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

=== 3.3 Mitigation Pathways ===

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'''Limiting human-caused global warming requires net zero anthropogenic CO 2 emissions. Pathways consistent with 1.5°C and 2°C carbon budgets imply rapid, deep, and in most cases immediate GHG emission reductions in all sectors ( '''''high confidence''''' ) . Exceeding a warming level and returning (i.e. overshoot) implies increased risks and potential irreversible impacts; achieving and sustaining global net negative CO 2 emissions would reduce warming ( '''''high confidence''''' ) .'''
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==== 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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==== 3.3.2 Net Zero Emissions: Timing and Implications ====

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'''From a physical science perspective, limiting human-caused global warming to a specific level requires limiting cumulative CO''' '''2''' '''emissions, reaching net zero or net negative CO''' '''2''' '''emissions, along with strong reductions of other GHG emissions (see Cross-Section Box.1). Global modelled pathways that reach and sustain net zero GHG emissions are projected to result in a gradual decline in surface temperature (''' '''''high confidence).''''' Reaching net zero GHG emissions primarily requires deep reductions in CO 2 , methane, and other GHG emissions, and implies net negative CO 2 emissions. '''[[#footnote-023|134]]''' Carbon dioxide removal (CDR) will be necessary to achieve net negative CO 2 emissions '''[[#footnote-022|135]]''' . Achieving global net zero CO 2 emissions, with remaining anthropogenic CO 2 emissions balanced by durably stored CO 2 from anthropogenic removal, is a requirement to stabilise CO 2 -induced global surface temperature increase (see 3.3.3). ( ''high confidence'' ). This is different from achieving net zero GHG emissions, where metric-weighted anthropogenic GHG emissions (see Cross-Section Box.1) equal CO 2 removal ( ''high confidence'' ). Emissions pathways that reach and sustain net zero GHG emissions defined by the 100-year global warming potential imply net negative CO 2 emissions and are projected to result in a gradual decline in surface temperature after an earlier peak ( ''high confidence'' ). While reaching net zero CO 2 or net zero GHG emissions requires deep and rapid reductions in gross emissions, the deployment of CDR to counterbalance hard-to-abate residual emissions (e.g., some emissions from agriculture, aviation, shipping, and industrial processes) is unavoidable ( ''high confidence'' ). { ''WGI SPM D.1,'' . ''WGI SPM D.1.1, WGI SPM D.1.8; WGIII SPM C.2, WGIII SPM C.3, WGIII SPM C.11, WGIII Box TS.6; SR1.5 SPM A.2.2'' }

'''In modelled pathways, the timing of net zero CO '''2''' emissions, followed by net zero GHG emissions, depends on several variables, including the desired climate outcome, the mitigation strategy and the gases covered''' '''''(''''' '''''high confidence).''''' Global net zero CO 2 emissions are reached in the early 2050s in pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot, and around the early 2070s in pathways that limit warming to 2°C (&gt;67%). While non-CO 2 GHG emissions are strongly reduced in all pathways that limit warming to 2°C (&gt;67%) or lower, residual emissions of CH 4 and N 2 O and F-gases of about 8 [5–11] GtCO 2 -eq yr ''-1'' remain at the time of net zero GHG, counterbalanced by net negative CO 2 emissions. As a result, net zero CO 2 would be reached before net zero GHGs. ( ''high confidence'' ). { ''WGIII SPM C.2, WGIII SPM C.2.3, WGIII SPM C.2.4, WGIII Table SPM.2, WGIII 3.3'' } (Figure 3.6)

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'''Figure 3.6: Total GHG, CO''' '''2''' '''and CH''' '''4''' '''emissions and timing of reaching net zero in different''' '''mitigation''' '''pathways. Top row:''' GHG, CO 2 and CH 4 emissions over time (in GtCO 2 eq) with historical emissions, projected emissions in line with policies implemented until the end of 2020 (grey), and pathways consistent with temperature goals in colour (blue, purple, and brown, respectively). '''Panel (a) (left)''' shows pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot (C1) and '''Panel (b) (right)''' shows pathways that limit warming to 2°C (&gt;67%) (C3). '''Bottom row: Panel (c)''' shows median (vertical line), ''likely'' (bar) and ''very likely'' (thin lines) timing of reaching net zero GHG and CO 2 emissions for global modelled pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot (C1) (left) or 2°C (&gt;67%) (C3) (right). { ''WGIII Figure SPM.5'' }

[https://www.ipcc.ch/figures/figure-3-6 ]

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==== 3.3.3 Sectoral Contributions to Mitigation ====

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'''All global modelled pathways that limit warming to 2°C (&gt;67%)''' '''or lower by 2100 involve rapid and deep and in most cases immediate GHG emissions reductions in all sectors (see also 4. 1, 4.5).''' Reductions in GHG emissions in industry, transport, buildings, and urban areas can be achieved through a combination of energy efficiency and conservation and a transition to low-GHG technologies and energy carriers (see also 4.5, Figure 4.4). Socio-cultural options and behavioural change can reduce global GHG emissions of end-use sectors, with most of the potential in developed countries, if combined with improved infrastructure design and access. ( ''high confidence'' ). { ''WGIII SPM C.3, WGIII SPM C.5, WGIII SPM C.6, WGIII SPM C.7.3, WGIII SPM C.8, WGIII SPM C.10.2'' }

'''Global modelled mitigation pathways reaching net zero CO''' '''2''' '''and GHG emissions include transitioning from fossil fuels without carbon capture and storage (CCS) to very low- or zero-carbon energy sources, such as renewables or fossil fuels with CCS, demand-side measures and improving efficiency, reducing non-CO''' '''2''' '''GHG emissions, and CDR''' '''[[#footnote-021|136]]''' . In global modelled pathways that limit warming to 2°C or below, almost all electricity is supplied from zero or low-carbon sources in 2050, such as renewables or fossil fuels with CO 2 capture and storage, combined with increased electrification of energy demand. Such pathways meet energy service demand with relatively low energy use, through e.g., enhanced energy efficiency and behavioural changes and increased electrification of energy end use. Modelled global pathways limiting global warming to 1.5°C (&gt;50%) with no or limited overshoot generally implement such changes faster than pathways limiting global warming to 2°C (&gt;67%). ( ''high confidence'' ) { ''WGIII SPM C.3, WGIII SPM C.3.2, WGIII SPM C.4, WGIII TS.4.2; SR1.5 SPM C.2.2'' }

'''AFOLU mitigation options, when sustainably implemented, can deliver large-scale GHG emission reductions and enhanced CO''' '''2''' '''removal; however, barriers to implementation and trade-offs may result from the impacts of climate change, competing demands on land, conflicts with food security and livelihoods, the complexity of land ownership and management systems, and cultural aspects (see 3.4.1).''' All assessed modelled pathways that limit warming to 2°C (&gt;67%) or lower by 2100 include land-based mitigation and land-use change, with most including different combinations of reforestation, afforestation, reduced deforestation, and bioenergy. However, accumulated carbon in vegetation and soils is at risk from future loss (or sink reversal) triggered by climate change and disturbances such as flood, drought, fire, or pest outbreaks, or future poor management.. ( ''high confidence'' ). { ''WGI SPM B.4.3; WGII SPM B.2.3, WGII SPM B.5.4; WGIII SPM C.9, WGIII SPM C.11.3, WGIII SPM D.2.3, WGIII TS.4.2, 3.4; SR1.5 SPM C.2.5; SRCCL SPM B.1.4, SRCCL SPM B.3, SRCCL SPM B.7'' }

'''In addition to deep, rapid, and sustained emission reductions, CDR can fulfil three complementary roles: lowering net CO''' '''2''' '''or net GHG emissions in the near term; counterbalancing ‘ hard-to-abate’ residual emissions (e.g., some emissions from agriculture , aviation, shipping, industrial processes) to help reach net zero CO''' '''2''' '''or GHG emissions, and achieving net negative CO''' '''2''' '''or GHG emissions if deployed at levels exceeding annual residual emissions (''' '''''high confidence)''''' '''.''' CDR methods vary in terms of their maturity, removal process, time scale of carbon storage, storage medium, mitigation potential, cost, co-benefits, impacts and risks, and governance requirements. ( ''high confidence'' ). Specifically, maturity ranges from lower maturity (e.g., ocean alkalinisation) to higher maturity (e.g., reforestation); removal and storage potential ranges from lower potential (&lt;1 Gt CO 2 yr ''-1'' , e.g., blue carbon management) to higher potential (&gt;3 Gt CO 2 yr ''-1'' , e.g., agroforestry); costs range from lower cost (e.g., –45 to 100 USD tCO 2 ''-1'' for soil carbon sequestration) to higher cost (e.g., 100 to 300 USD tCO 2 ''-1'' for direct air carbon dioxide capture and storage) ( ''medium confidence'' ). Estimated storage timescales vary from decades to centuries for methods that store carbon in vegetation and through soil carbon management, to ten thousand years or more for methods that store carbon in geological formations. ( ''high confidence'' ). Afforestation, reforestation, improved forest management, agroforestry and soil carbon sequestration are currently the only widely practiced CDR methods ( ''high confidence'' ). Methods and levels of CDR deployment in global modelled mitigation pathways vary depending on assumptions about costs, availability and constraints ( ''high confidence'' ). { ''WGIII SPM C.3.5, WGIII SPM C.11.1, WGIII SPM C.11.4'' }

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==== 3.3.4 Overshoot Pathways: Increased Risks and Other Implications ====

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'''Exceeding a specific remaining carbon budget results in higher global warming. Achieving and sustaining net negative global CO''' '''2''' '''emissions could reverse the resulting temperature exceedance (''' '''''high confidence)''''' '''''.''''' Continued reductions in emissions of short-lived climate forcers, particularly methane, after peak temperature has been reached, would also further reduce warming ( ''high confidence'' ). Only a small number of the most ambitious global modelled pathways limit global warming to 1.5°C (&gt;50%) without overshoot. { ''WGI SPM D.1.1, WGI SPM D.1.6, WGI SPM D.1.7; WGIII TS.4.2'' }

Overshoot of a warming level results in more adverse impacts, some irreversible, and additional risks for human and natural systems compared to staying below that warming level, with risks growing with the magnitude and duration of overshoot ( ''high confidence'' ). Compared to pathways without overshoot, societies and ecosystems would be exposed to greater and more widespread changes in climatic impact-drivers, such as extreme heat and extreme precipitation, with increasing risks to infrastructure, low-lying coastal settlements, and associated livelihoods ( ''high confidence'' ). Overshooting 1.5°C will result in irreversible adverse impacts on certain ecosystems with low resilience, such as polar, mountain, and coastal ecosystems, impacted by ice-sheet melt, glacier melt, or by accelerating and higher committed sea level rise ( ''high confidence'' ). Overshoot increases the risks of severe impacts, such as increased wildfires, mass mortality of trees, drying of peatlands, thawing of permafrost and weakening natural land carbon sinks; such impacts could increase releases of GHGs making temperature reversal more challenging ( ''medium confidence'' ). { ''WGI SPM C.2, WGI SPM C.2.1, WGI SPM C.2.3; WGII SPM B.6, WGII SPM B.6.1, WGII SPM B.6.2; SR1.5 3.6'' }

The larger the overshoot, the more net negative CO 2 emissions needed to return to a given warming level ( ''high confidence'' ). Reducing global temperature by removing CO 2 would require net negative emissions of 220 GtCO 2 (best estimate, with a ''likely'' range of 160 to 370 GtCO 2 ) for every tenth of a degree ( ''medium confidence'' ). Modelled pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot reach median values of cumulative net negative emissions of 220 GtCO 2 by 2100, pathways that return warming to 1.5°C (&gt;50%) after high overshoot reach median values of 360 GtCO 2 ( ''high confidence'' ). '''[[#footnote-020|137]]''' More rapid reduction in CO 2 and non-CO 2 emissions, particularly methane, limits peak warming levels and reduces the requirement for net negative CO 2 emissions and CDR, thereby reducing feasibility and sustainability concerns, and social and environmental risks ( ''high confidence'' ). { ''WGI SPM D.1.1; WGIII SPM B.6.4, WGIII SPM C.2, WGIII SPM C.2.2, WGIII Table SPM.2'' }

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