That's how many are expected to inhabit the world by 2050. Experts worry over looming food shortages.

The world is an odd place. A tight global food situation with record-high grain prices presents the possibility of increasing malnutrition, perhaps famine, in parts of Africa and South Asia. Yet an estimated 1.6 billion adults, about a quarter of the world's 6.7 billion people, are overweight, some of them obese.

As a result, chubby Americans are spending roughly $1 billion a year to lose a few pounds with special diets, treadmills, etc., while hundreds of millions in poor nations are scrambling to buy enough food to add a little weight. "You couldn't write any stranger fiction," says Joseph Chamie, former head of the United Nation's Population Division.

The possibility of a world food shortage is causing more and more concern. "It's likely to get worse in coming years," reckons Mr. Chamie, now research director at the Center for Migration Studies, a New York think tank.

His fear is partly based on the fact that the world's population is growing by about 78 million people a year, with projections of an additional 2.5 billion people by 2050 - a generation away.

"The most significant event of the 20th and 21st century is the growth of world population," Chamie says. "It has affected every life form on this planet."

There have been a few dramatic spikes in food prices in the past century. For instance, in 1972 the Soviet Union, anticipating a domestic crop failure, quietly cornered available grain supplies in the world, doubling prices of wheat, rice, and corn. Weather-related events have pushed up food prices at other times.

But these events were temporary. Using surplus stocks, emergency measures eased food shortages in some poor nations. A new crop restored an adequate supply.

The present situation, however, is different, says Lester Brown, president of the Earth Policy Institute in Washington. It's based on trends, not specific incidents. Longer-term trends include the growing world population and the desire of huge numbers of increasingly prosperous people in China, India, and elsewhere to eat more meat and eggs. A shorter-term problem, he says, is the growing use of corn and other foods to distill biofuels for cars, trucks, and other energy uses.

Unless the food-shortage situation is tackled seriously and quickly, the world faces increased social unrest, food riots, political instability, and more failed states, notes Mr. Brown. "Civilization is now at risk," he says.

Other observers are not quite that worried. "You could construct that scenario," says Mark Rosegrant, an expert at the International Food Policy Research Institution in Washington. "The probability of famine has gone up." And his economic models of world food demand and supply suggest high food prices are likely to continue "for a number of years."

Nonetheless, Mr. Rosegrant hopes that concerted, major investments to boost world food output will keep shortages down to the malnutrition level in some of the world's poorer nations. The solution includes improving farm infrastructure and technological boosts to farm yields - "a lot of small green revolutions, particularly in Africa," he says.

Not much can be done in the short term about the rise in the world's population. When President Bush first assumed office, there were 6 billion people in the world. When the next president's term ends in 2012, there likely will be 7 billion people, most of the additions in South Asia and Africa. India will add 500 million, reaching 1.6 billion. Africa's population, now 950 million, will grow by 1 billion.

Birthrates in most countries have been falling fast. But it takes decades to stabilize a nation's population. Chamie finds it unfortunate that roughly 200 million of the nearly 1 billion women of child-bearing age in the world have no access to modern family-planning methods. Lack of governmental safety nets for old age and a culture of pride and joy in large families also lead to high birth rates in much of Africa, most Arab nations, and some Asian nations.

Measures to restrain population growth, however, are controversial, notes Katarina Wahlberg, an analyst at Global Policy Forum in New York, a nongovernmental group that looks at issues concerning the United Nations. Her recent paper explaining the possibility of a "global food crisis" and other similar papers tend to ignore or deal only briefly with the population problem.

A rough calculation by Brown finds that just to feed the addition to the world population each year would take some 640 square miles of good new farmland. That's an area approximately the size of Greater London, or Los Angeles County, or 18 million football fields.

The problem is that although tropical forests in the Amazon region, Indonesia, and the Congo are being chopped down for timber and to create farmland, the amount of farmland around the world has been shrinking through desertification - not growing.

Moreover, the growth of yields from the world's grain fields has declined from 2.1 percent a year between 1950 and 1990 during the height of the "green revolution" to 1.2 percent a year since then.