The most discussed forecasts of the world population are those by the Population Division of the United Nations that are discussed here. But there are also other institutions working on population forecasts like the Austria-based Institute for Applied Systems Analysis (IIASA) that can be found here. We can see below that there are stark differences in the projected world population depending on the fertility rate. If the rate is low, then the world population will start to decline. However, if it is high, then the population will continue to increase rapidly with no decrease in sight.