I mean, it’s already swampy. Canada has the band of current habitability shown, followed by muskeg (frozen swamp) and boreal forest for most of it’s area, followed by a strip of tundra along the coast (and continuing into all those giant uninhabited islands). Swamps are actually carbon fixers, though; it’s the process of a swamp drying out and the exposed plant matter rotting that’s the problem.

The tundra bit has permafrost that will thaw and rot into pretty pure methane, which is bad, but that’s in line to become new boreal forest next rather than new farmland.