For me, if I'm speaking scientifically, the two guaranteed ones are the sun and the galaxy Andromeda; which will take roughly four billion years to happen. But right now I'm betting on the overpopulation issue that might very well be catastrophic to us all. Some would argue that global warming will do the job, and it could no doubt. Global warming, if not reversed soon will cause the following: More frequent and severe weather, higher death rates, dirtier air, higher wildlife extinction rates, more acidic oceans, and higher sea levels. Some of this is already coming into play... unfortunately. How do you think the world will end? I don't know if it's just me, but it seems like us Homo sapiens don't take shit seriously until it happens. Or is that just me? Edit: When I say "end" I don't necessarily mean the destruction of Earth, just things that would set us far back as a species or kill billions.