This paper explores the implications of assessing tacit knowledges of climatic change in extended peer communities, as applied in two European research projects on climate action. Post-normal science (PNS) proposes the extension of the peer community to co-produce better quality knowledge for decision-making on issues like climate change, where facts are uncertain, values in dispute, stakes high and decisions urgent. The paper has two aims. The first, more practical, is to explore methods for critically appraising tacit knowledges for climate action, using the example of two ongoing research projects. The second, more conceptual, is to improve practices and discourses surrounding tacit knowledge in current PNS praxis, with close consideration to the implications and challenges involved in including these forms of knowledge in decision making processes. By exploring theoretical perspectives on the topic of tacit knowledge, four challenges facing extended peer communities in engaging with tacit forms of knowledge have been identified: communication, representation, appropriation, and assessment.