Say you were in the US government and knew with certainty by secure and
non-public means that an incoming asteroid or cometary nucleus was less than a
week out from hitting the earth. No other country knows, just for the sake of
argument, all the astronomers have died in a horrible gardening accident...
This is a big one, a dino-killer size rock. At this point, you know it will hit
earth, but not exactly where. At the size we're suggesting, it really doesn't
matter: an ocean hit will send tsunamis about two states deep into both the
American coastlines, if it doesn't hit dry land. That's just for starters. You
can't stop it.

Do you bother even warning your people, since, assuming you still will somehow
be around yourselves to rule them, all you do is assure more survivors to drain
the remaining resources you have left after the strike? Because it's not like
you have anything you can really DO for them...

What if it's not a dino killer, just "real bad", still catastrophic, but in
some bizarre macabre sense "survivable". Do you warn then? At what size/ level
do you decide it's worth the extra mouths to feed after, balanced against the
number you need to have a working nation?

No, I don't know anything...... but I wouldn't sweat that crabgrass problem if
I were you;-)