It is clear that food not grown here is going to have to be replaced-- grown elsewhere. And, those new farmers will have to clear land & destroy habitat to do that. (I suppose that should've been 5c.: destruction of forest / grasslands / habitat.)
Since ours is among the most productive farmland in the world, chances are the new land will be less fertile. And so it'll take more land area and more work and tractor fuel, on average, to yield the same crop.
And the replacement's agricultural practices aren't likely to be as advanced and efficient as ours in general.
What the total affect of those factors is, I haven't calculated; I was just passing on that link (to the fellows who have, who estimate ethanol's net effect is to double CO2 emissions).
Best wishes, James Arthur