Prudent risk management requires consideration of bad-to-worst-case scenarios. Yet, for climate change, suchpotential futures are poorly understood. Could anthropo-genic climate change result in worldwide societal collapseor even eventual human extinction? At present, this is adangerously underexplored topic. Yet there are amplereasons to suspect that climate change could result in aglobal catastrophe. Analyzing the mechanisms for theseextreme consequences could help galvanize action, improveresilience, and inform policy, including emergency respon-ses. We outline current knowledge about the likelihood ofextreme climate change, discuss why understanding bad-to-worst cases is vital, articulate reasons for concern about cat-astrophic outcomes, define key terms, and put forward aresearch agenda. The proposed agenda covers four mainquestions: 1) What is the potential for climate change todrive mass extinction events? 2) What are the mechanismsthat could result in human mass mortality and morbidity? 3)What  are  human  societies’  vulnerabilities  to  climate-triggered risk cascades, such as from conflict, political insta-bility, and systemicfinancial risk? 4) How can these multiplestrands of evidence—together with other global dangers—be usefully synthesized into an“integrated catastropheassessment”?Itistimeforthescientific community to grap-ple with the challenge of better understanding catastrophicclimate change