I didn’t want to hijack the Could humanity survive another dinosaur killer? thread, but a key question to that post is, just what contingency plans does the US have for recovering from a gigaton-range global disaster (manmade or natural)?
For example, suppose that after the Event there was so much soot and dust in the upper atmosphere that it would not be possible to grow crops for years or decades. Whether humanity survived or not would be strongly dependent on whether the US (or any government) had an “ark” contingency plan to keep a group of people alive in a shelter for years, not just months, after the Event.
You would not only have to keep the people sheltered, and store enough food to feed them possibly for decades, but also to provide for how they and their descendents would recolonize the surface. Things like seeds of otherwise extinct plants. Farm animals. Maybe even a small zoo to try to preserve some megafauna other than cattle.
If there is or was any such Extinction Level Event planning, I’ve never heard of it. All I’ve ever heard of was short-term (<5 years) shelter for government officials, with the civilian population on it’s own, or even during the Reagan years suggestions that a nuclear war with the Soviet Union would be a “counterforce” exchange, that would be no worse, relatively speaking, then what Europe suffered during WWII.