I doubt overpopulation will lead to any global significant change--i.e. not a change that affects everyone equally. For one, human death rates are still very high so despite the exponential growth of our
wonderful species, it's still going to be a major factor in only very specific areas (i.e. Tokyo, Los Angeles, Africa, etc).
I don't imagine Iqualuit in Nunavut, Canada experiencing overpopulation issues anytime soon. (This is conjecture. If I'm wrong about that, as I don't live there, correct me)
After that, I don't think Nuclear war breaking out is very likely either, as the people with most nukes (i.e. America) are probably quite fixated on ensuring no one else's goes off. It's not a criticism or anything, I just believe they probably have many black ops missions focused on preventing any nuclear war/explosion.
The most likely event I think would be terrestrial. My reasoning is as follows:
1) Terrestrial events happen at a relatively consistent and predictable rate. (i.e. Earth quakes of large magnitudes happen at at x frequency/1000 years, super volcanoes explode once every x years, and so on)
2) There is relatively little we can do to stop it. If a volcano is going to explode, there's nothing we can do. If a methane gas deposit deep in the crust of the earth manages to work its way out from under the rock, float right up through the ocean and into the atmosphere to drastically change the atmosphere's composition... we're, well, boned.
Furthermore, we're due for plenty of dramatic events. Yellowstone park is building up, California's on a fault line, the magnetic field is slowly declining until it resets (I think something like 1000 years from now?), by probability's standards we're due for a large meteor, and so forth. These are things we can't escape.
EDIT:
I_am_a_Spoon said:
I love it how everyone always pictures themselves as a survivor. Who's to say that you won't become one of the very first zombies? I mean, for there to be zombie hordes, shitloads of people have to be infected in the first place, right?
I agree with all your points. Also, if a zombie infection arose, I wouldn't mind being a zombie if I could retain some cognition. If zombies outnumber people, then by sheer probability I'd basically be following the giant horde going from place A to place B looking for people to eat. Odds are I wouldn't be near the front lines and never see any people to eat. It'd just be me, walking around. Doesn't sound too bad.