Population growth

Population growth is the change in population over time, and can be quantified as the change in the number of individuals in a population per unit time. 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 specific time period 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:


 * $$\mathrm{Growth\ rate} = [(\mathrm{births} + \mathrm{immigration}) - (\mathrm{deaths} + \mathrm{emigration})] / \mathrm{population}$$

The crude death rate as defined above and applied to a whole population can give a misleading impression. For example, the number of deaths per 1000 people can be higher for developed nations than in less-developed countries, despite standards of health being better in developed countries.

It is usually measured in a fraction or percent in relation to the base population.

Human population growth rate
Population growth which exceeds the carrying capacity of an area or environment results in overpopulation. 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 (ie. population is growing more slowly than in the recent past, see External Links), although the last 50 years have seen a rapid increase in population due to medical advances and massive increase in agricultural productivity made by the Green Revolution.

The actual annual growth in the number of humans is in decline, from 87 million per annum in the late 1980s, down to 75 million per annum in 2006. Growth remains high in the Middle East, South Asia, Southeast Asia, Latin America, and primarily in Sub-Saharan Africa.

In some countries there is negative population growth (ie. net decrease in population over time), especially in Central and Eastern Europe (mainly due to low fertility rates) and Southern Africa (due to the high number of HIV-related deaths). Within the next decade, Japan and some countries in Western Europe are also expected to encounter negative population growth due to sub-replacement fertility rates.