Looks like I have iggernunce to spare.
What fuel is there to burn when a comet strikes a continent-sized ice cap? Trees and other stuff that has been crushed under the ice?
Not a whole comet, but fragments of a broken up comet (Shoemaker-Levy 9). Some fragments were a mile in diameter but most were air-bursters like the one over Russia a few years back (video on web, search). However, the order of magnitude was more akin to the Tunguska strike.
This was a process that happened over 100 years as the Earth passed through a dense part of the Taurids (every Fall). There so many that the heart of the bursts ignited the vegetation continent-wide (there is a black mat geological layer several feet thick composed of the ash). This is a roughly a tear drop shape with the bulbous end in Canada/Great lakes region stretching over the the UK and down to North Africa and the pointy tail in South America.
Some of the big fragments did hit the ice cap throwing huge ice clumps into the upper atmosphere, then hitting the ground shattered and rained down as ice-like razor blades slicing and dicing all living things; this process created the Carolina Basins and the Nebraska Rainwater Basin geological features. These ice shards helped extinguish the raging firestorms consuming vegetation, animals, and people.
None of this stuff should be seen as small or a one time incident - these were like a nuclear bombardment from space it was so devastating and traumatic for the survivors. The world literally changed and we forgot all of this, relegating it to myth and fable. The survivors embarked on a world-wide sky watch program using stone to tell the events coming from the heavens (Stone Henge, stone monuments, mounds, wooden posts all used to determine what might be in store and give warning).