The Day After Tomorrow/WMG

It was caused by Storm
nuff said

The Global warming effect in The Day After Tomorrow was ALIVE
CMON Everyone who relentlessly criticizes this movie all say the same things (Weather does NOT work that way, real captive wolves don't act like that, Ice Ages aren't that fast..Etc the list can go on and on). So this can only lead us to this wild conclusion. The radical effects created by "global warming" in The Day After Tomorrow either came from a elemental sentient force or all the natural phenomena portrayed in the film were somehow alive and capable of sentient thought unto themselves which leads to these 2 theories which are..
 * 1. The movie was a literal "Dont mess with Mother Nature" Green Aesop in where a unknown force/energy that really does control all of the natural forces and elements of the Earth basically just got sick and tired of all these annoying homo sapiens constantly screwing with the weather or atmosphere and simply decided to kick start all the consequences of actual global warming early as either a way of cleansing the earth of us polluting pests or simply a Harsh lesson in not taking nature for granted. And since practically ALL of the characters in the film seemed Too Dumb to Live no one realized that perhaps some other force might have been at work behind all this chaos.
 * 2. All of the weather based forces depicted in the film were each alive and sentient unto themselves and simply decided to gang up on the Human Race for the purpose to become the dominant force/form of life on Earth. This theory incorporates a little Fridge Horror in this because in most nature vs man films (The Birds, whatever giant attacking animal movie on Syfy..Etc) all involved some plot of some form of life on earth wanting to kill off another for total dominance to be the planets sole species (animals kill off Humans, animals kill off each other, plants kill off animals and so forth). So basically The Day After Tomorrow takes this to its logical extreme. The very earths atmosphere itself wants to kill off all sentient, animal and plant life in general in order for the WIND ITSELF to hold sole dominance over the world. Thats why the wave of "COLD" appears to chase the movies protagonists, thats why the events in the film take place over only a few days instead of decades or better yet centuries. This theory can also be somewhat interconnected with other weather related disaster films as well such as Twister and The Storm. And to go further there is a little Nightmare Fuel in this as well when you realize that most of the planets known to man outside of Earth do consist mainly of nothing BUT gas and air (especially Jupiter) meaning that there is a remote possibility that gaseous elements really ARE ALIVE and they've succeeded in bumping off the civilizations of other planets. Good luck sleeping while thinking about one day the force of the very air we breath may intentionally try to murder us All. Sleep Tight kids.

Cirno caused The Day After Tomorrow without even knowing.
Read this comic. Enough said.
 * Yes. Cirno is the only possible explanation for something as epic as The Day After Tomorrow. This must have taken something with more power than Chuck Norris to provoke. This leaves us with only one character...

The Littlest Cancer Patient is a weather-controlling mutant.
Peter is a mutant. His mutation, however, also triggered cellular reproduction instabilities - cancer, in other words. His powers were latent until he was subjected to chemotherapy, at which point his emotional trauma began to manifest.

The weather was caused by Thanos testing his new Cosmic Cube
He was mucking around with laws of thermodynamics for amusement.

Sauron is returning to life
His fiery powers turned to ice by his stay in the underworld, he transformed the world into ice, chased after the people who say the signs of his return with his great Icy Eye, and transformed ordinary wolves into wargs with his magic to send after them.
 * More likely it was the White Witch of Narnia, she's more well known for using wolves as her servants and inflicting cold weather on whole worlds.

This is an alternative future to The Day After
Because the world didn't become a nuclear wasteland, it became an icy wasteland twenty years later. This was somewhat better for the people on the East Coast of America, at least.

Parts of Argentina, Chile, South Africa, and Australia also got severely snowed under and frozen over.
Seriously, if "global warming" is gonna freeze half the landmass of the Northern Hemisphere, it needs to freeze half the landmass of the Southern Hemisphere as well.

At least this will restore balance to Antarctica -- sort of...
 * Since the global conveyor of heat shut down, what would be more likely to happen was the southern hemisphere, and the tropics especially, experiencing a rapid rise in temperature. Unless there is a good reason for the Earth to reflect a lot more sunlight into space (such as, in the long term, snow from an ice age), the average temperature of the Earth must stay the same. So if the average temperature on the northern hemisphere drops to 0 Farenheit / -30 Celsius, the southern hemisphere should have an average temperature of 140 Farenheit/60 Celsius.
 * Which would in turn lead to vast cloud cover forming, since most of the southern hemisphere is ocean which would evaporate like nobody's business, and I'm not a good enough climatologist to figure out the implications of cloud formation on that scale, though my money's on "permanent huricaine on regional scale". Or maybe a sharp decrease in temperature as the white clouds block sunlight. I'd run a computer simulation to model it and see what would happen, but the irony of running computer simulations on the implications of something directed by Captain Simulations Are Useless has been known to cause Divide by Zero error.

The wolves didn't attack because they were hungry.
They could sense the stupid of... Well, everyone, and didn't want there to be any survivors.

The ice storm leads into Battlefield 2142

 * Simple enough