Intergovernmental Panel on Climate Change (IPCC) member governments approve each report’s summary for policymakers (SPM) by consensus, discussing and agreeing on each sentence in a plenary session with scientist authors. A defining feature of IPCC assessment, the governmental approval process builds joint ownership of current knowledge by scientists and governments. The resulting SPM revisions have been extensively discussed in anecdotes, interviews, and perspectives, but they have not been comprehensively analyzed. We provide an in-depth evaluation of IPCC SPM revisions, establishing an evidential basis for understanding their nature. Revisions associated with governmental review and approval generally expand SPMs, with SPM text growing by 17 to 53% across recent assessment reports. Cases of high political sensitivity and failure to reach consensus are notable exceptions, resulting in SPM contractions. In contrast to recent claims, we find that IPCC SPMs are as readable, for multiple metrics of reading ease, as other professionally edited assessment summaries. Across reading-ease metrics, some SPMs become more readable through governmental review and approval, whereas others do not. In an SPM examined through the entire revision process, most revisions associated with governmental review and approval occurred before the start of the government-approval plenary session. These author revisions emphasize clarity, scientific rigor, and explanation. In contrast, the subsequent plenary revisions place greater emphasis especially on policy relevance, comprehensiveness of examples, and nuances of expert judgment. Overall, the value added by the IPCC process emerges in a multistage crucible of revision and approval, as individuals together navigate complex science-policy terrain.
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