Well the issue is, if it's adapt or die, a pretty fucking huge number of people will end up dying. Yes, with the earth being warmer, perhaps the net amount of usable land would increase. But if said usable land is in a different location than the current usable land there might be a few, you know, issues in picking up the entire agricultural infrastructure of the world and relocating it. Especially when you take into account things like national borders which countries tend to be a little touchy about.
So yeah, sure, we'll adapt, it won't be the end of humanity. But the transition is going to suck.
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