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

=== 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.

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[[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.

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==== 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]] .

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<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.

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