IPCC Explained

Structure and Process

The IPCC has three Working Groups. Working Group I covers the physical science basis (temperature trends, climate system mechanics, projections). Working Group II addresses impacts, adaptation, and vulnerability. Working Group III focuses on mitigation options and emission pathways. A Synthesis Report integrates findings across all three groups into a concise summary for policymakers.

Reports undergo multiple rounds of expert and government review. Authors must respond to every comment received, creating extensive response documents. Draft versions are reviewed by hundreds of external experts and government representatives. This open, documented process ensures thoroughness and transparency, though it also means IPCC conclusions tend to be conservative, representing confident consensus rather than cutting-edge findings.

Key Findings of AR6

The AR6 Working Group I report, published in 2021, stated that human influence on the climate system is "unequivocal" (upgraded from "clear" in AR5). It confirmed warming of 1.1 degrees since 1850-1900 and projected continued warming under all emission scenarios. Climate sensitivity was narrowed to 2.5-4.0 degrees per CO2 doubling. The report identified acceleration in ice loss, sea level rise, and extreme events, with improvements in regional climate projections and attribution science.

Working Group II (2022) documented widespread climate impacts across all regions and sectors, finding that 3.3 to 3.6 billion people live in contexts highly vulnerable to climate change. It identified adaptation gaps and limits, particularly for vulnerable communities and ecosystems. Working Group III (2022) outlined mitigation pathways showing that limiting warming to 1.5 degrees requires emissions to peak before 2025 and decline rapidly thereafter.

Confidence and Uncertainty Language

The IPCC uses calibrated language to express certainty. Likelihood terms have specific meanings: "virtually certain" means 99-100 percent probability, "very likely" 90-100 percent, "likely" 66-100 percent. Confidence levels combine evidence quality and agreement among studies. This framework allows precise communication of what is well-established versus uncertain, preventing overstatement while conveying the strength of scientific evidence.

Criticisms and Limitations

Common critiques include that the consensus process is inherently conservative (underestimating risks), that reports are outdated by publication time due to lengthy review cycles, and that Summary for Policymakers documents are negotiated line-by-line with governments. Some scientists argue IPCC projections have consistently underestimated ice loss, sea level rise, and extreme event intensification. Others note the process, while imperfect, remains the gold standard for scientific assessment.