The world's population of seven billion is set to rise to at least 10 billion by 2100, but could top 15 billion if birth rates are just slightly higher than expected, the United Nations said on Wednesday.

In a report ahead of ceremonies on October 31 to mark the seven billionth human alive today, the UN Population Fund (UNFPA) warned demographic pressure posed mighty challenges for easing poverty and conserving the environment.

New estimates see a global human tally of 9.3 billion at 2050, an increase over earlier figures, and more than 10 billion by century's end, UNFPA said.

But, it added, "with only a small variation in fertility, particularly in the most populous countries, the total could be higher: 10.6 billion people could be living on Earth by 2050 and more than 15 billion in 2100."

The 126-page document, "The State of the World Population 2011", highlights a surge that began with the post-World War II baby boom -- a numbers "bulge" that shows up in following generations as they in turn grow up and have children.

In contrast, prosperity, better education and access to contraception have slashed the global fertility rate to the point that some rich countries have to address a looming population fall.

Over the past six decades, fertility has declined from a statistical average of 6.0 children per women to about 2.5 today, varying from 1.7 in the most advanced economies to 4.2 in the least developed nations.

Even so, 80 million people each year are added to the world's population. People under 25 comprise 43 per cent of the total.

"Our record population can be viewed in many ways as a success for humanity -- people are living longer, healthier lives," said Babatunde Osotimehin, UNFPA's executive director.

"How did we become so many? How large a number can our Earth sustain?" he asked.

"These are important questions, but perhaps not the right ones for our times. When we look only at the big number, we risk being overwhelmed and losing sight of new opportunities to make life better for everyone in the future." 

The report highlighted these challenges:

- HELPING YOUTH: Having large numbers of young adults offers many poor countries the hope of rising from poverty.

But, warns the UNFPA, "this opportunity of a 'demographic dividend' is a fleeting moment that must be claimed quickly or lost." Finding jobs for this swelling sea of youngsters is essential.

The report notably quotes from a report by the UN's International Labour Organisation (ILO) which suggests the 23.4-percent youth unemployment in the Arab world was a major contributor to the uprisings there.

- GREEN WORRIES: The report cites environmental problems that are already pressing and set to intensify as demand grows for food, energy and homes.

Referring to a yardstick of sustainability used by the environmental thinktank Global Footprint Network, the report said it now takes the Earth 18 months to regenerate the natural resources that we use in a year.

"Climate change and rapid population growth are among the many factors contributing to the current drought and famine in the Horn of Africa, which has affected more than 12 million people," it says.

Future concerns focus especially on water stress. "Analysis suggests that the world will face a 40-percent global shortfall (in water) between forecast demand and available supply by 2030," says the report, citing Egypt -- hugely dependent on the Nile -- as a particular example.

- CITY FUTURES: The balance between rural and urban populations "has tipped irreversibly" towards cities in today's world of seven billion. The biggest urban agglomeration, as defined by the UNFPA, is Tokyo, with 36.7 million people, followed by Delhi, with 22 million, Sao Paulo, 20 million and Mumbai, with 20 million.

As the world's population expands, better urban planning, with closer involvement of residents, will be essential. Adequate housing, sanitation and green spaces should be incorporated in the shaping of cities rather than ad-hoc growth that leads to shanty towns.

- IMMIGRATION: In rich countries where populations are becoming top-heavy with the elderly, the task will be to meet growing demands for labour. Immigration, one of the options, needs to be orderly and managed so that migrants are better integrated and protected.

- FAMILY PLANNING: Dozens of countries are lagging in achieving the UN's Millennium Development Goal of providing universal access to reproductive health, said the report.

"A stable population is a sine qua non for accelerated, planned economic growth and development," said Osotimehin. 

FACTS & FIGURES

- Around 2,000 years ago, the world's population was around 300 million. Around 1800, it reached a billion. The second billion was notched up in 1927. The three billion mark was swiftly reached in 1959, rose to four billion in 1974, then accelerated to five billion in 1987, six billion in 1999 and seven billion in 2011.

- By 2050, there will be around 9.3 billion people and more than 10 billion by 2100. But this could be as high as 10.6 billion by 2050 and more than 15 billion in 2100 with only a small rise in fertility in high-population countries.

- Each year around 80 million are added to the world's population, a number roughly equivalent to the population of Germany, Vietnam or Ethiopia. People under 25 comprise 43 percent of the world's population.

- The main reason for the demographic surge of recent decades is the Baby Boom of the 1950s and 60s, which shows up in ensuing "bulges" when this generation reproduces.

- Average life expectancy rose from about 48 years in the early 1950s to about 68 in the first decade of the new millennium. Infant mortality fell by nearly two-thirds.

- Contraception, prosperity and changing cultural attitudes have also brought about a fall in fertility, from a statistical 6.0 children per woman to 2.5 over six decades.

- In more advanced economies, the average fertility rate today is about 1.7 children per woman, below the replacement level of 2.1. In the least developed countries, the rate is 4.2 births, with sub-Saharan African reporting 4.8.

- Asia accounts for 4.2 billion of the world's population. It is projected to reach 5.2 billion in 2052 before declining slowly. The biggest rate of increase is in Africa, whose population first surpassed a billion in 2009 and is expected to add another billion by 2044.

- China is the world's most populous country, with 1.35 billion, followed by India with 1.24 billion. In 2025, India will have 1.46 billion, overtaking China's 1.39 billion. China's population will decline to about 1.3 billion by 2050; India's will peak at 1.7 billion by 2060.

- Under the UN Millennium Goals, access to reproductive health should be universal by 2015. But there are still 46 countries where a fifth or more of women who are married or living in a union still have an unmet need for contraception, particularly in sub-Saharan Africa. Gender equality and women's empowerment are also keys to lowering birth rates.

SOURCE: The State of World Population 2011, published by the United Nations Population Fund (UNFPA)