What can happen when world population reaches 18 billion?
The world will not reach 18 billion people, for the same reason that you cannot sail a yacht in your bathtub.
Many scientists think Earth has a maximum carrying capacity of 9 billion to 10 billion people. This is based on mass refusal to give up eating meat. Around 10 billion people is the uppermost population limit where food is concerned. It's probably not quite even that many. The truth is that nobody really knows, but a world with 18 billion people would mean a lot more people starving and living in poverty, which would mean far more conflict. Things would be bad!
While it is said that all (current) 7.3 billion humans could fit inside of New York City, space isn't the issue. (And that's a terrible idea!) The real limiting factors are finite resources, especially food and water.
Other factors for limiting human growth on Earth are the nitrogen cycle, available quantities of phosphorus, atmospheric carbon concentrations, not to mention fossil fuels or whatever replaces them.
The good news is that the UN says we are a population close to being in decline. Globally, the fertility rate is falling to the replacement level of 2.1 children per woman, which is the rate at which children replace their parents and make up for those who die. If the global fertility rate does indeed reach replacement level by the end of the century, then the human population will stabilize between 9 billion and 10 billion.
Source: https://www.quora.com/What-can-happen-when-world-population-reaches-18-billion
