Americans' attitudes about the environment are highly polarized, but it is unclear why this is the case. We conducted five studies to examine this issue. Studies 1a and 1b demonstrated that liberals, but not conservatives, view the environment in moral terms and that this tendency partially explains the relation between political ideology and environmental attitudes. Content analyses of newspaper op-eds (Study 2a) and public-service announcements (Study 2b) found that contemporary environmental discourse is based largely on moral concerns related to harm and care, which are more deeply held by liberals than by conservatives. However, we found that reframing proenvironmental rhetoric in terms of purity, a moral value resonating primarily among conservatives, largely eliminated the difference between liberals' and conservatives' environmental attitudes (Study 3). These results establish the importance of moralization as a cause of polarization on environmental attitudes and suggest that reframing environmental discourse in different moral terms can reduce the gap between liberals and conservatives in environmental concern.