Forecast Sees Halt to Population Growth by End of Century

John Roach
for National Geographic News
August 6, 2001
The foreboding threat of world disaster from explosive population growth
could turn out to be overly alarmist, say the authors of a new
demographic study.

Their forecast shows there's a high chance
that the world's population will stop growing before the end of the 21st
century. It suggests that the total number of people may peak in 70
years or so at about 9 billion people, compared with 6.1 billion today.

The scientists say their prediction is more reliable than other population forecasts because they employed non-traditional but more rigorous methods of analysis. The study was conducted by the International Institute for Applied Systems Analysis (IIASA) in Laxenburg, Austria.

In their report, published in the August 2 issue of Nature, the authors attribute the rosier-than-usual outlook to successful efforts in the last few decades to curb fertility rates.

Now, they say, the time has come for society to think seriously about how to meet the needs of a stable but considerably larger world population in the decades ahead.

"We are going to have a stable population. So we have to make sure we have a sustainable environment and a sustainable economy to go with the sustainable population," said Warren Sanderson of IIASA, a co-author of the report.

The figures obtained in the new study are roughly in line with future population scenarios released by the United Nations in 1998.

But some population experts are at odds with IIASA's conclusions, arguing there is no guarantee that population growth will stabilize before the end of the 21st century.

"In the '60s we were panicked about population growth and did something about it," said Carl Haub of the Population Reference Bureau in Washington, D.C. "We find ourselves about halfway to this point of stable population growth. We will get there, but to call the game over in fifth inning isn't quite right."

Calculating Probable Outcomes

Determining accurately what the world's population will be in the distant future is impossible because fertility, mortality, and migration rates are highly uncertain. Because of this, some demographers who do population forecasting have adopted an approach—called probabilistic analysis—that helps account for uncertainty. "We don't know what future fertility will be, we don't know future life expectancy, but we can gather information about the ranges [of possible outcomes] they might be in," said Sanderson. From this, demographers calculate the probability, or likelihood, of certain trends or events occurring.

According to IIASA's forecast, there is an 85 percent probability that the world's population will have stopped growing by the year 2100, and a 60 percent probability that it will not have exceeded 10 billion before then.

The study also shows that the world population is likely to be much older in the future, with 34 percent of the population over the age of 60 by the end of the century. Addressing the needs of an older population will require a rethinking of programs providing social security, health, and education, said Sanderson.

Before the world stabilizes, however, there will be a growing demographic divide, the IIASA study warns. On one side will be countries with shrinking populations, such as the European states that were formerly part of Soviet Union; on the other side, countries with growing populations, such as Nigeria.

This demographic divide is likely to result in enormous stresses both within countries and between countries over immigration and other issues, said Sanderson.

Lots of Uncertainty

Although probability-based forecasting is an improvement over population scenarios that do not take uncertainty into account, probability analysis itself is uncertain to some extent, Nico Keilman, an economist at the University of Oslo in Norway, points out in an accompanying article in Nature.

Critics of the IIASA study say that no mathematical model can accurately measure the socioeconomic factors that affect population growth. Fertility decline, for example, which is the major factor in a slowdown of population growth, can happen in a variety of settings for unpredictable reasons, Haub explained.

Eastern Europe always had a higher fertility rate than Western Europe, he noted. But when the former Soviet Union broke up, fertility rates in those former Soviet countries plummeted unexpectedly.

"The actual outcome of world population growth will depend on how people's social behavior changes," said Haub. "That is really very difficult to predict using any kind of mathematical method."

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