# Population momentum

Population momentum is the proportion between the size of a stable (unchanging) population to the total size of the initial population that experiences a drastic shift in fertility to replacement rate (2.1 children per woman).[1] Demographers refer to population momentum as the size of the resulting stationary-equivalent population relative to the current size of the population.

Momentum occurs because older cohorts differ in absolute size from those cohorts currently bearing children. This impacts the immediate birth and death rates in the population that determine the natural rate of growth. For the a population to have an absolute zero amount of natural growth, the US National Library of Medicine National Institutes of Health suggests that three things must have to occur.

1.) Fertility rates would need to level off to the replacement rate (the net reproduction rate should be 1). If the fertility rate remains higher than the replacement rate, then the population would continue to grow.

2.) Next, the mortality rate stops declining meaning that it remains constant.

3.) Lastly, the age structure has to adjust to the new rates of fertility and mortality. This last step takes the longest to complete.[2]

## Implications of Population Momentum

Population momentum has implications for population policy for a number of reasons.

1. With respect to high-fertility countries in the developing world, a positive population momentum, meaning that the population is increasing, warns that these countries will continue to grow despite large and rapid declines in fertility.

2. With respect to lowest-low fertility countries in Europe, a negative population momentum implies that these countries may experience population decline even if they try to increase their rate of fertility to the replacement rate of 2.1. For example, some Eastern European countries show a population shrinkage even if their birth rates recovered to replacement level. The way a population momentum can become negative is if the fertility rate is under the replacement level for a long period of time.

3. Population momentum shows that replacement level fertility is a long-term concept rather than an indication of current population growth rates. Depending on the extant age structure, a fertility rate of two children per woman may correspond to short-term growth or decline.

## Calculation

Population pyramid of India showing the beginning of population momentum even though their birth rates have been declining since 1971

To calculate population momentum for population A, a theoretical population is constructed in which the birth rate for population A immediately becomes replacement level. Under such conditions, the population will eventually stabilize into a stationary population, with no year-to-year changes in age-specific rates or in total population. The population momentum is calculated by dividing this final total population number by the starting population.[3] Momentum, Ω, can be expressed as:

${\displaystyle \Omega =(be_{o})Q}$

${\displaystyle Q={1 \over rx}{R_{o}-1 \over R_{o}}}$

In the original equation, b is the crude birth rate. In the second equation, r is the growth rate, Ro is the Net Reproduction Rate of the non-changing population while eo is the life expectancy at birth and x is the unchanging population mean age at childbearing. Q is the total number of births per initial birth.[1]

## Causes

Population momentum is typically caused by a shift in the country's demographic transition.[4] When mortality rates drop, the young survive childhood and the aging population live longer. Fertility rates remain high, causing the overall population size to grow.[5] According to population momentum, even if high fertility rates were immediately replaced with replacement level fertility rates, the population would continue to grow due to the pre-childbearing population entering childbearing years.[4]

## Countries with a population momentum

The following list are countries that maintain growth despite a fertility rate of under 2.1. However, some of these countries also maintain growth because of immigration.

## Effects of Population Momentum on the Environment

Forests, wetlands and mangroves cover two thirds of potential agricultural land; they all rely on water. These percentages vary by region but fall along the range of 23% to 89%. If these areas were to be preserved, the amount of land with rainfed agricultural potential would amount to 550 million ha. The rapid population growth during the next 30 years leaves little time for sustainable development of the land and water resources. The constant demand for land resources and water can cause some regions that, in the future, will encounter troubles and hardships trying to acquire these resources. These regions may be forced to begin to rely on external resources. These regions include Western Asia, South-Central Asia and Northern Africa.[6] There has been a recent drive to develop global control and oversee systems to study the overlying causes and impacts of land and land use both nationally and internationally.[6]