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A crowded treain leaves Dhaka in 2007.
An overcrowded train leaves the rail station at Bangladesh's Dhaka Airport before a festival in 2007.

Photograph by Rafiqur Rahman, Reuters

Ker Than

for National Geographic News

Published October 28, 2011

The global population will reach seven billion sometime on Monday—at least symbolically—according to a United Nations report released this week. (See National Geographic magazine's yearlong series on our planet at seven billion.)

But why did the UN choose October 31? And why does the seventh-billion date differ from other projections? The U.S. Census Bureau, which also attempts global population projections, predicts humanity won't hit the seventh-billion milestone until March 12, 2012, for example.

7 Billion Video: Are You Typical?

 

In theory, estimating when the world population will reach seven billion is simple: Take the last known population numbers for each country from its last census, subtract the number of people you think have died, and add the number of people that you think will be born, as well as the population change expected from migration.

In practice, of course, it's more complicated.

For example, "some governments don't have strong statistical systems in place, or sometimes the [census] data is incomplete," said Richard Kollodge, senior editor of the UN's State of the World Population Report.

And in some cases, the UN has to factor in trends that are not reflected in a country's official census data.

For instance, the one-child policy in China has caused millions of parents to not report the birth of girls because of the culture's bias for males, said John Bongaarts, vice president of the New York-based nonprofit Population Council.

"We know that a large number of girls are missing from the census, because when you count the number of girls in school you find there are more than were born six years ago," Bongaarts said.

(Quiz: Population Seven Billion—Could We All Fit in One City?)

Seven-Billion Figure More Than an Educated Guess

The UN Population Division tries to account for all such complicating factors before crunching the numbers. (Read more about our population at seven billion.)

"To say it's an educated guess is oversimplifying it, because they're very meticulous about factoring in all sorts of things," the UN's Kollodge said.

The U.S. Census Bureau does much the same thing in their population analyses.

The difference between the two organizations estimates of October 31, 2011, and March 12, 2012 is partly a matter of timing, explained Daniel Goodkind, a demographer with the bureau's Population Division.

"The UN revises their country population estimates and projections every two years," he said.

"We do it on a flow basis. Typically, twice a year we do a certain chunk of countries, based in part on the data that's available. So our estimates are not always in sync with the UN."

(See pictures of Earth's seven billion people.)

Symbolic Date Highlights Overpopulation

Despite the care that goes into each projection, both organizations have made it clear that their milestone dates are symbolic.

"There is no system that monitors instantaneously the births and deaths that are actually occurring," Goodkind said.

Instead of picking a specific date, both organizations could have announced a range of dates for when the seven-billionth person is likely to be born. But they were smart not to do so, said Jack Goldstone, a policy expert at George Mason University in Virginia.

"It was a very good idea [to pick a specific date], because the advance of global population is an issue that everyone has to deal with and that everyone ignores," Goldstone said.

"By picking a specific day and a specific round number, the United Nations has succeeded in bringing more attention to this than I've seen in years."

(Download National Geographic's 7 Billion iPad app.)

Debating Seven-Billion Date Is "Missing the Point"?

The Population Council's Bongaarts said it doesn't really matter which projection you choose to believe.

"Worrying about whether the population is 6.95 or 7.05 billion is a bit missing the point," he said.

"The message is that we have a huge population and we should learn to pay attention to its impact."

The UN's Kollodge agreed. "[This] should be seen as a call to action to address humanity's big challenges now, while the opportunity lasts," he said.

"About 1.8 billion people are still living on [the equivalent of U.S.] $1.25 a day or less. Millions of young people have limited access to education and jobs. More than 900 million people are suffering from hunger.

"These are just some of the issues we really should be focusing on. Fixating on one big number obscures the more compelling concerns of ordinary people everywhere."

More 7-Billion Population Coverage

Three African Nations and the Population Boom

Pictures: Population 7 Billion

Video: 7 Billion: Are You Typical?

Blog: How Big is Seven Billion?

Blog: Can the World Feed Seven Billion?

Population 7 Billion: Biodiversity Game

Video: 7 Billion: World Party

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