Written by David Crary

Halloween this year brings with it a new spectre. As of Oct. 31 there will be 7 billion people sharing Earth’s land and resources, the UN Population Fund says.

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 John Bongaarts, of the Population Council, a New York-based research organization. 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 centre stage,” he said. “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.”

But as Mr. Osotimehin noted, population-related challenges vary dramatically around the world.

In Western Europe, Japan and Russia, it will be an ironic milestone amid worries about low birth rates and aging populations. In China and India, the two most populous nations, it’s an occasion to reassess policies that have already slowed once-rapid growth. In sub-Saharan Africa, the demographic news is mostly sobering as the region staggers under the double burden of the world’s highest birth rates and deepest poverty.