I guess it depends what you mean by the world ending. Our planet is pretty tough and has survived millions of years of threats like asteroids, volcanoes and earthquakes. Life is more fragile, and it is easier to wipe out life than wipe out the world – just think of the dinosaurs!
We’re probably one of the biggest threats to the world – if people don’t start taking climate change seriously, we could be in a lot of trouble as a species. During the cold war everyone was worried about nuclear war killing everyone. Then there are things like super-volcanoes like the one in the USA (Yellowstone), if it erupted it would change everything. Other things might be asteroids wiping us out.
If it was to happen today I think the most likely reason is going to be us. As Katherine said the planet is pretty resilient and has survived a lot in its long history and it is only since we’ve been around that things are starting to become problematic but again not for the planet itself just for everything living on it.
Climate change is a big problem and something that is happening now, but even still its going to take many years before all life is destroyed, more likely is human action resulting in a world war and many nuclear weapons being utilised. Having been to Hiroshima in Japan I got to see the devastating effects this can have on an area and is something I’d recommend anyone to go and visit if they can.