1) Mr. Freeze
You might not think that Mr. Freeze has it in him to cover the entire Eastern Seaboard with snow and ice, given that this Bat-rogue's #1 priority is somehow saving his fatally ill but cryogenically frozen wife Nora. But that doesn't take into account the fact that Mr. Freeze is crazypants. In some continuities Nora is already dead; in others she's just a random lady who Victor has convinced himself loves him. This is not a man who makes sensible decisions. Even at his most sane, Freeze would be more than willing to bring about a new ice age if he thought it would bring him the slightest chance of bringing back Nora; all it would take is a bad guy with a promise of a cure and Freeze would do whatever he said.
While the Malekith of Thor: The Dark World was after a macguffin called the Aether, but in the original Thor comics, Malekith was the wielder of the Casket of Ancient Winters, as seen in the first Thor film. In the comics, Malekith actually destroyed the casket, letting its magic escape and plunging the world into a brand-new ice age. I'm pretty confident this battle actually took places a couple of weeks ago, and we're now starting to see the repercussions.
3) The Winter Warlock
I know what half of you are thinking: Didn't this guy turn
good in the 1970 Rankin-Bass special Santa Claus Is Coming to Town? (The other
half of you are thinking: What the fuck is the 1970 Rankin-Bass special Santa
Claus Is Coming to Town?) And yes, Kris Kringle did give a toy train to the
Winter Warlock in order to get into Sombertown to deliver toys to the kids, and the
Winter Warlock became good and helpful and started calling himself Winter. But
what if he's reverted? What if he got tired of his toy train? What if now he
wants a bunch of Monster High dolls, and Kris Kringle hasn't gotten around to it yet? All I'm saying is that if you don't
want a Winter Warlock to plunge the world into a snow-pocalypse, maybe you should give him more than a toy train.
4) Captain Cold
Generally, The Flash's antagonist Captain Cold is more interested in crime than actual evil, and traditionally trying to kill The Flash is about as big picture as his plans get. Besides, his power is usually simply a gun that shoots cold at people, and he'd have a tough time extending that across the country. But in the New 52, Captain Cold has actual superpowers, including slowing the molecules around him, lowering temperatures, freezing things, and allowing him to catch the Flash. While this might still seem too limited to cause our current cold woes, remember, Cold frequently partners with the Weather Wizard — a team-up between those two could very easily cover the northeasst with snow and force us into our longjohns.
5) Professor Coldheart
Oh, you laugh, but the Care Bears' cartoon villain Professor Coldheart has already demonstrated his ability to freeze over entire towns in The Care Bears Battle the Freeze Machine, as well as a whole forest in the Care Bears TV episode "Forest of Misfortune." And that was back in the '80s; surely his freeze technology and capabilities have only increased since then. Don't pretend like Coldheart was too goofy to pull a stunt like this; the man captured several children and turned them into his slaves — clearly, there is nothing that's beneath him.
Much like Captain Cold, this Marvel villain spent most of his time running around with a freeze ray and taking on a variety of heroes, all of whom kicked his ass (actually, the first Blizzard was killed by Iron Man 2020 solely because he was in Iron Man's way). But a few months ago, as part of the "Infinity" storyline, Blizzard was exposed to the Terrigen Mists, turning him into one of the new Inhumans and giving him real ice powers. We don't know the scope of them yet, but since the Inhuman Black Bolt can literally destroy the planet if he clears his throat too loudly, anything is possible.
7) Jack Frost
I know the titular serial killer-turned-sentient snowman prefers to stick to murder (you're very welcome for me not writing "cold-blooded murder" there, by the way) but Frost has a variety of powers, including ice generation and the ability to split himself off into smaller autonomous mini-killers. It's not hard to imagine Jack Frost hiring a flight, jumping out at 25,000 feet, seeding himself amongst the clouds, and then burying North America in snow and ice. And then trying to stab everyone he meets with his carrot nose. Avoid carrots this winter, is all I'm saying.
8) Ice King
Yes, the Ice King lives in the future, where he spends most of hime chasing princesses and trying to convince Finn and Jake that they're all best friends together. But hardcore Adventure Time know that the Ice King is really a guy named Simon who found a magic crown prior to the apocalyptic Mushroom War that destroyed the world and set the stage for the land of Ooo. This crown gave him magic ice powers, but also possessed him, slowly driving him crazy and causing him to forget his true self not long after he found little Marceline in the wasteland. What if the real-life Simon Petrikov has just now found the crown? What if these insane temperatures and winter storms are his first acts of madness? Does this mean the Mushroom War is nearly upon us?! The idea is, of course, chilling.