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The Habitable Planet: A Systems Approach to Environmental Science 

Human Population Dynamics

What factors influence human population growth trends most strongly, and how does population growth or decline impact the environment? Does urbanization threaten our quality of life or offer a pathway to better living conditions? What are the social implications of an aging world population? Discover how demographers approach these questions through the study of human population dynamics.

Interactive Labs

Demographics Lab (Units 5, 13)

Baby boom. Overpopulation. Birth dearth. These terms all refer to human population growth, and can conjure images of environmental and economic peril. Which are real issues, and should they matter to us?

Demographers like the US Census Bureau make population projections based on mathematical models. In this lab you will explore a fully functional simulation, based on real demographic data. You will examine important demographic trends through a series of guided lessons. After completing these lessons you will understand the factors that control human population growth, recognize the sea-change in human history that is the “demographic transition,” and gain a sense of how population demographics has a very human impact in all areas of our habitable planet. launch lab


Disease Lab (Unit 5)Recently, new diseases, such as SARS, and the potential for a pandemic avian flu have raised international concerns about health. As populations grow (see the Demographics lab), especially in densely packed urban areas, there is increased risk of disease transmission. This lab will allow you to explore various types of diseases: “Kold” is similar to the common cold, “Impfluenza” resembles a typical influenza outbreak, and “Red Death” represents a fast-spreading epidemic with a high mortality rate (such as avian flu if it were to develop through human-to-human transmission). What factors come into play in the spread of these diseases, and what can we do to counter them? launch lab



Unit 5: Human Population Dynamics // Glossary

carrying capacity
The number of individuals an environment can support without significant negative impacts to the given organism and its environment.

demographic convergence
When the gaps narrow between developed and developing countries for major indicators such as fertility rates and life expectancies.

demographic dividend
A rise in the rate of economic growth due to a rising share of working age people in a population.

demographic transition
The pattern of population growth exhibited by the now-developed countries during the 19th and early 20th centuries.

dependency ratio
The ratio of non-workers (children and retirees) to workers in a human population: the higher the ratio, the greater the dependency load.

A measure of the capacity of an organism to produce offspring.

A measure of reproduction: the number of children born per couple, person, or population.

life expectancy
Term usually used at birth, indicating the average age that a newborn can be expected to attain.

When living organisms move from one biome to another. It can also describe geographic population shifts within nations and across borders.

The loss of members of a population through death.

population momentum
The impetus for continued expansion of the number of people in a country when the age structure is characterized by a large number of children. Even if birth control efforts are effective in the adult community and the number of new births per person decreases, the number of people in the country expands as the large population of children reach reproductive age.

replacement level
The number of children per woman necessary to keep population levels constant when births and deaths are considered together over time; estimated to be an average of 2.1 children for every woman.



Beyond the Habitable Planet: Light Pollution: Cities in the Dark About the Stars

by Kelly Korreck

Seeing Stars at Night

In this photograph, a composite of 150 one-minute time exposures, the peak of Mauna Kea volcano is visible in the distance – home of some of the world’s great astronomical observatories. As the Earth rotated over several hours, the stars traced circular paths in the sky.
Source: Peter Michaud, Gemini Observatory/AURA, NSF

If you live in a major metropolitan area, you have probably seen the Moon and a few bright stars. However, there are approximately 3,000 stars that your eye could detect if there were no light interference. Telescopes also need darkness to get the best images of astronomical objects. Hence, the latest research telescopes are located away from civilization in the mountains of Chile, Arizona, or on a dormant volcano in Hawaii. But with the ever-growing human population, sites to observe the night sky from the ground are becoming increasingly difficult to find.

City Lights from Space

A set of satellites operated by the Defense Meteorological Satellite Program (DMSP) collects images of the entire Earth at least once each day. Passing over the night side of the planet, visible light sensors can detect lights from cities, towns, industrial sites, gas flares, and ephemeral events such as fires and lightning-illuminated clouds. This image was compiled from nighttime data collected in 1994-1995.
Source: C. Mayhew & R. Simmon (NASA/GSFC), NOAA/ NGDC, DMSP Digital Archive

Human civilization needs outdoor lighting at night for safety, however, as seen in this satellite image, the amount of light scattered skyward from human habitation at night is bright enough to be seen from space. Not only is this light taking away the wonders of the night sky, the energy escaping into space from outdoor lighting fixtures is completely wasted.

Geospatial demographers, such as Deborah Balk, use satellite images of the Earth at night to estimate population densities.

Electrical Blackout Hides Cities

A failure of the electrical power distribution system darkened major cities in New York, Canada, and parts of the Midwest in August 2003.
Source: DMSP

It is striking to see the difference between two satellite images taken a day apart: one, on a normal night, and the other one night later, during a major blackout that darkened parts of the Eastern U.S. and Canada in August 2003! New York City, Albany, Toronto, Buffalo, and Ottawa almost disappeared as seen from space.

Lighting Fixtures that Save the Night for Stars

The International Dark Sky Association certifies outdoor lighting fixtures to ensure that their light is spread downward, not into the sky. This fixture has been given the IDA Fixture Seal of Approval (FSA).

Although lights are necessary at night, many communities and individuals are working together so that people can be safe but still enjoy the night sky. For example, installing lighting fixtures that direct the light downward instead of upward saves energy and decreases light pollution. The International Dark Sky Association works to reduce light pollution from all sources:


Kelly Korreck

Is an Astrophysicist in the High Energy Astrophysics Division at the Harvard-Smithsonian Center for Astrophysics. She received her Ph.D. in Space and Planetary Physics from the University of Michigan. She is part of the Hinode Solar satellite X-ray Telescope (XRT) team at the Smithsonian Astronomical Observatory. Her research interests include high energy processes in the sun and all over the universe- especially magnetic reconnection, particle heating and acceleration, and mass transfer.


Series Directory

The Habitable Planet: A Systems Approach to Environmental Science 


Harvard Smithsonian Center for Astrophysics in association with the Harvard University Center for the Environment. 2007.
  • Closed Captioning
  • ISBN: 1-57680-883-1