The key background event of my storyline is the eruption of the Yellowstone super volcano. This eruption forms the basis for the calendar where the year of the eruption is designated Y+0.
I try to set each of my stories or vignettes on this timeline, though there are some exceptions. The vignette Cold Contact is not on the timeline. The short story Morgan's Road is on the timeline to indicate the year in which Nils Carmike tells the story for the first time.
The Yellowstone eruption plays into some of the motivation or emotional state of the characters. It serves as a focal point event where almost everyone relates to it. It has the same affect as the attack on Pearl Harbor or 9/11 had on their respective generations.