I have two theories
1. The world will go on as planned
2. Some old rock band, maybe the rolling stones or the who or the beatles will be celebrating the 40th anniversary of one of their album releases and will host an anniversary tour. People will be stoked and everyone knows what happens when people get stoked. This anniversary tour will actually be the cover for a mass terrorist organization called Dagger Panties.
Dagger panties main goal is to infiltrate the pornographic database of the World. When they accomplish their task they will delete everything pornographic that has ever been posted on the internet. This even includes things like furniture porn.
This task will be accomplished at approximately 8:45 P.M on december 20th 2012, right as people are starting to crawl into bed with their laptops. Anyone with a wife, girlfriend, fleshlight, imagination, canteloupe, dog or grinder will be not affected. The rest of the world will turn to chaos though.
This in turn will trigger mass hysteria and the only thing that the governments of the world will be able to do is broadcast their home pornographic movies on public broadcasting networks to try and satisfy the insatiable need for porn that they realize their citizens really have.
This action will then trigger extreme religious groups to try and take over the government to stop the outrage that they are viewing even on their favourite public religious channels. These people will slowly build a bridge connecting all of the continents of the world and harness the power of the worlds animals.
And then... When penguins and polar bears are finally united in the wild for the first time each pair will transform into a giant earth destroying mega polarguin!
These animals are extremely dangerous, can breath fire, bite 6000 foot holes in the earths crust, shit cannon balls and can destroy the most impressive human achievements with their mind but their scariest power of all?
THEY HAVE THE FUCKING ABILITY TO RELEASE THE FUCKING TITANS FROM HERCULES
Now these titans and polarguins should be no match for the Armies of the world right? Think again, all of the worlds military leaders are too distracted laughing and fapping to their superiors home pornographic videos on public television until its too late! These titans only goal is to reach the top of mount olympus but because that doesnt exist on earth their search goes on forever slowly killing every human being that has ever existed. Earths only hope is Hercules but because he gave up his immortality to be with that dumb bitch Meg on earth he is no longer around to give us a hand and save us for all of eternity.
Either of those two options are entirely plausible but I'm sorta banking on option number one.