</sup> Geophysical uncertainties reported in these columns and TCRE uncertainty are not statistically independent, as uncertainty in TCRE depends on uncertainty in the assessment of historical temperature, non-CO <sub>2</sub> versus CO <sub>2</sub> forcing, and uncertainty in emissions estimates. These estimates cannot be formally combined, and these uncertainty variations are not directly additional to the spread of remaining carbon budgets due to TCRE uncertainty reported in columns three to seven.

<sup>e</sup> Recent emissions uncertainty reflects the ±10% uncertainty in the historical CO <sub>2</sub> emissions estimate since 1 January 2015.

Methodological improvements and new evidence result in updated remaining carbon budget estimates. The assessment in AR6 applies the same methodological improvements as in SR1.5, which uses a recent observed baseline for historic temperature change and cumulative emissions. Changes compared to SR1.5 are therefore small: the assessment of new evidence results in updated 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, after accounting for emissions since SR1.5. Meanwhile, remaining carbon budgets for limiting warming to 1.5°C would be about 300–350 GtCO <sub>2</sub> larger if evidence and methods available at the time of AR5 would be used. If a specific remaining carbon budget is exceeded, this results in a lower probability of keeping warming below a specified temperature level and higher irreversible global warming over decades to centuries, or alternatively a need for net negative CO <sub>2</sub> emissions or further reductions in non-CO <sub>2</sub> greenhouse gases after net zero CO <sub>2</sub> is achieved to return warming to lower levels in the long term. Links to chapters 5.5.2, 5.6, Box 5.2

Based on idealized model simulations that explore the climate response once CO <sub>2</sub> emissions have been brought to zero, the magnitude of the zero CO <sub>2</sub> emissions commitment (ZEC, see Glossary) is assessed to be ''likely'' smaller than 0.3°C for time scales of about half a century and cumulative CO <sub>2</sub> emissions broadly consistent with global warming of 2°C. However, there is ''low confidence'' about its sign on time scales of about half a century. For lower cumulative CO <sub>2</sub> emissions, the range would be smaller yet with equal uncertainty about the sign. If the ZEC is positive on decadal time scales, additional warming leads to a reduction in the estimates of remaining carbon budgets, and vice versa if it is negative. Links to chapters 4.7.1, 5.5.2

Permafrost thaw is included in estimates together with other feedbacks that are often not captured by models. Limitations in modelling studies combined with weak observational constraints only allow ''low confidence'' in the magnitude of these estimates (Section TS.3.2.2). Despite the large uncertainties surrounding the quantification of the effect of additional Earth system feedback processes, such as emissions from wetlands and permafrost thaw, these feedbacks represent identified additional risk factors that scale with additional warming and mostly increase the challenge of limiting warming to specific temperature levels. These uncertainties do not change the basic conclusion that global CO <sub>2</sub> emissions would need to decline to net zero to halt global warming. Links to chapters 5.4.8, 5.5.2, Box 5.1

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