Four planet-level dangers loom over humankind in the coming hundred years, requiring solutions that can only be achieved through planet-level strategies and instruments. Reversing climate change requires a concerted effort among all the world’s peoples to decarbonize their economies and energy systems as swiftly as possible, while ramping up new technologies for removing accumulated carbon dioxide. Nuclear weapons will continue to pose an existential threat as long as nations vie with each other in a zero-sum competition for power and dominance. Naturally occurring or bioengineered pandemics threaten human well-being, and can only be mitigated via a comprehensive system of global regulation. Artificial intelligence proffers many tantalizing benefits, but will also create extreme risks unless humankind finds ways to control the development of these powerful machines. The historical track record suggests that these challenges, while daunting, can realistically be surmounted by concerted action.