Editing IPCC:AR6/SYR/Longer-Report (section)

==== 3.3.1 Remaining Carbon Budgets ====

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'''Limiting global temperature increase to a specific level requires limiting cumulative net CO''' '''2''' emissions to within a finite carbon budget '''[[#footnote-031|126]] , along with strong reductions in other GHGs.''' For every 1000 GtCO 2 emitted by human activity, global mean temperature rises by ''likely'' 0.27°C to 0.63°C (best estimate of 0.45°C). This relationship implies that there is a finite carbon budget that cannot be exceeded in order to limit warming to any given level. { ''WGI SPM D.1, WGI SPM D.1.1; SR1.5 SPM C.1.3'' } . ( ''Figure 3.5'' )

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'''Figure 3.5: Cumulative past,''' '''projected, and committed''' '''emissions, and associated global''' '''temperature changes. Panel (a)''' Assessed remaining carbon budgets to limit warming ''more likely than not'' to 1.5°C, to 2°C with a 83% and 67% likelihood, compared to cumulative emissions corresponding to constant 2019 emissions until 2030, existing and planned fossil fuel infrastructures (in GtCO 2 ). For remaining carbon budgets, thin lines indicate the uncertainty due to the contribution of non-CO 2 warming. For lifetime emissions from fossil fuel infrastructure, thin lines indicate the assessed sensitivity range. '''Panel (b)''' Relationship between cumulative CO 2 emissions and the increase in global surface temperature. Historical data (thin black line) shows historical CO 2 emissions versus observed global surface temperature increase relative to the period 1850-1900. The grey range with its central line shows a corresponding estimate of the human-caused share of historical warming. Coloured areas show the assessed ''very likely'' range of global surface temperature projections, and thick coloured central lines show the median estimate as a function of cumulative CO 2 emissions for the selected scenarios SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5. Projections until 2050 use the cumulative CO 2 emissions of each respective scenario, and the projected global warming includes the contribution from all anthropogenic forcers. { ''WGI SPM D.1, WGI Figure SPM.10, WGI Table SPM.2; WGIII SPM B.1, WGIII SPM B.7, WGIII 2.7; SR1.5 SPM C.1.3'' }

[https://www.ipcc.ch/figures/figure-3-5 ]

'''The best estimates of the remaining carbon budget (RCB) from the beginning of 2020 for limiting warming to 1.5°C with a 50% likelihood''' '''[[#footnote-030|127]] is estimated to be 500 GtCO''' '''2''' ; for 2°C (67% likelihood) this is 1150 GtCO '''2''' . '''[[#footnote-029|128]]''' Remaining carbon budgets have been quantified based on the assessed value of TCRE and its uncertainty, estimates of historical warming, climate system feedbacks such as emissions from thawing permafrost, and the global surface temperature change after global anthropogenic CO 2 emissions reach net zero, as well as variations in projected warming from non-CO 2 emissions due in part to mitigation action. The stronger the reductions in non-CO 2 emissions the lower the resulting temperatures are for a given RCB or the larger RCB for the same level of temperature change. For instance, the RCB for limiting warming to 1.5°C with a 50% likelihood could vary between 300 to 600 GtCO 2 depending on non-CO 2 warming '''[[#footnote-028|129]]''' . Limiting warming to 2°C with a 67% (or 83%) likelihood would imply a RCB of 1150 (900) GtCO 2 from the beginning of 2020. To stay below 2°C with a 50% likelihood, the RCB is higher, i.e., 1350 GtCO 2 '''[[#footnote-027|130]]''' . { ''WGI SPM D.1.2, WGI Table SPM.2; WGIII Box SPM.1, WGIII Box 3.4; SR1.5 SPM C.1.3'' }

If the annual CO 2 emissions between 2020–2030 stayed, on average, at the same level as 2019, the resulting cumulative emissions would almost exhaust the remaining carbon budget for 1.5°C (50%), and exhaust more than a third of the remaining carbon budget for 2°C (67%) (Figure 3.5). Based on central estimates only, historical cumulative net CO 2 emissions between 1850 and 2019 (2400 ±240 GtCO 2 ) amount to about four-fifths '''[[#footnote-026|131]]''' of the total carbon budget for a 50% probability of limiting global warming to 1.5°C (central estimate about 2900 GtCO 2 ) and to about two-thirds '''[[#footnote-025|132]]''' of the total carbon budget for a 67% probability to limit global warming to 2°C (central estimate about 3550 GtCO 2 ). { ''WGI Table SPM.2; WGIII SPM B.1.3, WGIII Table 2.1'' }

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'''Table 3.1: Key''' '''characteristics of the''' '''modelled global''' '''emissions''' '''pathways.''' Summary of projected CO 2 and GHG emissions, projected net zero timings and the resulting global warming outcomes. Pathways are categorised (columns), according to their likelihood of limiting warming to different peak warming levels (if peak temperature occurs before 2100) and 2100 warming levels. Values shown are for the median [p50] and 5–95th percentiles [p5–p95], noting that not all pathways achieve net zero CO 2 or GHGs. ''WGIII Table SPM.2''

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'''1''' Detailed explanations on the Table are provided in WGIII Box SPM.1 and WGIII Table SPM.2. The relationship between the temperature categories and SSP/RCPs is discussed in Cross-Section Box.2. Values in the table refer to the 50th and [5–95th] percentile values across the pathways falling within a given category as defined in WGIII Box SPM.1. The three dots (…) sign denotes that the value cannot be given (as the value is after 2100 or, for net zero, net zero is not reached). Based on the assessment of climate emulators in AR6 WG I (Chapter 7, Box 7.1), two climate emulators were used for the probabilistic assessment of the resulting warming of the pathways. For the ‘Temperature Change’ and ‘Likelihood’ columns, the non-bracketed values represent the 50th percentile across the pathways in that category and the median [50th percentile] across the warming estimates of the probabilistic MAGICC climate model emulator. For the bracketed ranges in the “ likelihood” column, the median warming for every pathway in that category is calculated for each of the two climate model emulators (MAGICC and FaIR). These ranges cover both the uncertainty of the emissions pathways as well as the climate emulators’ uncertainty. All global warming levels are relative to 1850-1900.

'''2''' C3 pathways are sub-categorised according to the timing of policy action to match the emissions pathways in WGIII Figure SPM.4.

'''3''' Global emission reductions in mitigation pathways are reported on a pathway-by-pathway basis relative to harmonised modelled global emissions in 2019 rather than the global emissions reported in WGIII SPM Section B and WGIII Chapter 2; this ensures internal consistency in assumptions about emission sources and activities, as well as consistency with temperature projections based on the physical climate science assessment by WGI (see WGIII SPM Footnote 49). Negative values (e.g., in C5, C6) represent an increase in emissions. The modelled GHG emissions in 2019 are 55 [53–58] GtCO 2 -eq, thus within the uncertainty ranges of estimates for 2019 emissions [53-66] GtCO 2 -eq (see 2.1.1).

'''4''' Emissions milestones are provided for 5-year intervals in order to be consistent with the underlying 5-year time-step data of the modelled pathways. Ranges in square brackets underneath refer to the range across the pathways, comprising the lower bound of the 5th percentile 5-year interval and the upper bound of the 95th percentile 5-year interval. Numbers in round brackets signify the fraction of pathways that reach specific milestones over the 21st century. Percentiles reported across all pathways in that category include those that do not reach net zero before 2100.

'''5''' For cases where models do not report all GHGs, missing GHG species are infilled and aggregated into a Kyoto basket of GHG emissions in CO 2 -eq defined by the 100-year global warming potential. For each pathway, reporting of CO 2 , CH 4 , and N 2 O emissions was the minimum required for the assessment of the climate response and the assignment to a climate category. Emissions pathways without climate assessment are not included in the ranges presented here. See WGIII Annex III.II.5.

'''6''' Cumulative emissions are calculated from the start of 2020 to the time of net zero and 2100, respectively. They are based on harmonised net CO 2 emissions, ensuring consistency with the WG I assessment of the remaining carbon budget. { ''WGIII Box 3.4, WGIII SPM Footnote 50'' } .

'''In scenarios with increasing CO''' '''2''' '''emissions, the land and ocean carbon sinks are projected to be less effective at slowing the accumulation of CO''' '''2''' '''in the atmosphere''' '''''(''''' '''''high confidence).''''' While natural land and ocean carbon sinks are projected to take up, in absolute terms, a progressively larger amount of CO 2 under higher compared to lower CO 2 emissions scenarios, they become less effective, that is, the proportion of emissions taken up by land and ocean decreases with increasing cumulative net CO 2 emissions ( ''high confidence'' ). Additional ecosystem responses to warming not yet fully included in climate models, such as GHG fluxes from wetlands, permafrost thaw, and wildfires, would further increase concentrations of these gases in the atmosphere ( ''high confidence'' ). In scenarios where CO 2 concentrations peak and decline during the 21st century, the land and ocean begin to take up less carbon in response to declining atmospheric CO 2 concentrations ( ''high confidence'' ) and turn into a weak net source by 2100 in the very low GHG emissions scenario. ( ''medium confidence'' ) '''[[#footnote-024|133]]''' . { ''WGI SPM B.4, WGI SPM B.4.1, WGI SPM B.4.2, WGI SPM B.4.3'' }

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