Editing IPCC:AR6/WGIII/Chapter-3 (section)

== 3.5 Interaction Between Near-, Medium- and Long-term Action in Mitigation Pathways ==

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This section assesses the relationship between long-term climate goals and short- to medium-term emissions reduction strategies based on the mitigation pathway literature. After an overview of this relationship ( [[#3.5.1|Section 3.5.1]] ), it provides an assessment of what currently planned near-term action implies for limiting warming to 1.5°C–2°C ( [[#3.5.2|Section 3.5.2]] ), and to what extent pathways with accelerated action beyond current NDCs can improve the ability to keep long-term targets in reach ( [[#3.5.3|Section 3.5.3]] ).

The assessment in this section shows that if mitigation ambitions in NDCs announced prior to COP26 2 , [[#footnote-002|18]] are followed until 2030, leading to estimated emissions of 47–57 GtCO 2 -eq in 2030 [[#footnote-001|19]] (Section 4.2.2), it is no longer possible to limit warming to 1.5°C (&gt;50%) with no or limited overshoot ( ''high confidence'' ). Instead, it would entail high overshoot (typically &gt;0.1°C) and reliance on net negative CO 2 emissions with uncertain potential to return warming to 1.5°C (&gt;50%) by the end of the century. It would also strongly increase mitigation challenges to limit warming to 2°C (&gt;67%) ( ''high confidence'' ). GHG emissions reductions would need to abruptly increase after 2030 to an annual average rate of 1.4–2.0 GtCO 2 -eq during the period 2030–2050, around 70% higher than in mitigation pathways assuming immediate action 1 to limit warming to 2°C (&gt;67%). The higher post-2030 reduction rates would have to be obtained in an environment of continued buildup of fossil fuel infrastructure and less development of low-carbon alternatives until 2030. A lock-in to fossil fuel-intensive production systems (carbon lock-in) will increase the societal, economic and political strain of a rapid low-carbon transition after 2030 ( ''hig'' ''h confidence'' ).

The section builds on previous assessments in the IPCC’s ''Fifth Assessment Report'' (Clarke et al. 2014) and the ''IPCC Special Report on 1.5°C Warming'' ( [[#Rogelj--2018|Rogelj et al. 2018]] a). The literature assessed in these two reports has focused on delayed action until 2030 in the context of limiting warming to 2°C ( [[#den%20Elzen--2010|den Elzen et al. 2010]] ; [[#van%20Vuuren--2011|van Vuuren and Riahi 2011]] ; [[#Luderer--2013|Luderer et al. 2013]] , 2016; [[#Rogelj--2013a|Rogelj et al. 2013a]] ; [[#Kriegler--2015|Kriegler et al. 2015]] ; [[#Riahi--2015|Riahi et al. 2015]] ) and 1.5°C ( [[#Rogelj--2013b|Rogelj et al. 2013b]] ; [[#Luderer--2018|Luderer et al. 2018]] ; [[#Strefler--2018|Strefler et al. 2018]] ). Here we provide an update of these assessments drawing on the most recent literature on global mitigation pathways. New studies have focused, ''inter alia'' , on constraining near-term developments by peak warming limits ( [[#Rogelj--2019b|Rogelj et al. 2019b]] ; [[#Riahi--2021|Riahi et al. 2021]] ; [[#Strefler--2021b|Strefler et al. 2021b]] ) and updating assumptions about near- andmedium-term emissions developments based on national plans and long-term strategies ( [[#Roelfsema--2020|Roelfsema et al. 2020]] ) (Section 4.2). Several studies have explored new types of pathways with accelerated action bridging between current policy plans and the goal of limiting warming below 2°C ( [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#van%20Soest--2021a|van Soest et al. 2021a]] ) and looked at hybrid international policy regimes to phase in global collective action ( [[#Bauer--2020|Bauer et al. 2020]] ).

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<span id="between-long-term-climate-goals-and-near--to-medium-term-emissions-reductions"></span>
=== 3.5.1 Between Long-term Climate Goals and Near- to Medium-term Emissions Reductions ===

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The close link between cumulative CO 2 emissions and warming has strong implications for the relationship between near-, medium-, and long-term climate action to limit global warming. The AR6 WGI Assessment has estimated a remaining carbon budget of 500 (400) GtCO 2 from the beginning of 2020 onwards for staying below 1.5°C with 50% (67%) likelihood, subject to additional uncertainties about historic warming and the climate response, and variations in warming from non-CO 2 climate forcers ( [[#Canadell--2019|Canadell and Monteiro 2019]] ) (AR6 WGI Chapter 5, [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] ). For comparison, if current CO 2 emissions of more than 40 GtCO 2 are keeping up until 2030, more than 400 GtCO 2 will be emitted during 2021–2030, already exhausting the remaining carbon budget for 1.5°C by 2030.

The relationship between warming limits and near-term action is illustrated in Figure 3.29, using a set of 1.5°C–2°C scenarios with different levels of near-term action, overshoot and non-CO 2 warming contribution from a recent study ( [[#Riahi--2021|Riahi et al. 2021]] ). In general, the more CO 2 is emitted until 2030, the less CO 2 can be emitted thereafter to stay within a remaining carbon budget and below a warming limit. Scenarios with immediate action to observe the warming limit give the longest time to exhaust the associated remaining carbon budget and reach net zero CO 2 emissions (see light blue lines in Figure 3.29 and Cross-Chapter Box 3 in this chapter). In comparison, following projected NDC emissions until 2030 would imply a more pronounced drop in emissions from 2030 levels to net zero to make up for the additional near-term emissions (see orange lines in Figure 3.29). If such a drop does not occur, the remaining carbon budget is exceeded and net negative CO 2 emissions are required to return global mean temperature below the warming limit (see black lines in Figure 3.29) (Clarke et al. 2014; [[#Fuss--2014|Fuss et al. 2014]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a).

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[[File:17ae025f991eef9c87fc8471f037e387 IPCC_AR6_WGIII_Figure_3_29.png]]

'''Figure 3.29''' '''| Illustration of emissions and climate response in four mitigation pathways with different assumptions about near-term policy developments, global warming limit and non-CO''' 2 '''warming contribution drawn from Riahi''' '''et al.''' '''(2021).''' Shown are '''(a)''' CO 2 emissions trajectories, '''(b)''' cumulative CO 2 emissions, '''(c)''' effective non-CO 2 radiative forcing, and '''(d)''' the resulting estimate of the 67th percentile of global mean temperature response relative to 1850–1900. Light blue lines show a scenario that acts immediately on a remaining carbon budget of 900 GtCO 2 from 2020 without allowing net negative CO 2 emissions (i.e., temporary budget overshoot) (COFFEE 1.1, Scenario EN_NPi2020_900). Orange and black lines show scenarios drawn from the same model that follow the NDCs until 2030 and thereafter introduce action to stay within the same budget – in one case excluding net negative CO 2 emissions like before (orange lines; COFFEE 1.1., Scenario EN-INDCi2030_900) and in the other allowing for a temporary overshoot of the carbon budget until 2100 (black lines; COFFEE 1.1., Scenario EN-INDCi2030_900f). Light blue lines describe a scenario following the NDCs until 2030, and then aiming for a higher budget of 2300 GtCO 2 without overshoot (AIM/CGE 2.2, Scenario EN-INDCi2030_1200). It is drawn from another model which projects a lower anthropogenic non-CO 2 forcing contribution and therefore achieves about the same temperature outcome as the other two non-overshoot scenarios despite the higher CO 2 budget. Grey funnels include the trajectories from all scenarios that limit warming to 2°C (&gt;67%) (category C3). Historical CO 2 emissions until 2019 are from Chapter SM.2.1 EDGAR v6.0.

The relationship between warming limits and near-term action is also affected by the warming contribution of non-CO 2 greenhouse gases and other short-lived climate forcers ( [[#3.3|Section 3.3]] ; AR6 WGI [[IPCC:Wg3:Chapter:Chapter-6#6.7|Section 6.7]] ). The estimated budget values for limiting warming to 1.5°C–2°C already assume stringent reductions in non-CO 2 greenhouse gases and non-CO 2 climate forcing as found in 1.5°C–2°C pathways ( [[#3.3|Section 3.3]] and Cross-Working Group Box 1 in this chapter; AR6 WGI [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] and Box 5.2 in Chapter 5). Further variations in non-CO 2 warming observed across 1.5°C–2°C pathways can vary the median estimate for the remaining carbon budget by 220 GtCO 2 (AR6 WGI [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] ). In 1.5°C–2°C pathways, the non-CO 2 warming contribution differs strongly between the near, medium and long term. Changes to the atmospheric composition of short-lived climate forcers (SLCFs) dominate the warming response in the near term (AR6 WGI [[IPCC:Wg3:Chapter:Chapter-6#6.7|Section 6.7]] ). CO 2 reductions are combined with strong reductions in air pollutant emissions due to rapid reduction in fossil fuel combustion and in some cases the assumption of stringent air quality policies ( [[#Rao--2017b|Rao et al. 2017b]] ; [[#Smith--2020c|Smith et al. 2020c]] ). As air pollutants exert a net-cooling effect, their reduction drives up non-CO 2 warming in the near term, which can be attenuated by the simultaneous reduction of methane and black carbon ( [[#Shindell--2019|Shindell and Smith 2019]] ; [[#Smith--2020b|Smith et al. 2020b]] ) (AR6 WGI [[IPCC:Wg3:Chapter:Chapter-6#6.7|Section 6.7]] ). After 2030, the reduction in methane concentrations and associated reductions in tropospheric ozone levels tend to dominate so that a peak and decline in non-CO 2 forcing and non-CO 2 -induced warming can occur before net zero CO 2 is reached (Figure 3.29) ( [[#Rogelj--2018|Rogelj et al. 2018]] a). The more stringent the reductions in methane and other short-lived warming agents such as black carbon, the lower this peak and the earlier the decline of non-CO 2 warming, leading to a reduction of warming rates and overall warming in the near to medium term ( [[#Harmsen--2020|Harmsen et al. 2020]] ; [[#Smith--2020b|Smith et al. 2020b]] ). This is important for keeping warming below a tight warming limit that is already reached around mid-century as is the case in 1.5°C pathways ( [[#Xu--2017|Xu and Ramanathan 2017]] ). Early and deep reductions of methane emissions, and other short-lived warming agents such as black carbon, provide space for residual CO 2 -induced warming until the point of net zero CO 2 emissions is reached (see purple lines in Figure 3.29). Such emissions reductions have also been advocated due to co-benefits for, for example, reducing air pollution ( [[#Rao--2016|Rao et al. 2016]] ; [[#Shindell--2017a|Shindell et al. 2017a]] , 2018; [[#Shindell--2019|Shindell and Smith 2019]] ; [[#Rauner--2020a|Rauner et al. 2020a]] ; [[#Vandyck--2020|Vandyck et al. 2020]] ).

The relationship between long-term climate goals and near-term action is further constrained by social, technological, economic and political factors ( [[#Cherp--2018|Cherp et al. 2018]] ; [[#van%20Sluisveld--2018b|van Sluisveld et al. 2018b]] ; Aghion et al. 2019; [[#Mercure--2019|Mercure et al. 2019]] ; [[#Trutnevyte--2019b|Trutnevyte et al. 2019b]] ; [[#Jewell--2020|Jewell and Cherp 2020]] ). These factors influence path dependency and transition speed ( [[#Pahle--2018|Pahle et al. 2018]] ; [[#Vogt-Schilb--2018|Vogt-Schilb et al. 2018]] ). While detailed integrated assessment modelling of global mitigation pathways accounts for technology inertia ( [[#Bertram--2015a|Bertram et al. 2015a]] ; [[#Mercure--2018|Mercure et al. 2018]] ) and technology innovation and diffusion ( [[#Wilson--2013|Wilson et al. 2013]] ; [[#van%20Sluisveld--2018a|van Sluisveld et al. 2018a]] ; Luderer et al. 2021), there are limitations in capturing socio-technical and political drivers of innovation, diffusion and transition processes ( [[#Gambhir--2019|Gambhir et al. 2019]] ; [[#Köhler--2019|Köhler et al. 2019]] ; [[#Hirt--2020|Hirt et al. 2020]] ; [[#Keppo--2021|Keppo et al. 2021]] ). Mitigation pathways show a wide range of transition speeds that have been interrogated in the context of socio-technical inertia ( [[#Gambhir--2017|Gambhir et al. 2017]] ; [[#Kefford--2018|Kefford et al. 2018]] ; [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#Brutschin--2021|Brutschin et al. 2021]] ) vs accelerating technological change and self-enforcing socio-economic developments ( [[#Creutzig--2017|Creutzig et al. 2017]] ; [[#Zenghelis--2019|Zenghelis 2019]] ) ( [[#3.8|Section 3.8]] ). Diagnostic analysis of detailed IAMs found a lag of 8–20 years between the convergence of emissions pricing and the convergence of emissions response after a period of differentiated emission prices ( [[#Harmsen--2021|Harmsen et al. 2021]] ). This provides a measure of the inertia to changing policy signals in the model response. It is about half the time scale of 20–40 years observed for major energy transitions ( [[#Grubb--2021|Grubb et al. 2021]] ). Hence, the mitigation pathways assessed here capture socio-technical inertia in reducing emissions, but the limited modelling of socio-political factors may alter the extent and persistence of this inertia.

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<span id="implications-of-near-term-emission-levels-for-keeping-long-term-climate-goals-within-reach"></span>
=== 3.5.2 Implications of Near-term Emission Levels for Keeping Long-term Climate Goals Within Reach ===

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The implications of near-term climate action for long-term climate outcomes can be explored by comparing mitigation pathways with different near-term emissions developments aiming for the same climate target ( [[#Riahi--2015|Riahi et al. 2015]] ; [[#Vrontisi--2018|Vrontisi et al. 2018]] ; [[#Roelfsema--2020|Roelfsema et al. 2020]] ). A particular example is the comparison of cost-effective pathways with immediate action to limit warming to 1.5°C–2°C with mitigation pathways pursuing more moderate mitigation action until 2030. After the adoption of the Paris Agreement, near-term action was often modelled to reflect conditional and unconditional elements of originally submitted NDCs (2015–2019) ( [[#Fawcett--2015|Fawcett et al. 2015]] ; [[#Fujimori--2016a|Fujimori et al. 2016a]] ; [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#Vrontisi--2018|Vrontisi et al. 2018]] ; [[#Roelfsema--2020|Roelfsema et al. 2020]] ). The most recent modelling studies also include submission of updated NDCs or announcements of planned updates in the first half of 2021 ( [[#Network%20for%20Greening%20the%20Financial%20System--2021|Network for Greening the Financial System 2021]] ; [[#Riahi--2021|Riahi et al. 2021]] ). Emissions levels under NDCs announced prior to COP26 are assessed to range between 47–57 GtCO 2 -eq in 2030 (Section 4.2.2). This assessed range corresponds well to 2030 emissions levels in 2°C mitigation pathways in the literature that are designed to follow the original or updated NDCs until 2030. [[#footnote-000|20]] For the 139 scenarios of this kind that are collected in the AR6 scenario database and that still limit warming to 2°C (&gt;67%), the 2030 emissions range is 53 (45–58) GtCO 2 -eq (based on native model reporting) and 52.5 (47–56.5) GtCO 2 -eq, respectively (based on harmonised emissions data for climate assessment (Annex III.2.5.1); median and 5–95th percentile). This close match allows a robust assessment of the implications of implementing NDCs announced prior to COP26 for post-2030 mitigation efforts and warming outcomes based on the literature and the AR6 scenarios database.

Without a strengthening of policies beyond those that are implemented by the end of 2020, GHG emissions are projected to rise beyond 2025, leading to a median global warming of 3.2 [2.2 to 3.5] °C by 2100. Modelled pathways that are consistent with NDCs announced prior to COP26 until 2030 and assume no increase in ambition thereafter have lower emissions, leading to a median global warming of 2.8°C [2.1–3.4°C] by 2100.

The assessed emission ranges from implementing the unconditional (unconditional and conditional) elements of NDCs announced prior to COP26 implies an emissions gap to cost-effective mitigation pathways of 19–26 (16–23) GtCO 2 -eq in 2030 for limiting warming to 1.5°C (&gt;50%) with no or limited overshoot and 10–16 (6–14) GtCO 2 -eq in 2030 for limiting warming to 2°C (&gt;67%) (Cross-Chapter Box 4 in Chapter 4). The emissions gap gives rise to a number of mitigation challenges ( [[#Kriegler--2013a|Kriegler et al. 2013a]] , 2018a,b; [[#Luderer--2013|Luderer et al. 2013]] , 2018; [[#Rogelj--2013a|Rogelj et al. 2013a]] ; [[#Fawcett--2015|Fawcett et al. 2015]] ; [[#Riahi--2015|Riahi et al. 2015]] ; [[#Fujimori--2016b|Fujimori et al. 2016b]] ; [[#Strefler--2018|Strefler et al. 2018]] ; [[#Winning--2019|Winning et al. 2019]] ; [[#SEI--2020|SEI et al. 2020]] ; [[#UNEP--2020|UNEP 2020]] ): (i) larger transitional challenges post-2030 to still remain under the warming limit, in particular higher CO 2 emissions reduction rates and technology transition rates required during 2030–2050; (ii) larger lock-in into carbon-intensive infrastructure and increased risk of stranded fossil fuel assets ( [[#3.5.2.2|Section 3.5.2.2]] ); and (iii) larger reliance on CDR to reach net zero CO 2 more rapidly and compensate excess emissions in the second half of the century ( [[#3.5.2.1|Section 3.5.2.1]] ). All these factors exacerbate the socio-economic strain of implementing the transition, leading to an increased risk of overshooting the warming and a higher risk of climate change impacts ( [[#Drouet--2021|Drouet et al. 2021]] ).

The challenges are illustrated in Table 3.6 and Figure 3.30, surveying global mitigation pathways in the literature that were collected in the AR6 scenarios database. There is a clear trend of increasing peak warming with increasing 2030 GHG emission levels (Figure 3.30a,b). In particular, there is no mitigation pathway designed to follow the NDCs until 2030 that can limit warming to 1.5°C (&gt;50%) with no or limited overshoot. Our assessment confirms the finding of the ''IPCC'' ''Special Report on Global Warming of 1.5°C'' ( [[#Rogelj--2018|Rogelj et al. 2018]] ) for the case of NDCs announced prior to COP26 that pathways following the NDCs until 2030 ‘would not limit global warming to 1.5°C, even if supplemented by very challenging increases in the scale and ambition of emissions reductions after 2030’ (SR1.5 SPM). This assessment is now more robust than in SR1.5 as it is based on a larger set of 1.5°C–2°C pathways with better representation of current trends and plans covering a wider range of post-2030 emissions developments. In particular, a recent multi-model study limiting peak cumulative CO 2 emissions for a wide range of carbon budgets and immediate vs NDC-type action until 2030 established a feasibility frontier for the existence of such pathways across participating models ( [[#Riahi--2021|Riahi et al. 2021]] ).

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[[File:eaf007017fb9b2b2c4481e0d9d23c3b4 IPCC_AR6_WGIII_Figure_3_30.png]]

'''Figure 3.30 | Relationship between level of global GHG emissions in 2030 and selected indicators as listed in the panel titles for scenarios collected in the AR6 scenario database.''' Emissions data based on harmonised emissions used for the climate assessment. All scenarios that limit warming to 2°C (&gt;67%) or lower are coloured blue or red (see p67 peak warming in panel (b)). The large majority of blue-coloured scenarios act immediately on the temperature target, while red-coloured scenarios depict all those that were designed to follow the NDCs or lesser action until 2030 and orange-coloured scenarios comprise a small set of pathways with additional regulatory action beyond NDCs ( [[#3.5.3|Section 3.5.3]] ). Grey-coloured scenarios do not limit warming to 2°C (&gt;67%) due to temporary overshoot or towards the end of the century. Large markers denote the five Illustrative Mitigation Pathways (IMPs) (legend in Panel (h); [[#3.2|Section 3.2]] ). Shaded yellow areas depict the estimated range of 2030 emissions from NDCs announced prior to COP26 (Section 4.2.2). Dotted lines are inserted in some panels to highlight trends in the dependency of selected output variables on 2030 GHG emissions levels ( [[#3.5.2|Section 3.5.2]] ).

'''Table 3.6 | Comparison of key scenario characteristics for five scenario classes (see Table 3.''' '''2): (i) immediate action to limit warming to 1.5°C (&gt;50%) with no or limited overshoot, (ii) near team action following the NDCs until 2030 and returning warming to below 1.5°C (&gt;50%) by 2100 after a high overshoot, (iii) immediate action to limit warming to 2°C (&gt;67%), (iv) near term action following the NDCs until 2030 followed by post-2030 action to limit warming to 2°C (&gt;67%).''' Also shown are the characteristics for (v) the combined class of all scenarios that limit warming to 2°C (&gt;67%). The classes (ii) and (iv) comprise the large majority of scenarios indicated by red dots, and the classes (i) and (iii) comprise the scenarios depicted by blue dots in Figure 3.30. Shown are median and interquartile ranges (in brackets) for selected global indicators. Emissions ranges are based on harmonized emissions data for the climate assessment with the exception of land use CO 2 emissions for which uncertainty in historic estimates is large. Numbers are rounded to the nearest 5, with the exception of cumulative CCS, BECCS, and net negative CO 2 emissions rounded to the nearest 10.

{| class="wikitable"
|-
| rowspan="2"| '''Global indicators'''

| 1.5°C

| 1.5°C (&gt;50%) by 2100

| colspan="3"| 2°C (&gt;67%)

|-
| Immediate action, with no or limited overshoot (C1, 97 scenarios)

| '''NDCs until 2030, with overshoot before 2100 (subset of 42 scenarios in C2)'''

| '''Immediate action (C3a, 204 scenarios)'''

| NDCs until 2030 (C3b; 97 scenarios)

| All (C3; 311 scenarios)

|-
| Change in GHG emissions in 2030 (% rel to 2019)

| –45 (–50,–40)

| –5 (–5,0)

| –25 (–35,–20)

| –5 (–10,0)

| –20 (–30,–10)

|-
| in 2050 (% rel to 2019)

| –85 (–90,–80)

| –75 (–85,–70)

| –65 (–70,–60)

| –70 (–70,–60)

| –65 (–70,–60)

|-
| Change in CO 2 emissions in 2030 (% rel to 2019)

| –50 (–60,–40)

| –5 (–5,0)

| –25 (–35,–20)

| –5 (–5,0)

| –20 (–30,–5)

|-
| in 2050 (% rel to 2019)

| –100 (–105,–95)

| –85 (–95,–80)

| –70 (–80,–65)

| –75 (–80,–65)

| –75 (–80,–65)

|-
| Change in net land use CO 2 emissions in 2030 (% rel to 2019)

| –100 (–105,–95)

| –30 (–60,–20)

| –90 (–105,–75)

| –20 (–80,–20)

| –80 (–100,–30)

|-
| in 2050 (% rel to 2019)

| –150 (–200,–100)

| –135 (–165,–120)

| –135 (–185,–100)

| –130 (–145,–115)

| –135 (–180,–100)

|-
| Change in CH 4 emissions in 2030 (% rel to 2019)

| –35 (–40,–30)

| –5 (–5,0)

| –25 (–35,–20)

| –10 (–15,–5)

| –20 (–25,–10)

|-
| in 2050 (% rel to 2019)

| –50 (–60,–45)

| –50 (–60,–45)

| –45 (–50,–40)

| –50 (–65,–45)

| –45 (–55,–40)

|-
| Cumulative CCS until 2100 (GtCO 2 )

| 670 (520,900)

| 670 (540,860)

| 610 (490,900)

| 530 (440,720)

| 590 (480,820)

|-
| of which BECCS (GtCO 2 )

| 330 (250,560)

| 370 (280,590)

| 350 (240,450)

| 270 (240,400)

| 290 (240,430)

|-
| Cumulative net negative CO 2 emissions until 2100 (GtCO 2 )

| ''220 (70,430)''

| ''380 (300,470)''

| ''30 (0,130)''

| ''60 (20,210)''

| ''40 (10, 180)''

|-
| Change in primary energy from coal in 2030 (% rel to 2019)

| –75 (–80,–65)

| –10 (–20,–5)

| –50 (–65,–35)

| –15 (–20,–10)

| –35 (–55,–20)

|-
| in 2050 (% rel to 2019)

| –95 (–100,–80)

| –90 (–100,–85)

| –85 (–100,–65)

| –80 (–90,–70)

| –85 (–95,–65)

|-
| Change in primary energy from coal without CCS in 2030 (% rel to 2019)

| –75 (–80,–65)

| –10 (–20,–10)

| –50 (–65,–35)

| –15 (–20,–10)

| –35 (–55,–20)

|-
| in 2050 (% rel to 2019)

| –100 (–100,–95)

| –95 (–100,–95)

| –95 (–100,–90)

| –90 (–95,–85)

| –95 (–100,–90)

|-
| Change in primary energy from oil in 2030 (% rel to 2019)

| –10 (–25,0)

| 5 (5,10)

| 0 (–10,10)

| 10 (5,10)

| 5 (0,10)

|-
| in 2050 (% rel to 2019)

| –60 (–75,–40)

| –50 (–65,–35)

| –30 (–45,–15)

| –40 (–55,–20)

| –30 (–50,–15)

|-
| Change in primary energy from oil without CCS in 2030 (% rel to 2019)

| –5 (–20,0)

| 5 (5,10)

| 0 (–10,10)

| 10 (5,10)

| 5 (–5,10)

|-
| in 2050 (% rel to 2019)

| –60 (–75,–45)

| –50 (–65,–30)

| –30 (–45,–15)

| –40 (–55,–20)

| –35 (–50,–15)

|-
| Change in primary energy from gas in 2030 (% rel to 2019)

| –10 (–30,0)

| 15 (10,25)

| 10 (0,15)

| 15 (10,15)

| 10 (0,15)

|-
| in 2050 (% rel to 2019)

| –45 (–60,–20)

| –45 (–55,–30)

| –10 (–35,15)

| –30 (–45,–5)

| –15 (–40,10)

|-
| Change in primary energy from gas without CCS in 2030 (% rel to 2019)

| –20 (–30,–5)

| 15 (10,25)

| 5 (–5,10)

| 15 (10,15)

| 10 (0,15)

|-
| in 2050 (% rel to 2019)

| –70 (–80,–60)

| –60 (–70,–50)

| –35 (–50,–20)

| –40 (–60,–35)

| –40 (–55,–20)

|-
| Change in primary energy from nuclear in 2030 (% rel to 2019)

| 40 (10,70)

| 10 (0,25)

| 35 (5,50)

| 10 (0,30)

| 25 (0,45)

|-
| in 2050 (% rel to 2019)

| 90 (15,295)

| 100 (45,130)

| 85 (30,200)

| 75 (30,120)

| 80 (30,140)

|-
| Change in primary energy from modern biomass in 2030 (% rel to 2019)

| 75 (55,130)

| 45 (20,75)

| 60 (35,105)

| 45 (20,80)

| 55 (35,105)

|-
| in 2050 (% rel to 2019)

| 290 (215,430)

| 230 (170,420)

| 240 (130,355)

| 260 (95,435)

| 250 (115,405)

|-
| Change in primary energy from non–biomass renewables

in 2030 (% rel to 2019)

| 225 (155,270)

| 100 (85,145)

| 150 (115,190)

| 115 (85,130)

| 130 (90,170)

|-
| in 2050 (% rel to 2019)

| 725 (545,950)

| 665 (535,925)

| 565 (415,765)

| 625 (545,700)

| 605 (470,735)

|-
| Change in carbon intensity of electricity in 2030 (% rel to 2019)

| –75 (–80,–70)

| –30 (–40,–30)

| –60 (–70,–50)

| –35 (–40,–30)

| –50 (–65,–35)

|-
| in 2050 (% rel to 2019)

| –100 (–100,–100)

| –100 (–100,–100)

| –95 (–100,–95)

| –100 (–100,–95)

| –95 (–100,–95)

|-
| Change in carbon intensity of non–electric final energy consumption in 2030 (% rel to 2019)

| –15 (–15,–10)

| 0 (–5,0)

| –10 (–10,–5)

| 0 (–5,5)

| –5 (–10,0)

|-
| in 2050 (% rel to 2019)

| –50 (–55,–40)

| –35 (–40,–30)

| –30 (–35,–25)

| –30 (–40,–20)

| –30 (–35,–20)

|}

The 2030 emissions levels in the NDCS announced prior to COP26 also tighten the remaining space to limit warming to 2°C (&gt;67%). As shown in Figure 3.30b, the 67th percentile of peak warming reaches values above 1.7°C in pathways with 2030 emissions levels in this range. To still limit warming to 2°C (&gt;67%), the global post-2030 GHG emission reduction rates would need to be abruptly raised in 2030 from 0–0.7 GtCO 2 -eq yr –1 to an average of 1.4–2.0 GtCO 2 -eq yr –1 during the period 2030–2050 (Figure 3.30c), around 70% higher than in immediate mitigation pathways confirming findings in the literature ( [[#Winning--2019|Winning et al. 2019]] ). Their average reduction rate of 0.6–1.4 GtCO 2 yr –1 would already be unprecedented at the global scale and, with a few exceptions, national scale for an extended period of time ( [[#Riahi--2015|Riahi et al. 2015]] ). For comparison, the impact of COVID-19 on the global economy is projected to have lead to a decline of around 2.5–3 GtCO 2 of global CO 2 emissions from fossil fuels and industry in 2020 ( [[#Friedlingstein--2020|Friedlingstein et al. 2020]] ) ( [[IPCC:Wg3:Chapter:Chapter-2#2.2|Section 2.2]] ).

The increased post-2030 transition challenge in mitigation pathways with moderate near-term action is also reflected in the timing of reaching net zero CO 2 emissions (Figure 3.30d and Cross-Chapter Box 3 in this chapter). As 2030 emission levels and the cumulated CO 2 emissions until 2030 increase, the remaining time for dropping to net zero CO 2 and staying within the remaining carbon budget shortens (Figure 3.29). This gives rise to an inverted v-shape of the lower bound on the year of reaching net zero as a function of 2030 emissions levels. Reaching low emissions in 2030 facilitates reaching net zero early (left leg of the inverted v), but staying high until 2030 also requires reaching net zero CO 2 faster to compensate for higher emissions early on (right leg of the inverted v). Overall, there is a considerable spread of the timing of net zero CO 2 for any 2030 emissions level due to variation in the timing of spending the remaining carbon budget and the non-CO 2 warming contribution (Cross-Chapter Box 3 in this chapter).

There is also a profound impact on the underlying transition of energy and land use (Figure 3.30f–h and Table 3.6). Scenarios following NDCs until 2030 show a much smaller reduction in fossil fuel use, a slower growth in renewable energy use, and a smaller reduction in CO 2 and CH 4 land-use emissions in 2030 compared to immediate action scenarios. This is then followed by a much faster reduction of land-use emissions and fossil fuels, and a larger increase of nuclear energy, bioenergy and non-biomass renewable energy during the medium term in order to get close to the levels of the immediate action pathways in 2050. This is combined with a larger amount of net negative CO 2 emissions that are used to compensate the additional emissions before 2030. The faster transition during 2030–2050 is taking place from a greater investment in fossil fuel infrastructure and lower deployment of low-carbon alternatives in 2030, adding to the socio-economic challenges to realise the higher transition rates ( [[#3.5.2.2|Section 3.5.2.2]] ). Therefore, these pathways also show higher mitigation costs, particularly during the period 2030–2050, than immediate action scenarios ( [[#3.6.1|Section 3.6.1]] and Figure 3.34d) ( [[#Liu--2016|Liu et al. 2016]] ; [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#Vrontisi--2018|Vrontisi et al. 2018]] ). Given these circumstances and the fact the modelling of socio-political and institutional constraints is limited in Integrated Assessment Models (IAMs) ( [[#Gambhir--2019|Gambhir et al. 2019]] ; [[#Köhler--2019|Köhler et al. 2019]] ; [[#Hirt--2020|Hirt et al. 2020]] ; [[#Keppo--2021|Keppo et al. 2021]] ), the feasibility of realising these scenarios is assessed to be lower ( [[#Gambhir--2017|Gambhir et al. 2017]] ; [[#Napp--2017|Napp et al. 2017]] ; [[#Brutschin--2021|Brutschin et al. 2021]] ) (cf. [[#3.8|Section 3.8]] ), increasing the risk of an overshoot of climate goals.

<div id="_idContainer091" class="_idGenObjectStyleOverride-1"></div>

[[File:9795f8611b332f4ac06aeae409ba4fa5 IPCC_AR6_WGIII_Figure_3_34.png]]

'''Figure 3.34 |''' '''(a) Mean annual global consumption growth rate over 2020–2100 for the mitigation pathways in the AR6 scenarios database.''' '''(b)''' Global GDP loss compared to baselines (not accounting for climate change damages) in 2030, 2050 and 2100 for mitigation pathways with immediate global action. '''(c)''' Total discounted consumption loss (with a 3% discount rate) in mitigation scenarios with respect to their corresponding baseline (not accounting for climate change damages) as a function of cumulative CO 2 emissions until date of net zero CO 2 . '''(d)''' Comparison of GDP losses compared to baselines (not accounting for climate change damages) in 2030, 2050 and 2100 for pairs of scenarios depicting immediate action pathways and delayed action pathways. Source: AR6 Scenarios Database.

<div id="3.5.2.1" class="h3-container"></div>

<span id="overshoot-and-net-negative-co-2-emissions"></span>
==== 3.5.2.1 Overshoot and Net Negative CO 2 Emissions ====

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If near- to medium-term emissions developments deplete the remaining carbon budget, the associated warming limit will be overshot. Some pathways that return warming to 1.5°C (&gt;50%) by the end of the century show mid-century overshoots of up to 1.8°C median warming. The overshoot tends to be higher, the higher the 2030 emissions. Mitigation pathways with 2030 emissions levels in the NDCS announced prior to COP26 consistently overshoot 1.5°C by 0.15°C–0.3°C. This leads to higher risks from climate change impacts during the time of overshoot compared to pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot ( [[#Schleussner--2016a|Schleussner et al. 2016a]] ; [[#Mengel--2018|Mengel et al. 2018]] ; [[#Hofmann--2019|Hofmann et al. 2019]] ; [[#Lenton--2019|Lenton et al. 2019]] ; [[#Tachiiri--2019|Tachiiri et al. 2019]] ; [[#Drouet--2021|Drouet et al. 2021]] ). Furthermore, even if warming is reversed by net negative emissions, other climate changes such as sea level rise would continue in their current direction for decades to millennia (AR6 WGI Sections 4.6 and 5.6).

Returning warming to lower levels requires net negative CO 2 emissions in the second half of the century (Clarke et al. 2014; [[#Fuss--2014|Fuss et al. 2014]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a). The amount of net negative CO 2 emissions in pathways limiting warming to 1.5°C–2°C climate goals varies widely, with some pathways not deploying net negative CO 2 emissions at all and others deploying up to –600 to –800 GtCO 2 . The amount of net negative CO 2 emissions tends to increase with 2030 emissions levels (Figure 3.30e and Table 3.6). Studies confirmed the ability of net negative CO 2 emissions to reduce warming, but pointed to path dependencies in the storage of carbon and heat in the Earth System and the need for further research particularly for cases of high overshoot ( [[#Zickfeld--2016|Zickfeld et al. 2016]] , 2021; [[#Keller--2018a|Keller et al. 2018a]] ,b; [[#Tokarska--2019|Tokarska et al. 2019]] ). The AR6 WGI assessed the reduction in global surface temperature to be approximately linearly related to cumulative CO 2 removal and, with lower confidence, that the amount of cooling per unit CO 2 removed is approximately independent of the rate and amount of removal (AR6 WGI TS.3.3.2). Still there remains large uncertainty about a potential asymmetry between the warming response to CO 2 emissions and the cooling response to net negative CO 2 emissions ( [[#Zickfeld--2021|Zickfeld et al. 2021]] ). It was also shown that warming can adversely affect the efficacy of carbon dioxide removal measures and hence the ability to achieve net negative CO 2 emissions ( [[#Boysen--2016|Boysen et al. 2016]] ).

Obtaining net negative CO 2 emissions requires massive deployment of carbon dioxide removal (CDR) in the second half of the century, on the order of 220 (160–370) GtCO 2 for each 0.1°C degree of cooling (based on the assessment of the ''likely'' range of the transient response to cumulative CO 2 emissions in AR6 WGI [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] in Chapter 5, not taking into account potential asymmetries in the temperature response to CO 2 emissions and removals). CDR is assessed in detail in [[IPCC:Wg3:Chapter:Chapter-12#12.3|Section 12.3]] of this report (see also Cross-Chapter Box 8 in Chapter 12). Here we only point to the finding that CDR ramp-up rates and absolute deployment levels are tightly limited by techno-economic, social, political, institutional and sustainability constraints ( [[#Smith--2016|Smith et al. 2016]] ; [[#Boysen--2017|Boysen et al. 2017]] ; [[#Fuss--2018|Fuss et al. 2018]] , 2020; [[#Nemet--2018|Nemet et al. 2018]] ; [[#Hilaire--2019|Hilaire et al. 2019]] ; [[#Jia--2019|Jia et al. 2019]] ) ( [[IPCC:Wg3:Chapter:Chapter-12#12.3|Section 12.3]] ). CDR therefore cannot be deployed arbitrarily to compensate any degree of overshoot. A fraction of models was not able to compute pathways that would follow the mitigation ambition in unconditional and conditional NDCs until 2030 and return warming to below 1.5°C by 2100 ( [[#Luderer--2018|Luderer et al. 2018]] ; [[#Roelfsema--2020|Roelfsema et al. 2020]] ; [[#Riahi--2021|Riahi et al. 2021]] ). There exists a three-way trade-off between near-term emissions developments until 2030, transitional challenges during 2030–50, and long-term CDR deployment post-2050 ( [[#Sanderson--2016|Sanderson et al. 2016]] ; [[#Holz--2018|Holz et al. 2018]] ; [[#Strefler--2018|Strefler et al. 2018]] ). For example, [[#Strefler--2018|Strefler et al. (2018)]] find that if CO 2 emission levels stay at around 40 GtCO 2 until 2030, within the range of what is projected for NDCs announced prior to COP26, rather than being halved to 20 GtCO 2 until 2030, CDR deployment in the second half of the century would have to increase by 50–100%, depending on whether the 2030–2050 CO 2 emissions reduction rate is doubled from 6% to 12% or kept at 6% yr –1 . This three-way trade-off has also been identified at the national level ( [[#Pan--2020|Pan et al. 2020]] ).

In addition to enabling a temporary budget overshoot by net negative CO 2 emissions in the second half of the century, CDR can also be used to compensate – on an annual basis – residual CO 2 emissions from sources that are difficult to eliminate and to reach net zero CO 2 emissions more rapidly if deployed before this point ( [[#Kriegler--2013b|Kriegler et al. 2013b]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a). This explains its continued deployment in pathways that exclude overshoot and net negative CO 2 emissions ( [[#Riahi--2021|Riahi et al. 2021]] ). However, given the time scales that would likely be needed to ramp-up CDR to gigatonne scale ( [[#Nemet--2018|Nemet et al. 2018]] ), it can be expected to only make a limited contribution to reaching net zero CO 2 as fast as possible. In the vast majority (95%) of 1.5°C–2°C mitigation pathways assessed in this report, cumulative CDR deployment did not exceed 100 GtCO 2 until mid-century. This adds to the risk of excessively relying on CDR to compensate for weak mitigation action until 2030 by either facilitating massive net CO 2 emissions reduction rates during 2030–2050 or allowing a high temporary overshoot of 1.5°C until the end of the century. If international burden-sharing considerations are taken into account, the CDR penalty for weak action could increase further, in particular for developed countries ( [[#Fyson--2020|Fyson et al. 2020]] ). Further assessment of CDR deployment in 1.5°C–2°C mitigation pathways is found in [[#3.4.7|Section 3.4.7]] .

<div id="3.5.2.2" class="h3-container"></div>

<span id="carbon-lock-in-and-stranded-assets"></span>
==== 3.5.2.2 Carbon Lock-in and Stranded Assets ====

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There already exists a substantial and growing carbon lock-in today, as measured by committed emissions associated with existing long-lived infrastructure ( [[IPCC:Wg3:Chapter:Chapter-2#2.7|Section 2.7]] and Figure 2.31). If existing fossil fuel infrastructure would continue to be operated as historically, it would entail CO 2 emissions exceeding the carbon budget for 1.5°C ( [[IPCC:Wg3:Chapter:Chapter-2#2.7.2|Section 2.7.2]] and Figure 2.32). However, owner-operators and societies may choose to retire existing infrastructure earlier than in the past, and committed emissions are thus contingent on the competitiveness of non-emitting alternative technologies and climate policy ambition. Therefore, in mitigation pathways, some infrastructure may become stranded assets. Stranded assets have been defined as ‘assets that have suffered from unanticipated or premature write-downs, devaluations or conversion to liabilities’ ( [[#Caldecott--2017|Caldecott 2017]] ).

A systematic map of the literature on carbon lock-in has synthesized quantification of stranded assets in the mitigation pathways literature, and showed that (i) coal power plants are the most exposed to risk of becoming stranded, (ii) delayed mitigation action increases stranded assets, and (iii) sectoral distribution and the amount of stranded assets differ between countries ( [[#Fisch-Romito--2021|Fisch-Romito et al. 2021]] ). There is high agreement that existing fossil fuel infrastructure would need to be retired earlier than historically, used less, or retrofitted with CCS, to stay within the remaining carbon budgets of limiting warming to 1.5°C or 2°C ( [[#Johnson--2016|Johnson et al. 2016]] ; [[#Kefford--2018|Kefford et al. 2018]] ; [[#Pfeiffer--2018|Pfeiffer et al. 2018]] ; [[#Cui--2019|Cui et al. 2019]] ; [[#Fofrich--2020|Fofrich et al. 2020]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a). Studies estimate that cumulative early retired power plant capacities by 2060 can be up to 600 GW for gas and 1700 GW for coal ( [[#Iyer--2015a|Iyer et al. 2015a]] ; [[#Kefford--2018|Kefford et al. 2018]] ), that only 42% of the total capital stock of both operating and planned coal-fired powers plants can be utilised to be compatible with the 2°C target ( [[#Pfeiffer--2018|Pfeiffer et al. 2018]] ), and that coal-fired power plants in scenarios consistent with keeping global warming below 2°C or 1.5°C retire one to three decades earlier than historically has been the case ( [[#Cui--2019|Cui et al. 2019]] ; [[#Fofrich--2020|Fofrich et al. 2020]] ). After coal, electricity production based on gas is also projected to be phased out, with some capacity remaining as back-up ( [[#van%20Soest--2017a|van Soest et al. 2017a]] ). [[#Kefford--2018|Kefford et al. (2018)]] find USD541 billion worth of stranded fossil fuel power plants could be created by 2060, with China and India the most exposed.

Some publications have suggested that stranded long-lived assets may be even more important outside of the power sector. While stranded power sector assets by 2050 could reach up to USD1.8 trillion in scenarios consistent with a 2°C target, [[#Saygin--2019|Saygin et al. (2019)]] found a range of USD5–11 trillion in the buildings sector. [[#Muldoon-Smith--2019|Muldoon-Smith and Greenhalgh (2019)]] have even estimated a potential value at risk for global real estate assets up to USD21 trillion. More broadly, the set of economic activities that are potentially affected by a low-carbon transition is wide and includes also energy-intensive industries, transport and housing, as reflected in the concept of climate policy relevant sectors introduced in [[#Battiston--2017|Battiston et al. (2017)]] . The sectoral distribution and amount of stranded assets differ across countries ( [[#Fisch-Romito--2021|Fisch-Romito et al. 2021]] ). Capital for fossil fuel production and distribution represents a larger share of potentially stranded assets in fossil fuel-producing countries such as the United States and Russia. Electricity generation would be a larger share of total stranded assets in emerging countries because this capital is relatively new compared to its operational lifetime. Conversely, buildings could represent a larger part of stranded capital in more developed countries and regions such as the USA, EU or even Russia because of high market value and low turnover rate.

Many quantitative estimates of stranded assets along mitigation pathways have focused on fossil fuel power plants in pathways characterised by mitigation ambition until 2030 corresponding to the NDCs followed by strengthened action afterwards to limit warming to 2°C (&gt;67%) or lower ( [[#Bertram--2015a|Bertram et al. 2015a]] ; [[#Iyer--2015b|Iyer et al. 2015b]] ; [[#Lane--2016|Lane et al. 2016]] ; [[#Farfan--2017|Farfan and Breyer 2017]] ; [[#van%20Soest--2017a|van Soest et al. 2017a]] ; [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#Luderer--2018|Luderer et al. 2018]] ; [[#Cui--2019|Cui et al. 2019]] ; [[#Saygin--2019|Saygin et al. 2019]] ; [[#SEI--2020|SEI et al. 2020]] ). Pathways following NDCs announced prior to COP26 until 2030 do not show a significant reduction of coal, oil and gas use (Figure 3.30f–h and Table 3.6) compared to immediate action pathways. Stranded coal power assets are evaluated to be higher by a factor of two to three if action is strengthened after 2030 rather than now ( [[#Iyer--2015b|Iyer et al. 2015b]] ; [[#Cui--2019|Cui et al. 2019]] ). There is high agreement that the later climate policies are implemented, the higher the expected stranded assets and the societal, economic and political strain of strengthening action. Associated price increases for carbon-intensive goods and transitional macro-economic costs have been found to scale with the emissions gap in 2030 ( [[#Kriegler--2013a|Kriegler et al. 2013a]] ). At the aggregate level of the whole global economy, [[#Rozenberg--2015|Rozenberg et al. (2015)]] showed that each year of delaying the start of mitigation decreases the required CO 2 intensity of new production by 20–50 gCO 2 per USD. Carbon lock-in can have a long-lasting effect on future emissions trajectories after 2030. [[#Luderer--2018|Luderer et al. (2018)]] compared cost-effective pathways with immediate action to limit warming to 1.5°C–2°C with pathways following the NDCs until 2030 and adopting the pricing policy of the cost-effective pathways thereafter, and found that the majority of additional CO 2 emissions from carbon lock-in occur after 2030, reaching a cumulative amount of 290 (160–330) GtCO 2 by 2100 ( [[IPCC:Wg3:Chapter:Chapter-2#2.7.2|Section 2.7.2]] ). Early action and avoidance of investments in new carbon-intensive assets can minimise these risks.

The risk of stranded assets has implications for workers depending on those assets, asset owners, assets portfolio managers, financial institutions and the stability of the financial system. [[IPCC:Wg3:Chapter:Chapter-6|Chapter 6]] assesses the risks and implications of stranded assets for energy systems ( [[IPCC:Wg3:Chapter:Chapter-6#6.7.3|Section 6.7.3]] and Box 6.11) and fossil fuels ( [[IPCC:Wg3:Chapter:Chapter-6#6.7.4|Section 6.7.4]] ). The implications of stranded assets for inequality and Just Transition are assessed in [[IPCC:Wg3:Chapter:Chapter-17|Chapter 17]] ( [[IPCC:Wg3:Chapter:Chapter-17#17.3.2.3|Section 17.3.2.3]] ). [[IPCC:Wg3:Chapter:Chapter-15|Chapter 15]] assesses the literature on those implications for the financial system as well as on coping options (Sections 15.5.2 and 15.6.1).

On the other hand, mitigation, by limiting climate change, reduces the risk of destroyed or stranded assets from the physical impacts of climate change on natural and human systems, from more frequent, intense or extended extreme events and from sea level rise (O’Neill et al. 2020a). The literature on mitigation pathways rarely includes an evaluation of stranded assets from climate change impacts. [[#Unruh--2019|Unruh (2019)]] suggest that these are the real stranded assets of carbon lock-in and could prove much more costly.

<div id="3.5.3" class="h2-container"></div>

<span id="global-accelerated-action-towards-long-term-climate-goals"></span>
=== 3.5.3 Global Accelerated Action Towards Long-term Climate Goals ===

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A growing literature explores long-term mitigation pathways with accelerated near-term action going beyond the NDCs ( [[#Graichen--2017|Graichen et al. 2017]] ; [[#Jiang--2017|Jiang et al. 2017]] ; [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#Roelfsema--2018|Roelfsema et al. 2018]] ; [[#Fekete--2021|Fekete et al. 2021]] ; [[#van%20Soest--2021a|van Soest et al. 2021a]] ). Global accelerated action pathways are designed to transition more gradually from implemented policies and planned implementation of NDCs onto a 1.5°C–2°C pathway and at the same time alleviate the abrupt transition in 2030 that would be caused by following the NDCs until 2030 and strengthening towards limiting warming to 2°C thereafter ( [[#3.5.2|Section 3.5.2]] ). Therefore, they have sometimes been called bridging scenarios/pathways in the literature ( [[#IEA--2011|IEA 2011]] ; [[#Spencer--2015|Spencer et al. 2015]] ; [[#van%20Soest--2021a|van Soest et al. 2021a]] ). They rely on regionally differentiated regulatory and pricing policies to gradually strengthening regional and sectoral action beyond the mitigation ambition in the NDCs. There are limitations to this approach. The tighter the warming limit, the more likely it is that disruptive action becomes inevitable to achieve the speed of transition that would be required ( [[#Kriegler--2018a|Kriegler et al. 2018a]] ). Cost-effective pathways already have abrupt shifts in deployments, investments and prices at the time a stringent warming limit is imposed, reflecting the fact that the overall response to climate change has so far been misaligned with long-term climate goals ( [[#Fawcett--2015|Fawcett et al. 2015]] ; [[#Rogelj--2016|Rogelj et al. 2016]] ; [[#Schleussner--2016b|Schleussner et al. 2016b]] ; [[#Geiges--2020|Geiges et al. 2020]] ). Disruptive action can help to break lock-ins and enable transformative change ( [[#Vogt-Schilb--2018|Vogt-Schilb et al. 2018]] ).

The large literature on accelerating climate action was assessed in the ''IPCC Special Report on Global Warming of 1.5°C'' ( [[#de%20Coninck--2018|de Coninck et al. 2018]] ) and is taken up in this report primarily in Chapters 4, 13, and 14. Accelerating climate action and facilitating transformational change requires a perspective on socio-technical transitions ( [[#Geels--2016a|Geels et al. 2016a]] ; [[#Geels--2016b|Geels et al. 2016b]] ; Geels 2020), a portfolio of policy instruments to manage technological and environmental change ( [[#Fischer--2008|Fischer and Newell 2008]] ; [[#Goulder--2008|Goulder and Parry 2008]] ; Acemoglu et al. 2012, 2016), a notion of path dependency and policy sequencing ( [[#Pierson--2000|Pierson 2000]] ; [[#Meckling--2017|Meckling et al. 2017]] ; [[#Pahle--2018|Pahle et al. 2018]] ) and the evolvement of polycentric governance layers of institutions and norms in support of the transformation ( [[#Dietz--2003|Dietz et al. 2003]] ; [[#Leach--2007|Leach et al. 2007]] ; [[#Messner--2015|Messner 2015]] ). This subsection is focused on an assessment of the emerging quantitative literature on global accelerated action pathways towards 1.5°C–2°C, which to a large extent abstracts from the underlying processes and uses a number of stylised approaches to generate these pathways. A representative of accelerated action pathways has been identified as one of the Illustrative Mitigation Pathways (IMPs) in this assessment ( ''IMP-GS'' , Figure 3.31).

<div id="_idContainer085" class="Basic-Text-Frame"></div>

[[File:ceb4a9ca7f4f5083eb84dae4f7f24c8c IPCC_AR6_WGIII_Figure_3_31.png]]

'''Figure 3.31 |''' '''Comparison of (i) pathways with immediate action to limit warming to 2°C (&gt;67%) (Immediate, light blue), (ii) pathways following the NDCs until 2030 and limiting warming to 2°C (&gt;67%) thereafter (NDC; orange), and (iii) pathways accelerating near-term action until 2030 beyond NDC ambition levels and limiting warming to 2°C (&gt;67%) thereafter (accelerated) for selected indicators as listed in the panel titles, based on pathways from van Soest''' '''et al.''' '''(2021a).''' Low-carbon electricity comprises renewable and nuclear power. Indicator ranges are shown as box plots (full range, interquartile range, and median) for the years 2030, 2050 and 2100 (absolute values) and for the periods 2020–2030, 2030–2050 (change indicators). Ranges are based on nine models participating in [[#van%20Soest--2021a|van Soest et al. (2021a)]] with only seven models reporting emissions and climate results and eight models reporting carbon prices. The purple dot denotes the Illustrative Mitigation Pathway ''GS'' that was part of the study by van Soest et al.

One approach relies on augmenting initially moderate emissions-pricing policies with robust anticipation of ratcheting up climate action in the future ( [[#Spencer--2015|Spencer et al. 2015]] ). If announcements of strong future climate policies are perceived to be credible, they can help to prevent carbon lock-in as investors anticipating high future costs of GHG emissions would reduce investment into fossil fuel infrastructure, such as coal power plants ( [[#Bauer--2018b|Bauer et al. 2018b]] ). However, the effectiveness of such announcements strongly hinges on their credibility. If investors believe that policymakers could drop them if anticipatory action did not occur, they may not undertake such action.

Another approach relies on international cooperation to strengthen near-term climate action. These studies build on international climate policy architectures that could incentivise a coalition of like-minded countries to raise their mitigation ambition beyond what is stated in their NDC ( [[#Graichen--2017|Graichen et al. 2017]] ). Examples are the idea of climate clubs characterised by harmonised carbon and technology markets ( [[#Nordhaus--2015|Nordhaus 2015]] ; [[#Keohane--2017|Keohane et al. 2017]] ; [[#Paroussos--2019|Paroussos et al. 2019]] ; [[#Pihl--2020|Pihl 2020]] ) and the Powering Past Coal Alliance (PPCA) ( [[#Jewell--2019|Jewell et al. 2019]] ). [[#Paroussos--2019|Paroussos et al. (2019)]] find economic benefits of joining a climate club despite the associated higher mitigation effort, in particular due to access to technology and climate finance. [[#Graichen--2017|Graichen et al. (2017)]] find an additional reduction of 5–11 GtCO 2 -eq compared to the mitigation ambition in the NDCs from the successful implementation of international climate initiatives. Other studies assess benefits from international transfers of mitigation outcomes ( [[#Stua--2017|Stua 2017]] ; [[#Edmonds--2021|Edmonds et al. 2021]] ). [[#Edmonds--2021|Edmonds et al. (2021)]] find economic gains from sharing NDC emissions-reduction commitments compared to purely domestic implementation of NDCs. If reinvested in mitigation efforts, the study projects an additional reduction of 9 billion tonnes of CO 2 in 2030.

The most common approach relies on strengthening regulatory policies beyond current policy trends, also motivated by the finding that such policies have so far been employed more often than comprehensive carbon pricing ( [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#Roelfsema--2018|Roelfsema et al. 2018]] ; [[#Fekete--2021|Fekete et al. 2021]] ; [[#IEA--2021a|]] [[#IEA--2021|IEA 2021]] a ; [[#van%20Soest--2021a|van Soest et al. 2021a]] ). Some studies have focused on generic regulatory policies such as low-carbon support policies, fossil fuel-sunset policies, and resource-efficiency policies ( [[#Bertram--2015b|Bertram et al. 2015b]] ; [[#Hatfield-Dodds--2017|Hatfield-Dodds et al. 2017]] ). [[#Bertram--2015b|Bertram et al. (2015b)]] found that a moderate carbon price combined with a coal moratorium and ambitious low-carbon support policies can limit efficiency losses until 2030 if emissions pricing is raised thereafter to limit warming to 2°C. They also showed that all three components are needed to achieve this outcome. [[#Hatfield-Dodds--2017|Hatfield-Dodds et al. (2017)]] found that resource efficiency can lower 2050 emissions by an additional 15–20% while boosting near-term economic growth. The International Energy Agency ( [[#IEA--2021a|]] [[#IEA--2021|IEA 2021]] a ) developed a detailed net zero scenario for the global energy sector characterised by a rapid phase-out of fossil fuels, a massive clean energy and electrification push, and the stabilisation of energy demand, leading to 10 GtCO 2 lower emissions from energy use in 2030 than in a scenario following the announced pledges.

The Paris Agreement has spurred the formulation of NDCs for 2030 and mid-century strategies around the world (cf. Chapter 4). This is giving researchers a rich empirical basis to formulate accelerated policy packages taking national decarbonisation pathways as a starting point ( [[#Graichen--2017|Graichen et al. 2017]] ; [[#Jiang--2017|Jiang et al. 2017]] ; [[#van%20Soest--2017b|van Soest et al. 2017b]] ; [[#Waisman--2019|Waisman et al. 2019]] ). The concept is to identify good practice policies that had demonstrable impact on pushing low-carbon options or reducing emissions in a country or region and then consider a wider roll out of these policies taking into account regional specificities ( [[#den%20Elzen--2015|den Elzen et al. 2015]] ; [[#Fekete--2015|Fekete et al. 2015]] , 2021; [[#Kriegler--2018a|Kriegler et al. 2018a]] ; [[#Kuramochi--2018|Kuramochi et al. 2018]] ; [[#Roelfsema--2018|Roelfsema et al. 2018]] ). A challenge for this approach is to account for the fact that policy effectiveness varies with different political environments in different geographies. As a result, a global roll out of good practice policies to close the emissions gap will still be an idealised benchmark, but it is useful to understand how much could be gained from it.

Accelerated action pathways derived with this approach show considerable scope for narrowing the emissions gap between pathways reflecting the ambition level of the NDCs and cost-effective mitigation pathways in 2030. [[#Kriegler--2018a|Kriegler et al. (2018a)]] find around 10 GtCO 2 -eq lower emissions compared to original NDCs from a global roll out of good practice plus net zero policies and a moderate increase in regionally differentiated carbon pricing. [[#Fekete--2021|Fekete et al. (2021)]] show that global replication of sector progress in five major economies would reduce GHG emissions in 2030 by about 20% compared to a current policy scenario. These findings were found in good agreement with a recent model comparison study based on results from nine integrated assessment models (IAMs) ( [[#van%20Soest--2021a|van Soest et al. 2021a]] ). Based on these three studies, implementing accelerated action in terms of a global roll out of regulatory and moderate pricing policies is assessed to lead to global GHG emissions of 48 (38–52) GtCO 2 -eq in 2030 (median and 5–95th percentile based on 10 distinct modelled pathways). This closes the implementation gap for the NDCs, and in addition falls below the emissions range implied by implementing unconditional and conditional elements of NDCs by 2–9 GtCO 2 -eq. However, it does not close the emissions gap to immediate action pathways that limit warming to 2°C (&gt;67%), and, based on our assessment in [[#3.5.2|Section 3.5.2]] , emission levels above 40 GtCO 2 -eq in 2030 still have a very low prospect for limiting warming to 1.5°C (&gt;50%) with no or limited overshoot.

Figure 3.31 shows the intermediate position of accelerated action pathways derived by [[#van%20Soest--2021a|van Soest et al. (2021a)]] between pathways that follow the NDCs until 2030 and immediate action pathways limiting warming to 2°C (&gt;67%). Accelerated action is able to reduce the abrupt shifts in emissions, fossil fuel use and low-carbon power generation in 2030 and also limits peak warming more effectively than NDC pathways. But primarily due to the moderate carbon price assumptions (Figure 3.31b), the reductions in emissions and particular fossil fuel use are markedly smaller than what would be obtained in the case of immediate action. The assessment shows that accelerated action until 2030 can have significant benefits in terms of reducing the mitigation challenges from following the NDCs until 2030. But putting a significant value on GHG emissions reductions globally remains a key element of moving onto 1.5°C–2°C pathways. The vast majority of pathways that limit warming to 2°C (&gt;67%) or lower, independently of their differences in near-term emission developments, converge to a global mitigation regime putting a significant value on GHG emission reductions in all regions and sectors.

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