I've not worked out all the details yet, but I'm thinking a very large portal was opened, possibly to a frozen level of the abyss or hell and thats bringing in cold along with the fallout of the battle are producing a ripe environment for a short ice age to happen and then melt away. How long does it take a continental glacier to melt?
Along with your comment about the land bridge, I maybe should have made the land to the south more arid.
