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

=== Cross-Chapter Box 3 | Understanding Net Zero CO 2 and Net Zero GHG Emissions ===

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'''Authors:''' Elmar Kriegler (Germany), Alaa Al Khourdajie (United Kingdom/Syria), Edward Byers (Austria/Ireland), Katherine Calvin (the United States of America), Leon Clarke (the United States of America), Annette Cowie (Australia), Navroz Dubash (India), Jae Edmonds (the United States of America), Jan S. Fuglestvedt (Norway), Oliver Geden (Germany), Giacomo Grassi (Italy/European Union), Anders Hammer Strømman (Norway), Frank Jotzo (Australia), Alexandre Köberle (Brazil/United Kingdom), Franck Lecocq (France), Yun Seng Lim (Malaysia), Eric Masanet (the United States of America), Toshihiko Masui (Japan), Catherine Mitchell (United Kingdom), Gert-Jan Nabuurs (the Netherlands), Anthony Patt (the United States of America/Switzerland), Glen P. Peters (Norway/Australia), Andy Reisinger (New Zealand), Keywan Riahi (Austria), Joeri Rogelj (United Kingdom/Belgium), Yamina Saheb (France/Algeria), Jim Skea (United Kingdom), Detlef P. van Vuuren (the Netherlands), Harald Winkler (Republic of South Africa)

This Cross-Chapter Box surveys scientific, technical and policy aspects of net zero carbon dioxide (CO 2 ) and net zero greenhouse gas (GHG) emissions, with a focus on timing, the relationship with warming levels, and sectoral and regional characteristics of net zero emissions. Assessment of net zero GHG emissions additionally requires consideration of non-CO 2 gases and choice of GHG emission metrics used to aggregate emissions and removals of different GHGs (Cross-Chapter Box 2 in [[IPCC:Wg3:Chapter:Chapter-2|Chapter 2]] and Cross-Chapter Box 7 in Chapter 10). The following considers net zero CO 2 and GHG emissions globally, followed by regional and sectoral dimensions.

'''Net zero CO''' 2 '''emissions'''

'''Reaching net zero CO''' 2 '''emissions globally is necessary for limiting global warming to any level.''' At the point of net zero CO 2 , the amount of CO 2 human activity is putting into the atmosphere equals the amount of CO 2 human activity is removing from the atmosphere (see Annex I: Glossary). Reaching and sustaining net zero CO 2 emissions globally stabilizes CO 2 -induced warming. Reaching net zero CO 2 emissions and then moving to net negative CO 2 emissions globally leads to a peak and decline in CO 2 -induced warming (AR6 WGI Sections 5.5 and 5.6).

'''Limiting warming to 1.5°C (&gt;50%) or to 2°C (&gt;67%) requires deep, rapid, and sustained reductions of other greenhouse gases including methane alongside rapid reductions of CO''' 2 '''emissions to net zero.''' This ensures that the warming contributions from non-CO 2 forcing agents as well as from CO 2 emissions are both limited at low levels. The AR6 WGI estimated remaining carbon budgets until the time of reaching net zero CO 2 emissions for a range of warming limits, taking into account historical CO 2 emissions and projections of the warming from non-CO 2 forcing agents (Box 3.4 in [[#3.3|Section 3.3]] , AR6 WGI [[IPCC:Wg3:Chapter:Chapter-5#5.5|Section 5.5]] ).

'''The earlier global net zero CO''' 2 '''emissions are reached, the lower the cumulative net amount of CO''' 2 '''emissions and human-induced global warming, all else being equal''' (Figure 1a in this Cross-Chapter Box). For a given net zero date, a variation in the shape of the CO 2 emissions profile can lead to a variation in the cumulative net amount of CO 2 emissions until the time of net zero CO 2 and as a result to different peak-warming levels. For example, cumulative net CO 2 emissions until the time of reaching net zero CO 2 will be smaller, and peak warming lower, if emissions are reduced steeply and then more slowly compared to reducing emissions slowly and then more steeply (Figure 1b in this Cross-Chapter Box).

'''Net zero CO''' 2 '''emissions are reached between 2050–2055 (2035–2070) in global emissions pathways limiting warming to 1.5°C (&gt;50%) with no or limited overshoot, and between 2070–2075 (2055–…) in pathways limiting warming to 2°C (&gt;67%) as reported in the AR6 scenarios database''' (median five-year interval and 5–95th percentile ranges). [[#footnote-015|5]] The variation of non-CO 2 emissions in 1.5°C–2°C pathways varies the available remaining carbon budget which can move the time of reaching net zero CO 2 in these pathways forward or backward. [[#footnote-014|6]] The shape of the CO 2 emissions reduction profile also affects the time of reaching net zero CO 2 (Figure 1c in this Cross-Chapter Box). Global emission pathways that more than halve CO 2 emissions from 2020 to 2030 can follow this rapid reduction by a more gradual decline towards net zero CO 2 and still limit warming to 1.5°C with no or limited overshoot, reaching the point of net zero after 2050. The literature since SR1.5 included a larger fraction of such pathways than were available at the time of SR1.5. This is the primary reason for the small backward shift in the median estimate of reaching global net zero CO 2 emissions in 1.5°C pathways collected in the AR6 scenario database compared to SR1.5. This does not mean that the world is assessed to have more time to rapidly reduce current emissions levels compared to SR1.5. The assessment of emissions reductions by 2030 and 2040 in pathways limiting warming to 1.5°C (&gt;50%) with no or limited overshoot has not changed substantially. It only means that the exact timing of reaching net zero CO 2 after a steep decline of CO 2 emissions until 2030 and 2040 can show some variation, and the SR1.5 median value of 2050 is still close to the middle of the current range (Figure 1c in this Cross-Chapter Box).

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[[File:5f3aa58eb3e450fb0bc2fbb9f2c3fc10 IPCC_AR6_WGIII_CCBox_3_Figure_1.png]]

'''Cross-Chapter Box 3, Figure 1 | Selected global CO 2 emissions trajectories with similar shape and different net zero CO 2 date (a), different shape and similar net zero CO 2 date (b), and similar peak warming, but varying shapes and net zero CO 2 dates (c). Funnels show pathways limiting warming to 1.''' 5 °C (&gt;50%) with no or limited overshoot (light blue) and limiting warming to 2°C (&gt;67%) (beige). Historic CO 2 emissions from [[IPCC:Wg3:Chapter:Chapter-2#2.2|Section 2.2]] (EDGAR v6).

'''Pathways following emissions levels projected from the implementation of Nationally Determined Contributions (NDCs) announced prior to COP26 until 2030 would result in substantially (&gt;0.1°C) exceeding 1.5°C.''' They would have to reach net zero CO 2 around 5–10 years later [[#footnote-013|7]] than in pathways with no or limited overshoot in order to reach the net negative emissions that would then be required to return warming to 1.5°C (&gt;50%) after a high overshoot by 2100. Those high overshoot pathways have higher transient warming and higher reliance on net negative CO 2 emissions towards the end of the 21st century. As they need to reach net zero CO 2 emissions in only limited amount of time but from much higher 2030 emissions levels, their post-2030 CO 2 emissions reduction rates are substantially higher (by around 30%) than in pathways limiting warming to 1.5°C with no or limited overshoot. ( [[#3.5|Section 3.5]] ).

'''Pathways following emissions levels projected from the implementation of NDCs announced prior to COP26 until 2030 would have to reach net zero CO''' 2 '''around 5 years earlier''' [[#footnote-012|8]] '''than cost-effective pathways that''' '''limit warming to 2°C (&gt;67%).''' While cost-effective pathways take around 50–55 years to reach net zero CO 2 emissions, those pathways would only have 35–40 years left for transitioning to net zero CO 2 from 2030 onwards, close to the transition times that 1.5°C pathways are faced with today. Current CO 2 emissions and 2030 emission levels projected under the NDCs announced prior to COP26 are in a similar range (Sections 3.5 and 4.2).

'''Net zero greenhouse gas (GHG) emissions'''

'''The amount of CO''' 2 '''-equivalent emissions and the point when net zero GHG emissions are reached in multi-GHG emissions pathways depends on the choice of GHG emissions metric.''' Various GHG emission metrics are available for this purpose. [[#footnote-011|9]] GWP-100 is the most commonly used metric for reporting CO 2 -equivalent emissions and is required for emissions reporting under the Rulebook of the Paris Agreement. (Cross-Chapter Box 2 in Chapter 2, Annex I and Annex II.9)

'''For most choices of GHG emissions metric, reaching net zero GHG emissions requires net negative CO''' 2 '''emissions in order to balance residual CH''' 4 ''', N''' 2 '''O and F-gas emissions.''' Under foreseen technology developments, some CH 4 , N 2 O and F-gas emissions from, for example, agriculture and industry, will remain over the course of this century. Net negative CO 2 emissions will therefore be needed to balance these remaining non-CO 2 GHG emissions to obtain net zero GHG emissions at a point in time after net zero CO 2 has been reached in emissions pathways. Both the amount of net negative CO 2 emissions and the time lag to reaching net zero GHG depend on the choice of GHG emission metric.

'''Reaching net zero GHG emissions globally in terms of''' '''GWP-100''' '''leads to a reduction in global warming from an earlier peak.''' This is due to net negative CO 2 emissions balancing the GWP-100-equivalent emissions of short-lived GHG emissions, which by themselves do not contribute to further warming if sufficiently declining ( [[#Fuglestvedt--2018|Fuglestvedt et al. 2018]] ; [[#Rogelj--2021|Rogelj et al. 2021]] ). Hence, 1.5°C–2°C emissions pathways in the AR6 scenario database that reach global net zero GHG emissions in the second half of the century show warming being halted at some peak value followed by a gradual decline towards the end of the century (AR6 WGI Chapter 1, Box 1.4).

'''Global net zero GHG emissions measured in terms of''' '''GWP-100''' '''are reached between 2095 and 2100 (2050–…)''' [[#footnote-010|10]] '''in emission pathways limiting warming to 1.5°C (&gt;50%) with no or limited overshoot (median and 5–95th percentile).''' Around 50% of pathways limiting warming to 1.5°C (&gt;50%) with no or limited overshoot and 70% of pathways limiting warming to 2°C (&gt;67%) do not reach net zero GHG emissions in terms of GWP-100 before 2100. These pathways tend to show less reduction in warming after the peak than pathways that reach net zero GHG emissions. For the subset of pathways that reach net zero GHG emissions before 2100, including around 90% of pathways that return warming to 1.5°C after a high overshoot (&gt;0.1°C) by 2100, the time lag between reaching net zero CO 2 and net zero GHG is 12–14 (7–39) years and the amount of net negative CO 2 emissions deployed to balance non-CO 2 emissions at the time of net zero GHG is around -7 (–10 to –4) GtCO 2 (range of medians and lowest 5th to highest 95 percentile across the four scenario classes that limit median warming to 2°C or lower) ( [[#3.3|Section 3.3]] and Table 3.2).

'''Sectoral and regional aspects of net zero'''

'''The timing of net zero CO''' 2 '''or GHG emissions may differ across regions and sectors. Achieving net zero emissions globally implies that some sectors and regions must reach net zero CO''' 2 '''or GHG ahead of the time of global net zero CO''' 2 '''or GHG if others reach it later.''' Similarly, some sectors and regions would need to achieve net negative CO 2 or GHG emissions to compensate for continued emissions by other sectors and regions after the global net zero year. Differences in the timing to reach net zero emissions between sectors and regions depend on multiple factors, including the potential of countries and sectors to reduce GHG emissions and undertake carbon dioxide removal (CDR), the associated costs, and the availability of policy mechanisms to balance emissions and removals between sectors and countries ( [[#Fyson--2020|Fyson et al. 2020]] ; [[#Strefler--2021a|Strefler et al. 2021a]] ; [[#van%20Soest--2021b|van Soest et al. 2021b]] ). A lack of such mechanisms could lead to higher global costs to reach net zero emissions globally, but less interdependencies and institutional needs ( [[#Fajardy--2020|Fajardy and Mac Dowell 2020]] ). Sectors will reach net zero CO 2 and GHG emissions at different times if they are aiming for such targets with sector-specific policies or as part of an economy-wide net zero emissions strategy integrating emissions reductions and removals across sectors. In the latter case, sectors with large potential for achieving net negative emissions would go beyond net zero to balance residual emissions from sectors with low potential, which in turn would take more time compared to the case of sector-specific action. Global pathways project global AFOLU emissions to reach global net zero CO 2 the earliest, around 2030 to 2035 in pathways to limit warming to 2°C (&gt;67%) or lower, by rapid reduction of deforestation and enhancing carbon sinks on land, although net zero GHG emissions from global AFOLU are typically reached 30 years later, if at all. The ability of global AFOLU CO 2 emissions to reach net zero as early as in the 2030s in modelled pathways hinges on optimistic assumptions about the ability to establish global cost-effective mechanisms to balance emissions reductions and removals across regions and sectors. These assumptions have been challenged in the literature and the ''Special Report on Climate Change and Land'' (IPCC SRCCL).

'''The adoption and implementation of net zero CO''' 2 '''or GHG emission targets by countries and regions also depends on equity and capacity criteria.''' The Paris Agreement recognises that peaking of emissions will occur later in developing countries (Art. 4.1). Just transitions to net zero CO 2 or GHG could be expected to follow multiple pathways, in different contexts. Regions may decide about net zero pathways based on their consideration of potential for rapid transition to low-carbon development pathways, the capacity to design and implement those changes, and perceptions of equity within and across countries. Cost-effective pathways from global models have been shown to distribute the mitigation effort unevenly and inequitably in the absence of financial support mechanisms and capacity building ( [[#Budolfson--2021|Budolfson et al. 2021]] ), and hence would require additional measures to become aligned with

equity considerations ( [[#Fyson--2020|Fyson et al. 2020]] ; [[#van%20Soest--2021b|van Soest et al. 2021b]] ). Formulation of net zero pathways by countries will benefit from clarity on scope, roadmaps and fairness ( [[#Rogelj--2021|Rogelj et al. 2021]] ; [[#Smith--2021|Smith 2021]] ). Achieving net zero emission targets relies on policies, institutions and milestones against which to track progress. Milestones can include emissions levels, as well as markers of technological diffusion.

'''The accounting of anthropogenic carbon dioxide removal on land matters for the evaluation of net zero CO''' 2 '''and net zero GHG strategies.''' Due to the use of different approaches between national inventories and global models, the current net CO 2 emissions are lower by 5.5 GtCO 2 , and cumulative net CO 2 emissions in modelled 1.5°C–2°C pathways would be lower by 104–170 GtCO 2 , if carbon dioxide removals on land are accounted based on national GHG inventories. National GHG inventories typically consider a much larger area of managed forest than global models, and on this area additionally consider the fluxes due to human-induced global environmental change (indirect effects) to be anthropogenic, while global models consider these fluxes to be natural. Both approaches capture the same land fluxes, only the accounting of anthropogenic vs natural emissions is different. Methods to convert estimates from global models to the accounting scheme of national GHG inventories will improve the use of emission pathways from global models as benchmarks against which collective progress is assessed. ( [[IPCC:Wg3:Chapter:Chapter-7#7.2.2|Section 7.2.2]] .5).

'''Net zero CO''' 2 '''and carbon neutrality have different meanings in this assessment, as is the case for net zero GHG and GHG neutrality.''' They apply to different boundaries in the emissions and removals being considered. Net zero (GHG or CO 2 ) refers to emissions and removals under the direct control or territorial responsibility of the reporting entity. In contrast, (GHG or carbon) neutrality includes anthropogenic emissions and anthropogenic removals within and also those beyond the direct control or territorial responsibility of the reporting entity. At the global scale, net zero CO 2 and carbon neutrality are equivalent, as is the case for net zero GHG and GHG neutrality. The term ‘climate neutrality’ is not used in this assessment because the concept of climate neutrality is diffuse, used differently by different communities, and not readily quantified.

Table 3.2 summarises the key characteristics for all temperature categories in terms of cumulative CO 2 emissions, near-term emission reductions, and the years of peak emission and net zero CO 2 and GHG emissions. The table shows again that many pathways in the literature limit global warming to 2 '''°''' C (&gt;67%) or limit warming to 1.5°C (&gt;50%) with no or limited overshoot compared to pre-industrial levels. Cumulative net CO 2 emissions from the year 2020 until the time of net zero CO 2 in pathways that limit warming to 1.5°C (&gt;50%) with no or limited overshoot are 510 (330–710) GtCO 2 and in pathways that limit warming to 2°C (&gt;67%), 890 (640–1160) GtCO 2 (see also Cross-Chapter Box 3 in this chapter). Mitigation pathways that limit warming to 2°C (&gt;67%) compared to pre-industrial levels are associated with net global GHG emissions of 44 (32–55) GtCO 2 -eq yr –1 by 2030 and 20 (13–26) GtCO 2 -eq yr –1 in 2050. These correspond to GHG emissions reductions of 21% (1–42%) by 2030, and 64% (53–77%) by 2050 relative to 2019 emission levels. Pathways that limit global warming to 1.5°C (&gt;50%) with no or limited overshoot require a further acceleration in the pace of the transformation, with GHG emissions reductions of 43% (34–60%) by 2030 and 84% (73–98%) in 2050 relative to modelled 2019 emission levels. The likelihood of limiting warming to below 1.5°C (&gt;50%) with no or limited overshoot of the most stringent mitigation pathways in the literature (C1) has declined since SR1.5. This is because emissions have risen since 2010 by about 9 GtCO 2 yr –1 , resulting in relatively higher near-term emissions of the AR6 pathways by 2030 and slightly later dates for reaching net zero CO 2 emissions compared to SR1.5.

Given the larger contribution of scenarios in the literature that aim to reduce net negative emissions, emission reductions are somewhat larger in the short term compared to similar categories in the IPCC SR1.5. At the same time, the year of net zero emissions is somewhat later (but only if these rapid, short-term emission reductions are achieved). The scenarios in the literature in C1–C3 show a peak in global emissions before 2025. Not achieving this requires a more rapid reduction after 2025 to still meet the Paris goals ( [[#3.5|Section 3.5]] ).

'''Table 3.2 | GHG, CO''' 2 '''emissions and warming characteristics of different mitigation pathways submitted to the AR6 scenarios database and as categorised in the climate assessment.'''

{| class="wikitable"
|-
! colspan="3"| '''p50 [p5–p95]''' ''a''

! colspan="3"| '''GHG emissionsGt CO''' ''2'' '''-eq/yr''' ''g''

! colspan="3"| '''GHG emissions reductions from 2019%''' ''h''

! colspan="4"| '''Emissions milestones''' ''i,j''

! colspan="2"| '''Cumulative CO''' ''2'' '''emissionsGt CO''' ''2'' ''m''

! '''Cumulative net-negative CO''' ''2'' '''emissionsGt CO''' ''2''

! colspan="2"| '''Global mean temperature changes 50% probability''' ''n'' '''°C'''

! colspan="3"| '''Likelihood of peak global warming staying below (%)''' ''o''

! colspan="3"| '''Time when specific global warming levels are reached (with a 50% probability)'''

|-
! '''Category''' ''b, c, d'' '''[# path-ways]'''

! '''Category/ subset label'''

! '''WG I SSP &amp; WG III IPs/IMPs alignment''' ''e, f''

! '''2030'''

! '''2040'''

! '''2050'''

! '''2030'''

! '''2040'''

! '''2050'''

! '''Peak CO''' ''2'' '''emissions (% peak before 2100)'''

! '''Peak GHG emissions (% peak before 2100)'''

! '''Net-zero CO''' ''2'' '''(% net-zero pathways)'''

! '''Net-zero GHGs''' ''k, l'' '''(% net-zero pathways)'''

! '''2020 to net-zero CO''' ''2''

! '''2020–2100'''

! '''Year of net-zero CO''' ''2'' '''to 2100'''

! '''at peak warming'''

! '''2100'''

! '''&lt;1.5°C'''

! '''&lt;2°C'''

! '''&lt;3°C'''

! '''1.5°C'''

! '''2°C'''

! '''3°C'''

|-
! colspan="3"| Modelled global emissions pathways categorised by projected global warming levels (GWL). Detailed likelihood definitions are provided in SPM Box1.

The five illustrative scenarios (SSPx-yy) considered by AR6 WGI and the Illustrative (Mitigation) Pathways assessed in WGIII are aligned with the temperature categories and are indicated in a separate column. Global emission pathways contain regionally differentiated information. This assessment focuses on their global characteristics.

! colspan="3"| Projected median annual GHG emissions in the year across the scenarios, with the 5th–95th percentile in brackets.

Modelled GHG emissions in 2019: 55 [53–58] Gt CO 2 -eq.

! colspan="3"| Projected median GHG emissions reductions of pathways in the year across the scenarios compared to modelled 2019, with the 5th–95th percentile in brackets. Negative numbers indicate increase in emissions compared to 2019.

! colspan="2"| Median 5-year intervals at which projected CO 2 &amp; GHG emissions peak, with the 5th–95th percentile interval in square brackets. Percentage of peaking pathways is denoted in round brackets.

Three dots (…) denotes emissions peak in 2100 or beyond for that percentile.

! colspan="2"| Median 5-year intervals at which projected CO 2 &amp; GHG emissions of pathways in this category reach net-zero, with the 5th–95th percentile interval in square brackets. Percentage of net zero pathways is denoted in round brackets.

Three dots (…) denotes net zero not reached for that percentile.

! colspan="2"| Median cumulative net CO 2 emissions across the projected scenarios in this category until reaching net-zero or until 2100, with the 5th–95th percentile interval in square brackets.

! Median cumulative net-negative CO 2 emissions between the year of net-zero CO 2 and 2100. More net-negative results in greater temperature declines after peak.

! colspan="2"| Projected temperature change of pathways in this category (50% probability across the range of climate uncertainties), relative to 1850–1900, at peak warming and in 2100, for the median value across the scenarios and the 5th–95th percentile interval in square brackets.

! colspan="3"| Median likelihood that the projected pathways in this category stay below a given global warming level, with the 5th–95th percentile interval in square brackets.

! colspan="3"| Median 5-year intervals at which specific global warming levels are reached (50% probability), with the 5th–95th percentile interval in square brackets. Percentage of pathways is denoted in round brackets.

Three dots (…) denotes temperature does not exceed the GWL by 2100 for that percentile.

|-
| '''C1 [97]'''

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

|
| 31

[21–36]

| 17

[6–23]

| 9

[1–15]

| 43

[34–60]

| 69

[58–90]

| 84

[73–98]

| rowspan="4" colspan="2"| 2020–2025 (100%)

[2020–2025]

| rowspan="4"| 2050–2055 (100%)

[2035–2070]

| 2095–2100 (52%)

[2050–…]

| 510

[330–710]

| 320

[–210–570]

| –220

[–660-–20]

| 1.6

[1.4–1.6]

| 1.3

[1.1–1.5]

| 38

[33–58]

| 90

[86–97]

| 100

[99–100]

| 2030–2035 (91%)

[2030–…]

| …–… (0%)

[…–…]

| …–… (0%)

[…–…]

|-
| '''C1a [50]'''

| '''… with net-zero GHGs'''

| SSP1-1.9, IMP-SP

IMP-LD

| 33

[22–37]

| 18

[6–24]

| 8

[0–15]

| 41

[31–59]

| 66

[58–89]

| 85

[72–100]

| 2070–2075 (100%)

[2050–2090]

| 550

[340–760]

| 160

[–220–620]

| –360

[–680-–140]

| 1.6

[1.4–1.6]

| 1.2

[1.1–1.4]

| 38

[34–60]

| 90

[85–98]

| 100

[99–100]

| 2030–2035 (90%)

[2030–…]

| …–… (0%)

[…–…]

| …–… (0%)

[…–…]

|-
| rowspan="2"| '''C1b [47]'''

| rowspan="2"| '''… without net-zero GHGs'''

| IMP-Ren

| rowspan="2"| 29

[21–36]

| rowspan="2"| 16

[7–21]

| rowspan="2"| 9

[4–13]

| rowspan="2"| 48

[35–61]

| rowspan="2"| 70

[62–87]

| rowspan="2"| 84

[76–93]

| rowspan="2"| …–… (0%)

[…–…]

| rowspan="2"| 460

[320–590]

| rowspan="2"| 360

[10–540]

| rowspan="2"| –60

[–440–0]

| rowspan="2"| 1.6

[1.5–1.6]

| rowspan="2"| 1.4

[1.3–1.5]

| rowspan="2"| 37

[33–56]

| rowspan="2"| 89

[87–96]

| rowspan="2"| 100

[99–100]

| rowspan="2"| 2030–2035 (91%)

[2030–…]

| rowspan="2"| …–… (0%)

[…–…]

| rowspan="2"| …–… (0%)

[…–…]

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

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

| '''IMP-Neg'''

| rowspan="2"| 42

[31–55]

| rowspan="2"| 25

[17–34]

| rowspan="2"| 14

[5–21]

| rowspan="2"| 23

[0–44]

| rowspan="2"| 55

[40–71]

| rowspan="2"| 75

[62–91]

| colspan="2"| 2020–2025 (100%)

| rowspan="2"| 2055–2060 (100%)

[2045–2070]

| rowspan="2"| 2070–2075 (87%)

[2055–…]

| rowspan="2"| 720

[530–930]

| rowspan="2"| 400

[–90–620]

| rowspan="2"| –360

[–680-–60]

| rowspan="2"| 1.7

[1.5–1.8]

| rowspan="2"| 1.4

[1.2–1.5]

| rowspan="2"| 24

[15–42]

| rowspan="2"| 82

[71–93]

| rowspan="2"| 100

[99–100]

| rowspan="2"| 2030–2035 (100%)

[…–…]

| rowspan="2"| …–… (0%)

[…–…]

| rowspan="2"| …–… (0%)

[…–…]

|-
|
| [2020–2030]

| [2020–2025]

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

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

|
| rowspan="2"| 44

[32–55]

| rowspan="2"| 29

[20–36]

| rowspan="2"| 20

[13–26]

| rowspan="2"| 21

[1–42]

| rowspan="2"| 46

[34–63]

| rowspan="2"| 64

[53–77]

| colspan="2"| 2020–2025 (100%)

| rowspan="2"| 2070–2075 (93%)

[2055–…]

| rowspan="2"| …–… (30%)

[2075–…]

| rowspan="2"| 890

[640–1160]

| rowspan="2"| 800

[510–1140]

| rowspan="2"| –40

[–290–0]

| rowspan="2"| 1.7

[1.6–1.8]

| rowspan="2"| 1.6

[1.5–1.8]

| rowspan="2"| 20

[13–41]

| rowspan="2"| 76

[68–91]

| rowspan="2"| 99

[98–100]

| rowspan="2"| 2030–2035 (100%)

[…–…]

| rowspan="2"| …–… (0%)

[…–…]

| rowspan="2"| …–… (0%)

[…–…]

|-
|
| [2020–2030]

| [2020–2025]

|-
| '''C3a [204]'''

| '''… with action starting in 2020'''

| SSP1-2.6

| 40

[30–49]

| 29

[21–36]

| 20

[14–27]

| 27

[13–45]

| 47

[35–63]

| 63

[52–76]

| colspan="2"| 2020–2025 (100%)

[2020–2025]

| 2070–2075 (91%)

[2055–…]

| …–… (24%)

[2080–…]

| 860

[640–1180]

| 790

[480–1150]

| –30

[–280–0]

| 1.7

[1.6–1.8]

| 1.6

[1.5–1.8]

| 21

[14–42]

| 78

[69–91]

| 100

[98–100]

| 2030–2035 (100%)

[2030–2040]

| …–… (0%)

[…–…]

| …–… (0%)

[…–…]

|-
| '''C3b [97]'''

| '''… NDCs until 2030'''

| IMP-GS

| 52

[47–56]

| 29

[20–36]

| 18

[10–25]

| 5

[0–14]

| 46

[34–63]

| 68

[56–82]

| rowspan="3" colspan="2"| 2020–2025 (100%)

[2020–2030]

| 2065–2070 (97%)

[2055–2090]

| …–… (41%)

[2075–…]

| 910

[720–1150]

| 800

[560–1050]

| –60

[–300–0]

| 1.8

[1.6–1.8]

| 1.6

[1.5–1.7]

| 17

[12–35]

| 73

[67–87]

| 99

[98–99]

| 2030–2035 (100%)

[2030–2035]

| …–… (0%)

[…–…]

| …–… (0%)

[…–…]

|-
| '''C4 [159]'''

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

|
| 50

[41–56]

| 38

[28–44]

| 28

[19–35]

| 10

[0–27]

| 31

[20–50]

| 49

[35–65]

| 2080–2085 (86%)

[2065–…]

| …–… (31%)

[2075–…]

| 1210

[970–1490]

| 1160

[700–1490]

| –30

[–390–0]

| 1.9

[1.7–2.0]

| 1.8

[1.5–2.0]

| 11

[7–22]

| 59

[50–77]

| 98

[95–99]

| 2030–2035 (100%)

[2030–2035]

| …–… (0%)

[…–…]

| …–… (0%)

[…–…]

|-
| '''C5 [212]'''

| '''limit warming to 2.5°C (&gt;50%)'''

|
| 52

[46–56]

| 45

[37–53]

| 39

[30–49]

| 6

[–1–18]

| 18

[4–33]

| 29

[11–48]

| …–… (41%)

[2080–…]

| …–… (12%)

[2090–…]

| 1780

[1400–2360]

| 1780

[1260–2360]

| 0

[–160–0]

| 2.2

[1.9–2.5]

| 2.1

[1.9–2.5]

| 4

[0–10]

| 37

[18–59]

| 91

[83–98]

| 2030–2035 (100%)

[2030–2035]

| 2060–2065 (99%)

[2050–2095]

| …–… (0%)

[…–…]

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

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

| rowspan="2"| SSP2-4.5

Mod-Act

| rowspan="2"| 54

[50–62]

| rowspan="2"| 53

[48–61]

| rowspan="2"| 52

[45–57]

| rowspan="2"| 2

[–10–11]

| rowspan="2"| 3

[–14–14]

| rowspan="2"| 5

[–2–18]

| 2030–2035 (96%)

| 2020–2025 (97%)

| rowspan="5" colspan="2"| no net-zero

| rowspan="5"| no net-zero

| rowspan="2"| 2790

[2440–3520]

| rowspan="5"| no net-zero

| rowspan="5"| temperature does not peak by 2100

| rowspan="2"| 2.7

[2.4–2.9]

| rowspan="2"| 0

[0–0]

| rowspan="2"| 8

[2–18]

| rowspan="2"| 71

[53–88]

| rowspan="2"| 2030–2035 (100%)

[2030–2035]

| rowspan="2"| 2050–2055 (100%)

[2045–2060]

| rowspan="2"| …–… (0%)

[…–…]

|-
| colspan="2"| [2020–2090]

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

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

| rowspan="2"| SSP3-7.0

Cur-Pol

| rowspan="2"| 62

[53–69]

| rowspan="2"| 67

[56–76]

| rowspan="2"| 70

[58–83]

| rowspan="2"| –11

[–18–3]

| rowspan="2"| –19

[–31–1]

| rowspan="2"| –24

[–41––2]

| 2085–2090 (57%)

| 2090–2095 (56%)

| rowspan="2"| 4220

[3160–5000]

| rowspan="2"| 3.5

[2.8–3.9]

| rowspan="2"| 0

[0–0]

| rowspan="2"| 0

[0–2]

| rowspan="2"| 22

[7–60]

| rowspan="2"| 2030–2035 (100%)

[2030–2035]

| rowspan="2"| 2045–2050 (100%)

[2040–2055]

| rowspan="2"| 2080–2085 (100%)

[2070–2100]

|-
| colspan="2"| [2040–…]

|-
| '''C8 [29]'''

| '''exceed warming of 4°C (''' ≥ '''50%)'''

| SSP5-8.5

| 71

[69–81]

| 80

[78–96]

| 88

[82–112]

| –20

[–34-–17]

| –35

[–65-–29]

| –46

[–92-–36]

| colspan="2"| 2080–2085 (90%)

[2070–…]

| 5600

[4910–7450]

| 4.2

[3.7–5.0]

| 0

[0–0]

| 0

[0–0]

| 4

[0–11]

| 2030–2035 (100%)

[2030–2035]

| 2040–2045 (100%)

[2040–2050]

| 2065–2070 (100%)

[2060–2075]

|}

a Values in the table refer to the 50th and [5th–95th] percentile values across the pathways falling within a given category as defined in Box SPM.1. For emissions-related columns these values relate to the distribution of all the pathways in that category. Harmonised emissions values are given for consistency with projected global warming outcomes using climate emulators. Based on the assessment of climate emulators in AR6 WGI (WG1 Chapter 7, Box 7.1), two climate emulators are used for the probabilistic assessment of the resulting warming of the pathways. For the ‘Temperature change’ and ‘Likelihood’ columns, the single upper-row values represent the 50th percentile across the pathways in that category and the median [50th percentile] across the warming estimates of the probabilistic MAGICC climate model emulator. For the bracketed ranges, the median warming for every pathway in that category is calculated for each of the two climate model emulators (MAGICC and FaIR). Subsequently, the 5th and 95th percentile values across all pathways for each emulator are calculated. The coolest and warmest outcomes (i.e., the lowest p5 of two emulators, and the highest p95, respectively) are shown in square brackets. These ranges therefore cover both the uncertainty of the emissions pathways as well as the climate emulators’ uncertainty.

b For a description of pathways categories see Box SPM.1 and Table 3.1.

c All global warming levels are relative to 1850–1900. (See footnote n below and Box SPM.1 45 for more details.)

d C3 pathways are sub-categorised according to the timing of policy action to match the emissions pathways in Figure SPM.4. Two pathways derived from a cost-benefit analysis have been added to C3a, whilst 10 pathways with specifically designed near-term action until 2030, whose emissions fall below those implied by NDCs announced prior to COP26, are not included in either of the two subsets.

e Alignment with the categories of the illustrative SSP scenarios considered in AR6 WGI, and the Illustrative (Mitigation) Pathways (IPs/IMPs) of WGIII. The IMPs have common features such as deep and rapid emissions reductions, but also different combinations of sectoral mitigation strategies. See Box SPM.1 for an introduction of the IPs and IMPs, and [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-3 Chapter 3] for full descriptions. {3.2, 3.3, Annex III.II.2.4}

f The Illustrative Mitigation Pathway ‘Neg’ has extensive use of carbon dioxide removal (CDR) in the AFOLU, energy and the industry sectors to achieve net negative emissions. Warming peaks around 2060 and declines to below 1.5°C (50% likelihood) shortly after 2100. Whilst technically classified as C3, it strongly exhibits the characteristics of C2 high-overshoot pathways, hence it has been placed in the C2 category. See Box SPM.1 for an introduction of the IPs and IMPs.

g The 2019 range of harmonised GHG emissions across the pathways [53–58 GtCO 2 -eq] is within the uncertainty ranges of 2019 emissions assessed in [[IPCC:Wg3:Chapter:Chapter-2|Chapter 2]] [53–66 GtCO 2 -eq]. 49 (Figure SPM.1, Figure SPM.2, Box SPM.1)

h Rates of global emission reduction in mitigation pathways are reported on a pathway-by-pathway basis relative to harmonised modelled global emissions in 2019 rather than the global emissions reported in SPM Section B and Chapter 2; this ensures internal consistency in assumptions about emission sources and activities, as well as consistency with temperature projections based on the physical climate science assessment by WGI. 49 {Annex III.II.2.5} . Negative values (e.g., in C7, C8) represent an increase in emissions.

i Emissions milestones are provided for five-year intervals in order to be consistent with the underlying five-year time-step data of the modelled pathways. Peak emissions (CO 2 and GHGs) are assessed for five-year reporting intervals starting in 2020. The interval 2020–2025 signifies that projected emissions peak as soon as possible between 2020 and at latest before 2025. The upper five-year interval refers to the median interval within which the emissions peak or reach net zero. Ranges in square brackets underneath refer to the range across the pathways, comprising the lower bound of the 5th percentile five-year interval and the upper bound of the 95th percentile five-year interval. Numbers in round brackets signify the fraction of pathways that reach specific milestones.

j Percentiles reported across all pathways in that category include those that do not reach net zero before 2100 (fraction of pathways reaching net zero is given in round brackets). If the fraction of pathways that reach net zero before 2100 is lower than the fraction of pathways covered by a percentile (e.g., 0.95 for the 95th percentile), the percentile is not defined and denoted with ‘…’. The fraction of pathways reaching net zero includes all with reported non-harmonised, and/or harmonised emissions profiles that reach net zero. Pathways were counted when at least one of the two profiles fell below 100 MtCO 2 yr –1 until 2100.

k The timing of net zero is further discussed in SPM C2.4 and Cross-Chapter Box 3 in [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-3 Chapter 3] on net zero CO 2 and net zero GHG emissions.

l For cases where models do not report all GHGs, missing GHG species are infilled and aggregated into a Kyoto basket of GHG emissions in CO 2 -eq defined by the 100-year global warming potential. For each pathway, reporting of CO 2 , CH 4 , and N 2 O emissions was the minimum required for the assessment of the climate response and the assignment to a climate category. Emissions pathways without climate assessment are not included in the ranges presented here. {See Annex III.II.2.5 }

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

n Global mean temperature change for category (at peak, if peak temperature occurs before 2100, and in 2100) relative to 1850–1900, based on the median global warming for each pathway assessed using the probabilistic climate model emulators calibrated to the AR6 WGI assessment. 12 (See also Box SPM.1) {Annex III.II.2.5; WGI Cross-Chapter Box 7.1}

o Probability of staying below the temperature thresholds for the pathways in each category, taking into consideration the range of uncertainty from the climate model emulators consistent with the AR6 WGI assessment. The probabilities refer to the probability at peak temperature. Note that in the case of temperature overshoot (e.g., category C2 and some pathways in C1), the probabilities of staying below at the end of the century are higher than the probabilities at peak temperature.

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