Humanity will end when we fuck up the planet too much for us to survive, or create a super disease. The world itself will end in 1 of 2 ways. Meteor or the sun explodes. Those are the only ones I can see in both situations, nuking the world to death doesn't just seem that likely, specially as humanity actually would continue to live, like in New Zealand who I don't think anyone would fire a nuke at. They don't have many enemies, would you believe it. I don't even think North Korea would fire a nuke their way, and those people are out of their minds.