Complex sentence: Historically firestorms have occurred in a number of forests and cities, with nuclear winter researchers using both Hamburg and the less ferocious Hiroshima firestorms as the two principal cases where it is conceivable that the injection of soot into the stratosphere occurred.
Split into simpler sentences: Historically firestorms have occurred in a number of forests and cities. Nuclear winter researchers use both Hamburg and the less ferocious Hiroshima firestorms as the two principal cases where it is conceivable that the injection of soot into the stratosphere occurred.