联合国环境署执行主任在IPCC气候科学报告发布会的发言-42页_5mb

Summary of Climate Change 2021: The Physical Science Basis

Core Content

This document, the Summary for Policymakers (SPM) of the IPCC Sixth Assessment Report (AR6), presents key findings from the Working Group I (WGI) on the physical science basis of climate change. It provides an overview of the current state of the climate, possible future scenarios, climate information for regional risk assessment, and the potential for limiting future climate change.

Main Findings

A. The Current State of the Climate

• Human influence has warmed the atmosphere, ocean, and land with widespread and rapid changes in the climate system. This warming is unequivocal since around 1750, and well-mixed greenhouse gases (GHGs) are the main drivers.
• CO₂ concentrations in 2019 reached 410 ppm, CH₄ reached 1866 ppb, and N₂O reached 332 ppb, all higher than at any time in at least 2 million years (CO₂), 800,000 years (CH₄ and N₂O).
• The global surface temperature in 2011–2020 was 1.09 [0.95–1.20] °C higher than the 1850–1900 baseline, with land areas warming more than oceans.
• The rate of warming since 1970 is faster than any other 50-year period over the last 2000 years.
• Arctic sea ice area in 2011–2020 was the lowest since at least 1850, and marine heatwaves have doubled in frequency since the 1980s.
• Global mean sea level has risen faster since 1900 than in any preceding century for at least 3000 years.
• The global upper ocean has warmed since the 1970s, and CO₂ emissions are the main driver of ocean acidification.
• Glacier retreat and Arctic sea ice loss are unprecedented since the last 2000 years.
• Land biosphere changes such as shifts in climate zones and lengthened growing seasons are consistent with global warming.

B. Possible Climate Futures

• The scale of recent climate changes is unprecedented over many centuries to thousands of years, with human influence playing a central role in many observed changes.
• Heatwaves and extreme weather events have become more frequent and intense, and human-induced climate change is the main driver of these changes.
• Heavy precipitation events have increased since the 1950s, and droughts have also been influenced by human activities.
• Tropical cyclones have shifted in latitude, and compound extreme events (e.g., heatwaves and droughts occurring together) have increased in likelihood since the 1950s.

C. Climate Information for Risk Assessment and Regional Adaptation

• Regional climate changes are assessed for various inhabited regions across the globe, including North America, Europe, Africa, Asia, and Australasia.
• The SPM uses a visual representation (Figure SPM.3) to show observed changes and confidence in human contribution for each region.
• Hot extremes have increased in frequency and intensity in most land regions, with high confidence in human influence.
• Heavy precipitation has increased in many regions, with medium confidence in human contribution.
• Agricultural and ecological droughts have increased in some areas, with medium confidence in human influence.
• Tropical cyclones have shifted in latitude and increased in frequency, but long-term trends are low confidence.
• Compound extreme events have increased globally, with high confidence in human influence.

D. Limiting Future Climate Change

• The best estimate of equilibrium climate sensitivity (ECS) is 3°C, with a narrower range than in the Fifth Assessment Report (AR5).
• Human-caused radiative forcing in 2019 was 2.72 [1.96–3.48] W m⁻² relative to 1750, mainly due to increased GHG concentrations.
• Aerosol cooling has partially offset the warming from GHGs, but improvements in scientific understanding have led to a greater warming estimate in AR6.
• The IPCC AR6 WGI Interactive Atlas is a tool to explore these key findings and supporting information across the reference regions.

Key Information

• Confidence levels are indicated using the IPCC calibrated language, with high confidence for most climate changes attributed to human influence.
• Observational data and paleoclimate archives support the conclusion that human influence has warmed the climate at an unprecedented rate over the last 2000 years.
• Climate models and attribution studies have improved understanding of the role of human activities in climate change.
• Regional assessments show that most regions are experiencing increased frequency and intensity of extreme weather events due to human-induced climate change.
• Ocean warming, glacier retreat, sea level rise, and changes in precipitation patterns are all high confidence findings.

Supporting Evidence

• The scientific basis for key findings is detailed in chapter sections of the main report and integrated synthesis in the Technical Summary (TS).
• Figure SPM.1 and Figure SPM.2 provide visual summaries of global temperature changes and contributions to warming from various factors.
• Figure SPM.3 is a regional synthesis of observed changes and confidence levels in human contribution.

Conclusion

The SPM confirms that human activities have significantly warmed the climate and altered weather and climate extremes across the globe. The physical science basis supports the need for urgent action to limit future climate change and adapt to its impacts.