Nikis-Knight
Deity
My take: The Age of Ice was both bigger and smaller than the world. It was supernatural; the god who could create Ice from nothing (or more likely, destroy heat energy) broke the rules and started operating in the mortal realm. So Erebus wasn't the only place covered in ice, other mortal worlds, inhabited or not, were as well.If the Age of Ice literally means that the entire earth was covered by ice and we can infer that the ecogology of the planet operates according to the same laws of nature that govern ours, it would take many millions of years for jungles to re-emerge assuming some tiny pockets even survived on an all-permafrost biosphere.
But since the ice age was not a natural occurance, but the result of the will of a non-omnipotent god, the world was not an inpenetrable block of ice. Scattered pockets may have escaped Mulcarn's gaze, for a time, and had some semblance of their natural state reassert itself.
In particular, there was one area that Mulcarn could not afflict with perfect winter--the city of Braduk, in which part of his opposing god Bhall (opposite in power if not alignment anymore) remained, leaving an eternal flame that allowed the Orcs protection during the Age of Ice. Maybe enough for some jungles to survive; certainly enough for the orc population to boom in such a short time.