Top nine worst things that could happen to the world. A meteor could strike the globe like a million hydrogen bombs. Political instability might lead to nuclear holocaust. An animal-to-human supervirus might rip pandemically across continents. Everyone knows that in a few billion years the Sun will become a red giant and swallow the Earth, but here and now the lack of urgency surrounding climate change shows that many people find it hard to meaningfully own the idea of a catastrophically damaged planet. Only a matter of time.
#1. World War III
We have so many threats in this world (like Iraq, Syria, and North Korea) and many allies. In the past, people just invaded with guns, which is puny compared to those city destroying nuclear bombs. It can all start with the activation of a detonator.