This chapter concerns how three teachers in lower secondary school include climate change in school mathematics. Data was collected over a oneyear period, where the teachers organised several teaching activities such as fieldwork, posters, contribution to an exhibition, and dialogue and debates, to facilitate students’ critical mathematics competences through working with climate change. We apply a teacher perspective and focus on the role mathematics can play in formatting the understanding of climate change. A formatting power of mathematics is identified at three levels: (1) in teachers’ metareflections, (2) when the teachers use mathematics to format students’ understanding, and (3) when teachers facilitate students’ awareness of the formatting power of mathematics. The findings suggest that a complex issue like climate change brings forth an awareness of the formatting powers of mathematics.