A nuclear holocaust, can end all of humanity. It is a theoretical scenario where the mass detonation of nuclear weapons causes globally widespread destruction and radioactive fallout.
Such a scenario envisages large parts of the Earth becoming uninhabitable due to the effects of nuclear warfare, potentially causing the collapse of civilization and, in the worst case, extinction of humanity and/or termination of life on Earth.
The International Physicians for the Prevention of Nuclear War believe that nuclear war could indirectly contribute to human extinction via secondary effects, including environmental consequences, societal breakdown, and economic collapse.
It has been estimated that a relatively small-scale nuclear exchange between India and Pakistan involving 100 Hiroshima yield (15 kilotons) weapons, could cause a nuclear winter and kill more than a billion people. So, nuclear Armageddon is a chance no one should bet on.
A related area of inquiry is: if a future nuclear arms race someday leads to larger stockpiles or more dangerous nuclear weapons than existed at the height of the Cold War, at what point could war with such weapons result in human extinction?
Physicist Leo Szilard warned in the 1950s that a deliberate doomsday device could be constructed by surrounding powerful hydrogen bombs with a massive amount of cobalt. Cobalt has a half-life of five years, and its global fallout might, some physicists have posited, be able to clear out all human life via lethal radiation intensity.
Nuclear holocaust will always come with indirect risks such as a societal collapse following nuclear war that can make humanity much more vulnerable to other existential threats.
