Mankind appears likely to face two strikingly contrasting challenges in the not-too-distant future if current world population trends continue.

The most developed nations will be burdened with the large financial obligation of caring for their shrinking populations of mostly senior citizens while less-developed countries will face the challenging task of meeting the needs of a fast-growing army of children and teenagers.

"If the fertility rate remains high, it would lead to instability and slower social and economic progress in the least developing countries," said Stan Bernstein, senior policy adviser at the United Nations Population Fund (UNFPA).

At the same time, "there will be greater stress on the social security system in the developed countries," he added.

There has been a long history - extending back to the projections of Thomas Malthus in the early 19th century - of apocalyptic predictions of world social and economic catastrophe flowing out of rapid population growth. But, time and again, human ingenuity has risen to the challenge of producing the food and other goods needed to meet multiplying human needs.

Now, experts predict the world population will reach 9.1 billion by 2050, an increase of almost 50 percent from the current level of 6.5 billion people, and alarms are being raised again, this time with the added complications of the uncertain consequences of global warming and a projected population decline in the developed nations.

"The average population in the developed countries is growing at the rate of 0.2 percent a year, and it is 2.3 per year in the least developed countries," said Bernstein.

These projections mean that population in the least developed countries would double after 30 years, while it stayed flat in the developed countries. At the same time in Europe, the population is expected to decrease, he added.

"Clearly," he continued, the population increase "remains quite critical" in the least developed countries, while the population decline in developed nations has "negative consequences."

Bernstein describes the expected population shift as a "severe challenge" for the planet.

There will be a sharp increase in demand for services, including education, health care, transportation and housing, experts predict. Increased demand for food could set the stage for efforts at mass production accompanied by a decline in quality control. The impact on an already stressed environment of meeting these needs could be severe.

Some Asian nations are most threatened by a potential "population explosion" due to their high fertility rates -- the number of children that the average woman will have in her lifetime.

According to the Washington-based Population Reference Bureau's projections, India will replace China as the most populous country in with world by 2050. At that time, the Indian population will jump to 1.6 billion, followed by 1.4 billion Chinese. The United States is expected to rank third with 420 million, followed by Indonesia with 308 million.

While Asia wrestles to manage its exploding population, Europe will be struggling with a very different population challenge.

By 2050, the proportion of people over 60 in Europe will have doubled to 40 percent of the total population. People over 60 years old would constitute 41 percent of population in Germany, 38 percent in Belgium, 36 percent in Denmark, 44 percent in Spain and 38 percent in France, according to estimates by the European Union.

With a smaller proportion of workers struggling to support themselves and their parents, experts believe the European nations will have no choice but to cut health insurance and other social benefits and raise retirement ages. Austria took a first step, passing legislation in 2003 to cut benefits by 10 percent and to raise the retirement age from 60 to 65.

Concerned about the looming population decline, many European countries are struggling to increase their fertility rate by paying child allowances and giving incentives for couples to have more children, such as long paid maternity leave.

In Asia, Japan shares the European malaise. In that country, nearly 42 percent of the population is projected to be 60 or older by 2050, with 16 percent 80 or older, according to a paper on global demographic changes by professors David Bloom and David Cunning of Harvard's School of Public Health.

Despite those menacing numbers, some experts see a brighter future for mankind than the demographic projections suggest.

Cunning, for one, refuses to call aging a problem. "It is one of the main achievements of the mankind during the last century, for people to live longer," he said. He explained that it is less "costly" for governments to deal with large numbers of elderly than to deal with overpopulation.

Cunning expressed optimism that demographic changes will adapt well with economic adjustments 50 years from now, but "this depends on how key institutions would change," in reference to social security, economic openness, the labor market and economic incentives.

While many European countries are urged to amend their social security systems, raise the retirement age and allow people to work longer if they wish, Cunning explained, there is a need to open up the economies in Eastern Europe, introduce more economic incentives created in the northern African and Middle Eastern countries, and improve health services for the elderly population in Asia.

Also, there is a need to achieve economic growth in Latin America, where despite the introduced economic reform, "the problem of unemployment and poverty is still widespread," Cunning said.

Apart from the economic challenges from the global tides of demographic growth and decline, experts differ on the impact of more population on natural resources.

Some say that more people will use more resources and occupy more space. Accordingly, this side of the argument says in order to improve the living standards, there must be limits on the number of people on earth.

"The rebuttal to this argument is equally simple: People create resources," wrote N. Gregory Mankiw, a Harvard economics professor and author of Principles of Economics.

Cunning agrees, arguing strongly that increasing population would not necessarily mean depleting natural resources.

For "most countries" natural resources are not a problem. With technological advancement, these resources have been used properly. The price of natural resources is declining," said Cunning.

Still, even the optimistic experts concede that some areas of the globe - particularly sub-Saharan Africa - are likely to face severe hardship, due to continued high birth rates the plague of AIDS and limited economic development.

"As a result, traditionally high fertility rates and large family sizes have persisted in the face of improvements in infant and child mortality, and now the ravages of HIV/AIDS are depleting the working-age population," Bloom and Cunning wrote in their apaper.

Other countries are adjusting to deal with both high population growth and large numbers of immigrant workers.

The Arab region has become a major destination for labor migrants, one of the reasons behind the demographic changes worldwide.

According to U.N. estimates, the Arab region hosted one in every 10 international migrants in the world and one in every four migrants from the less developed regions.

By 2005, migrants represented 7.3 percent of the region's population, and three Arab countries were among the 20 countries with the largest number of migrants in the world: Saudi Arabia with 6.4 million, the United Arab Emirates with 3.2 million and Jordan with 2.2 million, U.N. statistics show.

Apart from migration, the Arab countries in general have a high percentage of population under the age of 15.

The Arab countries are among the regions where population figures are skyrocketing, and are described as young societies, while many developed countries are getting old, said Mohamed Al Sayed Saeed, deputy head of the Cairo-based Al Ahram Center for strategic studies.

Jumana Al Tamimi is an editor at The Gulf News in Dubai in the United Arab Emirates. She wrote this article while on a recent visit to The Sun as part of a program sponsored by the International Center for Journalists (ICFJ).