The medium scenario assumes that fertility rates stay below replacement levels, at the current level of 1.85, for no more than 100 years and then, return to replacement level of 2.05 by the year 2175. Growth would continue rapidly to 2050, peak at 9.22 billion in 2075, and then dip to remain fairly constant at around 8 billion people. The high scenario assumes that fertility rates stay 0.5 above the levels in the medium scenario so that eventually, fertility level is fixed at 2.35 children per woman. This high fertility rate will cause human population to grow rapidly to a size of 36 billion in 2300, with world popultion growing by a third in the space of 50 years between 2050 and 2100. The low scenario assumes that fertility stays 0.5 below the levels in the medium scenario and declines to a fixed rate of 1.85. This fertility rate will cause a decline in world population, bringing human population in 2300 to 2.3 billion with population cut by two-thirds between 2050 and 2300.
What these three scenarios do not take into account is the Earth's limited amount of food, fuel, and space resouces. Our planet does not have the food resources or the space for 36 billion people and it is unknown if it even has enough to support 8.9 billion people. The human species is a hundred times more numerous than it should be for an animal of its weight class and uses 20-40% of the solar energy captured by land plants, more than any other species (Wilson, 1992). More people means more land must be cleared for human development and new food technologies would need to be invented to feed everyone. There is hope however, because by changing human behavior to limit the number of children per couple, our society can choose which scenario to follow. In China where population growth is of serious concern, a one-child program was implemented in 1986, which limited couples to having one child in rural and urban areas, and has helped slow population growth (U.S. Library of Congress, 2003). As life in developing countires improves, fertility rates tend to decline, but many countires have a long way to come before such changes occur and this is why future population growth is an important topic for many countries around the world.