It is widely assumed that the more certain and precise the scientific knowledge-base for predicting and understanding climate change, the better defined and robust will be the policy measures undertaken in response. In this paper we argue to the contrary that in the case of Global Warming Potentials (GWPs) ambiguity in their precise meaning is a major reason why they have been developed and continue as scientific policy tools [although this is not how they are commonly represented in the reports of the Intergovernmental Panel on Climate Change (IPCC)]. We survey and analyse the range of opinion on GWPs with respect to their scientific stability and comprehensiveness and argue that the utility of GWPs has to be evaluated in terms of their symbolic, interactional and heuristic effects as well as with respect to their direct instrumental uses. In addition, we argue that scientific discussion of GWPs commonly incorporates elements of the social and policy contexts of their application and provide several examples from detailed discussions at the IPCC. We endeavour to account for the ambiguous identity of GWPs and draw out several implications from the findings of the paper for the construction and use of scientific tools in policy.