Editing IPCC:AR6/WGI/Chapter-1 (section)

== 1.1 Report and Chapter Overview ==

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The role of the Intergovernmental Panel on Climate Change (IPCC) is to critically assess the scientific, technical and socio-economic information relevant to understanding the physical science and impacts of human-induced climate change and natural variations, including the risks, opportunities and options for adaptation and mitigation. This task is performed through a comprehensive assessment of the scientific literature. The robustness of IPCC assessments stems from the systematic consideration and combination of multiple lines of independent evidence. In addition, IPCC reports undergo one of the most comprehensive, objective, open and transparent review and revision processes ever employed for science assessments.

Starting with the First Assessment Report (FAR; [[#IPCC--1990a|IPCC, 1990a]] ) the IPCC assessments have been structured into three Working Groups. Working Group I (WGI) assesses the physical science basis of climate change, Working Group II (WGII) assesses associated impacts, vulnerability and adaptation options, and Working Group III (WGIII) assesses mitigation response options. Each report builds on the earlier comprehensive assessments by incorporating new research and updating previous findings. The volume of knowledge assessed and the cross-linkages between the three Working Groups have substantially increased over time.

As part of its Sixth Assessment Cycle, from 2015 to 2022, the IPCC is producing three Working Group Reports, three targeted Special Reports, a Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, and a Synthesis Report. The AR6 Special Reports covered the topics of Global Warming of 1.5°C (SR1.5; [[#IPCC--2018|IPCC, 2018]] ), Climate Change and Land (SRCCL; [[#IPCC--2019a|IPCC, 2019a]] ) and The Ocean and Cryosphere in a Changing Climate (SROCC; [[#IPCC--2019b|IPCC, 2019b]] ). The SR1.5 and SRCCL are the first IPCC reports jointly produced by all three Working Groups. This evolution towards a more integrated assessment reflects a broader understanding of the interconnectedness of the multiple dimensions of climate change.

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=== 1.1.1 The AR6 WGI Report ===

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The Sixth Assessment Report (AR6) of the IPCC marks more than 30 years of global collaboration to describe and understand, through expert assessments, one of the defining challenges of the 21st century: human-induced climate change. Since the inception of the IPCC in 1988, our understanding of the physical science basis of climate change has advanced markedly. The amount and quality of instrumental observations and information from paleoclimate archives have substantially increased. Understanding of individual physical, chemical and biological processes has improved. Climate model capabilities have been enhanced, through the more realistic treatment of interactions among the components of the climate system, and improved representation of the physical processes, in line with the increased computational capacities of the world’s supercomputers.

This Report assesses both observed changes, and the components of these changes that are attributable to anthropogenic influence (i.e., human-induced), distinguishing between anthropogenic and naturally forced changes (Chapter 3, Sections [[#1.2.1.1|1.2.1.1]] and [[#1.4.1|1.4.1]] , and the Cross-Working Group Box on Attribution). The core assessment conclusions from previous IPCC reports are confirmed or strengthened in this report, indicating the robustness of our understanding of the primary causes and consequences of anthropogenic climate change.

The WGI contribution to AR6 is focused on physical and biogeochemical climate science information, with particular emphasis on regional climate changes. These are relevant for mitigation, adaptation and risk assessment in the context of complex and evolving policy settings, including the Paris Agreement, the global stocktake, the Sendai Framework and the Sustainable Development Goals (SDGs) Framework.

The core of this report consists of 12 chapters plus the [[IPCC:Wg1:Chapter:Atlas|Atlas]] (Figure 1.1), which can together be grouped into three categories (excluding this framing chapter):

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[[File:99f437809496408df2aaa7993979df85 IPCC_AR6_WGI_Figure_1_1.png|thumb|400x300px]]

'''Figure 1.1 |''' '''The structure of the AR6 WGI Report''' . Shown are the three pillars of the AR6 WGI, its relation to the WGII and WGIII contributions, and the cross-working-group AR6 Synthesis Report (SYR).

'''Large-scale Information (Chapters 2, 3 and 4).''' These chapters assess climate information from global to continental or ocean-basin scales. [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] presents an assessment of the changing state of the climate system, including the atmosphere, biosphere, ocean and cryosphere. [[IPCC:Wg1:Chapter:Chapter-3|Chapter 3]] continues with an assessment of the human influence on this changing climate, covering the attribution of observed changes, and introducing the fitness-for-purpose approach for the evaluation of climate models used to conduct the attribution studies. Finally, [[IPCC:Wg1:Chapter:Chapter-4|Chapter 4]] assesses climate change projections, from the near to the long term, including climate change beyond 2100, as well as the potential for abrupt and ‘low-likelihood, high-impact’ outcomes.

'''Process Understanding (Chapters 5, 6, 7, 8 and 9).''' These five chapters provide end-to-end assessments of fundamental Earth system processes and components: the carbon budget and biogeochemical cycles (Chapter 5), short-lived climate forcers and their links to air quality (Chapter 6), the Earth’s energy budget and climate sensitivity (Chapter 7), the water cycle (Chapter 8), and the ocean, cryosphere and sea level changes (Chapter 9). All these chapters provide assessments of observed changes, including relevant paleoclimatic information and understanding of processes and mechanisms as well as projections and model evaluation.

'''Regional Information (Chapters 10, 11, 12 and Atlas).''' New knowledge on climate change at regional scales is reflected in this report with four chapters covering regional information. [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-10 Chapter 10] provides a framework for assessment of regional climate information, including methods, physical processes, an assessment of observed changes at regional scales, and the performance of regional models. [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-11 Chapter 11] addresses extreme weather and climate events, including temperature, precipitation, flooding, droughts and compound events. [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-12 Chapter 12] provides a comprehensive, region-specific assessment of changing climatic conditions that may be hazardous or favourable (hence influencing climate risk) for various sectors to be assessed in WGII. Lastly, the [[IPCC:Wg1:Chapter:Atlas|Atlas]] assesses and synthesizes regional climate information from the whole report, focussing on the assessments of mean changes in different regions and on model assessments for the regions. It also introduces the online Interactive Atlas, a novel compendium of global and regional climate change observations and projections. It includes a visualization tool, which combines various warming levels and scenarios on multiple scales of space and time.

Embedded in the chapters are '''Cross-Chapter Boxes''' that highlight cross-cutting issues. Each chapter also includes an '''Executive Summary (ES),''' and several '''Frequently Asked Questions (FAQs)''' . To enhance traceability and reproducibility of report figures and tables, detailed information on the input data used to create them, as well as links to archived code, are provided in The '''Input Data Tables''' in chapter '''Supplementary Material''' . Additional metadata on the model input datasets is provided via the report website ( https://www.ipcc.ch/assessment-report/ar6/ ).

The AR6 WGI Report includes a '''Summary for Policymakers (SPM)''' and a '''Technical Summary (TS)''' . The integration among the three IPCC Working Groups is strengthened by the inclusion of The '''Cross-Working-''' '''Group Glossary''' .

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=== 1.1.2 Rationale for the New AR6 WGI Structure and Its Relation to the Previous AR5 WGI Report ===

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The AR6 WGI report, as a result of its scoping process, is structured around topics such as large-scale information, process understanding and regional information (Figure 1.1). This represents a rearrangement relative to the structure of the WGI contribution to the IPCC Fifth Assessment Report (AR5; [[#IPCC--2013a|IPCC, 2013a]] ), as summarized in Figure 1.2. The AR6 approach aims at a greater visibility of key knowledge developments that are potentially relevant for policymakers, including climate change mitigation, regional adaptation planning based on a risk management framework, and the global stocktake.

Two key subjects presented separately in AR5, paleoclimate and model evaluation, are now distributed among multiple AR6 WGI chapters. Various other cross-cutting themes are also distributed throughout this Report. A summary of these themes and their integration across chapters is described in Table 1.1.

'''Table 1.1 |''' '''Cross-cutting themes in AR6 WGI, and the main chapters that deal with them.''' Bold numbers in the table indicate the chapters that have extensive coverage.

{| class="wikitable"
|-
! '''Thematic Focus'''

! '''Main Chapters;''' Addi tional Chapters

|-
| Aerosols

| '''2, 6, 7, 8, 9, 10,''' '''11;''' 3, 4, Atlas

|-
| Atmospheric Circulation

| '''3, 4,''' '''8;''' 2, 5, 10, 11

|-
| Biosphere

| '''2, 3, 5, 11, Cross-Chapter Box''' '''5.1;''' 1, 4, 6, 8

|-
| Carbon Dioxide Removal (CDR)

| '''4, 5;''' 8

|-
| Cities and Urban Aspects

| '''10, 11, 12;''' 2, 8, 9, Atlas

|-
| Climate Services

| '''12, Atlas, Cross-Chapter''' '''Box 12.2;''' 1, 10

|-
| Climatic Impact-Drivers

| '''12, Annex VI;''' 1, 9, 10, 11, Atlas

|-
| CO <sub>2</sub> Concentration Levels

| '''1, 2, 5, Cross-Chapter Box''' '''1.1;''' 12, Atlas

|-
| Coronavirus Pandemic (COVID-19)

| '''Cross-Cha''' '''pter Box 6.1;''' 1

|-
| Cryosphere

| '''2, 3, 9;''' 1, 4, 8, 12, Atlas

|-
| Deep Uncertainty

| '''9;''' 4, 7, 8, Cross-Chapter Box 11.2, Cross-Chapter Box 12.1

|-
| Detection and Attribution

| '''3, 10, 11, Cross-Working Group Box: Attribution;''' 5, 6, 8, 9, 12, Atlas

|-
| Emergence

| '''1''' ''', 10, 12;''' 8, 11

|-
| Extremes and Abrupt Change

| '''11, 12;''' 1, 5, 7, 8, 9, 10, Atlas, Cross-Chapter Box 12.1

|-
| Global Warming Hiatus

| '''Cross-Chapter''' '''Box 3.1;''' 10, 11

|-
| Land Use

| '''5;''' 2, 7, 8, 10, 11

|-
| Limits of Habitability

| '''9, 12;''' 11

|-
| Low-Likelihood, High-Impact/High Warming

| '''1, 4, 11;''' 7, 8, 9, 10, Cross-Chapter Box 1.1, Cross-Chapter Box 1.3, Cross-Chapter Box 4

|-
| Model Evaluation

| '''1, 3, 9, 10, 11,''' '''Atlas;''' 5, 6, 8

|-
| Modes of Variability

| '''1, 2, 3, 4, 8, 9, Annex IV;''' 7, 10, 11, 12, Atlas

|-
| Monsoons

| '''8;''' 3, 4, 9, 10, 11, 12, Atlas

|-
| Natural Variability

| '''1, 2, 3, 4,''' '''9, 11;''' 5, 8, 10

|-
| Ocean

| '''3, 5, 9;''' 1, 2, 4, 7, 12, Atlas

|-
| Paleoclimate

| '''1, 2;''' 3, 5, 7, 8, 9, Atlas, Box 11.3

|-
| Polar Regions

| '''9, 12, At''' '''las;''' 2, 3, 7, 8

|-
| Radiative Forcing

| '''7;''' 1, 2, 6, 11

|-
| Regional Case Studies

| '''10, 11, Atlas;''' 12, Box 8.1, Box 11.4, Cross-Chapter Box 12.2

|-
| Risk

| '''1, 11, 12, Cross-Chapter Box 1.3;''' 4, 5, 9, Cross-Chapter Box 12.1

|-
| Sea Level

| '''9, 12;''' 1, 2, 3, 4, 7, 8, 10, 11, Atlas

|-
| Short-Lived Climate Forcers (SLCFs)

| '''6, 7;''' 1, 2, 4, Atlas

|-
| Solar Radiation Modification (SRM)

| '''4, 5;''' 6, 8

|-
| Tipping Points

| '''5, 8, 9;''' 4, 11, 12, Cross-Chapter Box 12.1

|-
| Values and Beliefs

| '''1, 10;''' 12

|-
| Volcanic Forcing

| '''2, 4, 7, 8;''' 1, 3, 5, 9, 10, Annex III

|-
| Water Cycle

| '''8, 11;''' 2, 3, 10, Box 11.1

|}

[[File:bba317ed389879a33672c9472ea68473 IPCC_AR6_WGI_Figure_1_2.png|thumb|400x300px]]

'''Figure 1.2 |''' '''Main relations between AR5 WGI and AR6 WGI chapters.''' The left-hand column shows the AR5 WGI chapter categories. The central column lists the AR5 WGI chapters, with the colour code indicating their relation to the AR6 WGI structure shown in Figure 1.1: Large-Scale Information (purple), Process Understanding (gold), Regional Information (light blue) and Whole-Report Information (dark blue). AR5 WGI chapters depicted in white have their topics distributed over multiple AR6 WGI chapters and categories. The right-hand column explains where to find related information in the AR6 WGI report.

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=== 1.1.3 Integration of AR6 WGI Assessments With Other Working Groups ===

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Integration of assessments across the chapters of the WGI Report, and with WGII and WGIII, occurs in a number of ways, including work on a common Glossary, risk framework (Cross-Chapter Box 1.3), scenarios and projections of future large-scale changes, and the presentation of results at various global warming levels ( [[#1.6|Section 1.6]] ).

Chapters 8 to 12, and the Atlas, cover topics also assessed by WGII in several areas, including regional climate information and climate-related risks. This approach produces a more integrated assessment of impacts of climate change across Working Groups. In particular, [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-10 Chapter 10] discusses the generation of regional climate information for users, the co-design of research with users, and the translation of information into the user context (in particular directed towards WGII). [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-12 Chapter 12] provides a direct bridge between physical climate information (climatic impact-drivers) and sectoral impacts and risk, following the chapter organization of the WGII Assessment. Notably, Cross-Chapter Box 12.1 draws a connection to representative key risks and Reasons for Concern (RFC).

The science assessed in Chapters 2 to 7, such as the carbon budget, short-lived climate forcers (SLCFs) and emissions metrics, are topics in common with WGIII, and relevant for the mitigation of climate change. This includes a consistent presentation of the concepts of carbon budget and net zero emissions targets within chapters, in order to support integration in the Synthesis Report. Emissions-driven emulators (simple climate models), summarized in Cross-Chapter Box 7.1, are used to approximate large-scale climate responses of complex Earth System Models (ESMs) and have been used as tools to explore the expected global surface air temperature (GSAT) response to multiple scenarios consistent with those assessed in WGI for the classification of scenarios in WGIII. [[IPCC:Wg1:Chapter:Chapter-6|Chapter 6]] provides information about the impact of climate change on global air pollution, relevant for WGII, including Cross-Chapter Box 6.1 on the implications of the recent coronavirus pandemic (COVID-19) for climate and air quality. Cross-Chapter Box 2.3 in [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] presents an integrated cross-Working Group discussion of global temperature definitions, with implications for many aspects of climate change science.

In addition, [https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-1 Chapter 1] sets out a shared terminology on cross-cutting topics, including climate risk, attribution and storylines, as well as an introduction to emissions scenarios, global warming levels and cumulative carbon emissions as an overarching topic for integration across all three Working Groups.

All these integration efforts are aimed at enhancing the bridges and ‘handshakes’ among Working Groups, enabling the final cross-Working Group exercise of producing the integrated Synthesis Report.

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=== 1.1.4 Chapter Preview ===

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The main purposes of this chapter are: (i) to set the scene for the WGI Assessment and to place it in the context of ongoing global changes, international policy processes, the history of climate science and the evolution from previous IPCC assessments, including the Special Reports prepared as part of the Sixth Assessment Cycle; (ii) to describe key concepts and methods, relevant developments since AR5, and the modelling framework used in this Assessment; and (iii) together with the other chapters of this report, to provide context and support for the WGII and WGIII contributions to AR6, particularly on climate information to support mitigation, adaptation and risk management.

The chapter comprises seven sections (Figure 1.3). [[#1.2|Section 1.2]] describes the present state of Earth’s climate, in the context of reconstructed and observed long-term changes and variations caused by natural and anthropogenic factors. It also provides context for the present Assessment by describing recent changes in international climate change governance and fundamental scientific values. The evolution of knowledge about climate change and the development of earlier IPCC assessments are presented in [[#1.3|Section 1.3]] . Approaches, methods and key concepts of this Assessment are introduced in [[#1.4|Section 1.4]] . New developments in observing networks, reanalyses, modelling capabilities and techniques since AR5 are discussed in [[#1.5|Section 1.5]] . The three main ‘dimensions of integration’ across Working Groups in AR6, that is, emissions scenarios, global warming levels and cumulative carbon emissions, are described in [[#1.6|Section 1.6]] . The Chapter closes with a discussion of opportunities and gaps in knowledge integration in [[#1.7|Section 1.7]] .

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[[File:31d1d2abe6f4069a2673d23ff7e1a698 IPCC_AR6_WGI_Figure_1_3.png|thumb|400x300px]]

'''Figure 1.3 |''' '''Visual guide to Chapter 1''' '''.'''

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