At those levels of scarcity, the economics of farming will likely change enough that hydroponics could (would?) become far more commonplace. That drastically cuts down on the amount of land needed, as you're no longer limited to ground surface area for growing crops.

In fact, there's a growing trend of combining fish farming with hydroponic vegetable farming; circulating water between the plants and fish tanks, monitoring nutrient and bacteria levels, can allow for a non-vegetarian diet.

Granted, they're not going to be the tastiest fish- think tilapia, not salmon. There's a CSA farm doing exactly that in a relatively densely populated suburb not too far from me.