There would be no gain on their part for starting a nuclear war. All wars are fought with a calculated gain. If there is no gain to be found, then it simply isn't done.
Also, if NK actually attacked. They'd have China coming down on them. Personally, it's my estimate that China will eventually have to take care of NK because they really did screw it up. I don't want to get into much reasoning but NK is a scare on China's record. Also, China is too smart to help such a small country because they'd lose the business they have with the US and other countries which they really need.
Even though North Korea has the largest military by percent of population the US still out preforms them. North Korea hasn't been in a war since the ceasefire for the Korean War. So, all their troops have no combat experience, while US troops now have plenty of combat experience.
If NK were to make a point by launching nukes at the US. We'd have a problem. However, they wouldn't live to see what that problem is. Also, one nuke isn't going to speed up global warming you need 100 million raise the temputer a few degrees (F). So, we might loose a huge urban center but that'll only make us mad and an angry American is never a good thing for the enemy. Examples: Revolution, 1812, Spainish-American, Mexican-American, Chase of Pancho Via (sp? it's late here), WWI, WWII, War on Terror.
Although, we will have to deal with NK eventually and probably should've delt with them a lot eariler. Reason we haven't is because they have enough artillery aimed at Souel to flatten it. We will probably fight Iran before we fight North Korea.