What's on my mind is the 4th of July fireworks offered by North Korea, leading to thinking about Tom Lehrer's song, "Who's Next," on nuclear proliferation, leading to a line by Sergeant Yanek, the teacher of the course I took in the early 1960s in CBR: Chemical, Biological, and Radiological warfare.
Another student in the course tried to waste some time and/or was really interested and asked Sergeant Yanek about concerns publicized on the upcoming test by the USSR of some 50 megaton or 100 megaton or whatever really big "device" and Yanek said that there was some worry that an explosion that large might crack the crust of the Earth or get the planet wobbling a bit on its axis, which in turn could crack the crust .... And the student said, "You don't sound too concerned," and Yanek paused a beat and said, "Well, I probably shouldn't say this since the motto of the course is 'Survive, Struggle, and Prevail,' but the way I figure it, by the time the Russians set off their bombs and we set off ours, and the English and French and Chinese and maybe Israelis and God-knows-who-else set off theirs, WE'RE ALL GONNA DIE!! So, no, I'm not worried about no 50- or 100-megaton Russian superbomb."
A North Korean bomb that can go on an ICBM is an issue, but I live next to a US West Coast deep-water port with container-ship traffic, and if the DPRK or anyone else can make a bomb small enough to fit into a shipping container and not be too obvious, well, "they" can take out a lot of Americans, including, even in California, a lot of Republicans.
In terms of existential threats, the North Koreans aren't an issue. The Americans and Russians are: between us we have enough warheads to risk nuclear winter or at least put big parts of human culture back to the late Medieval. And our leaders really need to keep working on that (things were worse during the Cold War).
It would make me very unhappy — more exactly, very dead — if Port Hueneme, CA, and a big part of Ventura County got reduced to a rapidly expanding ball of white-hot plasma; but H. sapiens and the USA could take the loss. A serious thermonuclear exchange, and "WE'RE ALL GONNA DIE!!" or at least risk species death. And we came close to that in my lifetime: not just the Cuban Missile crisis, but on the quiet day of 28 September 1983, when the balloon almost went up, and a whole lot else, because of a computer glitch.
Whoops. Messrs. Trump and Putin et al. need to stop messing around.