We’re growing and growing: World's population to reach 7 billion this month

But in sub-Saharan Africa, the demographic news is mostly sobering as the region staggers under the double burden of the world's highest birthrates and deepest poverty. The regional population of nearly 900 million could reach 2 billion in 40 years at current rates, accounting for about half of the projected global population growth over that span.

"Most of that growth will be in Africa's cities, and in those cities it will almost all be in slums where living conditions are horrible," said John Bongaarts of the Population Council, a New York-based research organisation.

Experts say most of Africa - and other high-growth developing nations such as Afghanistan and Pakistan - will be hard-pressed to furnish enough food, water and jobs for their people, especially without major new family-planning initiatives.

"Extreme poverty and large families tend to reinforce each other," says Lester Brown, the environmental analyst who heads the Earth Policy Institute in Washington. "The challenge is to intervene in that cycle and accelerate the shift to smaller families."

The International Water Management Institute has predicted that by 2025 about 1.8 billion people will live in places suffering from severe water scarcity.

According to demographers, the world's population didn't reach 1 billion until 1804, and it took 123 years to hit the 2 billion mark in 1927. Then the pace accelerated - 3 billion in 1959, 4 billion in 1974, 5 billion in 1987, 6 billion in 1998.

Looking ahead, the UN projects that the world population will reach 8 billion by 2025, 10 billion by 2083. But the numbers could be much higher or lower, depending on such factors as access to birth control, infant mortality rates and average life expectancy - which has risen from 48 years in 1950 to 69 years today.

"Overall, this is not a cause for alarm - the world has absorbed big gains since 1950," said Mr Bongaarts, a vice president of the Population Council. But he cautioned that strains are intensifying: rising energy and food prices, environmental stresses, more than 900 million people undernourished.

"For the rich, it's totally manageable," Mr Bongaarts said. "It's the poor, everywhere, who will be hurt the most."

The executive director of the UN Population Fund, former Nigerian health minister Babatunde Osotimehin, describes the 7 billion milestone as a call to action - especially in the realm of enabling adolescent girls to stay in school and empowering women to control the number of children they have.

"It's an opportunity to bring the issues of population, women's rights and family planning back to center stage," he said in an interview. "There are 215 million women worldwide who need family planning and don't get it. If we can change that, and these women can take charge of their lives, we'll have a better world."