As many physicists & astronomers think, natural phenomenon called Nuclear Winter can happen as a climatic effect of nuclear war.
Carl Edward Sagan, an american astrophysicist, has proposed a theory, that detonating large numbers of nuclear weapons has a profound and severe effect on the climate causing cold weather and reduced sunlight for a period of months or even years, especially over flammable targets such as cities, where large amounts of smoke and soot would be ejected into the Earth's stratosphere...
Gamardah Fungus now want you to think about it. Not about theoretical doomsday event caused by gods, meteorites, ancient people prejudices or alien invaders. But about human behavoir. We can cause an end of our civilization by ourselves.