Climate change is one of the most pressing issues of the 21st Century. Mathematics is involved at every level of understanding climate change, including the description, prediction and communication of climate change. As a highly complex issue, climate change is an example of "post-normal" science -- it is urgent, complex and involves a high degree of uncertainty. This kind of science requires the participation of citizens much more than traditional science. This implies a role for mathematics education in educating current and future citizens to contribute to the kind of dialogue that is needed. In this paper, I argue that critical mathematics education offers a perspective from which to conceptualise how mathematics teaching and learning might undertake this role, drawing in particular on the idea of the "formatting power" of mathematics and the importance of reflective knowing in relation to the mathematics of climate change.