Population density is not a problem. Even at a relatively low population density equivalent to that of Los Angeles, you could fit the entire population of the

Population density is not a problem. Even at a relatively low population density equivalent to that of Los Angeles, you could fit the entire population of the world 3.4 times over in the contiguous United States. Agriculture is a potential problem, but technological advances are improving the situation. If everyone switched to vastly more efficient plant-based diets, it wouldn't be a problem at all.