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Chapter 3
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I
Introduction
A
Understanding Global Warming of 1.5°C*
B
Projected Climate Change, Potential Impacts and Associated Risks
C
Emission Pathways and System Transitions Consistent with 1.5°C Global Warming
D
Strengthening the Global Response in the Context of Sustainable Development and Efforts to Eradicate Poverty
+
Core Concepts Central to this Special Report
+
Acknowledgements
+
Citation
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1.1
Assessing the Knowledge Base for a 1.5°C Warmer World
1.1.1
Equity and a 1.5°C Warmer World
1.1.2
Eradication of Poverty
1.1.3
Sustainable Development and a 1.5°C Warmer World
1.2
Understanding 1.5°C: Reference Levels, Probability, Transience, Overshoot, and Stabilization
1.2.1
Working Definitions of 1.5°C and 2°C Warming Relative to Pre-Industrial Levels
1.2.1.1
Definition of global average temperature
1.2.1.2
Choice of reference period
1.2.1.3
Total versus human-induced warming and warming rates
1.2.2
Global versus Regional and Seasonal Warming
1.2.3
Definition of 1.5°C Pathways: Probability, Transience, Stabilization and Overshoot
1.2.3.1
Pathways remaining below 1.5°C
1.2.3.2
Pathways temporarily exceeding 1.5°C
1.2.3.3
Impacts at 1.5°C warming associated with different pathways: transience versus stabilisation
1.2.4
Geophysical Warming Commitment
1.3
Impacts at 1.5°C and Beyond
1.3.1
Definitions
1.3.2
Drivers of Impacts
1.3.3
Uncertainty and Non-Linearity of Impacts
1.4
Strengthening the Global Response
1.4.1
Classifying Response Options
1.4.2
Governance, Implementation and Policies
1.4.3
Transformation, Transformation Pathways, and Transition: Evaluating Trade-Offs and Synergies Between Mitigation, Adaptation and Sustainable Development Goals
1.5
Assessment Frameworks and Emerging Methodologies that Integrate Climate Change Mitigation and Adaptation with Sustainable Development
1.5.1
Knowledge Sources and Evidence Used in the Report
1.5.2
Assessment Frameworks and Methodologies
1.6
Confidence, Uncertainty and Risk
1.7
Storyline of the Report
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Mitigation pathways compatible with 1.5°C in the context of sustainable development
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2.1
Introduction to Mitigation Pathways and the Sustainable Development Context
2.1.1
Mitigation Pathways Consistent with 1.5°C
2.1.2
The Use of Scenarios
2.1.3
New Scenario Information since AR5
2.1.4
Utility of Integrated Assessment Models (IAMs) in the Context of this Report
2.2
Geophysical Relationships and Constraints
2.2.1
Geophysical Characteristics of Mitigation Pathways
2.2.1.1
Geophysical uncertainties: non-CO
2
forcing agents
2.2.1.2
Geophysical uncertainties: climate and Earth system feedbacks
2.2.2
The Remaining 1.5°C Carbon Budget
2.2.2.1
Carbon budget estimates
2.2.2.2
CO
2
and non-CO
2
contributions to the remaining carbon budget
2.3
Overview of 1.5°C Mitigation Pathways
2.3.1
Range of Assumptions Underlying 1.5°C Pathways
2.3.1.1
Socio-economic drivers and the demand for energy and land in 1.5°C pathways
2.3.1.2
Mitigation options in 1.5°C pathways
2.3.1.3
Policy assumptions in 1.5°C pathways
2.3.2
Key Characteristics of 1.5°C Pathways
2.3.2.1
Variation in system transformations underlying 1.5°C pathways
2.3.2.2
Pathways keeping warming below 1.5°C or temporarily overshooting it
2.3.3
Emissions Evolution in 1.5°C Pathways
2.3.3.1
Emissions of long-lived climate forcers
2.3.3.2
Emissions of short-lived climate forcers and fluorinated gases
2.3.4
CDR in 1.5°C Pathways
2.3.4.1
CDR technologies and deployment levels in 1.5°C pathways
2.3.4.2
Sustainability implications of CDR deployment in 1.5°C pathways
2.3.5
Implications of Near-Term Action in 1.5°C Pathways
2.4
Disentangling the Whole-System Transformation
2.4.1
Energy System Transformation
2.4.2
Energy Supply
2.4.2.1
Evolution of primary energy contributions over time
2.4.2.2
Evolution of electricity supply over time
2.4.2.3
Deployment of carbon capture and storage
2.4.3
Energy End-Use Sectors
2.4.3.1
Industry
2.4.3.2
Buildings
2.4.3.3
Transport
2.4.4
Land-Use Transitions and Changes in the Agricultural Sector
2.5
Challenges, Opportunities and Co-Impacts of Transformative Mitigation Pathways
2.5.1
Policy Frameworks and Enabling Conditions
2.5.2
Economic and Investment Implications of 1.5°C Pathways
2.5.2.1
Price of carbon emissions
2.5.2.2
Investments
2.5.3
Sustainable Development Features of 1.5°C Pathways
2.6
Knowledge Gaps
2.6.1
Geophysical Understanding
2.6.2
Integrated Assessment Approaches
2.6.3
Carbon Dioxide Removal (CDR)
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3.1
About the Chapter
3.2
How are Risks at 1.5°C and Higher Levels of Global Warming Assessed in this Chapter?
3.2.1
How are Changes in Climate and Weather at 1.5°C versus Higher Levels of Warming Assessed?
3.2.2
How are Potential Impacts on Ecosystems Assessed at 1.5°C versus Higher Levels of Warming?
3.3
Global and Regional Climate Changes and Associated Hazards
3.3.1
Global Changes in Climate
3.3.2
Regional Temperatures on Land, Including Extremes
3.3.2.1
Observed and attributed changes in regional temperature means and extremes
3.3.2.2
Projected changes in regional temperature means and extremes at 1.5°C versus 2°C of global warming
3.3.3
Regional Precipitation, Including Heavy Precipitation and Monsoons
3.3.3.1
Observed and attributed changes in regional precipitation
3.3.3.2
Projected changes in regional precipitation at 1.5°C versus 2°C of global warming
3.3.4
Drought and Dryness
3.3.4.1
Observed and attributed changes
3.3.4.2
Projected changes in drought and dryness at 1.5°C versus 2°C
3.3.5
Runoff and Fluvial Flooding
3.3.5.1
Observed and attributed changes in runoff and river flooding
3.3.5.2
Projected changes in runoff and river flooding at 1.5°C versus 2°C of global warming
3.3.6
Tropical Cyclones and Extratropical Storms
3.3.7
Ocean Circulation and Temperature
3.3.8
Sea Ice
3.3.9
Sea Level
3.3.10
Ocean Chemistry
3.3.11
Global Synthesis
3.4
Observed Impacts and Projected Risks in Natural and Human Systems
3.4.1
Introduction
3.4.2
Freshwater Resources (Quantity and Quality)
3.4.2.1
Water availability
3.4.2.2
Extreme hydrological events (floods and droughts)
3.4.2.3
Groundwater
3.4.2.4
Water quality
3.4.2.5
Soil erosion and sediment load
3.4.3
Terrestrial and Wetland Ecosystems
3.4.3.1
Biome shifts
3.4.3.2
Changes in phenology
3.4.3.3
Changes in species range, abundance and extinction
3.4.3.4
Changes in ecosystem function, biomass and carbon stocks
3.4.3.5
Regional and ecosystem-specific risks
3.4.3.6
Summary of implications for ecosystem services
3.4.4
Ocean Ecosystems
3.4.4.1
Observed impacts
3.4.4.2
Warming and stratification of the surface ocean
3.4.4.3
Storms and coastal runoff
3.4.4.4
Ocean circulation
3.4.4.5
Ocean acidification
3.4.4.6
Deoxygenation
3.4.4.7
Loss of sea ice
3.4.4.8
Sea level rise
3.4.4.9
Projected risks and adaptation options for oceans under global warming of 1.5°C or 2°C above pre-industrial levels
3.4.4.10
Framework organisms (tropical corals, mangroves and seagrass)
3.4.4.11
Ocean foodwebs (pteropods, bivalves, krill and fin fish)
3.4.4.12
Key ecosystem services (e.g., carbon uptake, coastal protection, and tropical coral reef recreation)
3.4.5
Coastal and Low-Lying Areas, and Sea Level Rise
3.4.5.1
Global / sub-global scale
3.4.5.2
Cities
3.4.5.3
Small islands
3.4.5.4
Deltas and estuaries
3.4.5.5
Wetlands
3.4.5.6
Other coastal settings
3.4.5.7
Adapting to coastal change
3.4.6
Food, Nutrition Security and Food Production Systems (Including Fisheries and Aquaculture)
3.4.6.1
Crop production
3.4.6.2
Livestock production
3.4.6.3
Fisheries and aquaculture production
3.4.7
Human Health
3.4.7.1
Projected risk at 1.5°C and 2°C of global warming
3.4.8
Urban Areas
3.4.9
Key Economic Sectors and Services
3.4.9.1
Tourism
3.4.9.2
Energy systems
3.4.9.3
Transportation
3.4.10
Livelihoods and Poverty, and the Changing Structure of Communities
3.4.10.1
Livelihoods and poverty
3.4.10.2
The changing structure of communities: migration, displacement and conflict
3.4.11
Interacting and Cascading Risks
3.4.12
Summary of Projected Risks at 1.5°C and 2°C of Global Warming
3.4.13
Synthesis of Key Elements of Risk
3.5
Avoided Impacts and Reduced Risks at 1.5°C Compared with 2°C of Global Warming
3.5.1
Introduction
3.5.2
Aggregated Avoided Impacts and Reduced Risks at 1.5°C versus 2°C of Global Warming
3.5.2.1
RFC 1 – Unique and threatened systems
3.5.2.2
RFC 2 – Extreme weather events
3.5.2.3
RFC 3 – Distribution of impacts
3.5.2.4
RFC 4 – Global aggregate impacts
3.5.2.5
RFC 5 – Large-scale singular events
3.5.3
Regional Economic Benefit Analysis for the 1.5°C versus 2°C Global Goals
3.5.4
Reducing Hotspots of Change for 1.5°C and 2°C of Global Warming
3.5.4.1
Arctic sea ice
3.5.4.2
Arctic land regions
3.5.4.3
Alpine regions
3.5.4.4
Southeast Asia
3.5.4.5
Southern Europe and the Mediterranean
3.5.4.6
West Africa and the Sahel
3.5.4.7
Southern Africa
3.5.4.8
Tropics
3.5.4.9
Small islands
3.5.4.10
Fynbos and shrub biomes
3.5.5
Avoiding Regional Tipping Points by Achieving More Ambitious Global Temperature Goals
3.5.5.1
Arctic sea ice
3.5.5.2
Tundra
3.5.5.3
Permafrost
3.5.5.4
Asian monsoon
3.5.5.5
West African monsoon and the Sahel
3.5.5.6
Rainforests
3.5.5.7
Boreal forests
3.5.5.8
Heatwaves, unprecedented heat and human health
3.5.5.9
Agricultural systems: key staple crops
3.5.5.10
Agricultural systems: livestock in the tropics and subtropics
3.6
Implications of Different 1.5°C and 2°C Pathways
3.6.1
Gradual versus Overshoot in 1.5°C Scenarios
3.6.2
Non-CO2 Implications and Projected Risks of Mitigation Pathways
3.6.2.1
Risks arising from land-use changes in mitigation pathways
3.6.2.2
Biophysical feedbacks on regional climate associated with land-use changes
3.6.2.3
Atmospheric compounds (aerosols and methane)
3.6.3
Implications Beyond the End of the Century
3.6.3.1
Sea ice
3.6.3.2
Sea level
3.6.3.3
Permafrost
3.7
Knowledge Gaps
3.7.1
Gaps in Methods and Tools
3.7.2
Gaps in Understanding
3.7.2.1
Earth systems and 1.5°C of global warming
3.7.2.2
Physical and chemical characteristics of a 1.5°C warmer world
3.7.2.3
Terrestrial and freshwater systems
3.7.2.4
Ocean Systems
3.7.2.5
Human systems
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4.1
Accelerating the Global Response to Climate Change
4.2
Pathways Compatible with 1.5°C: Starting Points for Strengthening Implementation
4.2.1
Implications for Implementation of 1.5°C-Consistent Pathways
4.2.1.1
Challenges and Opportunities for Mitigation Along the Reviewed Pathways
4.2.1.2
Implications for Adaptation Along the Reviewed Pathways
4.2.2
System Transitions and Rates of Change
4.2.2.1
Mitigation: historical rates of change and state of decoupling
4.2.2.2
Transformational adaptation
4.2.2.3
Disruptive innovation
4.3
Systemic Changes for 1.5°C-Consistent Pathways
4.3.1
Energy System Transitions
4.3.1.1
Renewable electricity: solar and wind
4.3.1.2
Bioenergy and biofuels
4.3.1.3
Nuclear energy
4.3.1.4
Energy storage
4.3.1.5
Options for adapting electricity systems to 1.5°C
4.3.1.6
Carbon dioxide capture and storage in the power sector
4.3.2
Land and Ecosystem Transitions
4.3.2.1
Agriculture and food
4.3.2.2
Forests and other ecosystems
4.3.2.3
Coastal systems
4.3.3
Urban and Infrastructure System Transitions
4.3.3.1
Urban energy systems
4.3.3.2
Urban infrastructure, buildings and appliances
4.3.3.3
Urban transport and urban planning
4.3.3.4
Electrification of cities and transport
4.3.3.5
Shipping, freight and aviation
4.3.3.6
Climate-resilient land use
4.3.3.8
Sustainable urban water and environmental services
4.3.3.7
Green urban infrastructure and ecosystem services
4.3.4
Industrial Systems Transitions
4.3.4.1
Energy efficiency
4.3.4.2
Substitution and circularity
4.3.4.3
Bio-based feedstocks
4.3.4.4
Electrification and hydrogen
4.3.4.5
CO2 capture, utilization and storage in industry
4.3.5
Overarching Adaptation Options Supporting Adaptation Transitions
4.3.5.1
Disaster risk management (DRM)
4.3.5.2
Risk sharing and spreading
4.3.5.3
Education and learning
4.3.5.4
Population health and health system adaptation options
4.3.5.5
Indigenous knowledge
4.3.5.6
Human migration
4.3.5.7
Climate services
4.3.6
Short-Lived Climate Forcers
4.3.7
Carbon Dioxide Removal (CDR)
4.3.7.1
Bioenergy with carbon capture and storage (BECCS)
4.3.7.2
Afforestation and reforestation (AR)
4.3.7.3
Soil carbon sequestration and biochar
4.3.7.4
Enhanced weathering (EW) and ocean alkalinization
4.3.7.5
Direct air carbon dioxide capture and storage (DACCS)
4.3.7.6
Ocean fertilization
4.3.8
Solar Radiation Modification (SRM)
4.3.8.1
Governance and institutional feasibility
4.3.8.2
Economic and technological feasibility
4.3.8.3
Social acceptability and ethics
4.4
Implementing Far-Reaching and Rapid Change
4.4.1
Enhancing Multilevel Governance
4.4.1.1
Institutions and their capacity to invoke far-reaching and rapid change
4.4.1.2
International governance
4.4.1.3
Sub-national governance
4.4.1.4
Interactions and processes for multilevel governance
4.4.2
Enhancing Institutional Capacities
4.4.2.1
Capacity for policy design and implementation
4.4.2.2
Monitoring, reporting, and review institutions
4.4.2.3
Financial institutions
4.4.2.4
Co-operative institutions and social safety nets
4.4.3
Enabling Lifestyle and Behavioural Change
4.4.3.1
Factors related to climate actions
4.4.3.2
Strategies and policies to promote actions on climate change
4.4.3.3
Acceptability of policy and system changes
4.4.4
Enabling Technological Innovation
4.4.4.1
The nature of technological innovations
4.4.4.2
Technologies as enablers of climate action
4.4.4.3
The role of government in 1.5°C-consistent climate technology policy
4.4.4.4
Technology transfer in the Paris Agreement
4.4.5
Strengthening Policy Instruments and Enabling Climate Finance
4.4.5.1
The core challenge: cost-efficiency, coordination of expectations and distributive effects
4.4.5.2
Carbon pricing: necessity and constraints
4.4.5.3
Regulatory measures and information flows
4.4.5.4
Scaling up climate finance and de-risking low-emission investments
4.4.5.5
Financial challenge for basic needs and adaptation finance
4.4.5.6
Towards integrated policy packages and innovative forms of financial cooperation
4.5
Integration and Enabling Transformation
4.5.1
Assessing Feasibility of Options for Accelerated Transitions
4.5.2
Implementing Mitigation
4.5.2.1
Assessing mitigation options for limiting warming to 1.5˚C against feasibility dimensions
Enabling conditions for implementation of mitigation options towards 1.5˚C
4.5.3
Implementing Adaptation
4.5.3.1
Feasible adaptation options
4.5.3.2
Monitoring and evaluation
4.5.4
Synergies and Trade-Offs between Adaptation and Mitigation
4.6
Knowledge Gaps and Key Uncertainties
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5.1
Scope and Delineations
5.1.1
Sustainable Development, SDGs, Poverty Eradication and Reducing Inequalities
5.1.2
Pathways to 1.5°C
5.1.3
Types of Evidence
5.2
Poverty, Equality and Equity Implications of a 1.5°C Warmer World
5.2.1
Impacts and Risks of a 1.5°C Warmer World: Implications for Poverty and Livelihoods
5.2.2
Avoided Impacts of 1.5°C versus 2°C Warming for Poverty and Inequality
5.2.3
Risks from 1.5°C versus 2°C Global Warming and the Sustainable Development Goals
5.3
Climate Adaptation and Sustainable Development
5.3.1
Sustainable Development in Support of Climate Adaptation
5.3.2
Synergies and Trade-Offs between Adaptation Options and Sustainable Development
5.3.3
Adaptation Pathways towards a 1.5°C Warmer World and Implications for Inequalities
5.4
Mitigation and Sustainable Development
5.4.1
Synergies and Trade-Offs between Mitigation Options and Sustainable Development
5.4.1.1
Energy Demand: Mitigation Options to Accelerate Reduction in Energy Use and Fuel Switch
5.4.1.2
Energy Supply: Accelerated Decarbonization
5.4.1.3
Land-based agriculture, forestry and ocean: mitigation response options and carbon dioxide removal
5.4.2
Sustainable Development Implications of 1.5°C and 2°C Mitigation Pathways
5.4.2.1
Air pollution and health
5.4.2.2
Food security and hunger
5.4.2.3
Lack of energy access/energy poverty
5.4.2.4
Water security
5.5
Sustainable Development Pathways to 1.5°C
5.5.1
Integration of Adaptation, Mitigation and Sustainable Development
5.5.2
Pathways for Adaptation, Mitigation and Sustainable Development
5.5.3
Climate-Resilient Development Pathways
5.5.3.1
Transformations, equity and well-being
5.5.3.2
Development trajectories, sharing of efforts and cooperation
5.5.3.3
Country and community strategies and experiences
5.6
Conditions for Achieving Sustainable Development, Eradicating Poverty and Reducing Inequalities in 1.5°C Warmer Worlds
5.6.1
Finance and Technology Aligned with Local Needs
5.6.2
Integration of Institutions
5.6.3
Inclusive Processes
5.6.4
Attention to Issues of Power and Inequality
5.6.5
Reconsidering Values
5.7
Synthesis and Research Gaps
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