I hope you're joking. Pretty much all the potentially usable land in Canada is already in use, regardless of climate change. The issue with Canadian agricultural lands isn't climate it's soil quality. Almost everything else isn't going to be arable because it's thin soils over granite bedrock (Canadian shield). If you go really far north there's areas in permafrost but that's a) arid and b) would be poor fertility anyways. Likely to desertify if it actually got that warm.
It isn't about just fertility, it can have other uses, and its also possible to make it more fertile (although it is a poor choice as farmland). Las Vegas for example is very inhospitable, as was the ancient Persepolis.