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7 billion people? How do they know?

A counter approaches the 7 billion mark at a National Geographic exhibit in a subway station in Bucharest, Romania, highlighting global population density.
A counter approaches the 7 billion mark at a National Geographic exhibit in a subway station in Bucharest, Romania, highlighting global population density.Daniel Mihailescu / AFP - Getty Images

Head-counters around the world are marking Monday as the day when the world's population hits the 7 billion mark. It's a date that has served as the focus for musings on the problems and possibilities facing our species and our planet. But how do experts know that Oct. 31 is the precise day that the world's 7 billionth human being will be born?

"The answer is, we don't," said Omar Gharzeddine, a spokesman for the U.N. Population Fund. Even though the United Nations gathered the statistics pointing to the Day of 7 Billion, U.N. officials freely admit that Oct. 31 is merely the date that popped out of their population projections, and will serve as a symbolic rather than a statistically precise milestone.

Every five years, the U.N.'s Population Division updates its country-by-country projections of demographic trends, and the computer models for 2010 were combined to yield a projection of Oct. 31. In the report, World Population Prospects, the U.N. analysts emphasize that there could be a 1 to 2 percent overall margin of error in the global tally, which translates into plus or minus six months or more for reaching the 7 billion mark.

Some folks are planning to identify a specific baby in India's Uttar Pradesh state or Russia's Kaliningrad region as the 7 billionth human on the planet, but Gharzeddine told me that the United Nations isn't giving official status to such publicity efforts. "There's no way that the U.N. or anyone could know where or at what minute on the 31st the 7 billionth baby will be born," he said.

The Day of 7 Billion could well be revised, even years later. That was the case for the Day of 6 Billion, Gharzeddine pointed out. "The U.N. marked the '6 billionth' [person] in 1999, and then a couple of years later the Population Division itself reassessed its calculations and said, actually, no, it was in 1998," he told me.

This time around, a lot of population experts suspect that we're actually months away from hitting the 7 billion mark. The U.S. Census Bureau, for example, projects that the milestone won't be reached until March 12, 2012. And researchers at the International Institute for Applied Systems Analysis point to a time frame between February 2012 and July 2014.

Those same researchers say that the tally for the world's total population is "not the issue" that experts should be worrying about. Instead, they say the United Nations and other groups involved in global development should focus on imbalances in the distribution of various populations by age, education and health status.

Gharzeddine agreed that population policy should be about more than the big number. "It's a good occasion to highlight all these issues," he said. Among the issues on his list:

An estimated 1.8 billion people are between 10 and 24 years of age, meaning that this is the biggest generation of young people in history. But 90 percent of those youths live in the developing world and are in danger of missing out on the economic opportunities of the 21st century.  

About 215 million women live in areas of the world where access to family planning and contraception is restricted. That's one of the factors between the wide disparity in fertility rates, which range from 1.6 births per woman in east Asia to five births per woman in some parts of Africa.

What does the future hold? It's taken 13 years to go from 6 billion to 7 billion, but the United Nations estimates that we'll hit 8 billion by 2023, 9 billion by 2041 and 10 billion at some point after 2081. If you think there's a lot of uncertainty surrounding the Day of 7 Billion, hold onto your hats: Relatively small increases in fertility rates could cause a doubling of the current population by 2100 (to 15.8 billion), while a small decrease could result in fewer people than we have today (6.2 billion by 2100).

More about global demographics:

For more about the Day of 7 Billion, click on over to the U.N. Population Fund, 7 Billion Actions and the 7 Billionth Person Project.

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