You might be right Brian- and you might be wrong.
Having been nay sayed so many times myself, I dislike throwing stones at other peoples contributions. I have been simply dismissed as a liar and an idiot so many times I have lost count.

So with that said, please do not take my criticism the wrong way and consider it an opportunity to prepare for such criticisms.

There is a difference between an H-bomb and a first generation fission bomb. In fact, there is so big a difference in the heat pulse and radiation generated over a city that.……your comparison is not convincing to me unless you can provide more numbers. Not that I am trained to interpret such data and I would be easy to fool but you seem like the kind of person who would not blow smoke up my skirt. I may be missing alot here so if the following factors were taken into account please delete this comment.

There is no mention of the natural gas, automobiles (several million per city) and prevalence of alloys that burn quite well along with above ground gasoline storage. When ignited (aluminum) and the amount of matter that will jack up the heat such as asphalt, carpeting, plastic furniture, paint, and on and on- is not a good comparison with a 1945 Japanese city. High rise buildings and other structures that survive or collapse into perforated wreckage also act like blast furnaces efficiently consuming their contents- including people. I may be engaging in hyperbole but it seems like comparing a match to a cup of gasoline. H-bombs clouds also go really high and would this not much more efficiently transport smoke into the upper atmosphere?

While our nuclear arsenal has decreased dramatically there is the offset of improved quality control and electronics that mean many more of the submarine based missiles, land based missiles, and even whatever bombers manage to get off the ground will actually incinerate cities.

But.……events like Krakatoa are in your favor when doubting nuclear winter. Even so, after watching the movie “The Road” I do not have a good feeling about what will happen if civilization collapses. Movies do not usually impress me but that one was different. We might self-destruct so completely that there will be very few survivors and who knows what will happen to them? It is a possible existential threat in the sense that it will leave the human race vulnerable to another disaster such as a virulent disease that might put the nail in the coffin. Such things happen.

Thanks for your time reading this.

http://nextbigfuture.com/2010/04/historical-supervolcanoes-and.html

this indicates that the climate effects also seem to be less than the nuclear winter people say. So my view is that all out nuclear war will not trigger nuclear winter (to the level specified in the research) and will not kill all people. But that still means you should avoid nuclear war and if their were millions of nuclear devices hthen that woujld be getting to existential risk. Also, there is no upper limit on nuclear fusion power. So if one were to use nanotech to sort out the deuterium on a gas giant then fusion it, that would be an existential threat.

I am also saying that steps can be taken that would mitigate nuclear weapon effects. The same kind of steps that would limit citywide fires and limit earthquake damage. How you build things matters a lot. See the difference between Haiti and Chile against earthquakes. Why do people think that how you build things is not equally as important against nuclear devices. I also feel that non-proliferation is not good as the only strategy. It like going into a boxing match with the strategy of never getting hit.