I hope to never see a country blown off the face of the earth by one of those things. N. Korea, like Japan has people that have nothing to do with the government decisions. Nuking N Korea would affect the world. Everybody that has weapons would be on the button thinking they are next. Putting that aside, the radiation fallout in the trade winds puts Alaska in the path for the fallout, them Canada, then down the coast.
I do not believe that civilization will be wiped out in a war fought with the atomic bomb. Perhaps two-thirds of the people of the earth will be killed.
There have been 711 nuclear tests, not underground. Why would a bomb in North Korea cause such distant contamination, when all the others didn't do that much? Most of the poisoning occurred just down wind. I'm not for nuclear bombing anyone, but I'm not terrified of a limited exchange. This problem is China's making, they need to get off their won, and take North Korea down. Besides, if you light off a nuke next to China, their not going to be very happy. I could imagine how we'd feel if China nuked Mexico.
We were too soft on North Korea, now we have a hostile nuclear power to deal with.