In Cormac McCarthy's post-apocalyptic novel The Road, one reason why the man and his son are continually driven south is because the nuclear cataclysmic event has covered the sky in ash and toxic particles. The air is so full of ash and toxic particles that no sunlight shows through, not even in the day time, killing all plants and animals, completely changing the earth's biosphere. Plus, lack of sunlight naturally equates to a lack of warmth as well. We see McCarthy paint a description of the lack of sunlight and warmth in the very opening paragraph:
Nights dark beyond darkness and the days more gray each one than what had gone before. Like the onset of some cold glaucoma dimming away the world. (Ch. 1)
The man is also well aware that winter is fast approaching; he suspects the month is October. He knows that they will not survive another cold winter in the area, so the only hope for survival is to head south.
As the novel progresses, snow starts falling more heavily, pushing them farther south. One reason why the snow presents a threat to their survival, aside from making the environment too cold, is that it buries their supplies, as we see when one time the father emerges after a snow storm and cannot find their cart.
Cannibalism has also become a trend among the remaining humans because human beings are the only animals still in existence, except for dogs, which means human beings and dogs provide the only source of meat. The cold itself does not seem to be the main reason why all animals, except dogs and humans, have disappeared, but it is one reason. The other reason would be the cataclysmic event. Plus, due to the cataclysmic event blocking out the sun and making the world unbearably cold, as the remaining humans must press south for survival, cannibalistic humans must also press south.