Question: What is nuclear winter?

James M Monk answered on 13 Jun 2011:

During the cold war, if the USA and the Soviet Union had both detonated their huge nuclear stockpiles then the debris from the explosions would have covered the Earth’s upper atmosphere in a layer of dust/ash. The amount of sunlight reaching the Earth’s surface would have been reduced, so the climate would (temporarily) have become cooler. Nuclear war may then be followed by famine as the reduced sunlight causes agriculture to fail.

Volcanic eruptions can cause a similar effect. The eruption of Krakatoa spread dust into the atmosphere around the globe and caused more severe winters.