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

== 3.1 Introduction ==

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=== 3.1.1 Assessment of Mitigation Pathways and Their Compatibility With Long-term Goals ===

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[https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-3 Chapter 3] takes a long-term perspective on climate change mitigation pathways. Its focus is on the implications of long-term targets for the required short- and medium-term system changes and associated greenhouse gas (GHG) emissions. This focus dictates a more global view and on issues related to path-dependency and up-scaling of mitigation options necessary to achieve different emissions trajectories, including particularly deep mitigation pathways that require rapid and fundamental changes.

Stabilising global average-temperature change requires reducing CO 2 emissions to net zero. Thus, a central cross-cutting topic within the chapter is the timing of reaching net zero CO 2 emissions and how a ‘balance between anthropogenic emissions by sources and removals by sinks’ could be achieved across time and space. This includes particularly the increasing body of literature since the ''IPCC Special Report on Global Warming of 1.5°C'' (SR1.5) which focuses on net zero CO 2 emissions pathways that avoid temperature overshoot and hence do not rely on net negative CO 2 emissions. The chapter conducts a systematic assessment of the associated economic costs as well as the benefits of mitigation for other societal objectives, such as the Sustainable Development Goals (SDGs). In addition, the chapter builds on SR1.5 and introduces a new conceptual framing for the assessment of possible social, economic, technical, political, and geophysical ‘feasibility’ concerns of alternative pathways, including the enabling conditions that would need to fall into place so that stringent climate goals become attainable.

The structure of the chapter is as follows: [[#3.2|Section 3.2]] introduces different types of mitigation pathways as well as the available modelling. [[#3.3|Section 3.3]] explores different emissions trajectories given socio-economic uncertainties and consistent with different long-term climate outcomes. A central element in this section is the systematic categorisation of the scenario space according to key characteristics of the mitigation pathways (including e.g., global average-temperature change, socio-economic development, technology assumptions, etc.). In addition, the section introduces selected Illustrative Mitigation Pathways (IMPs) that are used across the whole report. [[#3.4|Section 3.4]] conducts a sectoral analysis of the mitigation pathways, assessing the pace and direction of systems changes across sectors. Among others, this section aims at the integration of the sectoral information across AR6 WGIII chapters through a comparative assessment of the sectoral dynamics in economy-wide systems models compared to the insights from bottom-up sectoral models (from Chapters 6 to 11). [[#3.5|Section 3.5]] focuses on the required timing of mitigation actions, and the implication of near-term choices for the attainability of a range of long-term climate goals. After having explored the underlying systems transitions and the required timing of the mitigation actions, [[#3.6|Section 3.6]] assesses the economic implications, mitigation costs and benefits; and [[#3.7|Section 3.7]] assesses related co-benefits, synergies, and possible trade-offs for sustainable development and other societal (non-climate) objectives. [[#3.8|Section 3.8]] assumes a central role in the chapter and introduces a multidimensional feasibility metric that permits the evaluation of mitigation pathways across a range of feasibility concerns. Finally, methods of the assessment and knowledge gaps are discussed in [[#3.9|Section 3.9]] , followed by Frequently Asked Questions (FAQs).

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=== 3.1.2 Linkages to Other Chapters in the Report ===

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[https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-3 Chapter 3] is linked to many other chapters in the report. The most important connections exist with [[IPCC:Wg3:Chapter:Chapter-4|Chapter 4]] on mitigation and development pathways in the near to mid-term; with the sectoral chapters (Chapters 6–11); with the chapters dealing with cross-cutting issues (Chapters 12 and 17, e.g., feasibility); and finally also with AR6 WGI and WGII.

Within the overall framing of the AR6 report, [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-3 Chapter 3] and [[IPCC:Wg3:Chapter:Chapter-4|Chapter 4]] provide important complementary views of the required systems transitions across different temporal and spatial scales. While [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-3 Chapter 3] focuses on the questions concerning the implications of the long-term objectives for the medium-to-near-term transformations, [[IPCC:Wg3:Chapter:Chapter-4|Chapter 4]] comes from the other direction, and focuses on current near-term trends and policies (such as the Nationally Determined Contributions – NDCs) and their consequences with regards to GHG emissions. The latter chapter naturally focuses much more on the regional and national dimensions, and the heterogeneity of current and planned policies. Bringing together the information from these two chapters enables the assessment of whether current and planned actions are consistent with the required systems changes for the long-term objectives of the Paris Agreement.

Important other linkages comprise the collaboration with the ‘sectoral’ Chapters 6 to 11 to provide an integrated cross-sectoral perspective. This information (including information also from the sectoral chapters) is taken up ultimately also by [[IPCC:Wg3:Chapter:Chapter-5|Chapter 5]] on demand/services and [[IPCC:Wg3:Chapter:Chapter-12|Chapter 12]] for a further assessment of sectoral potential and costs.

Linkages to other chapters exist also on the topic of feasibility, which are informed by the policy, the sectoral and the demand chapters, the technology and finance chapters, as well as [[IPCC:Wg3:Chapter:Chapter-4|Chapter 4]] on national circumstances.

Close collaboration with WGI permitted the use of AR6-calibrated emulators, which assure full consistency across the different working groups. Linkages to WGII concern the assessment of macroeconomic benefits of avoided impacts that are put into the context of mitigation costs as well as co-benefits and trade-offs for sustainable development.

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=== 3.1.3 Complementary Use of Large Scenario Ensembles and a Limited Set of Illustrative Mitigation Pathways (IMPs) ===

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The assessment of mitigation pathways explores a wide scenario space from the literature within which seven Illustrative Pathways (IPs) are explored. The overall process is indicated in Figure 3.5a.

For a comprehensive assessment, a large ensemble of scenarios is collected and made available through an interactive [https://data.ece.iiasa.ac.at/ar6/ AR6 Scenarios Database] [[#footnote-016|4]] . The collected information is shared across the chapters of AR6 and includes more than 3000 different pathways from a diverse set of studies. After an initial screening and quality control, scenarios were further vetted to assess if they sufficiently represented historical trends (Annex III.II.3.1). Subsequently, the climate consequences of each scenario were assessed using the climate emulator (leading to further classification). The assessment in [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-3 Chapter 3] is, however, not limited to the scenarios from the database, and wherever necessary other literature sources are also assessed in order to bring together multiple lines of evidence.

In parallel, based on the overall AR6 assessment, seven illustrative pathways (IP) were defined representing critical mitigation strategies discussed in the assessment. The seven pathways are composed of two sets: (i) one set of five Illustrative Mitigation Pathways (IMPs) and (ii) one set of two reference pathways illustrative for high emissions. The IMPs are on the one hand representative of the scenario spac but also help to communicate archetypes of distinctly different systems transformations and related policy choices. Subsequently, seven scenarios were selected from the full database that fitted these storylines of each IP best. For these scenarios more strict vetting criteria were applied. The selection was done by first applying specific filters based on the storyline followed by a final selection (Box 3.1 and Figure 3.5a).

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=== Box 3.1 | Illustrative Mitigation Pathways (IMPs) ===

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The literature shows a wide range of possible emissions trajectories, depicting developments in the absence of new climate policies or showing pathways consistent with the Paris Agreement. From the literature, a set of five Illustrative Mitigation Pathways (IMPs) was selected to denote implications of choices on socio-economic development and climate policies, and the associated transformations of the main GHG-emitting sectors (Figure 3.5b). The IMPs include a set of transformative pathways that illustrate how choices may lead to distinctly different transformations that may keep temperature increase to below 2°C (&gt;67%) or 1.5°C. These pathways illustrate the implications of a focus on renewable energy such as solar and wind; reduced energy demand; extensive use of CDR in the energy and the industry sectors to achieve net negative emissions and reliance on other supply-side measures; strategies that avoid net negative carbon emissions, and gradual strengthening. In addition, one IMP explores how climate policies consistent with keeping limit warming to 1.5C (&gt;50%) can be combined with a broader shift towards sustainable development. These IMPs are used in various chapters, exploring for instance their implications for different sectors, regions, and innovation characteristics (Figure 3.5b).

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