Aug. 1, 2001 -- People may finally be nearing their limit.

Despite a growth burst that more than doubled the global human population over the past 50 years, a study released today predicts it will peak at 9 billion by the year 2070 and then begin to decline.

"People thought for many years that we would breed ourselves out of existence," says Warren Sanderson, a professor of economics and history at State University of New York at Stonybrook and co-author of the study appearing in this week's Nature. "They thought we'd produce so many children, there would be no standing room left on the planet. But now it seems our population will peak.

"And that's an optimistic message."

The study is the first to pinpoint an end to the burgeoning population. The scientists estimate there is an 85 percent chance the species will taper to about 8.4 billion by the year 2100. The current world population is counted at 6.1 billion.

Previous demographic studies by the United Nations had projected higher populations of 9.32 billion by the year 2050, with no decline in growth. Part of the reason for the different predictions is the new study anticipates the number of children born per woman will go down.

Forecasts Never Certain

Like the weather, population is tricky to predict since it is dependent on so many independent factors, including disease, wars and social trends. Demographers in the 1930s, for example, never predicted the post-World War II baby boom that caused populations to spike in the United States. Later, they also missed that U.S. birth rates would drop significantly by the 1970s.

In order to reflect the uncertainty of population prediction, the researchers, based at the International Institute for Applied Systems Analysis in Laxenburg, Austria, framed their findings in probabilities.

The team determined there is a 20 percent chance the population will peak by the year 2050, a 55 percent chance it will peak by 2075 and an 85 percent chance numbers will reach their limit in the next century. Sanderson says this method can provide policy makers with a more precise estimate of what to expect.

But others argue the predictions could be dangerously misleading even though they are framed in probabilities.

"It's a mistake to think we can stop worrying about population growth," says Robert Engelman, vice president for research for Population Action International, a Washington-based advocacy group. "The population growth problem isn't solved until we've solved it."

While most developed countries, particularly in Europe, have shown signs of declining, or soon-to-be declining populations, developing nations in sub-Saharan Africa and in western Asia still have high population growth. According to U.N. findings, Chad's population is growing by 3.3 percent every year, Yemen's population is expanding at the same rate and Madagascar is growing by 3 percent annually.

Part of the reason, says Engelmen, is women in these countries often have many children, starting at a young age. He argues it's important for the United States and other countries to promote family planning programs around the world in order to encourage lower global birth rates.

The Population Divide

In order to maintain a stable population, women must average 2.1 children each, a figure known as the replacement fertility rate. Two children replace the woman and her partner and the 0.1 figure reflects infant mortality rates.

Sanderson says the United States is now exactly at this fertility rate and so is maintaining a stable population. One possible reason for the United States' steady fertility rate, says Engelman, could be its high immigration rates and its high populations of people from cultures where families are traditionally large.

But the United States is the exception among developed countries. Sanderson and his team calculate that the European portion of the former Soviet Union has already peaked and is now declining. China, Japan and most western European countries have replacement fertility rates below 2.1 and are expected to decline in coming years.

The demographers point out that the increasing contrast between growing and shrinking nations is sure to pose global immigration tensions in the future. People from increasingly crowded and economically strained countries will be more and more likely to seek migration to developed countries where populations are shrinking.

"Migration is going to be a big battle," says Sanderson. "We've already begun to see the problems in countries like Germany and Austria where a lot of people want zero migration. This is only the tip of the iceberg."

More demand for health care for the elderly could be another consequence of changing populations. The study expects those 60 or older to increase in number from about 10 percent of the population to about 22 percent by the year 2050. In the next 100 years, they could make up 34 percent of the population.

Team Cites Flaw in U.N. Estimates

The team of demographers based their estimates on forecasting by experts, on historical figures of birth, mortality and fertility rates and on the margins of error of previous population forecasts.

U.N. officials, meanwhile, say they are sticking by their own, less-optimistic population forecast.

"The first most rapid growth is over, but we're still growing," Joseph Chamie, director of the U.N.'s Population Division, told The Associated Press.

Sanderson argues a major flaw with the U.N.'s findings is their estimates assume fertility rates will not drop below 2.1 children per women.

"The evidence shows women are already not having enough children to replace themselves," says Sanderson. "We may as well wake up and smell the coffee and begin focusing on how to live sustainably with the number of people we will have in the next century."