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):
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.
A volcano ash is often black. The mass of already mentioned Pinatubo's dust, exceeds the mass of all human artifacts on the planet. If everything we have, go in smoke, it has less mass than the said dust, airborne in 1991.
Now you're just making stuff up. According to this http://www.thefreelibrary.com/Mt.+Pinatubo's+cloud+shades+global+climate.-a012467057 , there were 20 million tons of SO2 ejected into the atmosphere. The number of cars in the world is about 800 million cars on the road http://en.wikipedia.org/wiki/Automobile, mass about 1.5 tons, so we're ahead on those alone. Even if we're generous, and include the "10 billion metric tonnes (10 cubic kilometres) of magma" in Pinatubo (most of which is not relevant for the current discussion), I haven't started ... (read more)