When we think about overpopulation, the first think that comes to mind is the loss of fresh water. I recall reading that by the year 2025 many parts of the world will be facing fresh water crisis, due to the higher numbers and all the contaminated water sources that humans are causing. But, in time, the very factors that overpopulation creates will decrease our numbers. Overpopulation will result in epidemics that will decrease our numbers.
Some aquifers, such as in Saudi Arabia, are non-renewable, meaning that once it’s gone, it’s gone. Others renew and re-load through groundwater replenishment and percolation.
There are probably regions in the world where overpopulation puts tremendous stress on the water supply. There is probably an equation that can (and has already been) made where overpopulation outstrips nature’s capacity to supply adequate fresh water. This is especially true in Sub-Saharan Africa and India. Don’t know what China’s situation is.