U.N. Reports 7 Billion Humans, but Others Don’t Count on It

Feeling claustrophobic? You’re not alone. According to United Nations demographers, 6,999,999,999 other Earthlings potentially felt the same way on Monday when the world’s population topped seven billion. But if you’d rather go by the United States Census Bureau’s projections, you’ve got some breathing room. The bureau estimates that even with the world’s population increasing by 215,120 a day, it won’t reach seven billion for more than four months.

How do the dueling demographic experts reconcile a difference, as of Monday, of 28 million, which is more than all the people in Saudi Arabia?

They don’t.

“No one can know the exact number of people on the globe,” Gerhard Heilig, chief of the population estimates and projections section of the United Nations Population Division, acknowledges.

Even the best individual government censuses have a margin of error of at least 1 percent, he said, which would translate in the global aggregation to “a window of uncertainty of six months before or six months after Oct. 31.” An error margin of even as little as 2 percent would mean that Monday’s seven billion estimate actually was 56 million off (which is more people than were counted in South Africa).

The Census Bureau’s global population clock gives the pretense of greater precision. It projects that about 255 people are born every minute (about 367,000 a day) while about 106 die (roughly 153,000 a day). At that rate, the world’s natural increase would be about 78.5 million a year, or well more than the entire population of France, Britain or Thailand.

“We don’t use a population clock,” said Mr. Heilig. “It’s a bit silly.”

The two agencies begin with censuses and other vital statistics from more than 228 countries and other political entities, then project births and deaths, estimate the migration of refugees and project mortality rates from AIDS and other epidemics. Differences in interpreting the individual figures and how they fit together account for the overall disparities. Generally, the bureau’s projections lag behind those of the United Nations by up to a year (the population will reach eight billion in either 2026 or 2025, they figure, respectively).

“Realistically, the uncertainty is at least 2 percent and that’s for the 75 percent of the world for which we have recent official counts or estimates,” Joel E. Cohen, head of the Laboratory of Populations at Rockefeller University and Columbia University, said Monday. “Now, world population is estimated to be growing by about 1.1 percent per year. Hence each percent uncertainty in total count translates into almost one year uncertainty in the date by which the population grows past a given threshold. Bottom line: world population passes seven billion some time in the last year or two or the next year or two, most likely.”

Professor Cohen added, though, “Today’s as good a day as any to be aware of the problems of the world’s population and to begin to take action to solve them.’

Daniel Goodkind, a demographer in the Census Bureau’s Population Division, said the different estimates were still “remarkably close.”

“Although birth rates and death rates have both declined sharply since the 1960s,” he said, “death rates have declined more rapidly than birth rates. The cumulative effect of the excess of births over deaths in recent decades has led to a successive attainment of billion-person milestones every 12 or 13 years.”

Dr. Goodkind said the bureau revises its projections on a continuing basis, while the United Nations makes revisions every two years. Even so, the Census Bureau projects that the world population will hit seven billion next March 12 — well within the United Nations’ six-month, 1 percent window of uncertainty.

So who’s right? “We’re not exactly in synch, but we’re pretty close,” Dr. Goodkind said. “I’m not a betting man.”

Copyright 2011 The New York Times Company. Reprinted from The New York Times of Monday, October 31, 2011.