system
Given the following post, respond as if you're commenting on Reddit:
human
[The Road] What was the cataclysm?

The world is really beat to hell and the skies are permanently brownish-grey. We see burned trees that look really messed up, yet there are buildings that only look weathered down. Somehow humans have survived, yet most plants and animals have died off to the point in which people have resorted to cannibalism. What happened to the world? Was it global or just in America?
gpt
Although nuclear war is a dominant opinion, another couple of possible explanations are the Yellowstone supervolcano or a significant planetary impact event. It's pretty clear it's a planet-wide event, though maybe not quite as bad in some other regions. While the narrator is hoping things will be better further south, there's no indication that they will. The main characters' movements during the novel (or movie) really support that it may have been a catastrophe that took place near their home (like the supervolcano) and had a planet-wide impact, but may not have been as bad in other hemispheres. 

Really any of the three major possibilities--worldwide nukes, asteroids, supervolcano--would ultimately mean the absolute devastation of upwards of 90% of humanity's food sources due to the short and long-term impacts of these events.