5. Global Nuclear War
As more countries develop nuclear weapons capability, the chance of a country or terrorist group using a nuclear weapon increases. A Scientific American article (below) estimates that a single detonation or small-scale nuclear exchange is a 50/50 probability over the next fifteen years. A full-scale global nuclear war, however, is much less likely since the end of the Cold War, with only a one-in-thirty chance over the next ten years. Despite arms reductions, there are enough warheads left to kill a half billion people, followed by another billion deaths from radiation, disease and starvation. Although the climactic impact would be global, isolated countries, like Australia, might be less vulnerable, allowing for survival of civilization. Biological warfare is much harder to define but would probably have similar probability and consequences to a pandemic.
Reference: Scientific American, Sep 2010: Laying Odds on the Apocalypse; The Effects of Global Thermonuclear War.
6. Super Volcanoes
Super Volcanoes are extremely dangerous but the probability is low over the next 100 years, and, currently, there’s nothing we can do about them.