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 counting the trucks and trains and the 300 000 tons super tankers, all the smaller ships, the roads and the railways, etc... We'll reach 10 billion tons long before we have to start counting the largest mass in human artifacts: the buildings.
Here http://pubs.usgs.gov/fs/1997/fs113-97/ they say, the Pinatubo ejected 5 cubic kilometers of ash into the air.
1 tone of (combustible) artifacts per every person in on Earth - went up in smoke - would be the same mass. A very unrealistic assertion - but still nothing like "decade of nuclear winter".
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.