<p>This article investigates how the affordances of blogs facilitate epistemic action in climate science communication. In social media research, affordances are frequently employed to analyse the relationship between communication technologies and users across sociocultural contexts. However, the term is often applied inconsistently, which obscures how specific affordances relate to evident communicative outcomes. This issue is especially significant in online science communication, where misinformation and contestation are frequent. Drawing on Gibson’s original conceptualization of affordances as real, relational, and invariant possibilities for action, the article argues that blogs represent a distinct social media environment in which certain affordances become epistemically salient when enacted in knowledge-oriented practices. Through qualitative analysis of semi-structured, in-depth interviews with 12 mainstream scientist-bloggers and 26 readers of widely read climate science blogs, the study explores user interactions with blogging platforms, including writing, commenting, linking, moderating content, and sustaining discussion. The analysis identifies five affordances – extensive writing, editability, persistence, content extension, and interactivity – that collectively support sustained argumentation, evidence-based discussion, critique, and clarification. These affordances underpin the formation of epistemic communicative environments that differ from those enabled by other social media platforms. The article advances social media and science communication research by developing the concept of epistemic affordances as an analytically generative framework for understanding how social media can support the communication of complex and socially contested scientific knowledge, such as that concerning global warming and climate change.</p>