5 billion and counting ...

For the first time in history, there are more than 5 billion members of the human family. And this number is expected to double again before the global population stabilizes late in the next century. In the 15 minutes that it takes to read these two pages, more than 2,200 people will be added to the world's population. Each week, we add the equivalent of another Houston; each year, another Mexico.

But the problem of population goes far deeper than numbers. Analysts agree that many advances of the 20th century, such as falling death rates and rising incomes, could be reversed unless more is done to control population growth.

The current expansion of human settlements poses unprecedented environmental challenges. For example, the clearing of tropical forests for new cropland is thought to contribute to the gradual warming of the earth's atmosphere, known as the greenhouse effect.

``The fact that we've reached 5 billion means that we've managed to overcome high mortality,'' says Dr. Nafis Sadik, the new head of the United Nations Fund for Population Activities. But, she adds, it also underscores the need for effective family planning.

The UN will observe the ``Day of the 5 Billion'' on next Saturday. It is promoted as a day to ``contemplate a future in which population will eventually reach 10.2 billion.''

UN officials work with three population scenarios, each showing the level that world population will stabilize at under different circumstances. The middle-range projection is 10.2 billion, which assumes steady progress in bringing down global birthrates through education and family planning.

So far, actual population growth has fallen in this middle range - despite ups and downs along the way. The unanticipated success of China's controversial family planning program, for instance, is offset by failures in other regions.

A balance between births and deaths is the ultimate objective. In recent years, nations throughout the world made strides in health and sanitation which helped boost the number of babies that survive as well as the length of individual lives.

In most industrialized countries, this pattern led to a natural decline in births. As incomes rise and knowledge about family planning spreads, families tend to limit the number of children they have. Indeed, some Western European countries have even begun to see a natural decline in their populations (related story, Page 23).

Most developing countries have made significant progress in slowing their growth rates, stirring hope that they would follow in the footsteps of the industrialized nations.

But analysts now worry that a number of countries may be stuck in a ``demographic trap'' - where rapid growth outstrips the ability to advance socially and economically. Nigeria's population, for example, is expected to surge from 100 million to 532 million before it levels off in the middle of the next century. That would give that nation a population equal to all the people living in Africa today.

``We're putting enormous stress on the world's natural systems,'' says Lester Brown, president of the Worldwatch Institute, a Washington-based research group. ``And in some parts of the world, those systems are deteriorating.''

The result, says Mr. Brown, is economic decline. In parts of Latin America and Africa, for instance, real incomes have fallen in recent years. ``Declining living standards in rapid population growth areas is not hypothetical anymore, it's happening,'' says Brown.

The link between population and economic prosperity remains controversial, however. Some economists argue that a growing population boosts a nation's economy - providing a larger pool of workers and expanded markets for manufactured goods. This was certainly the case for Europe and North America during the Industrial Revolution, when populations grew rapidly along with wealth.

But, compared with today, the population surge that fed the factories of the Industrial Revolution was a blip on the screen. The key elements behind today's growth include:

Urbanization. By the year 2010, more than half the world's population will live in urban centers. Cities in developing countries are mushrooming especially fast - with expansion fueled by migration from the countryside and the natural growth of urban populations. In 1950, only three of the world's 10 largest cities were in the developing world; by the year 2000, all but two of them will be in developing regions.

Age. About half the people in the world are under age 24. This has ominous implications for the future, since these younger people will start having children in coming years. As a result, even if birthrates fall rapidly, growth is now inevitable well into the next century.

Expansion in developing regions. Nine out of 10 babies born today are in the developing world. This tends to make these countries even younger on average than the world total. It would be a mistake, however, to view these nations as a single bloc. Some countries, such as South Korea, have dramatically cut population growth and appear on the verge of joining the ranks of industrialized nations.

The most disturbing trends, meanwhile, are in Africa, South Asia, and Central America. These regions have rapid population growth coupled with quickly deteriorating resource bases.

``One doesn't need to be an ecologist to know that if you're cutting down 27 trees for every one planted - as is the case in Africa - you'll eventually denude the continent,'' says Werner Fornos, president of the Population Institute, a Washington-based group that advocates population control.

The key challenge is to feed these ever-growing numbers of people. Countries such as Brazil have plenty of land, but cultivating it will lead to massive deforestation.

Sharon Camp, vice-president of the Population Crisis Committee, says the combination of environmental decay and regional food shortages is putting the world on a collision course with famine. But careful planning and vigorous population control could avert disaster, she says. ``What happened in the last population doubling is that a lot of countries lost what little wiggle room they had left,'' Dr. Camp says.

A 1983 study by the UN's Food and Agriculture Organization found that, even using the most advanced farming techniques, 19 out of 117 nations will not be able to feed themselves in the year 2000. If farmers use less intense farming, the number of countries that are facing shortages jumps to 65.

Such dire projections have helped make family planning a top priority in many countries. ``Contraceptive supplies that used to last for months in some developing countries are being cleared out overnight,'' says Camp. ``It's spreading like a prairie fire.''

Meanwhile, the Reagan administration has dramatically altered the United States policy on population control.

At an international population conference in 1984, the United States announced that population was a ``neutral'' factor in development. The US has also withdrawn support from the UN's Fund for Population Activities and the International Planned Parenthood Federation, citing these groups' involvement with programs that offer abortion as a family planning option.

No one knows exactly when the world's population clicked past the 5 billion mark.

But that hasn't stopped a number of groups that wanted to publicize the event. The Washington-based Population-Environment Balance organization was first through the gate, heralding last July as the turning point. Others tended to favor March or April of this year.

The United Nations designated July 11 as the ``Day of the 5 Billion,'' reflecting official UN projections that the 5billionth baby will be born this month.

``This is what the statisticians call the fallacy of misplaced precision,'' says Sharon Camp, vice-president of the Population Crisis Committee. What's important, she says, is what these rapidly growing numbers mean in terms of the human condition.

Observers say it would have been nice if all groups agreed on a single date, but that was impossible. The UN, for instance, uses official population figures, which some nations juggle for political purposes.

Carl Haub, a demographer with the Population Reference Bureau, says one of the things the UN didn't factor in was the recent jump in China's birthrate. That nation - which accounts for one-fifth of the world's population - has a highly effective birth control program.

Even a slight increase in such a large country can substantially alter global demographic trends, says Mr. Haub.