Nuclear-armed countries are a threat to people everywhere partly because of the destructive power of single weapons – one weapon is enough to destroy a small city – and partly because of the growing ability of nations to launch missiles across the globe. There are now about fifteen thousand nuclear weapons, mainly in Russia and theUnited States but also about one thousand in Britain, China, France, India, Israel, North Korea and Pakistan. Nuclear war is an assault on the global climate system caused by smoke from fires ignited by the bombs used against urban centers. As the smoke rapidly spreads globally in the stratosphere it will absorb sunlight, preventing it from reaching the surface, which will reduce surface temperatures and rainfall. The sunlight absorbed by the smoke will also heat the stratosphere, destroying the global ozone layer that shields us from harmful ultraviolet radiation from the Sun. The deaths from these environmental changes would likely be a factor of 10 or more larger than the direct casualties from the explosions – potentially threatening the bulk of the human population – and would not be limited to the combatants.