Global warming is bollocks, jfc. They've been on about it for well over a decade now, our summers are still shit, and Pacific nations haven't been reclaimed by the ocean yet.

I'd say it'll be one of the following...

1. Economic collapse leading to WWIII... possibly nuclear, but even without nukes, the economic consequences of a war between USA & Europe vs Russia & China would be terrible for all parties.

2. Volcanic winter - Yellowstone Park being the prime candidate, but there's Katla in Iceland too which has huge potential, not to mention the chain of volcanoes around Indonesia.

3. Disease

4. Extraterrestial event - massive meteor. Most ET events would be devastating for the impact area only, it would take a monster like the one that supposedly wiped out the dinosaurs to fuck the world up.