This study explores the nature and significance of a crucial form of global environmental expertise: that which relates to conducting global environmental assessments with the aim of influencing decision-making. Drawing on the theory of expertise, which conceptualizes expertise as a social position defined by epistemic practice, this study focuses on expertise in the context of global environmental challenges—particularly relating to climate change and the IPCC—highlighting the expertise required to address this kind of complex and multifaceted issue. This type of expertise allows for a synthesis of the current state of environmental challenges, the proposal of options for action, and communication of these findings to decision-makers and society at large. This expertise shapes knowledge that is much broader than a single disciplinary field, encompassing both ecological and social dynamics, and allows for the development of recommendations for action. This study finds that such expertise embodies a distinct epistemic practice with four key characteristics that distinguish it from more narrowly defined forms of expertise and introduces the term “mega-expertise” to capture the character and position of this kind of expertise. This study concludes by reflecting on the broader implications of this form of expertise, considering its relationship to more traditional, disciplinary scientific expertise.