The whole of mankind could live in Jacksonville county florida if we lived in the highest density seen in tokyo, it wouldn't be comfortable at all though. Overpopulation is about resources not lack of space to put people. The US could reasonably and sustainably fit 3 times its current population but we'd live as densely as central europe and most americans don't like ultra dense living. If the left and right could see eye to eye everyone would see the solution to overpopulation and global warming is deporting all blacks to africa and withdrawing all non blacks, allow them to starve down to a sustainable population over the next 5 years then invade and carve up the continent between current global powers. You could do a more aggressive biomass redistribution, throw all the billions dead into deserts and the sea and plant a bunch trees and plants by dropping seeds from planes.