A Nuclear summer is a hypothetical scenario resulting from nuclear warfare that would follow a nuclear winter, caused by aerosols inserted into the atmosphere that would prevent sunlight from reaching lower levels or the surface. In this scenario, following the settling out of most of the aerosols in 1–3 years, the cooling effect would be overcome by a heating effect from greenhouse warming, which would raise surface temperatures rapidly by many degrees, enough to cause the death of much if not most of the life that had survived the cooling, much of which is more vulnerable to higher-than-normal temperatures than to lower-than-normal temperatures. The nuclear detonations would release CO2 and other greenhouse gases from burning, followed by more released from decay of dead organic matter. The detonations would also insert nitrogen oxides into the stratosphere that would then deplete the ozone layer around the Earth. This layer screens out UV-C radiation from the Sun, which causes genetic damage to life forms on the surface. As the temperature rises, the amount of water in the atmosphere would increase, causing further greenhouse warming of the surface, and if it rose enough, it could cause the sublimation of methane clathrate deposits on the sea floor, releasing huge amounts of methane, a greenhouse gas, into the atmosphere, perhaps enough to trigger runaway climate change.
Other more simplistic versions of the hypothesis exist: that Nuclear winter might give way to a nuclear summer. The high temperatures of the nuclear fireballs could destroy the ozone gas of the middle stratosphere.