Human Population: Fundamentals of Growth Population Growth and Distribution
Why has world population grown at such different rates throughout history? Population change results from the interaction of three variables: births, deaths, and migration. This relationship is summarized by a formula known as the balancing equation. The difference between births and deaths in a population produces the natural increase (or decrease) of a population. Net migration is the difference between the number of persons entering a geographic area (immigrants) and those leaving (emigrants). Natural increase usually accounts for the greatest amount of growth in a population, especially within a short period of time. For the world, growth occurs only when there are more births than deaths; for individual countries, migration is also a factor.
Since 1900, both birth and death rates in the more developed countries have continued to fall in tandem, with a few interruptions. A worldwide influenza pandemic in 1918 caused the death of between 20 million and 40 million people and produced a temporary increase in the death rate. A slight increase in birth rates occurred after World Wars I and II. In the 1980s, birth and death rates in the more developed world fell to historic lows. However, the total fertility rate (TFR) in many more developed countries are well below replacement levels of two children per couple. In addition, poor economic conditions in the countries of the former Soviet Union have led to a serious decline in birth rates and increase in death rates, contributing to declining population size in some of these countries.
The Demographic TransitionDemographers have attempted to explain the experience of these more developed countries as a demographic transition from high birth rates and death rates to the current low levels. This process tends to occur in three stages. First, birth and death rates are both high, so little growth occurs. Second, death rates fall due to improved living conditions, while birth rates remain high. During this period population grows rapidly. The third stage of the transition is reached when fertility falls and closes the gap between birth and death rates, resulting again in a slower pace of population growth. The chart "Population growth through natural increase" is a crude representation of this transition. All the more developed countries have entered this third stage of the demographic transition. A few have gone on to a fourth stage in which death rates are higher than birth rates, and the population declines.
Components of Population Change
In contrast to the more developed countries, the less developed countries — in Asia, Africa, and Latin America — had both higher birth and death rates in the 1900s than Europe and North America had in the 1700s, and these higher rates have continued throughout the 20th century. In most less developed countries, the mortality revolution did not begin in earnest until after World War II, and it followed a different pattern than that in European countries. Birth and death rates were higher at the start of the demographic transition than they had been in Europe or North America. Death rates fell rapidly in less developed countries through the introduction of medical and public health technology; antibiotics and immunization reduced deaths from infectious diseases; and insecticides helped control malaria. These changes did not result from economic development within the countries, but were a result of international foreign aid.
In the second stage of the demographic transition of these regions, mortality declines led to continued population growth. Birth rates even increased as a result of the better health enjoyed by the population. With declining mortality and increasing fertility rates, the population growth of the less developed countries achieved an unparalleled 2.5 percent per year in the 1960s. Overall, mortality rates in the less developed countries fell much faster than during the demographic transition in the more developed countries. As a result, there developed a large gap in the percentage of growth between these two regions. Since 1970, birth rates have fallen, but the death rate has fallen faster. The population growth rate is still high, about 1.9 percent annually in 2000. While the patterns of fertility decline have varied dramatically throughout the less developed world, many countries are well into the transition process. Even in sub-Saharan Africa, where birth rates remained high through much of the 1980s and 1990s, fertility rates in most countries are declining.Source: Untied Nations, World Population Prospects, The 1998 Revision
The chart "Future of world population growth" illustrates three scenarios for population change, depending on levels of fertility. World population is projected to increase to 7.8 billion by 2025, and to reach 8.9 billion by 2050, according to the medium scenario where fertility stabilizes at 2.1 children per women. This projection does not correspond with the doubling time of 51 years associated with the annual growth rate in 2000. The projection assumes that the growth rate will drop slightly by 2020 and continue declining as the century progresses. If the growth rate does fall and the world population reaches 11 billion by 2100, the population will have doubled in about 100 years.
Because most of the world's population growth is likely to continue to be in less developed countries, Asia will continue to hold the majority of the world's people. Africa and Latin America will gain larger shares than they have at present. The population of these regions may increase by 100 percent by 2100, according to moderate projections. In 2100, nearly 90 percent of world population could live in countries currently considered less developed, compared with about 80 percent today.
Birth rate (or crude birth rate): The number of live births per 1,000 population in a given year. Not to be confused with the growth rate.
Death rate (or crude death rate): The number of deaths per 1,000 population in a given year.
Demographic transition: The historical shift of birth and death rates from high to low levels in a population. The decline of mortality usually precedes the decline in fertility, thus producing rapid population growth during the transition period.
Emigration: The process of leaving one country to take up permanent or semipermanent residence in another.
Growth rate: The number of persons added to (or subtracted from) a population in a year due to natural increase and net migration; expressed as a percentage of the population at the beginning of the time period.
Immigration: The process of entering one country from another to take up permanent or semipermanent residence
Less developed countries: Less developed countries include all countries in Africa, Asia (excluding Japan), and Latin America and the Caribbean, and the regions of Melanesia, Micronesia, and Polynesia.
More developed countries: More developed countries include all countries in Europe, North America, Australia, New Zealand, and Japan.
Mortality: Deaths as a component of population change.
Rate of natural increase: The rate at which a population is increasing (or decreasing) in a given year due to a surplus (or deficit) of births over deaths, expressed as a percentage of the base population.
Total fertility rate (TFR): The number of children women are having today. The average number of children that would be born alive to a women during her childbearing years if she conformed to the age-specific fertility rates of a given year.
The dependency ratio is a measure used to indicate the ratio of people in the "dependent" ages (under 15 and ages 65 and older) per 100 people in the "economically productive" ages (15–64 years of age). The formula for the dependency ratio is:
Human Population: Fundamentals of Growth
The world has experienced unprecedented urban growth in recent decades. In 2000, about 47 percent of the world's population lived in urban areas, about 2.8 billion. There are 411 cities over 1 million. More developed nations are about 76 percent urban, while 40 percent of residents of less developed countries live in urban areas. However, urbanization is occurring rapidly in many less developed countries. It is expected that 60 percent of the world population will be urban by 2030, and that most urban growth will occur in less developed countries.
What is an urban area? An urban area may be defined by the number of residents, the population density, the percent of people not dependent upon agriculture, or the provision of such public utilities and services as electricity and education. Some countries define any place with a population of 2,500 or more as urban; others set a minimum of 20,000. There are no universal standards, and generally each country develops its own set of criteria for distinguishing urban areas. The United States defines urban as a city, town, or village with a minimum population of 2,500 people. The classification of metropolitan includes both urban areas as well as rural areas that are socially and economically integrated with a particular city.
When comparing countries it is often helpful to look beyond the proportion of populations that are rural or urban and instead consider the size of cities. Countries differ markedly in the distribution of their urban population. For example, many urban dwellers in Africa live in cities of fewer than 10,000 residents. In Argentina, 90 percent of the 2000 population was urban, and 38 percent of these people lived in just one city, Buenos Aires. In 2000, 39 percent of the world's urbanites lived in agglomerations of 1 million or more inhabitants, and 15 percent resided in agglomerations of 5 million or more. Only 8 percent of Americans live in cities of 1 million or more.
During the 19th and early 20th centuries, urbanization resulted from and contributed to industrialization. New job opportunities in the cities spurred the mass movement of surplus population away from the countryside. At the same time, migrants provided cheap, plentiful labor for the emerging factories. While the proportion increased through rural to urban migration, high death rates in the cities slowed urban growth. Cities were unhealthy places because of crowded living conditions, the prevalence of contagious diseases, and the lack of sanitation. Until the mid-1800s, the number of deaths exceeded births in many large European cities. Migration accounted for as much as 90 percent of city growth during this period.
Urbanization in most less developed countries in the past 50 years contrasts sharply with the experience of the more developed countries. Death rates have fallen faster in urban areas because of greater access to health services. Because birth rates are relatively high in most less developed countries, the rates of natural increase are also quite high in cities. Migration also fuels urban growth in less developed countries as people leave the countryside in search of better jobs.
The chart "Growth of urban agglomerations" shows population growth in selected cities. New York and London are typical of large cities in more developed countries that arose in the 1800s and early 1900s, reached their current size mid-century, and have since experienced slow growth or decline. Cities in some less developed countries, such as Mexico City, grew very rapidly between 1950 and 1980, and are growing more slowly now. Many Asian and African cities, such as Lagos and Bombay, are experiencing very rapid growth now and are projected to continue at this pace.
As the population increases, more people will live in large cities. Many people will live in the growing number of cities with over 5 million habitants known as megacities. As the map "Largest urban agglomerations" shows, just eight cities had populations of 5 million or more in 1950, two of them in less developed countries. Megacities numbered 41 in 2000. By 2015, 59 megacities will exist, 48 in less developed countries.
By the turn of the century, cities of 10 million and larger will be more common. In 1950, only one city had more than 10 million inhabitants (see table, "Top 10 Largest Urban Agglomerations" below). By 2015, 23 cities are projected to hold over 10 million people; all but four will be in less developed countries.
Top 10 largest urban agglomerations in 1950, 2000, 2015
Source: United Nations, World Urbanization Prospects, The 1999 Revision.
TermsDeath rate (or crude death rate): The number of deaths per 1,000 population in a given year.
Megacities: A city with a population of 10 million or more residents.
Metropolitan area: A large concentration of population, usually an area with 100,000 or more people. The area typically includes an important city with 50,000 or more inhabitants and the administrative areas bordering the city that are socially and economically integrated with it.
Urban: Countries differ in the way they classify population as "urban" or "rural." Typically, a community or settlement with a population of 2,000 or more is considered urban. A listing of country definitions is published annually in the United Nations Demographic Yearbook.
Urban agglomeration: Urban agglomerations are areas of 1 million population or more. The concept of agglomeration defines the population contained within the contours of contiguous territory inhabited at urban levels of residential density without regard to administrative boundaries.
Urbanization: Growth in the proportion of a population living in urban areas.
Find the column on the World Population Data Sheet showing the percent of population residing in urban areas. Also examine the list of the largest cities found in the table "Top 10 Largest Agglomerations." For the 10 largest cities, calculate the proportion of the country's population living in that city in 2000. For example, 18.4 million people reside in Mexico City; this is 18.5 percent of Mexico's population.
Why are megacities increasing so rapidly in less developed countries? What are some implications of rapid growth in these cities?
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