That would make the most sense, but it doesn't get any warmer once he's dead so....
. My first game, I was sure I was gonna hear ice breaking up in the river once he was banished back to wherever but....nope. Maybe it's a years-long winter like in GoT?
I think you're on to something. The Northern Hemisphere had a mini ice age in the 1700s, and 1753 was known in New England as the "Year Without a Summer." There are cycles. It just stinks when bad roads and famine and disappearances and war gang up on people all at once and it's cold too.
I'm a terrible liar. I did find the key fragments and got Fairel's treasure, and gave up on the Wastes notes. Then, I checked my shard collection to verify I had what I needed to finish the temple, and I haven't finished the Plains, and I haven't found shards on the Coast. I want to do Solasian early this time, so no demon pictures today.
Have some surprisingly pretty views I took in the Exalted Plains.

This one's funny. I was trying to climb on top of a roof at Fort Revasan, and fell off. The view was nice where I landed.
Made it.
Talk about cool. I was trying to sight in the infiltrator near the steps, and paused to catch this bee from the jar I just threw.
The bees didn't track him, but the fire staff did.
First time I ever completed my wine collection. Staff party at Skyhold at dark-thirty.
And, finally, proof positive that you don't even have to attack the barrier directly to destroy it. I lobbed a bees grenade at the arcane horror, and the barrier was gone before I reached him, and the rest of the team was fighting where I was a couple minutes ago, out of direct sight range. Continuous damage from the bees was enough to break the barrier. Good to know.