A growing body of research points to the role social norms may play in both maintaining carbon intensive lifestyles and soliciting changes towards more sustainable ways of living. However, despite highlighting the importance of pro-environmental social norms, such literature has said far less about the processes by which such norms might develop. We present a new approach to conceptualising social norms that focuses on understanding their dynamics within social interaction, by positioning interpersonal confrontation as a potential mechanism of change. We examine the normative dynamics of environmentalism by comparing the costs of interpersonally confronting climate change disregard with those associated with confronting racism. In two experimental studies, we presented participants with scenarios describing a person confronting (versus not confronting) contentious comments in each domain. We identified social costs to interpersonal confrontation of climate change disregard but not racism, as indicated by reduced ratings of perceived warmth of and closeness to the confronter (Study 1), and this effect was mediated by the perceived morality of the issue in question (Study 2). Our findings highlight how wider social constructions of (im)morality around climate change impact upon social interactions in ways that have important implications for processes of social (and ultimately environmental) change.