It's simple enough.
Souplex? Really? REALLY???
Religion? The laws of the universe breaking for no reason other than someone(s) says so? Doubt it.
Okay, I can see people worrying about nukes, but enough to destroy human life on all landmasses? It would be highly improbable to even get that much, let alone fire simultaneously with accurate coordinates to hit in the exact right spot.
The environment. Sure, global warming and what-not. But water EXPANDS when frozen. That means the poles melting wouldn't effect the waterline that much. And unless all plants died for no apparent reason, the atmosphere would be essentially the same, save the massive hole in the Ozone.
Space is BIG. There's a LOT of stuff in it. There's even MORE nothingness than that. Something the size of Jupiter, say, would still have trouble colliding with a peble like us. Science has proven that the sun still has plenty of Melenia (sp?) to make light for us.
Aliens? Do you need some pixie dust on the plasma reactor, or Pegasus wings stuck to the thrusters to get here?
And the world will end some day. Just look at these signs. The economy, war, the media, and so on.
Which leads me to my choice. The scientists. It may be to learn about the world we live in, but those cheeky bastards are too curious!