Risk assessment techniques are regarded as key devices for managing adaptation to climate change; this paper examines their use in the first UK Climate Change Risk Assessment. The conceptual framework is derived from the sociology of knowledge, which treats policy makers as co-producers of knowledge, in interaction with scientists and consultants. The paper considers the framing of the problem, the creation of metrics and their limits, and the validity and legitimacy of such tools in conditions of high uncertainty. While recognising the potential contribution of risk assessment to managing complexity and assessing priorities, it argues that over-reliance on such instruments may risk understating the potential for climate-related disasters, while unintentionally increasing the real risk of failure to act effectively to adapt to inevitable change. Reliance on socio-technical devices entails a top-down approach to policy, which limits engagement with the public in deliberation about social priorities and purposes. Consequently politicians and civil servants may focus on the 'wrong' variables, misunderstand the assumptions and values built in to risk assessments, misuse their outputs, and possibly fail to act on the precautionary principle.