A global catastrophic risk or a doomsday scenario is a hypothetical event that could damage human well-being on a global scale, even endangering or destroying modern civilization. An event that could cause human extinction or permanently and drastically curtail humanity's existence or potential is known as an "existential risk."
Artist's impression of a major asteroid impact. An asteroid caused the extinction of the non-avian dinosaurs.
Human extinction is the hypothetical end of the human species, either by population decline due to extraneous natural causes, such as an asteroid impact or large-scale volcanism, or via anthropogenic destruction (self-extinction), for example by sub-replacement fertility.
Nuclear war is an often-predicted cause of the extinction of humankind.
Castle Romeo nuclear test on Bikini Atoll
Placard against omnicide, at Extinction Rebellion (2018)