There are three IPCC working groups: one focusing on physical climate science, one on impacts, adaptation and vulnerability and one on mitigation. Each working group publishes a report, then there’s a report synthesising the working group reports. The synthesis report in this assessment cycle will also consider three special reports that were published in 2018 and 2019 on global warming of 1.5 °C, climate change and land and ocean and cryosphere. It is important to note that the reports are an assessment of the existing scientific literature on these topics: they are not the product of original research.

Each working group report consists of 12 to 18 chapters. Each chapter has two to three so-called coordinating lead authors, or CLAs, who coordinate the preparation of the chapter as a whole. They form a team with between six and 15 additional lead authors.

Governments or organizations that are accredited observers to the IPCC nominate these lead authors and the IPCC Bureau, consisting of the IPCC chair and vice-chairs, and the working group co-chairs and vice-chairs, selects them. A so-called chapter scientist supports each author team by conducting thorough literature searches and reviews where needed, and by taking care of logistical matters. SEI’s Zoha Shawoo for instance assumed this role for the chapter on climate-resilient development pathways of the upcoming report on adaptation.

For any expertise not represented in the author teams, the CLAs can invite experts to contribute relevant input. For example, the team working on the Africa chapter of the upcoming impacts and adaptation report felt they were lacking expertise on climate finance and asked Georgia Savvidou and SEI Affiliate Aaron Atteridge to contribute on that. On the area of transboundary climate risk, Magnus Benzie contributed to the Europe chapter. Sukaina Bharwani contributed to the chapter on decision-making options for managing risk and Jon Ensor to the chapter on poverty, livelihoods and sustainable development.

In addition to her role as chapter scientist, Zoha Shawoo also contributed to the chapter on climate-resilient development pathways. Contributing authors are, however, not officially part of the author team and do not have to sign off on the chapter, nor are they accountable for it.

Everybody considering themselves an expert in the field can request access to the first- and second-order draft chapters and submit review comments. No fewer than 51 387 comments were submitted to the second order draft of the physical climate science report last August and the upcoming report on adaptation’s second-order draft received 40 293 comments. All these comments, as well as the responses by the authors, are available in the public domain once the report has been published. No one should be able to question the IPCC report’s credibility and legitimacy on account of any comment falling by the wayside.

Each chapter has two or three review editors. Their role is to ensure that each chapter team does justice to each and every review comment they receive and I was one of the two review editors for the adaptation report’s chapter on decision-making options for managing risk.

Each chapter undergoes three rounds of reviews.  The so-called zero-order draft doesn’t go outside the IPCC and only receives internal review. The first-order draft is reviewed by external experts, just like any article does that gets submitted to a scientific journal, though there are many more reviewers for chapters in an IPCC report than there are for journal articles. In addition to experts, the governments also have their say about the second-order draft, which adds policy relevance. The final draft is then the basis for the report’s Summary for Policymakers (SPM), which governments approve line by line.

The “I” in IPCC stands for intergovernmental. It is governments that are the actual members of the IPCC, not the authors. Report preparation starts with governments formally approving the outline and ends with the approval of the SPM and the acceptance of the rest of the report. Governments own the reports. That intergovernmental ownership is very important within the context of the global climate negotiations under the UNFCCC. The IPCC reports provide a collective foundation for these negotiations, which cannot then be undermined by negotiators challenging the scientific basis on which all countries have agreed. Other global negotiation processes, such as those on biodiversity and desertification, don’t have the benefit of an equivalent process.

This intergovernmental ownership also means that the IPCC reports are important in setting the policy agenda: for example, in terms of what governments prioritize in their domestic policies and actions. In addition, the reports set the academic agenda. Topics mentioned in the report as relevant but still uncertain are bound to become a key area for further research and research funders are likely to prioritize funding work on this topic.

They literally must approve each and every line of the SPM. Every government delegation in the room or online gets asked whether they have any objections to a particular sentence. Sometimes they can take days to agree on a single paragraph or figure caption. The approval session for the SPM is normally scheduled for an entire week, or two now that the approval process is conducted virtually due to the Covid-19 pandemic.

The CLAs of each chapter are present in the approval session. As scientists, they will not agree to text a government may propose that is not in line with the scientific assessment in the relevant chapter of the report. Scientists and governments need to then work together in smaller groups to find a way that addresses a government’s concern while remaining true to the actual science in the report.

Where it is not possible to find common ground, sometimes an entire paragraph may get deleted from the SPM. The scientific assessment on which this paragraph is based will still be in the underlying chapter, but not in the SPM.

That happened for example at a session I attended for the report on impacts and adaptation that was part of the fourth assessment, in 2007. The paragraph spelled out projected impacts of climate change in North America. The US delegation would not accept the language as it was, nor accept any alternatives. After two days, the only way to move forward was to drop the entire paragraph from the SPM.