It does not have to be global to bring the thread of extinction. A local conflict with a few tactical nuclear weapons, say between Pakistan and India, would be

It does not have to be global to bring the thread of extinction. A local conflict with a few tactical nuclear weapons, say between Pakistan and India, would be an existential thread to humanity.[0] [0] http://www.scientificamerican.com/article/local-nuclear-war/ http://www.scientificamerican.com/article/local-nuclear-war/