The idea that an all-out nuclear war will trigger a civilization-ending nuclear winter has been on shaky ground for a while now. It appears the likelihood of it happening to such a degree was based on the political/idealist desire to reduce the nuclear stockpile. [1][2]

For a more realistic view of nuclear war and its survivability, check out [2] (with a foreword by Edward Teller and author's note by Eugene Wigner of Wigner's friend fame).

[1] https://en.wikipedia.org/wiki/Nuclear_winter#Criticism_and_d... [2] Kearny, C. Nuclear war survival skills