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

==== 3.4.1.2 Sectoral Emissions Strategies and the Timing of Net Zero ====

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Mitigation pathways show differences in the timing of decarbonisation (Figure 3.20) and the timing of net zero (Figure 3.19) across sectors and regions ( ''high confidence'' ); the timing in a given sector depends on the cost of abatement in it, the availability of CDR options, the scenario design, near-term emissions levels, and the amount of non-CO 2 abatement ( [[#Yeh--2017|Yeh et al. 2017]] ; [[#Emmerling--2019|Emmerling et al. 2019]] ; [[#Rogelj--2019a|Rogelj et al. 2019a]] ,b; [[#Johansson--2020|Johansson et al. 2020]] ; [[#Azevedo--2021|Azevedo et al. 2021]] ; [[#Ou--2021|Ou et al. 2021]] ; [[#van%20Soest--2021b|van Soest et al. 2021b]] ) (Cross-Chapter Box 3 in this chapter). However, delaying emissions reductions, or more limited emissions reductions in one sector or region, involves compensating reductions in other sectors or regions if warming is to be limited ( ''high confidence'' ) ( [[#Price--2017|Price and Keppo 2017]] ; [[#Grubler--2018|Grubler et al. 2018]] ; [[#Rochedo--2018|Rochedo et al. 2018]] ; [[#van%20Soest--2021b|van Soest et al. 2021b]] ).

At the time of net zero global CO 2 emissions, emissions in some sectors are positive and some negative. In cost-effective mitigation pathways, the energy supply sector typically reaches net zero CO 2 before the economy as a whole, while the demand sectors reach net zero CO 2 later, if at all ( [[#Pietzcker--2014|Pietzcker et al. 2014]] ; [[#Price--2017|Price and Keppo 2017]] ; [[#Luderer--2018|Luderer et al. 2018]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a,b; [[#Méjean--2019|Méjean et al. 2019]] ; [[#Azevedo--2021|Azevedo et al. 2021]] ) ( [[IPCC:Wg3:Chapter:Chapter-6#6.7|Section 6.7]] ). CO 2 emissions from transport, industry, and buildings are positive, and non-CO 2 GHG emissions are also positive at the time of global net zero CO 2 emissions (Figure 3.20).

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[[File:927d1ee874869f6ec709f85ea62d4591 IPCC_AR6_WGIII_Figure_3_19.png]]

'''Figure 3.19''' | '''Decade in which sectoral CO''' 2 '''emissions first reach net negative values.''' Each panel is a different temperature level. The colours indicate the decade in which CO 2 emissions go negative; the y-axis indicates the share of scenarios achieving net zero in that decade. Only scenarios that pass the vetting criteria are included ( [[#3.2|Section 3.2]] ). Scenarios achieving net zero prior to 2020 are excluded.

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[[File:eed9917ff2503f981437ef52849baf62 IPCC_AR6_WGIII_Figure_3_20.png]]

'''Figure 3.20''' | '''Greenhouse gas (GHG) emissions, including CO''' 2 '''emissions by sector and total non-CO''' 2 '''GHGs in 2050 (top left), 2100 (top middle), year of global net zero CO''' 2 '''(top right), cumulative CO''' 2 '''emissions from 2020–2100 (bottom left), and cumulative CO''' 2 '''emissions from 2020 until the year of net zero CO''' 2 '''for scenarios that limit warming to below 2°C.''' Scenarios are grouped by their temperature category. ‘Industry’ includes CO 2 emissions associated with industrial energy use only; sectors shown in this figure do not necessarily sum to total CO 2 . In this, and other figures in [[#3.4|Section 3.4]] , unless stated otherwise, only scenarios that pass the vetting criteria are included ( [[#3.2|Section 3.2]] ). Boxes indicate the interquartile range, the median is shown with a horizontal black line, while vertical lines show the 5–95% interval.

So, while pathways indicate some flexibility in emissions reductions across sectors, all pathways involve substantial CO 2 emissions reductions in all sectors and regions ( ''high confidence'' ) ( [[#Luderer--2018|Luderer et al. 2018]] ; [[#Rogelj--2018|Rogelj et al. 2018]] a,b; [[#Méjean--2019|Méjean et al. 2019]] ; [[#Azevedo--2021|Azevedo et al. 2021]] ). Projected CO 2 emissions reductions between 2019 and 2050 in 1.5°C (&gt;50%) pathways with no or limited overshoot are around 77% for energy demand, with a 5–95% range of 31–96%, [[#footnote-008|12]] 115% for energy supply (90–167%), and 148% for AFOLU (94–387%). In pathways that limit warming to 2°C (&gt;67%), projected CO 2 emissions are reduced between 2019 and 2050 by around 49% for energy demand, 97% for energy supply, and 136% for AFOLU (Sections 3.4.2–3.4.6). Almost 75% of GHG reductions at the time of net zero GHG are from the energy system, 13% are from AFOLU CO 2 , and 13% from non-CO 2 (Figure 3.21). These reductions are achieved through a variety of sectoral strategies, illustrated in Figure 3.21 (Figure 3.21b), and described in Sections 3.4.2 to 3.4.7; the primary strategies include declines in fossil energy, increases in low-carbon energy use, and CDR to address residual emissions.

'''Table 3.4 | Energy and emissions characteristics of the pathways by climate category for 2030, 2050, 2100.''' Source: AR6 scenarios database.

{| class="wikitable"
|-
! '''p50'''

'''(p5–p95)''' ''a''

! colspan="2"| '''Global Mean Surface Air Temperature change'''

! colspan="3"| '''Low-carbon share of Primary Energy''' ''d, e''

'''[%]'''

'''2020 = 16 (12–18)'''

! colspan="3"| '''Energy &amp; Industrial Processes Index'''

'''2020 = 100'''

! colspan="3"| '''Final energy demand'''

'''[EJ/yr]'''

'''2020 = 419 (367–458)'''

! colspan="3"| '''Final energy intensity of GDP Index'''

'''2020 = 100'''

! colspan="3"| '''Electricity share in final energy'''

'''[%]'''

'''2020 = 20 (18–25)'''

! colspan="3"| '''CO2 intensity of electricity'''

'''[Mt CO''' ''2'' '''/TWh]'''

'''2020 = 469 (419–538)'''

! colspan="3"| '''Non-energy GHG emissions'''

'''[Gt CO''' ''2'' '''-eq]'''

'''2020 = 18 (15–21)'''

! colspan="4"| '''Fossil CCS (2100)'''

'''[Gt CO''' ''2'' ''']'''

'''2020 = 0 (0–0)'''

|-
! '''Category [# pathways]''' ''b, c''

! '''Category/ subset'''

! '''WG1 SSP &amp; IPs alignment'''

! '''2030'''

! '''2050'''

! '''2100'''

! '''2030'''

! '''2050'''

! '''2100'''

! '''2030'''

! '''2050'''

! '''2100'''

! '''2030'''

! '''2050'''

! '''2100'''

! '''2030'''

! '''2050'''

! '''2100'''

! '''2030'''

! '''2050'''

! '''2100'''

! '''2030'''

! '''2050'''

! '''2100'''

! '''2030'''

! '''2050'''

! '''2100'''

! '''2020–2100'''

|-
| rowspan="2"| '''C1 [97]'''

| rowspan="2"| '''limit warming to 1.5°C (&gt;50%) with no or limited overshoot'''

| rowspan="2"| IMP-SD, IMP-LD,IMP-Ren, SSP1-1.9

| 32

| 68

| 75

| 65

| 8

| –3

| 399

| 410

| 612

| 71

| 46

| 26

| 27

| 52

| 66

| 99

| –5

| –4

| 10

| 5

| 2

| 1

| 2

| 3

| 196

|-
| (17–48)

| (25–86)

| (19–98)

| (49–75)

| (–8–24)

| (–20–8)

| (293–447)

| (325–540)

| (321–818)

| (59–81)

| (34–60)

| (14–45)

| (23–35)

| (40–64)

| (50–78)

| (4–215)

| (–66–11)

| (–104–1)

| (5–13)

| (1–9)

| (–2–9)

| (0–5)

| (0–13)

| (0–16)

| (3–882)

|-
| rowspan="2"| '''C2 [133]'''

| rowspan="2"| '''return warming to 1.5°C (&gt;50%) after a high overshoot'''

| rowspan="2"| IMP-Neg

| 24

| 57

| 86

| 79

| 18

| –14

| 458

| 442

| 675

| 76

| 44

| 23

| 25

| 45

| 61

| 218

| 0

| –1

| 13

| 6

| 1

| 0

| 3

| 1

| 280

|-
| (11–35)

| (19–77)

| (25–97)

| (66–94)

| (2–37)

| (–25–0)

| (372–504)

| (345–561)

| (415–819)

| (64–88)

| (35–63)

| (15–45)

| (20–29)

| (34–56)

| (49–73)

| (99–353)

| (–75–16)

| (–118–3)

| (10–19)

| (2–9)

| (–7–7)

| (0–4)

| (0–13)

| (0–16)

| (7–831)

|-
| rowspan="2"| '''C3 [311]'''

| rowspan="2"| limit warming to 2°C (&gt;67%)

|
| 24

| 51

| 73

| 84

| 31

| –1

| 446

| 448

| 625

| 77

| 50

| 26

| 24

| 42

| 60

| 248

| 5

| –8

| 12

| 7

| 5

| 0

| 3

| 5

| 266

|-
|
| (16–32)

| (29–75)

| (34–94)

| (70–95)

| (9–47)

| (–19–8)

| (356–491)

| (344–540)

| (421–788)

| (65–88)

| (36–62)

| (18–41)

| (20–29)

| (30–54)

| (43–72)

| (93–375)

| (–72–51)

| (–105–5)

| (6–18)

| (3–12)

| (–1–8)

| (0–3)

| (0–12)

| (0–15)

| (7–773)

|-
| rowspan="2"| '''C3a [204]'''

| rowspan="2"| '''… with action starting in 2020'''

| rowspan="2"| SSP2-2.6

| 21

| 39

| 71

| 92

| 45

| –3

| 459

| 489

| 641

| 76

| 45

| 22

| 23

| 35

| 56

| 322

| 24

| –14

| 13

| 9

| 2

| 0

| 2

| 6

| 279

|-
| (14–24)

| (24–63)

| (34–91)

| (80–100)

| (26–64)

| (–21–9)

| (379–497)

| (362–601)

| (450–796)

| (71–87)

| (39–65)

| (19–41)

| (19–28)

| (23–44)

| (44–69)

| (227–381)

| (–48–112)

| (–117–7)

| (8–19)

| (3–12)

| (–1–9)

| (0–2)

| (0–9)

| (0–16)

| (7–684)

|-
| rowspan="2"| '''C3b [97]'''

| rowspan="2"| '''… NDCs until 2030'''

| rowspan="2"| IMP-GS

| 21

| 31

| 67

| 92

| 66

| 9

| 466

| 519

| 680

| 77

| 51

| 23

| 23

| 32

| 53

| 341

| 107

| –3

| 15

| 10

| 4

| 0

| 1

| 5

| 200

|-
| (12–24)

| (22–44)

| (42–84)

| (84–102)

| (50–84)

| (–13–32)

| (389–499)

| (435–585)

| (383–812)

| (74–88)

| (45–66)

| (18–40)

| (19–28)

| (19–41)

| (40–65)

| (257–418)

| (14–208)

| (–73–34)

| (10–19)

| (5–15)

| (–1–11)

| (0–1)

| (0–7)

| (0–15)

| (5–730)

|-
| rowspan="2"| '''C4 [159]'''

| rowspan="2"| '''limit warming to 2°C (&gt;50%)'''

|
| 20

| 25

| 47

| 94

| 82

| 47

| 467

| 551

| 701

| 79

| 55

| 26

| 23

| 29

| 48

| 354

| 216

| 28

| 17

| 13

| 8

| 0

| 0

| 4

| 47

|-
|
| (11–23)

| (14–36)

| (28–65)

| (87–101)

| (67–92)

| (21–78)

| (410–508)

| (471–632)

| (432–910)

| (75–89)

| (50–70)

| (20–42)

| (19–28)

| (19–38)

| (30–56)

| (257–469)

| (69–317)

| (–20–166)

| (11–20)

| (9–17)

| (2–12)

| (0–0)

| (0–4)

| (0–16)

| (0–536)

|-
| rowspan="2"| '''C5 [212]'''

| rowspan="2"| '''limit warming to 2.5°C (&gt;50%)'''

|
| 17

| 19

| 29

| 98

| 94

| 73

| 492

| 599

| 804

| 85

| 64

| 33

| 24

| 29

| 41

| 414

| 311

| 185

| 19

| 19

| 16

| 0

| 0

| 0

| 0

|-
|
| (11–21)

| (8–29)

| (8–51)

| (91–101)

| (80–101)

| (56–106)

| (434–540)

| (513–701)

| (557–983)

| (76–91)

| (54–76)

| (27–48)

| (20–28)

| (23–35)

| (29–50)

| (311–538)

| (130–499)

| (12–461)

| (13–24)

| (14–25)

| (9–26)

| (0–0)

| (0–2)

| (0–8)

| (0–221)

|-
| rowspan="2"| '''C6 [97]'''

| rowspan="2"| '''limit warming to 3°C (&gt;50%)'''

| SSP2-4.5

| 13

| 13

| 29

| 102

| 106

| 91

| 540

| 696

| 941

| 89

| 73

| 47

| 26

| 31

| 43

| 463

| 425

| 189

| 20

| 21

| 20

| 0

| 0

| 0

| 0

|-
| Mod-Act

| (11–17)

| (9–20)

| (14–45)

| (99–103)

| (104–109)

| (87–95)

| (413–574)

| (504–856)

| (692–1136)

| (88–92)

| (64–79)

| (25–51)

| (22–30)

| (28–35)

| (35–50)

| (372–514)

| (352–484)

| (142–441)

| (19–25)

| (20–29)

| (13–31)

| (0–0)

| (0–0)

| (0–2)

| (0–38)

|-
| rowspan="2"| '''C7 [164]'''

| rowspan="2"| '''limit warming to 4°C (&gt;50%)'''

| SSP3-7.0

| 32

| 68

| 75

| 65

| 8

| –3

| 399

| 410

| 612

| 71

| 46

| 26

| 27

| 52

| 66

| 99

| –5

| –4

| 10

| 5

| 2

| 1

| 2

| 3

| 196

|-
| Cur-Pol

| (17–48)

| (25–86)

| (19–98)

| (49–75)

| (–8–24)

| (–20–8)

| (293–447)

| (325–540)

| (321–818)

| (59–81)

| (34–60)

| (14–45)

| (23–35)

| (40–64)

| (50–78)

| (4–215)

| (–66–11)

| (–104–1)

| (5–13)

| (1–9)

| (–2–9)

| (0–5)

| (0–13)

| (0–16)

| (3–882)

|-
| rowspan="2"| '''C8 [29]'''

| rowspan="2"| '''exceed warming of 4°C (≥50%)'''

| SSP5-8.5

| 24

| 57

| 86

| 79

| 18

| –14

| 458

| 442

| 675

| 76

| 44

| 23

| 25

| 45

| 61

| 218

| 0

| –1

| 13

| 6

| 1

| 0

| 3

| 1

| 280

|-
|
| (11–35)

| (19–77)

| (25–97)

| (66–94)

| (2–37)

| (–25–0)

| (372–504)

| (345–561)

| (415–819)

| (64–88)

| (35–63)

| (15–45)

| (20–29)

| (34–56)

| (49–73)

| (99–353)

| (–75–16)

| (–118–3)

| (10–19)

| (2–9)

| (–7–7)

| (0–4)

| (0–13)

| (0–16)

| (7–831)

|}

a Values in the table refer to the 50th and (5–95th) percentile values.

b See category descriptions in Table 3.1.

c The warming profile of ''IMP-Neg'' peaks around 2060 and declines thereafter to below 1.5°C (50% likelihood) shortly after 2100. Whilst technically classified as a C3, it strongly exhibits the characteristics of C2 high-overshoot scenarios.

d Primary Energy as calculated in ‘Direct Equivalent’ terms according to IPCC reporting conventions.

e Low-carbon energy here defined to include: renewables (including biomass, solar, wind, hydro, geothermal, ocean); fossil fuels when used with CCS; and, nuclear power.

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[[File:dcb652b7ddcd28456a52dd4c77eba9b8 IPCC_AR6_WGIII_Figure_3_21.png]]
'''Figure 3.21 | Left panel: Greenhouse gas (GHG) emissions reductions from 2019 by sector at the year of net zero GHG for all scenarios that reach net zero GHG.''' Emissions reductions by sector for direct (demand) and indirect (upstream supply) are shown as the percent of total GHG reductions. '''Right panel:''' key indicators in 2050 for the IMPs. Definitions of significant and very significant are defined relative to 2019 and vary between indicators, as follows: fossil energy (significant &gt;10%, very significant &gt;50%), renewables (&gt;150 EJ yr –1 , &gt;200 EJ yr –1 ), bioenergy (&gt;100%, &gt;200%), BECCS (&gt;2.0 GtCO 2 yr –1 , &gt;3.5 GtCO 2 yr –1 ), AFOLU (&gt;100% decline, &gt;130% decline), energy crops (&gt;150 million ha, &gt;400 million ha), forest (&gt;5% increase, &gt;15% increase). Source: AR6 Scenarios Database.

In the context of mitigation pathways, only a few studies have examined solar radiation modification (SRM), typically focusing on Stratospheric Aerosol Injection ( [[#Arinoa--2016|Arinoa et al. 2016]] ; [[#Emmerling--2018a|Emmerling and Tavoni 2018a]] ,b; [[#Heutel--2018|Heutel et al. 2018]] ; [[#Helwegen--2019|Helwegen et al. 2019]] ; [[#Rickels--2020|Rickels et al. 2020]] ; [[#Belaia--2021|Belaia et al. 2021]] ). These studies find that substantial mitigation is required to limit warming to a given level, even if SRM is available ( [[#Moreno-Cruz--2017|Moreno-Cruz and Smulders 2017]] ; [[#Emmerling--2018b|Emmerling and Tavoni 2018b]] ; [[#Belaia--2021|Belaia et al. 2021]] ). SRM may reduce some climate impacts, reduce peak temperatures, lower mitigation costs, and extend the time available to achieve mitigation; however, SRM does not address ocean acidification and may involve risks to crop yields, economies, human health, or ecosystems (AR6 WGII Chapter 16; AR6 WGI TS and Chapter 5; SR1.5 SPM; and Cross-Working Group Box 4 in [[IPCC:Wg3:Chapter:Chapter-14|Chapter 14]] of this report). There are also significant uncertainties surrounding SRM, including uncertainties on the costs and risks, which can substantially alter the amount of SRM used in modelled pathways ( [[#Tavoni--2017|Tavoni et al. 2017]] ; [[#Heutel--2018|Heutel et al. 2018]] ; [[#IPCC--2018|IPCC 2018]] ; [[#Helwegen--2019|Helwegen et al. 2019]] ; [[#NASEM--2021|NASEM 2021]] ). Furthermore, the degree of international cooperation can influence the amount of SRM deployed in scenarios, with uncoordinated action resulting in larger SRM deployment and consequently larger risks/impacts from SRM ( [[#Emmerling--2018a|Emmerling and Tavoni 2018a]] ). Bridging research and governance involves consideration of the full range of societal choices and ramifications ( [[#Sugiyama--2018|Sugiyama et al. 2018]] ). More information on SRM, including the caveats, risks, uncertainties, and governance issues is found in AR6 WGI Chapter 4; AR6 WGIII Chapter 14; and Cross-Working Group Box 4 in [[IPCC:Wg3:Chapter:Chapter-14|Chapter 14]] of this report.

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