Planet Earth is beset by myriad environmental problems attributable to the unsustainable nature of human activities. These problems include climate change, species loss, pervasive pollution, and ecosystem degradation, all of which are examples of "post-normal situations": they generate urgent, complex problems involving a high degree of risk and uncertainty. Mathematics is essential to our understanding of post-normal situations, to describe them, to make predictions about how they will progress, and to communicate about them. For citizens to participate in democratic debate about how to respond to these challenges and develop more sustainable ways of living, they need to engage with this mathematics at some level, as well as understand its role in making possible contemporary industrialised ways of life. I argue that critical mathematics education offers a perspective with which to conceptualise how mathematics teaching and learning might educate future citizens to participate in post-normal science, drawing in particular on the idea of the "formatting power" of mathematics and the importance of reflective knowing.