Just my own point of view.... not official at all....
The Ice Wall should disappear after duration is exceeded. The fact that it may melt in a warmer than freezing environment is irrelevant and actually, I would have the ice wall begin melting immediately (or almost instantaneously in some volcanic environments) - and if the spell is costed as being non-permanent, then the effect should be the same for all types.
The caster is summoning material from another location to create the wall - and after the spell duration is over it returns to where it came from.
If you want the spell to be permanent there is always scaling available. Of course, a permanent ice wall in a tropical environment simply becomes a puddle over time.