Complex sentence: While it is believed that the effects of the mass of soot emitted by 100 firestorms (one to five teragrams) would have been detectable with technical instruments in WWII, only five percent of that would not have been possible to observe at that time.
Split into simpler sentences: While it is believed that the effects of the mass of soot emitted by 100 firestorms (1 to 5 "Tg" Teragrams) would have been detectable with technical instruments in WWII. Only 5% of that would not have been possible to observe at that time.