Very clear numbers:
5 million square kilometers of bush burnt. That's 5*10^12 m2. Every year you can expect at least 1 kg of wood growth per square meter. That's 10^7 J accumulated per year on every square meter. But let say, it is all that it's there.
This gives you 5*10^19 J released by the biggest Australian bush fire. Many times more than your estimation for the atom bombs aggregate energy release.
Pure and simple, do you object this numbers?
you picked one set of numbers, calculated something, then didn't like it, changed from 1 to 10 megajoules per square metre, that's not how you do it if you are thinking straight.
Regarding 'my estimate', once again: 500 megatons is total testing, 10 ... 100x the figure you picked, total 5000 .. 50 000 megaton , the 1 megaton of tnt is 4E15 j , times 5E3 = 2E19 , times 5E4 , 2E20 .
Top it off by the smoke not going into stratosphere from a bush fire because there's too little intensity, it doesn't even burn all at once, so there's nothing whatsoever even comp...
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.