I've recently been looking for a collection of young adult science fiction that I had in the early 1990s. I don't remember many details about the book, except for one story:
A brother and sister are trying to survive in a post-apocalyptic winter. Nobody knows exactly what caused the winter; one day an object just appeared, blocking the sun. The brother and sister encounter a man in the woods, there is some exchange of dialogue, and we find out (I think) that the man is 1) a future version of the brother and 2) responsible for causing the artificial winter for reasons the I'm pretty sure involve the sister's future death.
I remember the entire collection being good, but this story was by far my favorite. I would love to find it again. Any ideas?