# Population growth

Past and projected population growth on different continents. The vertical axis is logarithmic and its scale is millions of people.

Population growth is the change in population over time, and can be quantified as the change in the number of individuals in a population using "per unit time" for measurement. The term population growth can technically refer to any species, but almost always refers to humans, and it is often used informally for the more specific demographic term population growth rate (see below), and is often used to refer specifically to the growth of the population of the world.

Simple models of population growth include the Malthusian Growth Model and the logistic model.

## Population growth rate

In demographics and ecology, Population growth rate (PGR) is the fractional rate at which the number of individuals in a population increases. Specifically, PGR ordinarily refers to the change in population over a unit time period, often expressed as a percentage of the number of individuals in the population at the beginning of that period. This can be written as the formula:

$\mathrm{Growth\ rate} = \frac{(\mathrm{population\ at\ end\ of\ period}\ -\ \mathrm{population\ at\ beginning\ of\ period})} {\mathrm{population\ at\ beginning\ of\ period}}$

(In the limit of a sufficiently small time period.)

The above formula can be expanded to: growth rate = crude birth rate - crude death rate + net immigration rate, or ∆P/P = (B/P) - (D/P) + (I/P) - (E/P), where P is the total population, B is the number of births, D is the number of deaths, I is the number of immigrants, and E is the number of emigrants.

This formula allows for the identification of the source of population growth, whether due to natural increase or an increase in the net immigration rate. Natural increase is an increase in the native-born population, stemming from either a higher birth rate, a lower death rate, or a combination of the two. Net immigration rate is the difference between the number of immigrants and the number of emigrants.

The most common way to express population growth is as a ratio, not as a rate. The change in population over a unit time period is expressed as a percentage of the population at the beginning of the time period. That is:

$\mathrm{Growth\ ratio} = \mathrm{Growth\ rate} \times 100%.$

A positive growth ratio (or rate) indicates that the population is increasing, while a negative growth ratio (or rate) indicates population decline. A growth ratio of zero indicates that there were the same number of people at the two times -- net difference between births, deaths and migration is zero. However, a growth rate may be zero even when there are significant changes in the birth rates, death rates, immigration rates, and age distribution between the two times. [1] Equivalently, percent death rate = the average number of deaths in a year for every 100 people in the total population.

A related measure is the net reproduction rate. In the absence of migration, a net reproduction rate of more than one indicates that the population of women is increasing, while a net reproduction rate less than one (sub-replacement fertility) indicates that the population of women is decreasing.

## Human population growth rate

Annual population growth rate in percent, as listed in the CIA World Factbook (2006 estimate).[2]

Population exceeding the carrying capacity of an area or environment is called overpopulation. It may be caused by growth in population or by reduction in capacity. Spikes in human population can cause problems such as pollution and traffic congestion, though these can be addressed by technological and economic changes. Conversely, such areas may be considered "underpopulated" if the population is not large enough to maintain an economic system (see population decline).

Globally, the growth rate of the human population has been steadily declining since peaking in 1962 and 1963 at 2.20% per annum. In 2007 the growth rate was 1.19% per annum. The last one hundred years have seen a rapid increase in population due to medical advances and massive increase in agricultural productivity[3] made by the Green Revolution.[4][5][6]

The actual annual growth in the number of humans fell from its peak of 87.5 million per annum in 1989, to a low of 76.4 million per annum in 2002, at which it stabilised and has started to slowly rise again to 79.4 million per annum in 2007, and 80.2 million per annum in 2009. Growth remains high in the Middle East, South Asia, Southeast Asia, Latin America, and primarily in Sub-Saharan Africa.[7] According to projections by the U.S. Census Bureau, the annual world population growth will peak in 2011 at 80.9 million.[8]

Some countries experience negative population growth, especially in Central and Eastern Europe (mainly due to low fertility rates and emigration) and Southern Africa (due to the high number of HIV-related deaths). Japan's population began decreasing in 2005[9] and some Western European countries are also expected to encounter negative population growth.

## Population growth rate in the United States

Population growth in the United States is unique in all the world. It is a country with a high Human Development Index (HDI) and is one of the world's wealthiest countries. The overwhelming number of countries with high HDI also have sub-replacement total fertility rates (Israel and Saudi Arabia are big exceptions). The United States has replacement total fertility rate, and in addition has high immigration, both legal and illegal. As a result the population growth rate is expected to remain constant for the foreseeable future. The change of growth rate relative to Latin America and Caribbean is shown in the graph.

Projected regional growth rates using birth, death, immigration, and emigration rates. Latin America and Caribbean will be growing slower than US by 2024