[Toddsterpatriot]

Depends on the size of the comet of course, but anything a mile or more in diameter would extinguish civilization and most (over 99 percent) of humanity. The EMP would take care of most or all of the comm networks worldwide.

It's unclear that a comet strike would generate an EMP.

In the book, they estimate the impact of the hot fudge sundae would be equivalent to 647,000 megatons. "About three thousand Krakatoas" and "would vaporize about sixty million cubic kilometers of water" for a water strike. But the Hammer is ten times larger.

[SunkenCiv]

It's not unclear at all. It would. Early in the 20th century, a midair detonation over Lake Michigan knocked out at least six municipal power generators of the time, sent a surge up the Grand River channel in Grand Haven, lit the surrounding surface of the Earth for miles, and was so evident people ran out of their homes into the night, sure that the world was ending, as far inland as Battle Creek. This was a small, transient event that left no other traces.