en.m.wikipedia.org/wiki/Nuclear_winter

Nuclear winter is a severe and prolonged global climatic cooling effect that is hypothesized[1][2] to occur after widespread firestorms following a large-scale nuclear war.[3] The hypothesis is based on the fact that such fires can inject soot into the stratosphere, where it can block some direct sunlight from reaching the surface of the Earth. It is speculated that the resulting cooling would lead to widespread crop failure and famine.