Their model used 100 Hiroshima-size bombs
2 Mt. 1.5% of the Tzar bomb, which exploded at Novaya Zemlya one day. Nothing much.
Enough to make as much fire as in Black Thursday bush fire? Which didn't caused a decade long "nuclear winter"?
Novaya Zemlya is tundra (ie, "lichen, sedge, sometimes grass, and if you're lucky, scattered dwarf shrubs sitting on permafrost") and glaciers. The Tsar Bomba went off in October. That's October in the Arctic Circle, by the way.
We shouldn't be surprised at this vast asymmetry between "models of nuclear warfare targeting cities in populated areas" and "one very large nuclear bomb, set off as a test, in the Arctic during the beginning of local 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.