Remember "we'll turn the food production down to force them to go to war to loot for food, otherwise, they will die"?
I'm not really aware of all the details of your intents, though, so maybe I'm wrong. Maybe I'm way off. But there is still a good chance that, indeed, they WILL die, and then they'll come whine about it.
I'd like to say that even a sudden targetting of half the continent wouldn't need instantly killing the realms without giving the players a chance to move elsewhere. The changes could be gradual in effect, instead of in geography. You could take up half of the continent, and then slowly apply things like dwindling production and shortage of materials (no building). If the new recruits won't work on gold, then what's gold good for anyways? Have the regions remain in existance, but with no more value to them. Perhaps give all nobles there the option to recruit troops directly from the regions themselves, much as the hero mechanic, and not just at the capital with the morale cost. The progress of the cold would not be in territory covered, but, for example, in the loss of population to the cold, until the regions are all uninhabited. That way, the players still have time to prepare, there's no nasty surprises about where the line will eventually be drawn or not, and there's no (less) feeling cheated on the part of the first affected.
Maybe you could just poll them, to. "Would you rather the changes be quick, or gradual?" Having a say in their loss might make them feel less powerless, and thus less resentful.
