There isn't one. Maximal theoretical damage (not practical) would not end civilisation.

The nuclear winter calculations by Sagan et al noticed that some fuels create more problematic smoke than other fuels. A lot of trees and wood structures would be ignited in a nuclear war, but the smoke from those fires falls out of the atmosphere relatively quickly compared to smoke from burning oil tanks, which Sagan et al said would contribute most of the cooling effect.

Then Saddam lit thousands of oil wells on fire as a big fuck-you to the miltary coalition arrayed against him, more oil than would be set on fire by any nuclear war. The cooling effect was detectable, but hardly serious.