Nuclear War

The single most likely way the world could end is a large-scale nuclear war triggered by great-power confrontation, regional conflict escalation, or miscalculation during a crisis. Unlike slower global threats, nuclear arsenals already exist, are operational, and can be launched within minutes. Rising rivalry among nuclear-armed states, modernization of arsenals, cyber risks to warning and command systems, and the speed of modern warfare all increase the chance that a conventional conflict could spiral into strategic exchange.

A full nuclear war would not only kill hundreds of millions quickly through blast, fire, and radiation, but could also collapse global civilization through infrastructure destruction, electromagnetic disruption, supply-chain failure, famine, disease, and social breakdown. Even a more limited exchange could inject enough soot into the atmosphere to trigger severe global cooling and agricultural collapse, threatening billions far beyond the countries directly attacked.

Other existential threats such as climate change, pandemics, or AI-related harms are serious, but nuclear war remains the most immediate and concentrated mechanism for abrupt worldwide destruction because the capability, incentives, and pathways to catastrophe already exist.