You are saying, that the total energy amount of all the nuclear weapons is 4*10^18 J. Not 40 times less as I have said.
Even in that case, the wildfire which has covered 5 million square kilometers and consumed 1 kg of dry grass and wood was a bigger event. Especially since I gave 10 times smaller energy per kilogram of wood or dry grass as it is. It's 10^7 J/kg not 10^6 as I have calculated. Not to mention, 1 kg per square meter is a very low number.
But it matters only a little. This calculation is not very exact, but quite enough to see the main point.
I still don't see the main point. Wildfires don't put stuff into stratosphere very well, its not very concentrated, and it ignores burnable stuff in the cities.
For the calculations, just do them carefully one time ok? I don't know full yield, the 1000mt is just example. Your own estimate for yield of arsenal, vs yield of testing, was arsenal = 10..100x the testing. You need to pick your numbers, and stick to them all the way through without fitting them after you arrive at something you don't like. That is just math. If you can't do that why you think your opinion on nuke war results is at all coupled to the actual results in any way?
Just a reminder that some of the old threats are still around (and hence that AI is not only something that can go hideously badly, but also some thing that could help us with the other existential risks as well):
http://blog.practicalethics.ox.ac.uk/2012/03/old-threats-never-die-they-fade-away-from-our-minds-nuclear-winter/
EDIT: as should have been made clear in that post (but wasn't!), the existential risks doesn't come from the full fledged nuclear winter directly, but from the collapse of human society and fragmentation of the species into small, vulnerable subgroups, with no guarantee that they'd survive or ever climb back to a technological society.