We entered the twentieth century with a population of less than two billion people and now, as we stand at the century's exit, world population is just over six billion. Six billion. It's hard to grasp what that means. I chose the following sites because they help explain both the number and its implications.
"The number of young people under age twenty-five has more than doubled since the 1950s: from 1.3 billion to 2.9 billion in 2000. The decisions that today's youth make about when to have children and how many to have will determine future population growth. In sub-Saharan Africa, for instance, childbearing rates have remained stubbornly high, and childbearing is common among teenagers." This comprehensive population study includes links to additional articles (both online and print) and graphs (click on Population Story: Presentation Graphics).