For us Americans, note that 260° C = 500° F. I haven't tried the experiment, but given how quickly various foods (including fresh rosemary sprigs and other live plant matter) blacken in an oven that temperature, I wouldn't be surprised at all if it set leaves and small branches on fire in less than 20 minutes. The smaller branches could then act as kindling for larger ones. Good points about water and ice, though. I'm curious now to see the assumptions in the old models. How much fiery molten rock raining from the sky does it take to melt and evaporate a heavy layer of snow?
Agreed, kindling would easily set on fire, and assuming a heavy density it could certainly start a forest fire. However, the effect would be no different than dropping a cigarette. When you get to that hair-trigger it's going to happen anyway. I assume the disaster is in the 'global' forest fire that would have ensued, keeping temperatures higher than habitable in major areas of the planet and then producing a cooling effect due to the soot emitted.
However, from my experience soot is emitted more by poor burning not good burning, so could the soot layer that was laid down have been made by a big poor-burn of fuel, rather than a major forest fire. I've seen people burning damp leaves and it emits huge plumes of soot.
