The primary problem with nuclear war is that it isn't obvious that humans can get back to our current tech level without the now consumed resources (primarily fossil fuels) that we've used to bootstrap ourselves up to our current tech level. If that's an issue, then any event that effectively pushes the tech level much below 1900 is about the same as an existential risk, it will just take longer for something else to then finish us off. There's been some discussion on LW about how possible it is to get back to current tech levels without the non-renewables to bootstrap us up, and there doesn't seem to be any real consensus on the matter. This should probably be on the list of things that someone at FHI should spend some time examining.
Yes, this is the general reason why global catastrophic risks (including global warming, global pandemics and so on) shade into existential risks.
In fact, my personal view of x-risk is that these are the most likely and worrying failure modes : some catastrophe cripples human civilization and technology, we never really recover, and then much further in the future die off from some natural extinction event.
The FHI's mini advent calendar: counting down through the big five existential risks. The first one is an old favourite, forgotten but not gone: nuclear war.
Nuclear War
Current understanding: medium-high
Most worrying aspect: the missiles and bombs are already out there
It was a great fear during the fifties and sixties; but the weapons that could destroy our species lie dormant, not destroyed.
But nuclear weapons still remain the easiest method for our species to destroy itself. Recent modelling have confirmed the old idea of nuclear winter: soot rising from burning human cities destroyed by nuclear weapons could envelop the world in a dark cloud, disrupting agriculture and the food supplies, and causing mass starvation and death far beyond the areas directly hit. And a creeping proliferation has spread these weapons to smaller states in unstable areas of the world, increasing the probability that nuclear weapons could get used, leading to potential escalation. The risks are not new, and several times (the Cuban missile crisis, the Petrov incident) our species has been saved from annihilation by the slimmest of margins. And yet the risk seems to have slipped off the radar for many governments: emergency food and fuel reserves are diminishing, and we have few “refuges” designed to ensure that the human species could endure a major nuclear conflict.