Quantitative risk analysis has long underpinned public trust in experts who interpret and address environmental disputes. However, in the uncertain and complex context of responding to a changing climate, it has been argued that trust in scientific expertise must be warranted on norms other than experts’ competence in quantifying risk. Nowhere is this more evident than in the case of solar geoengineering. Much like abrupt climate change, solar geoengineering involves potential nonlinear or threshold responses, where both the triggering points and the system’s reactions are poorly understood, leaving risks inadequately characterized. In this paper, we examine climate model experiments with solar geoengineering to understand how expertise is justified in the absence of reliable risk assessments. We argue that these experts enact a specific norm of competence, which holds that trust should be warranted not on their ability to quantify risk, but to render unknown unknowns into known unknowns.