IN a world of complexity and change, at least three facts stand out with stark simplicity:
* The number of people living on the earth is growing more rapidly than ever before in human history and will continue to do so for at least four more decades, according to United Nations estimates.
* Unless fertility rates - that is, the average number of children per family - fall significantly by the end of the decade, the world's population could expand to more than three times its current 5.4 billion before stabilizing a century or more from now.
* Without major technological breakthroughs and changes in patterns of consumption, even the most optimistic population-growth projections are likely to be accompanied by increases in poverty, hunger, and environmental degradation.
"Whether we look at population, the environment, or development, the next 10 years will be critical for our future," says Nafis Sadik, executive director of the United Nations Population Fund. "The decisions we make or don't take will widen or narrow our options for a century to come. They could decide the fate of the earth as a home for human beings."
The raw data on population growth tell a compelling story. Every day the human family grows by a quarter of a million people. Every four days the equivalent of a new Detroit or Dallas is added to the earth. Every eight months a new Germany. Every decade, at just under 1 billion new inhabitants, the equivalent of a new Africa and South America combined. Demographers say the decade that it will take to reach the next billion is an instant in time compared with thousands of years needed to reach the first, in the early 1800s.
Beyond the sheer scope of population growth, the future will be profoundly influenced by its distribution:
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