I would put this down to Aether too. Apart from the disruption Bahamut breaking free and the pieces of Dalamud raining down on the land caused, even prior to that the proximity of Dalamud was pulling Aether out of the land. If something caused a shift to large concentrations of Ice Aether in Coerthas, it would lead to the temperature change. Its the same logic on why we have floating islands. If a particular type of Aether is particularly strong in an area it can have rather drastic effects of the surroundings.