How many people will inhabit the planet before population growth finally levels off? The figure most commonly used is 9 billion by 2050, up from 6.7 billion today — an extraordinary number, considering that there were only 1 billion humans in 1830. (Right now there are a billion people just between the ages of 10 and 19.) But there’s a growing debate about this projection and what it means.

Optimists cite plunging fertility rates in some countries as evidence that Earth’s human passenger list will not reach 9 billion. Pessimists see a chance of zooming well past that mark, and they add that with all the signs of strained resources (what’s the price of oil today?), this trajectory will lead to some hard knocks. Some say we’ve already shot over the edge of the cliff and, like Wile E. Coyote in the old cartoons, simply haven’t noticed.

Now the Worldwatch Institute, a nonpartisan Washington research group studying the issue for many years, has come out with a fresh analysis concluding that there is so much variability in fertility rates that we can’t know with any confidence how many people the future holds. The study notes, for example, that the fertility rate is rising in the United States, violating the age-old dictum that rich countries don’t make lots of babies. The group sees hints that global numbers could well go higher.

The condition of women’s lives remains a central influence on fertility trends. A summary of the trends report by Robert Engelman, Worldwatch’s vice president for programs “World Population Growth: Fertile Ground for Uncertainty,” has been posted. The full report costs $10. Some highlights are below:

Although the average woman worldwide is giving birth to fewer children than ever before, an estimated 136 million babies were born in 2007. Global data do not allow demographers to be certain that any specific year sets a record for births, but this one cer­tainly came close. The year’s cohort of babies propelled global population to an estimated 6.7 billion by the end of 2007. The seeming contradiction between smaller-than-ever families and near-record births is easily explained. The number of women of childbearing age keeps growing and global life expectancy at birth continues to rise. These two trends explain why population continues growing despite declines in family size. There were 1.7 billion women aged 15 to 49 in late 2007, compared with 856 million in 1970. The average human being born today can expect to live 67 years, a full decade longer than the average newborn could expect in 1970. Only the future growth of the reproductive-age population is readily predictable, however: all but the youngest of the women who will be in this age group in two decades are already alive today. But sustaining further declines in childbearing and increases in life expectancy will require continued efforts by governments to improve access to good health care, and both trends could be threatened by environmental or social deterioration. The uncertain future of these factors makes population growth harder to predict than most people realize.

There’s more on this coming in a new book by Mr. Engelman, which I just received and will read shortly and explore here. It’s called “More: Population, Nature and What Women Want.”

My colleague Nicholas Kristof is working on a book on women in poor places, which I’m sure will do much to reveal the challenges that impede the progress of that half of the human race.

When girls in deep poverty have to spend their days toting water or collecting firewood instead of going to school, that’s a population problem in the end, as well as a simple, and avoidable, waste of human potential.

Via NY Times