Wasteland.

Cameron, a teenage boy, thinks nothing of it when the temperature in Chicago plummets, leaving him and his sister alone in their house at night. Faulty thermostat, he supposes, though the sun has turned gray in the sky, and ice is creeping across the floor. Things go wrong fast, leaving their home in pieces, and everything is up to Cam to keep himself and his sister alive in a world frozen over.
Mia didn't mean to kill millions of people. It wasn't her fault, really. She blames herself, though it was her father, a renowned scientist, who put together the project to stop global warming. She just pressed the button on the missile, and later, wondered why everyone seemed tense.
They both have different goals; Cameron to make some kind of life in the icy wilderness, and stay true to the boy he was before, and Mia: To set the world right again, and forgive herself for what she's done. But it's only one, icicle-covered planet, and they're bound to run into each other eventually.