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The IPCC: How Climate Science Is Assessed
Science by consensus, not by committee
The Intergovernmental Panel on Climate Change (IPCC) does not conduct original research. It assesses the state of scientific knowledge by systematically reviewing and synthesizing thousands of peer-reviewed studies. Its reports represent the most comprehensive, rigorously reviewed statements of what the scientific community collectively understands about climate change. This lesson explains how the IPCC works, how to read its findings, and why its assessments carry unique authority.
What the IPCC Is
The IPCC was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). It has 195 member governments. The IPCC's mandate is to provide governments with regular scientific assessments of climate change, its causes, potential impacts, and response options. Its reports are produced by volunteer scientists (authors and review editors drawn from the global research community) and are subject to multiple rounds of expert and government review before approval.
The IPCC does not make policy recommendations. It assesses the scientific evidence and presents findings with explicit statements of confidence. Policy decisions remain the responsibility of governments. This separation is deliberate and important: the IPCC's authority rests on its scientific credibility, not its advocacy role.
The Assessment Cycle
The IPCC produces Assessment Reports approximately every 6-7 years. The most recent, the Sixth Assessment Report (AR6), was completed in 2023. Each Assessment Report consists of contributions from three Working Groups and a Synthesis Report:
- Working Group I (WGI): The Physical Science Basis. Assesses the physical science of the climate system and climate change, including observed changes, attribution, and projections of future climate.
- Working Group II (WGII): Impacts, Adaptation and Vulnerability. Assesses the sensitivity, vulnerability, and exposure of natural and human systems to climate change, and options for adaptation.
- Working Group III (WGIII): Mitigation of Climate Change. Assesses methods and technologies for reducing GHG emissions and removing CO₂ from the atmosphere.
- Synthesis Report (SYR). Integrates the findings of all three Working Groups and any Special Reports in the assessment cycle.
Analogy: The IPCC as a systematic meta-review
Imagine you need to understand the best treatment for a complex medical condition. Rather than reading every individual study yourself, you hire a team of the world's leading specialists to read all the studies, evaluate their quality, identify patterns, and synthesize the evidence into a single authoritative assessment. That is roughly what the IPCC does for climate science. Each AR6 assessment drew on over 14,000 scientific papers, reviewed by thousands of expert reviewers.
How to Read IPCC Confidence Language
The IPCC uses a carefully calibrated language for expressing the strength of scientific evidence. Understanding these terms is essential for correctly interpreting IPCC findings. Two dimensions are used:
Confidence levels reflect the strength of evidence and the degree of agreement among scientists: very low, low, medium, high, and very high. These are typeset in italics in IPCC reports (for example: high confidence).
Likelihood terms reflect probability assessments where specific probability ranges can be assigned:
| Term | Probability |
|---|---|
| Virtually certain | 99-100% |
| Very likely | 90-100% |
| Likely | 66-100% |
| About as likely as not | 33-66% |
| Unlikely | 0-33% |
| Very unlikely | 0-10% |
| Exceptionally unlikely | 0-1% |
These terms are italicized in IPCC reports specifically so that readers can distinguish calibrated probability language from ordinary usage. When the AR6 states that it is "virtually certain" that global surface temperature has increased since the late 19th century, this means the probability is assessed at 99-100%: essentially a scientific fact.
The Review Process
The production of an IPCC assessment report involves multiple stages of expert and government review. After a first draft is produced by author teams, it undergoes expert review by other scientists who submit written comments. Authors revise and respond to each comment, keeping a record of how comments were addressed. A revised draft then undergoes government review, where representatives from member nations submit further comments. The Summary for Policymakers (SPM) undergoes a line-by-line approval process at a plenary session involving representatives of all 195 member governments.
This government approval process is sometimes misunderstood as political interference with the science. In practice, governments can only accept, reject, or request clarification of statements in the SPM; they cannot change the underlying scientific findings in the full report chapters. The SPM must accurately reflect the full report. Any weakening of language requires scientific justification. The process is transparent and recorded.
AR6 by the numbers
The IPCC AR6 cycle (2018-2023) involved approximately 700 authors and review editors across the three Working Groups, drawn from 90 countries. The reports referenced over 14,000 scientific papers in Working Group I alone. More than 78,000 expert review comments were submitted for the WGI report. The Synthesis Report SPM was approved line-by-line over nine days in March 2023 in Interlaken, Switzerland. This scale of scientific synthesis is without precedent in any other field.
Special Reports Between Assessment Cycles
Between full Assessment Reports, the IPCC produces Special Reports on focused topics. Three Special Reports were produced during the AR6 cycle:
- Global Warming of 1.5°C (SR1.5, 2018): Assessed the impacts of 1.5°C warming and pathways to achieve it, directly informing the enhanced ambition of the Paris Agreement.
- Climate Change and Land (SRCCL, 2019): Assessed the relationship between land use, land-use change, food security, and climate.
- The Ocean and Cryosphere in a Changing Climate (SROCC, 2019): Focused on sea level rise, ocean warming and acidification, and ice sheet dynamics.
Special Reports are particularly influential because they respond to specific policy needs, as SR1.5 did in relation to the Paris Agreement's 1.5°C target, and are produced on a faster cycle than full assessments.
The IPCC has faced criticism from multiple directions. Some critics argue it is too conservative (that the consensus process leads to understatement of risks because it requires broad agreement, potentially underweighting findings from individual cutting-edge studies). Other critics, primarily from fossil fuel interests and climate denial movements, have argued it overstates risks or is politically captured.
Independent analyses have generally found the opposite of conservative bias: in several cases, including sea level rise and Arctic sea ice loss, observed changes have exceeded IPCC projections, suggesting the consensus process may indeed be cautious. The scientific community generally regards IPCC reports as the most reliable synthesis of climate knowledge available, while recognizing they represent knowledge as of the literature cutoff date, which can be two or more years before publication.
Key Takeaways
- 1The IPCC assesses existing peer-reviewed science rather than conducting original research; its reports synthesize thousands of studies across hundreds of authors
- 2AR6 consists of three Working Group reports (physical science, impacts and adaptation, mitigation) plus a Synthesis Report, covering the full arc from causes to consequences to solutions
- 3IPCC uses specific calibrated language for confidence (very low to very high) and likelihood (exceptionally unlikely to virtually certain), both typeset in italics to distinguish from ordinary usage
- 4The SPM is approved line-by-line by 195 governments, but governments cannot change the underlying scientific findings, only the language used to communicate them
- 5Special Reports (SR1.5, SRCCL, SROCC) between assessment cycles respond to specific policy needs on a faster timetable