This article advances the third generation of expertise scholarship by exploring how expertise is produced through socio-material practices within global climate governance. The Paris Agreement commits international climate policy to be grounded in the ‘best available science,’ yet the United Nations Framework Convention on Climate Change (UNFCCC) lacks a formal mechanism for scientific uptake. Focusing on the expert groups established under the UNFCCC’s Warsaw International Mechanism for Climate Change Loss and Damage, the study shows that expertise is the outcome of intertwined material arrangements and social relations. Through interviews and ethnographic observations, the analysis shows how terms of reference and expert rosters formally structure who may participate, while informal relational practices enable the loss and damage Committee to selectively recruit and socialise experts. These dynamics stabilise and legitimise a contested policy area, shaping what becomes recognised as ‘global loss and damage expertise’. However, these practices also create exclusions, privileging actors aligned with UN working cultures, those perceived as politically acceptable and those able to self-fund participation. Such practices risk limiting representational diversity and constrain expert’s autonomy in shaping their work. The findings highlight the need for greater transparency and resourcing of expert groups to broaden participation and address knowledge gaps.