On this date in 1999, the population of the Earth reached six billion.

Though no one is able to do an accurate census of every living human on the planet, the figure of six billion, five hundred million (6,500,000,000) is the most current estimate of world population as of this date in 2006. World population first topped an estimated 6 billion on this date in 1999. Different countries have different rates of population growth; some are slowly declining, while others are increasing rapidly. In the industrialized world, rates of population growth are slow, and in some European nations, the populations are actually shrinking—with every two deaths being "replaced" by fewer than two births. In the developing world, however, populations are continually growing, and this accounts for much of the increase in the world’s population. Two countries, India and China, account for approximately one third of the world’s people, having a combined population of over two billion. Laymen and experts differ over the implications of the growth of the human population. Some consider it cause for alarm, others believe that the rate of growth will stabilize as countries of the third world continue to develop (in much the same way as has occurred in the industrialized world) and still others believe that increased population will ultimately prove a boon for human productivity.

Xpeditions
In Top Ten Cities (3–5), students identify and study the most populous cities in their state, province or country. Students are also introduced to population distributions as they are asked to create a map and indicate population density in different cities on the map by stacking objects to create three-dimensional bar graphs.

Population Pyramids and Us (6–8) introduces students to a tool demographers use to examine the structure of a population and the potential for growth in that population. Students brainstorm questions that could be answered through an understanding of a population's gender and age distribution, and then work in pairs or small groups to research and create population periods of a given location.

Population Pasta (K–12) encourages students to experiment with ways of representing information graphically through charts, graphs and diagrams.

In Population Issues in China and India (9–12), students examine population control practices of India and China. Students gather population statistics for these two countries, read about population issues in both places and determine whether India should adopt a one-child policy like the one implemented by the Chinese government.

Illuminations
In The Next Billion (6–8), students predict when world population will reach seven billion. To do so, they use an on-line counter that simulates the changing world population. They time the counter to find how long it takes for the population to increase by, say, 50 or 100 people, and then use that measurement to predict how long it would take for the population to increase by one billion. Students discuss the reliability of their predictions, compare them to past trends and discuss social factors that can affect population growth.

The unit Accessing and Investigating Population Data (3–5) offers activities in which students can use census data available on the Web to examine questions about population.

Science NetLinks
The Demographics of Mortality (9–12) is designed to help use the Internet to explore population distribution and survivorship curves. By the time students enter high school, they should be able to locate information in reference books, computer databases and other sources. In this activity, students utilize these research skills to explore some of the factors that can affect the size and rate of growth of human populations.

Population Dynamics (9–12) helps students investigate the causes and consequences of population growth and the environmental factors that contribute to it.

In the first of a five-lesson unit, Urban Ecosystems 1: Cities are Urban Ecosystems (6–8), students are introduced to the study of ecosystems and models that are used by urban ecologists. Throughout the unit, a series of lessons that include art, music and drama, students use their experiences with their families and local urban ecosystems to study concepts such as fossil fuel consumption, distribution of natural and man-made resources and ways of tracking or predicting population changes.