Actually, it would take far more than a couple deployed nuclear weapons to cause a so-called "nuclear winter", and it would be incredibly difficult for humanity to take the steps necessary to even invoke nuclear war.
Thus, global climate change is probably a larger threat than nuclear war could ever be. I'm uncertain as to why some Christians deny the concept of global warming, some claim that God would never eradicate the planet we inhabit, but how do you know it isn't God's way of punishing humanity when our society ends up excessively greedy/centralized around money?
The environment isn't limitless, and by always consuming to unhealthy degrees I do believe we've invoked this global climate change. I have yet to see Al Gore's documentary as it isn't playing anywhere near me, but I've read up quite a bit about global warming, and you can even begin to see the beginnings of a global shift now.
The past seven years have been the warmest on record, there have been increased hurricanes due to warming waters (we've never manage to make it to "Z" before), dryer summers/wetter winters in the Midwest, Tennessee and Illinois virtually becoming part of tornado alley, coral decaying globally... All of these side-effects of global warming were predicted far before they began occurring, and just look around, we're living in the middle of this now.