国会研究服务部-温室气体排放情景：背景、问题和政策相关性（英文）-2021.6-34页_1mb

Summary of Greenhouse Gas Emissions Scenarios: Background, Issues, and Policy Relevance

Core Content

This report provides an overview of greenhouse gas (GHG) emissions scenarios and their role in climate change policy analysis. It focuses on Integrated Assessment Models (IAMs), which are used to project future emissions and evaluate the implications of different policy options. The report discusses how these models are employed in the context of the Paris Agreement's temperature goals—limiting warming to 1.5°C or 2°C above preindustrial levels by 2100—and explores the technological, economic, and social pathways necessary to achieve these targets.

Main Views and Key Information

1. Purpose and Use of Emissions Scenarios

• Scenario analysis is used to explore possible futures and understand uncertainties in climate change projections.
• GHG emissions scenarios are based on socioeconomic, environmental, and technological trends.
• These scenarios are used as inputs to climate models to assess how changes in GHG concentrations affect the global climate.

2. Integrated Assessment Models (IAMs)

• IAMs are complex models that integrate the human system (economy, population, energy use) with the biophysical earth system (climate, carbon cycle).
• There are two main types:
  • Simple IAMs with limited economic detail and aggregated spatial representation.
  • Detailed IAMs with higher spatial resolution and process-based representation of energy systems and other sectors.
• The report focuses on detailed, process-based IAMs that are used to simulate future emissions and climate impacts.

3. Temperature Goals and Emissions Pathways

• The Paris Agreement sets a target of limiting global warming to 1.5°C or 2°C by 2100.
• To meet the 2°C target, global CO₂ emissions would need to reach net-zero between 2080 and 2100.
• To meet the 1.5°C target, emissions would generally peak around 2020 and reach net-zero by 2060.
• The later the peak, the sharper the required reductions later in the century.

4. Key Technologies and Strategies

• IAMs suggest that achieving these targets requires the scaling up of renewable energy, nuclear power, energy efficiency, and electrification.
• Negative emissions technologies, such as bioenergy with carbon capture and storage (BECCS), are critical in balancing positive emissions.
• By 2050, assumed negative emissions could be half to more than double the level of positive CO₂ emissions from energy, transport, and industry.

5. Energy Demand and Electrification

• Global energy demand is expected to increase significantly by 2050, potentially twice current levels, due to electrification trends.
• Energy intensity (energy per unit of GDP) is projected to decline by 25% to 33% in 1.5°C and 2°C consistent scenarios compared to the baseline.

6. Role of IAMs in Policymaking

• IAMs are used to compare policy options and evaluate emissions reduction trajectories.
• They help in identifying trade-offs, such as competition for land use between energy production and agriculture.
• IAMs can inform legislative decisions, including incentives for technology development and policy design.

7. IPCC and Scenario Development

• The IPCC has used emissions scenarios since its First Assessment Report (1990).
• RCPs (Representative Concentration Pathways) are used to represent different levels of radiative forcing in 2100.
• SSPs (Shared Socioeconomic Pathways) provide socioeconomic narratives that influence GHG emissions over time.
• RCP 2.6 and RCP 1.9 are consistent with 1.5°C and 2°C warming targets.
• RCP 8.5 is considered less plausible due to its unrealistic assumptions about future coal use.

8. Limitations of IAMs

• IAMs have limitations in predicting future technologies and their economic viability.
• They may not fully capture non-technical factors such as social behavior and policy implementation.
• The choice of technologies and fuels depends on model structure and objective functions.

Conclusion

The report emphasizes that while IAMs are valuable tools for understanding the long-term implications of climate change and evaluating policy options, they are not perfect and have limitations. They provide plausible pathways for achieving 1.5°C and 2°C warming targets, but real-world outcomes depend on technological innovation, economic conditions, and policy choices. The RCPs and SSPs framework helps in assessing different scenarios and their climate and socioeconomic impacts, offering a foundation for informed policy decisions.