She is a well-educated, professional woman, working in an office tower in central Nairobi, Kenya. Because of her status and education, the price required to marry her is bound to be high. Although dowries are often now paid in cash, she expects hers will be paid in the traditional method of cows and goats, and that the wedding will take place in the village she came from.

“I’m a traditional girl,” she explains.

It could take a long time for any suitor to accumulate the capital needed to pay – or at least down-pay – her dowry. She’s fine with that.

“We [women] are getting married later,” one of her colleagues explains. “We want an education, job security, and a nice place to live… This also means that we can’t have as many kids, even if we want them.”

These remarks offer a window on one of the most compelling questions of our time: how many people will fill the Earth? The United Nations Population Division projects that numbers will swell to more than 11 billion by the end of this century, almost 4 billion more than are alive today. Where will they live? How will we feed them? How many more of us can our fragile planet withstand?

But a growing body of opinion believes the UN is wrong. We will not reach 11 billion by 2100. Instead, the human population will top out at somewhere between 8 and 9 billion around the middle of the century, and then begin to decline.

Facebook Twitter Pinterest Niamey market in Niger, where the fertility rate, at 7.24, is the highest in the world. The population is set to double in 17 years. Photograph: Danita Delimont/Alamy Stock Photo/Alamy Stock Photo

Jørgen Randers, a Norwegian academic who decades ago warned of a potential global catastrophe caused by overpopulation, has changed his mind. “The world population will never reach nine billion people,” he now believes. “It will peak at 8 billion in 2040, and then decline.”

Similarly, Prof Wolfgang Lutz and his fellow demographers at Vienna’s International Institute for Applied Systems Analysis predict the human population will stabilise by mid-century and then start to go down.

A Deutsche Bank report has the planetary population peaking at 8.7 billion in 2055 and then declining to 8 billion by century’s end.

The UN discounts the claims of these experts, relying on the authority of experience. “We imagine that countries that currently have higher levels of fertility and lower levels of life expectancy will make progress in the future in a similar manner, at a similar speed, to what was experienced by countries in the past,” John Wilmoth, director of the UN Population Division, says. “It’s all grounded in past experience.”

But the dissident demographers think this is wrong, primarily because the UN is failing to account for an accelerating decline in fertility as a result of urbanisation. In 2007, for the first time in human history, the majority of people in the world lived in cities. Today, it’s 55%. In three decades, it will be two-thirds.

A lot happens when people move from the countryside into the city. First, a child changes from being an asset – another pair of shoulders to work in the fields – to a burden – another mouth to feed.

Even more important, a woman who moves to a city has greater access to media, to schools, to other women. She demands greater autonomy. And many women who are able to exercise control over their own bodies decide to have fewer children.

Facebook Twitter Pinterest Ipanema Beach, Rio de Janeiro. Brazil’s fertility rate has fallen from 6.21 in 1960 to 1.73. Photograph: Ingo Roesler/Getty Images

“The brain is the most important reproductive organ,” Lutz asserts. “Once a woman is socialised to have an education and a career, she is socialised to have a smaller family. There’s no going back.”

Religious and familial pressures to settle down and make babies also recede in the city; friends and co-workers, who are largely indifferent to one another’s reproductive choices, become more important.

Already, almost two dozen countries are getting smaller every year, from Poland to Cuba to Japan, which lost almost 450,000 people in 2018. In these countries, women have fewer than the 2.1 babies that they must produce, on average, for a population to remain stable. The population decline would be even steeper were it not for steadily increasing life expectancy.

The fertility rate in the UK is 1.7. Most population growth in the UK today is the result of international immigration, according to the Office of National Statistics. Without immigrants, Great Britain would eventually enter an era of population decline.

More old people and fewer young people place an increased strain on society’s ability to generate the wealth and taxes needed to fund, among other things, healthcare for the old.

As people move to cities, marginal farmland reverts to bush, a natural carbon sink and a boon to wildlife

The really big news, however, is found in the large countries of the developing world, where the great majority of people live. There, declines in birth rates have been simply astonishing. China, the world’s largest country, has a fertility rate of 1.5, lower than Britain’s. India, soon to overtake China as the world’s most populous nation, is at the replacement rate of 2.1 and falling. Brazil, the fifth most populous country, has a fertility rate of 1.8.

Africa remains the cradle of overpopulation, with fertility rates far above replacement. If the human population truly is heading towards 11 billion people, as the UN predicts, then the African story in this century will be grim; the continent will remain largely poor and rural. Women will be forced to have child after child, swelling the numbers of humanity in the one place on Earth that can least easily sustain them.

But this is far too pessimistic a prognosis. Parts of Africa are making great strides in empowering women and reducing the number of children they have. Kenya is one example, though not the only one.

The horrific attack by the Islamist terrorist group al-Shabaab on a hotel and business complex earlier this month brought home once again the challenges facing this sub-Saharan nation of 50 million.

Only about a quarter of its people earn a salary from either a private- or public-sector employer, which is the very definition of a modern workforce. Half the population doesn’t believe it gets enough to eat and about a third reports sometimes going to bed hungry.

On the other hand, over 75% of the population have mobile device subscriptions. In the past three decades, the country’s urban population has more than doubled to 32%. And as it urbanises, Kenya’s fertility rate plummets: from 8 in 1960, according to World Bank figures, to 3.4 today, according to a new study of global fertility rates published last November in the Lancet.

Facebook Twitter Pinterest Women working in a textile factory in Guangdong province, China. There were 15.2m births in China last year, down from 17.2m in 2017. Photograph: Qilai Shen/Corbis via Getty Images

Almost as many girls as boys sat last year for the exams that permit students to graduate from primary school (at the age of 14, after eight years of formal education). On average, the girls scored better.

Many Kenyan women live two lives at the same time. The first is immemorial, agricultural, subsistent and patriarchal. However in her back pocket, she has a mobile phone. And though she hasn’t told her parents yet, she’s planning to move to the city.

Elsewhere the fertility rate figures are less encouraging: Niger, 7; Mali, 6; Nigeria, 5. But even there, changes are happening: Nigeria’s fertility rate was almost seven in 1980.

Women make up 61% of the members of Rwanda’s parliament, the highest proportion of any government. The fertility rate in that country has plummeted from 8 to 4 in the past 30 years. Sub-Saharan Africa is the fastest urbanising part of the world, with annual urban population increases of 4%, twice the global average.

With any luck, Africa in this century will feature urbanisation, better-educated girls and women, and falling fertility. Not everywhere, and not all at once, but in more places than not, and sooner rather than later.

Facebook Twitter Pinterest Shibuya crossing in Tokyo, Japan. The country’s population shrank by a record 448,000 in 2018. Photograph: Danita Delimont/Getty Images/Gallo Images

From Malthusian predictions at global conferences to the latest dystopian offering from Hollywood, pessimists predict a future of overcrowding, scarcity, conflict and possible collapse. But the premise is probably false. We need to prepare, not for the consequences of a population boom, but a population bust. A child born this decade will probably reach middle age in a world where population growth has stalled, and may already have begun to shrink. There could be much about this world to admire. It may be cleaner, safer, quieter. Urbanisation produces a marked decrease in carbon emissions per person – people using public transport, for example, rather than travelling by car – and as people move to the city, marginal farmland reverts to bush, a natural carbon sink and a boon to wildlife.

Economically, however, things could be more challenging, as societies struggle to grow with fewer young workers and taxpayers. Automation will help, but robots don’t buy refrigerators or a smart dress for the office party. Consumption remains the bedrock of any economy.

Population decline is not a good thing or a bad thing. But it is a big thing. It’s time to look it in the eye.

Darrell Bricker is CEO of the global polling firm Ipsos Public Affairs. John Ibbitson is writer-at-large at the Toronto Globe and Mail. Their book Empty Planet: The Shock of Global Population Decline is published by Little, Brown on 5 February (£20). To order a copy for £17.60 go to guardianbookshop.com. Free UK p&p on all online orders over £15