Fred Pearce is the author of “The Coming Population Crash: And Our Planet’s Surprising Future” and an environment consultant at New Scientist magazine.

Alan Weisman once famously imagined “The World Without Us,” in which Earth heals itself after humanity’s mysterious disappearance. Now, in “Countdown,” he engages with the real future of our overpopulated and over-exploited world — a world with us. He asks in particular how many people our planet can sustain. What is its carrying capacity?

The result is a hugely impressive piece of reportage, a cacophony of voices from across the world, including me at one point. But does he answer the question he sets for himself? Despite enjoying his journey, I don’t think so.

Everywhere he visits has population problems. In the Philippines, the Catholic Church has long vetoed family-planning provision, and the country’s main export is people. In male-dominated Pakistan, girls are kept out of school and indoors to breed. In Israel and the Palestinian territories, some groups seem to be breeding for war.

But Weisman finds hope. Not, perhaps, in China’s one-child straitjacket — though it will deliver that country to population peak in a decade or so — but in women taking charge of their lives and reproduction. He meets them in the back streets of Bangkok; in upcountry Uganda; in Niger, which has the world’s highest fertility rate; and in Italy, where the comical sophistry of one cardinal on what family-planning methods are permissible cannot disguise how women have ignored the Vatican’s teachings to make their country among the least fecund places on the planet. Silvio Berlusconi notwithstanding.

“Countdown: Our Last, Best Hope for a Future on Earth?” by Alan Weisman (Little, Brown)

Weisman’s biggest good-news story, however, comes from another supposed heartland of theocracy: Iran. In 1987, a month after its bloody conflict with Iraq ended, Ayatollah Khomeini ordered the development of what Weisman calls the best voluntary family-planning program in the world. It has since cut the average family size from eight children to fewer than two.

Across the world, he talks to women who want fewer children and men who want more. He understands a big truth: Solving the population problem requires helping women take greater control of their reproductive lives, rather than snatching it from them.

The world has gone through tumultuous demographic change. Better health care and sanitation mean that for the first time in history, most kids get to grow up. For a while, women carried on having the five or six children they once needed to secure the next generation. As a result, world population quadrupled in the 20th century. But that era is passing. Women have got wise to the new reality, even if many men have not. Today, they have, on average, fewer than 2.5 children each — close to replacement level.

That reproductive revolution doesn’t fix the environmental mess we have gotten our planet into. Everywhere he goes, Weisman reports on the problems that fast-rising populations have created: rivers running dry, coral reefs without fish, disappearing forests, changing climates and eroding soils. But defusing the population bomb gives us a shot at cleaning up the mess.

Other demographic problems will emerge, exemplified by the dramatic aging of Japan, where super-low fertility rates have combined with world-record life expectancies and a reluctance to import migrant workers. Some say Japan’s aging is responsible for two decades of economic stagnation in that country. In perhaps his most intriguing chapter, Weisman asks Japanese economists if they can find a way out for their country — and by extension, the rest of the world.

Thus he raises a critical question: What would a planet with a stable population and ecologically sustainable use of resources look like? Where should we be headed? However, just as the discussion gets interesting, Weisman starts to duck out. He endorses the famous formula in Stanford ecologist Paul Ehrlich’s 1968 bestseller, “The Population Bomb”: that the impact of humans on the planet is a combination of our numbers, what we consume and the technology we use to produce what we consume. But while he is good on the impacts and on human numbers, he is sketchy on the rest.

There is, as he quotes Ehrlich, “no condom for consumption.” But lifestyles do change in unexpected ways. We are driving less these days. And better technology means we are doing more with less. In the rich world, our per capita consumption of a range of key resources, from cement and steel to water and fertilizer, is starting to fall.

Such changes make Weisman’s determination to figure out the world’s carrying capacity somewhat futile. He says early on, “This will likely be the century that determines what the optimum human population is for our planet.” Poppycock. There is no such number. When we were hunter-gatherers, we maxed out at perhaps 1 million. Farming got us to 1 billion or so. Now we have industrialization, a revolution still in progress.

Weisman is probably right to say that, at 7 billion people, we have overshot for now. But who knows what the greener and smarter technologies of the future may allow? Who knows what could be achieved if we used what we have already? Ehrlich’s doomsday prediction of billions starving in the 1980s failed to spot that the green revolution would double world food production in the final 30 years of the 20th century. Today, solar energy, electric cars, drip irrigation and a few other easily available technologies could dramatically increase the world’s carrying capacity.

Weisman, however, makes no such predictions. Forecasting what humans might do is harder than working out how nature would respond to our absence. He is gloomy, warning that “technological leaps have yet to solve anything without causing other unforeseen problems.” Sure. But maybe the first farmers were chastised on similar grounds. Innovating is what our species does. We are problem-solvers, for better or worse. The countdown continues.

Fred Pearce is the author of “The Coming Population Crash: And Our Planet’s Surprising Future” and an environment consultant at New Scientist magazine.


Our Last, Best Hope for a Future on Earth?

By Alan Weisman

Little, Brown. 513 pp. $28