NEW YORK — In a cramped office on the 19th floor of Two United Nations, Danan Gu, a nerdy population analyst, found 7 million children in China who didn’t officially exist.
They materialized in front of him, on a desktop computer. Gu pulled up a chart from China’s 2000 census, showing children age 1 and under. Then he clicked on a Ministry of Education report that recorded students in the same age group about 10 years later.
The numbers didn’t add up: 32 million students were in the schools, but only 25 million were counted in the census a decade earlier. The discrepancy didn’t surprise Gu, who emigrated from China and studied demographics at Duke University. Under the one-child policy, “parents hide children all the time,” he said.
Conjuring up uncounted people by cross-referencing census data with surveys and other statistics is a common trick of the demographics trade, one of several methods used recently by researchers at the U.N. Population Division to arrive at their landmark population projection: 7 billion people inhabiting Earth on Oct. 31.
Looking deeper into the cloudy future, the United Nations expects the world’s population to hit 9 billion in 2050, and, if current fertility levels ease downward as expected, surpass 10 billion at the start of the next century. Mortality rates have a lesser impact on population estimates because life expectancy is rising.
If fertility rates don’t decline, particularly in Africa and parts of Asia, the population could reach 27 billion next century, said the U.N. division’s chief, Gerhard K. Heilig. He called that projection a highly unlikely scenario that must nonetheless be considered.
The words could and would dominate population reports. The United Nations has no way of knowing for certain when the 7 billion milestone will be reached. Oct. 31 is a symbolic date based on population estimates dating back five years, according to an explanation on the division’s Web site. Projections have at least a 1 percent margin of error, meaning the population can be reached six months before a target date or six months after.
The United Nations is regarded in most quarters as the gold standard of population projection, but not everyone agrees with its estimates. A population clock on the U.S. Population Reference Bureau’s Web site, spinning faster than numbers on a gas pump, says humans blew past 7 billion weeks ago, reproducing at 25,000 about every 10 seconds.
Wolfgang Lutz, founding director of the Wittgenstein Centre for Demography and Human Capital, challenged the United Nations’ fertility estimates. The International Institute for Applied System Analysis, which collaborates with Lutz’s group, projected that the population won’t reach 7 billion until July next year at the earliest, or January 2013 at the latest.
“We’re constantly challenged, on a daily basis,” Heilig said. “I get between 10 and 15 questions or concerns, people wanting to know something. We try to follow up whatever people complain about. That’s why we do it [recalculate population projections] every second year.”
The United Nations’ count to 7 billion started 10 months ago on the drab 19th floor of a Manhattan skyscraper built in 1976. Five demographers, including Gu, were each assigned 40 countries, territories and areas, including the Holy See.
Gu and another analyst tackled his native China. Patrick Gerland, a senior analyst who emigrated from France and was educated at Princeton, was assigned some of the tougher nations, such as Afghanistan, which hasn’t had a census since the Soviet Union invaded in 1979. Kirill F. Andreev, a Russian native and expert on mortality, collected data for the United States.
The building’s windows feature picturesque views of the Hudson River. But Gu, like some of his colleagues, worked with his back to the view. The analysts were framed in their offices by gray metal bookcases piled high with papers.
For Afghanistan, Gerland, who sits within shouting distance of Gu, had to use every resource in his arsenal of numbers. The numbers would have to tell him how Afghans live, migrate, give birth and die.
Without a more recent census, he reached for data provided by a demographic survey financed by the U.S. Agency for International Development. In the past 50 years, surveys and questionnaires have created rich sources of information on people in developing countries, he said.
In addition, agencies such as the U.S. Centers for Disease Control and Prevention collect information while providing health care to tens of thousands of people worldwide. The United Nations Children’s Fund, UNICEF, collects data from families when it gives vaccinations.
Most nations have a statistical office with birth certificates, child mortality records and death certificates that help paint a portrait of their populations. Some Scandinavian nations go even further, providing population registers, a sort of national I.D.
“We can’t do this in an office without getting information from somewhere,” Gerland said.
There’s one vexing problem that U.N. analysts are always hard pressed to resolve, even for a developed nation such as the United States: how to account for citizens who hide from census takers, and for the undocumented immigrants, estimated to total more than 11 million in the United States alone.
Migrant workers in Arab nations such as Qatar and the United Arab Emirates also are a demographer’s nightmare. They comprise more than half the populations of those countries, and no one knows how they come and go, or with whom they have children in foreign lands or at home.
“It’s forensic kind of work,” said Gerland, the analyst. “Pieces of information have to make sense. You have to resolve it.”
“The work,” said Gu, “is very hard.”
Heilig, the division chief, said the analysts are supported by staff, and their work is reviewed by other offices in the United Nations and by commissions representing Latin America and the Caribbean. Errors are corrected.
The U.N. Population Division is one of the oldest units in the Secretariat, organized in 1949 when developing countries clamored for population reports. As Europe’s colonies were liberated, the new governments wanted to know the state of their populations.
The first U.N. count started with an estimate of 2.5 billion in 1950. Today the findings are packaged into compact discs, wall charts and books and given to the World Health Organization, the Food and Agricultural Organization, the World Bank and other offices in the U.N. system to help determine aid needed mostly in developing nations.
On a white board in his office, Heilig wrote worrisome notes on how the population is shifting. Even if things go as expected, Asia’s population boom will decline after 2050, but Africa’s will increase by 1.5 billion. African nations such as Nigeria, Senegal, Uganda and Rwanda are rife with ethnic and tribal groups that find strength in numbers. In 20 years, the population of today’s least developed countries will surpass the population of more developed regions.
“I think fertility will decline in Africa, the question is how fast,” said John Bongaarts, vice president at the Population Council, an international non-profit group that conducts research on poverty and AIDS. If it doesn’t, “you can end up with a very large number of people living in terrible conditions and slums.”
Bongaarts paused for a second and voiced a thought that no population projection can precisely answer. “Who knows what will happen.”