There is now clear scientific consensus that, without immediate and decisive action, the world risks climate catastrophe. This has fueled climate emergency declarations among activist groups and, increasingly, among local, state, and supranational governments. But what exactly counts as a "climate catastrophe" and what does catastrophic climate change portend for contemporary societies? This book argues that climate change is politically catastrophic insofar as it threatens to undermine the material conditions that make justice - and by extension stable democratic government - possible. It then uses the lens of catastrophe to bring into focus pressing questions concerning how to navigate trade-offs between fairness and precautionary efficacy in the design of climate policy, the permissibility of authoritarian climate emergency powers, and the nature and role of climate disobedience. Apart perhaps from the spectre of nuclear annihilation, human civilization has never had to reckon with a threat so final and encompassing as that of climate catastrophe. Much as some have argued that "supreme necessity" alters the contours of what is permissible in war, this book starts from the premise that the credible threat of politically catastrophic climate change upends many of the most basic and widely shared assumptions in liberal and democratic thought.