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20th may have been last century where young outnumber old

Nov. 15, 2003 - It took from the beginning of time until 1950 to put the first 2.5 billion people on the planet. Yet in the next half-century, an increase that exceeds the total population of the world in 1950 will occur.

So writes Joel E. Cohen, Ph.D., Dr.P.H., professor and head of the Laboratory of Populations at The Rockefeller University and Columbia University, in a Viewpoint article in the November 14 issue of the journal Science.

In "Human Population: The Next Half-Century," Cohen examines the history of human population and how it might change by the year 2050.

By then, the earth's present population of 6.3 billion is estimated to grow by 2.6 billion. "There are some things we can reasonably know and other things we cannot know," Cohen says about population projections. "By examining population size and distribution, it is possible to get a feeling for possible challenges to our future well-being. It is possible to get a sense of the larger picture."

What can be reasonably predicted? The world's population will be growing at a slower rate than it is today, especially in the richer, developed countries, but it will be larger by 2 to 4 billion people. It will also be more urban, especially in the underdeveloped countries. And it will be more elderly. However, exactly how international migration and family structures will change demographers cannot say.

"I also do not know whether we will inflict a doomsday on ourselves by warfare, disease or catastrophe. Our future depends on choices - on the choices we have made in the past and those we will make in the future," adds Cohen. "We cannot continue the exceptional growth of this last half century without experiencing consequences."

The demographic projections that Cohen cites assume that fertility rates will continue to decline and that more effective preventions and treatments against HIV and AIDS will be implemented and major catastrophes such as biological warfare, severe climate change, or thermonuclear holocaust will not be inflicted on the human population and the planet. These assumptions underlie the United Nations Population Division's urbanization forecasts and its online database, World Population Prospects: The 2002 Revision. In the Science article, Cohen reports such statistical information as the following:

• history of human population: It took from the beginning of time until about 1927 to put the first 2 billion people on the planet; less than 50 years to add the next 2 billion people (by 1974); and just 25 years to add the next 2 billion (by 1999). In the most recent 40 years, the population doubled.

• birth rates: The global total fertility rate fell from five children per woman per lifetime in 1950 to 2.7 children in 2000, a result of worldwide efforts to make contraception and reproductive health services available, as well as other cultural changes. Encouraging as this is, if fertility remains at present levels instead of continuing to decline, the population would grow to 12.8 billion by 2050 instead of the projected 8.9 billion.

• urbanization: In 1800, roughly 2 percent of people lived in cities; in 1900, 12 percent; in 2000, more than 47 percent. In 1900, not one metropolitan region had 10 million people or more. By 1950, one region did - New York. In 2000, 19 urban regions had 10 million people or more. Of those 19, only four (Tokyo, Osaka, New York, and Los Angeles) were in industrialized countries.

• poor, underdeveloped regions: Despite higher death rates, the population of poor countries grows six times faster than that of rich countries.

• population density: The world's average population density is expected to rise from 45 people per square kilometer in the year 2000 to 66 people per square kilometer by 2050. Assuming 10 percent of land is arable, population densities per unit of arable land will be roughly 10 times higher, posing unprecedented problems of land use and preservation for the developing world.

• aging population: The 20th century will probably be the last when younger people outnumbered older ones. By 2050, there will be 2.5 people aged 60 years or older for every child 4 years old or younger, a shift that has serious implications for health care spending for the young and old.

Although it is not possible to predict how global demographics will affect families or international migration, Cohen points out that three factors set the stage for major changes in families: fertility falling to very low levels; increasing longevity; and changing mores of marriage, cohabitation and divorce.

In a population with one child per family, no children have siblings, Cohen explains. In the next generation, the children of those children have no cousins, aunts, or uncles.

If people are between ages 20 and 30 on the average when they have children and live to 80 years of age, they will have decades of life after their children have reached adulthood, and their children will have decades of life with elderly parents, Cohen also points out. Cohen's article kicks off a four-week long series titled "The State of the Planet," which examines key issues of our planet's well-being. Cohen was asked to initiate the series because "population is people and people matter."