Grand Central Station has served New York City since 1871, when its p
​ opulation​ was just under
1 million people. Today, New York City has about 8.5 million inhabitants!
Human populations have a major effect on the environment. In the United States, the human
population has risen from about 2.5 million in 1776 to more than 300 million in 2016. But are
humans distributed evenly across the 3.8 million square miles of the United States? Not at all!
Most people live clustered together in towns and cities.
A human ​population​ is a group that consists of one species: ​Homo sapiens​. A population of
humans is defined by the area in which it exists. The world population includes all humans
currently alive across the globe. Every individual on Earth is simultaneously a member of the
world population and a member of smaller, more specific populations. Explore the images to
better understand how these populations are related.
Human ​populations​ change over time, and scientists who study ​demography​ think about the
causes and effects of such changes. Government officials also gather and use many types of
data about populations. Explore the chart to learn more about different terms associated with
demographic data.
f you have flown in an airplane or looked at a map or satellite image that shows where different
cities are located, you may have noticed that the human p
​ opulation​is not evenly distributed over
large areas. Instead, individuals are clustered into groups (towns and cities) surrounded by
emptier space (rural areas). Scientists study how populations are spread out in this way by
looking at the p
​ opulation distribution.
Scientists who study human ​populations​ also look at p
​ opulation density​. Population density is
typically reported as the number of individuals per square kilometer or square mile. Urban
populations have much greater density than rural populations. In developing nations, urban
areas are often very crowded.
Scientists can observe the effects of differences in human ​population density​ on various places
on Earth. One way is by tracking aerosols. An ​aerosol​ is commonly considered air pollution.
Aerosols can arise from natural sources or from human activity. The interactive map in the
Earth Observatory Virtual Lab uses green areas to represent aerosols of larger particle sizes,
such as those from windblown pollens, dirt, and dust. Aerosols of smaller particle sizes, which
are more associated with human-produced pollution, are represented in red.
Populations, human and otherwise, undergo constant change. The study of ​population
dynamics​ involves examining how ​population​ variables — including population size,
composition, and more — change over time.
Scientists study the ages and number of each gender in the population, the rates of births and
deaths, the distribution of people in a region, and even income, living conditions, and health of
the people. They look for patterns and relationships among all these variables. For example, the
growth rate​ is related to the ​birthrate​ and the ​death rate​. You can predict a population's growth
rate using this equation:
Growth rate = Birthrate – Death rate
Scientists who study human ​populations​ have observed certain patterns in their p
​ opulation
dynamics​. One pattern is that there is a relationship among factors such as b
​ irthrate​, ​infant
mortality​ rate, level of industry, and standard of living. Explore the tabs to learn about two kinds
of populations.
Scientists who study human populations have observed certain patterns in their population
dynamics. One pattern is that there is a relationship among factors such as birthrate, infant
mortality rate, level of industry, and standard of living. Explore the tabs to learn about two kinds
of population
Study the graph of the history of human ​population growth​. It took thousands of years for the
world p
​ opulation​ to reach 1 billion people. And it has taken a briefer and briefer period to add
each additional billion people. In fact, you can see by the sharp rise in the curve at the right side
of the graph that most of the growth of the human population has taken place over the past few
hundred years! Compare this graph with the two model graphs in the passage below.
If you were to move to the region you chose, the population there would increase. Likewise, the
population of your current home region would decrease. Human populations are affected by
individuals entering the population (​immigration​) and individuals leaving the population
(​emigration​).
In d
​ eveloped nations​, people have access to transportation and are less tied to the farming
lifestyles their families may have had in the past. Thus, many individuals move from place to
place within the nation, or even to other developed nations, to find opportunities. In ​developing
nations​, poverty and lack of opportunity often keep people from moving. Few people immigrate
into developing nations
Population growth can be positive (an increase in a ​population​) or negative (a decrease in a
population). The types of ​population growth​ (change) and their causes are shown here:
Positive population growth: Births + Immigrants > Deaths + Emigrants
Negative population growth: Births + Immigrants < Deaths + Emigrants
Zero population growth: Births + Immigrants = Deaths + Emigrants
Scientists use these relationships, along with existing data and predictions of future patterns of
behavior, to estimate population growth through the next 50 years.
Select this ​link​ to open a graph that shows projected increases in world population and
projected changes in the populations of individual regions. Use the graph to answer the
questions.
A population's ​age structure​ can predict some of the problems a country will have. For example,
an overabundance of children or elderly people means that a large part of the p
​ opulation​ cannot
hold a job. The income earned by workers must be spread thinly to support those who are either
too young or too old to work.
Developing nations​ tend to have high b
​ irthrates​, and thus children represent a large percentage
of the population. In contrast, the aging population in a ​developed nation w
​ ith little or no growth
will increase out of proportion with the population of working-age adults.
Human populations undergo continuous change. One pattern that often occurs as a nation
develops industry is called a d
​ emographic transition​. The high b
​ irthrates​ and d
​ eath rates​ of the
preindustrialized population shift to low birthrates and death rates as the country becomes
industrialized. P
​ er capita​ income rises, and ​infant mortality d
​ rops.
Because of their high birthrates, the populations of developing countries have a high percentage
of young people. As the country industrializes, the ratio of adults to children rises. Explore the
chart to see how demographic transition progresses through four distinct stages.
Earth's human population is constantly increasing. But for all ​populations​ — not just that of
humans — there is a limit to how much they can grow. This limit is called ​carrying capacity​. A
population can briefly exceed its carrying capacity, but the deaths of members of the population
will eventually bring the number back down.
We know that Earth's human ​population​ has not yet reached its global ​carrying capacity​,
because the world population is still increasing. However, some regions of the world have
populations that are experiencing death by famine. Such an occurrence demonstrates that for
that region at that time, carrying capacity has been exceeded.
The ​United Nations​ has used existing data and p
​ opulation dynamics​ analysis to produce three
projections of world population through the next 50 years. The researchers used three different
sets of assumptions, resulting in the three different projections, which are shown in the graph.