Nowhere is the need for handling complexity more pertinent than in addressing environmental issues. Our study explores students' situated constructs of complexity in unfolding discourses on socio-scientific issues. Students' dialogues in two group-work episodes are analysed in detail, with tools from Systemic Functional Linguistics. We identify the significance of intertextuality in students' realizations of low- and high-complexity discourses. In the high-complexity event, we show how students take on different roles and use modality and projection as grammatical resources for opening up, for different positions, multiple voices, and various contextual resources. Successful handling of complexity is construed by the interplay between students' roles in the discourse and resources in language for making multiple voices present. In the high-complexity event, the handling of complexity is guided by the students' sense of purpose. Handling complexity is demanding, and explicit scaffolding is necessary to prevent a potentially complex challenge from being treated as a simple one.