As a space for circulating knowledge, the Internet provides the public at large with opportunities for voicing their ideas and for interactions between experts and non-specialists. Online controversies over global warming abound, taking on different forms according to the socio-technical systems through which they evolve. This article focuses on three online interaction systems: forums, comments pages on press articles and discussions between Wikipedia contributors. Although forums and comments pages have rules that can identify levels of knowledge among participants, the lack of common aims and rules prevents the production of shared knowledge. Wikipedia uses rules, both human and automated, that allow experts and non-specialists to interact with a view to producing knowledge.