The global community remains significantly off track in achieving the United Nations Sustainable Development Goals (SDGs), prompting calls for not only stronger political commitments but also more effective and credible expert guidance. This paper contributes to this discourse by examining a critical form of global environmental expertise: the conductof global environmental assessments. Such expertise has become a foundational component of the institutional architecture underpinning global sustainability governance. Focusing on the Intergovernmental Panel on Climate Change (IPCC), one of the most prominent and widely recognized international expert bodies, this study examines the challenges that researchers face when synthesizing research findings and making them policyrelevant within the IPCC’s assessment work. The empirical material consists of an interview study (N = 18) conducted with experts involved in the IPCC, from its first assessment (1990) to the most recent one (2023). The data were analyzed thematically using NVivo.The analysis reveals four key internal challenges that the IPCC must address to enhanceits advisory capacity: epistemic hierarchies, leadership and management dynamics, the complexities of formulating recommendations, and inequities in recognition and reward systems. By identifying and analyzing these challenges, the paper contributes to ongoing discussions about the IPCC’s future development and offers broader insights into the evolving role of scientific expertise in global environmental governance.