Overview

The Intergovernmental Panel on Climate Change (IPCC) Assessment Reports synthesize global climate science. AR5 (2013-2014) declared >95% certainty humans caused warming; AR6 (2021-2023) stated “unequivocal” human influence, warning 1.5°C breach likely by 2030s without unprecedented action.

IPCC Structure

UN body established 1988 reviews thousands of peer-reviewed papers, involving 200+ scientists per report. Three Working Groups: WG1 (physical science), WG2 (impacts/adaptation), WG3 (mitigation). Governments approve summary language (word-by-word consensus)—criticized as watering down science, praised for political buy-in. Awards: 2007 Nobel Peace Prize (shared with Al Gore).

AR5 (2013-2014)

• WG1 (Sept 2013): “Extremely likely” (95-100%) human activities caused warming since 1950
• Key findings: Warming 0.85°C (1880-2012), oceans absorbing 90% heat, sea level rise accelerating, Arctic ice declining, extreme events increasing frequency/intensity
• Carbon budget: 1,000 GtCO2 remaining for 66% chance staying below 2°C (2014 baseline)—~20 years of current emissions
• Projections: 1.5-4.5°C warming by 2100 depending on emissions scenarios (RCPs)

AR6 (2021-2023)

• WG1 (Aug 2021): “Unequivocal” human influence, upgraded from AR5’s “extremely likely”—no more room for debate
• Warming: 1.1°C (1850-1900 to 2011-2020), unprecedented in 2,000+ years
• 1.5°C breach: Likely crossed 2030-2035 under all scenarios (including aggressive cuts)
• Tipping points: Ice sheet collapse, Amazon dieback, permafrost thaw, AMOC slowdown risks increasing
• Attribution science: Specific events (heatwaves, floods, fires) now linkable to climate change with confidence
• WG2 (Feb 2022): 3.3-3.6 billion people “highly vulnerable,” irreversible impacts, adaptation limits approaching
• WG3 (Apr 2022): Emissions must peak before 2025, halve by 2030 for 1.5°C—current policies lead to 3.2°C warming
• Synthesis Report (Mar 2023): “Code red for humanity”—António Guterres; window closing rapidly

Key Concepts

• RCPs/SSPs: Representative Concentration Pathways (AR5) / Shared Socioeconomic Pathways (AR6)—emissions scenarios ranging from aggressive mitigation to continued rise
• Carbon budget: Total CO2 emissions compatible with temperature limits; shrinking rapidly
• Attribution: Statistical methods linking extreme weather events to climate change
• Tipping points: Irreversible thresholds (ice sheet collapse, coral reef death, forest dieback)

Political & Public Impact

Reports timed with UN climate negotiations (COPs). AR5 informed Paris Agreement (2015). AR6 coincided with Glasgow COP26 (2021). Activists (Greta Thunberg, Extinction Rebellion) cite IPCC as scientific authority. Deniers attack IPCC as “alarmist,” seek to discredit consensus. Reality: IPCC understates risks (consensus conservatism, government approval dampening urgency).

Sources: IPCC AR5 WG1/2/3 reports, IPCC AR6 WG1/2/3 + Synthesis, SPM (Summary for Policymakers), press coverage, COP negotiations