Migrant workers travel home to Mali and Niger, 2006, on a two to three week journey across the Sahara from Libya

If you type the words “population clock” into a search engine, a ten-digit number appears. Its first figure represents 6bn, and its second, 900m; two and a half times a second, the units column ticks by. It’s hard to credit that each click represents one human birth—where? Karachi? Nairobi? Birmingham? Sometime on 31st October, the clock will show that the total number of people on the planet has reached 7,000,000,000.

And then? “There is a common myth that the peak will be 9bn,” said Professor Malcolm Potts, of the school of public health at the University of California, Berkeley. In May, the United Nations Population Division (UNPD) raised its projections, reckoning there would be 9.3bn people in 2050, and 10.1bn—still rising—at the end of the century.

But the UNPD has a “low” estimate, too: that there might be just 6.2bn people in 2100. And it has a high one: of 15.8bn, still rising, at that date. Where the figure falls between those poles will help shape the 21st century: which countries are rich, which powerful, which have revolutions and wars, what happens to migration, pollution and climate change. The peak will depend partly on what governments do now to slow the rate of growth.

Not much, if you look at the past two decades. Population policy grabbed attention in the 1960s and 1970s. But a stormy 1994 UN conference in Cairo fused together half a dozen strands of opposition to efforts to slow the rate of growth. Since then, many governments, rich and poor, have been loath to try again.

True, some of the silence is due to success. The total is rising, but the rate of growth is slowing. In October 2009, the Economist magazine carried the cover line: “Falling fertility: how the population problem is solving itself.” Matt Ridley, author of The Rational Optimist, notes that “most economists are now more worried about the effects of imploding populations than they are about exploding ones.”

But discussion of how to slow growth has also been the casualty of a 20-year taboo: coyness, or aversion, on a global scale. As the developing world found its voice, the debate sounded ugly, like rich countries telling poor ones how many babies to have. Environmentalists have also chosen to attack rich countries…

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