Apocalypse literally means a revelation, an uncovering and disclosure of knowledge (see Wikipedia article). Our use of the word to mean a prediction of catastrophic events comes from the content of the New Testament Book of Revelation, which contains some pretty dire predictions about the future and the coming end of the world.
Apocalyptic writers describe various scenarios of the ending of the world, whereas post-apocalyptic writing explores what it will be like to survive. I didn't set out to write either - I merely started with an interesting idea and followed it to its conclusion. The trigger for the idea was the debate about plastic carrier bars clogging up the environment, the idea was that wherever there is a source of food, life will find a way to use it. The conclusion was that the life form that learned the trick first would be able to grow explosively, because of the vast underground supplies we've left for it.
Whether you believe my scenario or not, there is no doubt that change will have to come. Even if we're not afflicted by a natural catastrophe, such as an asteroid strike or supervolcano eruption, we've got more immediate risks to worry about, for example climate change and population growth. In this section I look at some of the threats, both natural and man-made.