The foreboding threat of world disaster from explosive population growthcould turn out to be overly alarmist, according the authors of a newdemographic study. Their forecast shows there’s a high chance that the world’s population will stop growing before the end of the 21st century. Itsuggests that the total number of people may peak in 70 years or so at about9 billion people, compared with 6.1 billion today.

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

In their report, published in the August 2 issue of Nature, the authorsattribute their optimistic outlook to successful efforts in the last fewdecades to curb fertility rates. Now, they say, the time has come forsociety to think seriously about how to meet the needs of a stable butconsiderably larger world population in the decades ahead.

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

The figures obtained in the new study are roughly in line with futurepopulation scenarios released by the United Nations in 1998. But somepopulation experts disagree with IIASA’s conclusions, arguing there is noguarantee that population growth will stabilize before the end of the 21stcentury.

“In the 1960s we were panicked about population growth and did somethingabout it,” says Carl Haub of the Population Reference Bureau in Washington,D.C. “We find ourselves about halfway to this point of stable populationgrowth. We will get there, but to call the game over in fifth inning isn’tquite right.”

Determining accurately what the world’s population will be in the distantfuture is impossible because fertility, mortality, and migration rates arehighly uncertain. Because of this, some demographers who do populationforecasting have adopted an approach-called probabilistic analysis-thathelps account for uncertainty. “We don’t know what future fertility will be,we don’t know future life expectancy, but we can gather information aboutthe ranges [of possible outcomes] they might be in,” says Sanderson. Fromthis, demographers calculate the probability, or likelihood, of certaintrends or events occurring.

According to IIASA’s forecast, there is an 85 percent probability that theworld’s population will have stopped growing by the year 2100, and a 60percent 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 inthe future, with 34 percent of the population over the age of 60 by the endof the century. Addressing the needs of an older population will require arethinking of programs providing social security, health, and education.

Before the world stabilizes, however, there will be a growing demographicdivide, the IIASA study warns. On one side will be countries with shrinkingpopulations, such as Western Europe and the European states that wereformerly part of Soviet Union; on the other side, countries with growingpopulations, such as Nigeria. This demographic divide is likely to result inenormous stresses both within countries and between countries overimmigration and other issues. There is also the tragic specter of a virtualdepopulation in parts of Africa, due to AIDS.

Critics of the IIASA study say that no mathematical model can accuratelymeasure the socioeconomic factors that affect population growth. Fertilitydecline, for example, which is the major factor in a slowdown of populationgrowth, can happen in a variety of settings for unpredictable reasons.Eastern Europe always had a higher fertility rate than Western Europe, butwhen the Soviet Union broke up, fertility rates in those former Sovietcountries plummeted unexpectedly.

“The actual outcome of world population growth will depend on how people’ssocial behavior changes,” says Haub. “That is really very difficult topredict using any kind of mathematical method.”

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