Aside from the limited availability of freshwater, there are indeed constraints on the amount of food that Earth can produce, just as Malthus argued more than 200 years ago. Even in the case of maximum efficiency, in which all the grains grown are dedicated to feeding humans (instead of livestock, which is an inefficient way to convert plant energy into food energy), there’s still a limit to how far the available quantities can stretch. “If everyone agreed to become vegetarian, leaving little or nothing for livestock, the present 1.4 billion hectares of arable land (3.5 billion acres) would support about 10 billion people,” Wilson wrote.
Doesn’t even mention abrupt climate change impacts.
GMO and large agriculture will not be the solution, according to a recent study by the United Nations in 2011. The answer will be small, local, agriculture. Altho how a place like Manhattan does that is not clear.