To what extent has the Intergovernmental Panel on Climate Change (IPCC) succeeded in its ambition to shape a more diverse environmental expertise? In what ways are diversity important to the IPCC? What purposes does diversity serve in the IPCC’s production of global environmental assessments and thus environmental knowledge in general? These questions are explored by analyzing quantitative demographic data of the latest two assessment cycles (AR5 and AR6) and qualitative data from a semi-structured interview study with IPCC experts. The analysis shows that there have been improvements in diversity in recent years across measures of gender (women comprising 34% of authors in AR6 compared to 21% in AR5), regional representation and the proportion of authors from developing countries (35% in AR6 compared to 31% in AR5). These improvements have not, however, been distributed evenly when looking at the seniority of authors, nor when comparing across working groups, with WGI (the physical science) remaining much less diverse (28% female authors) than WGII (impacts) (41% female authors) and WGIII (mitigation) (32% female authors). The interviews suggest that rather than viewing diversity as a challenge it should be viewed as an opportunity to build capacity. Distinctions between scientific expertise and ‘diversity of voice’ need to be reconsidered in terms of both the substantive and instrumental value that a diverse range of knowledge, experience and skills add to the process of the scientific assessment of climate knowledge. In the concluding discussion, three points are raised: (i) the issue of diversity will probably grow in importance due to the fact that the complex task of transforming society has increasingly come into focus; (ii) the issue of diversity will be crucial for IPCC to maintain and develop its capacity to make assessments; (iii) the issue of diversity should not be reduced to simply a means for improving the process of making assessments, but should also improve the outcomes of the assessments.