Progress in slowing the increase in people on the earth is critical to finding solutions for global problems in the 21st century

IN a world of complexity and change, at least three facts stand out with stark simplicity:

* The number of people living on the earth is growing more rapidly than ever before in human history and will continue to do so for at least four more decades, according to United Nations estimates.

* Unless fertility rates - that is, the average number of children per family - fall significantly by the end of the decade, the world's population could expand to more than three times its current 5.4 billion before stabilizing a century or more from now.

* Without major technological breakthroughs and changes in patterns of consumption, even the most optimistic population-growth projections are likely to be accompanied by increases in poverty, hunger, and environmental degradation.

"Whether we look at population, the environment, or development, the next 10 years will be critical for our future," says Nafis Sadik, executive director of the United Nations Population Fund. "The decisions we make or don't take will widen or narrow our options for a century to come. They could decide the fate of the earth as a home for human beings."

The raw data on population growth tell a compelling story. Every day the human family grows by a quarter of a million people. Every four days the equivalent of a new Detroit or Dallas is added to the earth. Every eight months a new Germany. Every decade, at just under 1 billion new inhabitants, the equivalent of a new Africa and South America combined. Demographers say the decade that it will take to reach the next billion is an instant in time compared with thousands of years needed to reach the first, in the early 1800s.

Beyond the sheer scope of population growth, the future will be profoundly influenced by its distribution:

* Population growth is occurring in regions of the world least able to respond to the demand for food, housing, education, and employment opportunities that it will create. Ninety-five percent of world population growth will occur in the developing world. Africa, where birthrates are highest, is now growing 13 times faster than Europe, where population may actually decline in the years ahead.

* Population growth will be concentrated in the most vulnerable areas of the developing world: cities. In 1950, only one third-world city had a population of 5 million. By the year 2000, there will be 46, all taxed to or beyond the limit to provide needed services and maintain political stability. A historic turning point will be reached in 1997 when a majority of the world's population will live in urban areas, according to the UN Population Division.

The runaway population growth that began in the 1940s and peaked during the 1960s is a historical aberration. Largely the product of improvements in public health and sanitation, it will taper off during the last half of the next century. Exactly where it levels off could determine whether upward trends in economic growth and longevity will be reversed.

Demographers have been heartened by steady declines in fertility that have occurred since the late 1960s in response to economic development and the rapid spread of family-planning programs around the world.

But lower fertility is not an immediate panacea. With so many people in their reproductive years - one-half will be under 25 in the year 2000 - the world's population will actually grow for the next several decades by larger numbers than ever before.

According to UN estimates, if fertility is reduced during the 1990s to replacement level - that is, to the two children a couple need to replace themselves - there is a good chance population growth will begin to level off by mid-century at just twice today's total. If fertility rates remain high, the population could expand far beyond the earth's capacity to provide for it.

"There's no question that population growth is eventually going to come to a halt," says Stanford University demographer Paul Erlich. "The question is whether it's going to end because we manage to humanely lower birthrates or because we exceed the earth's carrying capacity."

The fact that most of today's 5 billion people have escaped famine is credited partly to the "green revolution" of the 1960s. The dramatic increases in world food production that resulted from breakthroughs in plant technology, irrigation, and farm management are a hint, say some economists, of the limitless possibilities offered by technology and human ingenuity to keep up with the needs of an expanding world population.

But as the growth in human numbers rolls relentlessly past the finite productivity of the green revolution, the future seems less promising. Since the mid-1980s, increases in food production in some regions have fallen behind population growth. In the poorer nations of Latin America, harvest deficits have been widening for a decade. In sub-Saharan Africa, which was once self-sufficient in agriculture, food production is growing at only half the rate of population, leaving an estimated one-third of the co ntinent undernourished.

"Population growth is clearly outpacing agricultural potential in Africa," says Stephen Vosti, a research fellow at the International Food Policy Research Institute in Washington. "It is a Malthusian train crash waiting to happen," he adds, referring to the predictions of Thomas Malthus, the 19th-century British economist who warned that population growth would someday outpace food supplies.

Population growth has also contributed to the worsening poverty that now affects hundreds of millions of people living in Africa, Asia, and Latin America.

Many economists and demographers warn that population growth combined with inefficient agriculture, inappropriate government policies, and outdated technologies has brought the world to critical thresholds of sustainability. Demographers point to the contribution of relentless population growth to deforestation, overgrazing, and the degradation of the very agricultural lands needed to sustain such growth.

Finding solutions to the global problems that will dominate the agenda of the 21st century will be nearly impossible without progress on the population front.

"There's no possible solution to the global problems of poverty and environmental degradation that does not include action to reduce projected population-growth rates," says former World Bank president Robert McNamara.

Environmental problems, such as global warming and depletion of the ozone layer, are not caused by population growth but by the extravagant use of resources in the rich industrialized countries, where 75 percent of all energy is consumed. But if even the most optimistic population projections materialize, the sheer weight of human numbers will soon make the third world a major burden on the global as well as local environment.

"For the most part, global pollution is a consequence of the use of resources and the waste of developing countries. But the simplest projections into the future tell you that's not going to be the case 10 years from now, even if per-capita consumption rates stay the same," says George Zeidenstein, president of the Population Council, a private research group in New York. "We're at a transitional moment."