Earlier quoted context omitted.
The nuclear war scenario, according to all known information about nuclear war policies, is not “chunks of the world becoming unlivable.” The principals in a nuclear conflict do not appear to even have a method to launch just a few nukes in response to a nuclear attacks: they will launch thousands of warheads at hundreds of cities.
I suppose thousands of warheads could be launched at hundreds of cities, and that could make chunks of the world uninhabitable and kill millions climate change will happen, and it will do that, left unchecked
Daniel Ellsberg, the top nuclear war planner at RAND during the Cold War, claims that the Joint Chiefs gave an estimate to Kennedy in 1961 that they'd expect 600,000,000 deaths from the US's war plan alone.
That's 600 million people:
1. At 1960s levels of urban populations (especially in China, things have changed quite a lot -- and yes the plan was to attack China... every moderately large city in China, in fact!)
2. Using 1960s nuclear weapons
3. Not including deaths from the Russian response (and now Chinese), at the time estimated to be 50 - 90 million Americans
4. Before the global nuclear winter