The Road is indeed dark. But then, I like dark movies. I always wonder how the world came to be in such a state (in the movie) though I suspect something such as the Yellowstone caldera going all splodey.
Did the book tell about the cause of the crappy weather and conditions?
No, because the book is one big long metaphor for McCarthy being relatively old, and having a young son at an old age and knowing he'll die of old age before his son reaches adulthood. So the actual "whys and hows" aren't important.
However, when McCarthy was pressed in interviews and such, he did have vague ideas the disaster was a nuclear war, or at least "man made" somehow. However, even with some reasonable suspension of disbelief, that's bunk. Your idea of the Yellowstone caldera going up, or perhaps asteroid/comet impact is more in line with what it would take to create the conditions in "The Road".
Full scale nuclear war, even at the peak of deployable/deliverable warheads in the 1980's, and if distributed to create maximum casualties and infrastructure collapse worldwide, and as much fire, smoke, and ash as possible, rather than an attempt at more rational tactical or strategic goals, there'd be a LOT of empty wild areas left relatively untouched. And climatalogical models refined since Carl Sagan proposed "Nuclear Winter", from evidence such as volcanic eruptions, and even the oil fires of the first Iraq war, show the particulate cooling from a 100% nuclear exchange wouldn't make much difference in world climate.
And the land would bounce back very quickly, much like how the animal populations of the Cheyrnobyl exclusion zone, except for some hot spots, the cessation of human activity, agriculture, roads etc. has been more beneficial than the radiation harmful.
I think they've also found that most animals in nature don't normally live long enough to suffer the effects of long term low-dose exposure.
