Man's Behaviour Many of the above fall in this category, of corse, but here are some more. War is another original Horseman of the Apocalypse. The weapons get more deadly. Over-population is surely going to happen at some point. Populations hit by war recover inside a generation.
Deadly diseases seem to have no effect on world population, which inevitably increases.
We are already using a substantial part of the world's surface to grow food for us or our animals, and we're over-fishing the sea. Even if we all turn vegetarian, and eat much less, as long as the population grows, at some point we will run out of food. Or we might get mass movement of population because of catastrophe or famine or inequality. Added to over-population, we get consumerism . So we are looking at a future of not only increasing population, but an increasingly greedy popluation.