I don't believe it will be something direct like the oil running out or war. I'm betting on something bigger, something on a cosmic level. The sun dying out or the big crunch are viable, but it feels like they are too far away to concern us. I'm obviously no expert but I think that something could mess up the earths orbit and the temerature gets either extremely cold or warm. Also something could happen to the moon, which would mess up the tides and there would be floods all over the place. It would also obstruct earth's rythm creating irregularities in the weather and such.