Nicholas Kristof is not the kind of person you would expect to be a slave owner. As a columnist on that most august of newspapers, the New York Times, he belongs to an elite within an elite, the embodiment of journalistic seriousness. Yet there he was, in 2004, blithely forking out $150 (£96) for Srey Neth and $203 for another teenager, Srey Momm; handing over the money to a brothel keeper in exchange for a receipt and complete dispensation to do with the two girls as he would. Nick Kristof: double Pulitzer prize winner, bestselling author, slave owner. But that, as is made clear in his new book, written with his wife Sheryl WuDunn, is just the start of it.

At the time of his purchase, Kristof had been travelling to a wild and dangerous part of north-western Cambodia, and had checked into an $8-a-night hotel-cum-brothel in the town of Poipet. He arranged to see Neth, who had been in the brothel for a month, having been sold to its owner by her own cousin. Thin and fragile, she had no idea how old she was, but looked to Kristof about 14. Her virginity had been auctioned to a Thai casino manager who later died of Aids, and now she was on offer to local punters at a premium price by dint of her youth and light skin.

Kristof arranged to buy her, as well as Momm from a different brothel. Momm was a frail girl further down the line of misery, having been forced into prostitution five years previously. Amid floods of tears and rage, she pleaded with Kristof to be bought, freed and taken back to her village on the other side of Cambodia. He took both girls back to their villages and, with the help of an American charity, attempted to ease them back into society.

The story of Neth and Momm is just a small indication of the lengths Kristof and WuDunn are prepared to go to expose the injustices that they see in the modern world. Buying up child prostitutes is pretty extreme, but no more than the message they are seeking to deliver in their groundbreaking book, Half the Sky.

In it, they argue that the world is in the grip of a massive moral outrage no less egregious in scale or in the intensity of despair than the African slave trade of the 18th and 19th centuries or the genocides of the 20th. They believe this outrage is a key factor behind many of the most pressing economic and political issues today, from famine in Africa to Islamist terrorism and climate change. Yet they say the phenomenon is largely hidden, invisible to most of us and passing relatively unreported. At worst it is actively tolerated; at best it is ignored.

The fodder of this latterday trade in human suffering is not African people, but women. Which is why they call it "gendercide". If the supreme moral challenge of the 19th century was slavery, and of the 20th century the fight against totalitarianism, then, they write, "in this century the paramount moral challenge will be the struggle for gender equality in the developing world".

The contention is as startling as the idea of a Pulitzer prize-winning journalist buying up prostitutes. I put it to them that, to some people, the claim will seem overblown. After all, you don't go lightly comparing the plight of women in developing countries today with slavery or, by implication, the Holocaust.

"This idea is a couple of decades in gestation," Kristof says. "Over those years, we reluctantly came to the conclusion that this really is the greatest moral challenge of this century."

A 16-year-old Cambodian girl hides after being rescued from a brothel were she was forced to work. Photograph: Rob Elliott/AFP

Then WuDunn chimes in: "When you hear that 60 to 100 million females are missing in the current population, we thought that number compares in the scope and size. And then you compare the slave trade at its peak in the 1780s, when there were 80,000 slaves transported from Africa to the New World, and you see there are now 10 times that amount of women trafficked across international borders, so you start to think you are talking about comparable weight."

Their disturbing conclusion seems all the more powerful for being reached at the end of what they call a "journey of awakening", which began in such different places. Kristof is the product of the west coast who grew up on a sheep and cherry farm in Oregon, the son of liberal academics. WuDunn is east coast-cum-Orient; a third-generation Chinese American formed through the fusion of her family's Chinese roots and the urbanity of New York City.

The hotch-potch of influences is reflected in their suburban New York home where we meet, a large house set in its own thick greenery in an area of real estate where residents are surely more familiar with hedge funds and stock fluctuations than fistulas or honour killings. It feels a little incongruous, the horror of what we're discussing amid the tranquility of the setting. But the gulf is lessened by the fact that they have surrounded themselves with mementoes of their many travels: African wooden dolls, Asian carvings and a large box bed from China covered in furs that fills the living room.

WuDunn and Kristof date the start of their journey to Tiananmen Square, which they covered for the New York Times, having been sent to China by the paper as a young married couple shortly before the 1989 protests erupted (they won a joint Pulitzer for their troubles). "We were horrified by what we saw in Tiananmen," WuDunn says. "But then we went roaming in the countryside a year or two afterwards and started finding all this stuff we had never heard about: the infanticide, with 30 million baby girls missing in the Chinese population."

They realised with a jolt that, every week, as many infant girls were dying in China through lack of access to health care as the up-to-800 protesters who died in Tiananmen. Kristof received a further shock in 1996 when he came face-to-face with girls being trafficked for sex in Cambodia. "I went to a village outside Phnom Penh where a very young teenage girl was having her virginity auctioned. Instead of helping her, the police were there to ensure that, if she escaped, she would be returned to her owners. The main difference from 19th-century slavery was that all these girls would be dead of Aids by their twenties. I was really shaken by how open it was, how blatant. It wasn't underground slavery, it was like what a cutting plantation would have been 150 years earlier."

The discovery of such horrendous abuse got them wondering about the nature of the journalism they were engaged in. Here they were, along with the rest of the "serious" press, debating weighty geopolitical issues of the day. Yet this huge injustice was going on under their noses, largely unreported, dismissed as "women's issues" by the mainstream media. "We've thought a lot about the failure to see this," says WuDunn. "Partly, it's because the news is defined by what happens on a particular day, and a lot of the most important things in the world don't happen on a particular day . . ."

"And it's partly that our definition of what constitutes news is a legacy of the perspective of middle-aged men," adds Kristof. "It may well be that one major reason why high-school girls drop out of school around the world is that they have trouble managing menstruation, and probably one reason nobody has cottoned on to this is that people who run aid organisations and write about it have never menstruated."

Gradually, they began to see this great global disaster more clearly, discovering that, every year, at least two million girls worldwide disappear because of discrimination. They began to investigate and chronicle its various forms, from sexual slavery to honour killings of women deemed to have disgraced the family, to rape as an extension of war, to genital mutilation, to the less violent but no less damaging exclusion of women from health services and education. They widened their net from China to India, Korea, Japan and then Africa.

Runa, 20, a prostitute in Bangladesh. The scar on her face was made by her pimp. Photograph: Sven Torfinn/Panos

"Over the years we began to think about the thread between all of this," WuDunn says. "We realised there was a societal attitude that doesn't allow women to be active members of society, that doesn't treat them like human beings. That's the link: the disregard of women as human beings."

The dawning recognition that they were confronted by nothing less than a modern form of global slavery, with women as its victims, has had profound personal and professional implications for them both. Above all, it has demanded a rethinking of the function of their writing. If you are convinced you have stumbled across an enormous moral outrage, you cannot merely cast light on the subject. You have to do something to stop it. You have to effect change.

That is what makes their book – named after the Chinese saying that women hold up half the sky – so unusual, not just in its searing and heart-rending contents but in its steely determination and sense of purpose. As the Washington Post's reviewer put it, this is a "call to arms, a call for help, a call for contributions, but also a call for volunteers".

From the opening pages, WuDunn and Kristof make an unashamed pitch for the reader's support and engagement. No on-the-one-hand-this and on-the-other-hand-that. "We hope to recruit you," the authors write, "to join an incipient movement to emancipate women . . . Just open your heart and join in."

At the end of the book, in similar vein, they give a list of action points that readers can take within 10 minutes to make a difference. And they set us a personal challenge: will we join a historical movement to eradicate sex slavery, honour killings and acid attacks, or are we content to remain detached bystanders? It is the 21st-century equivalent of that ultimately probing 20th-century question: "What did you do in the war, Daddy?"

The ambition to inspire us to action, to foment what they call a modern abolitionist movement, informs every page of the book. It's not just evident in the direct appeals they make to readers; it is also subliminally present in the way they manage their information. Specifically, they wanted to avoid a numbing effect where readers would become so overwhelmed by the grimness and apparent hopelessness of the lives women lead that they would sink into depression, rather than leap into action.

So WuDunn and Kristof studied psychological papers on what gels people to participate, and discovered that statistics are particularly bad as motivational tools. By contrast, focus on individuals is key. In one experiment, research subjects were told to donate $5 to alleviate world hunger. They were offered the choice of giving the money to Rokia, a girl in Mali aged seven, or to reduce hunger among 21 million Africans. Most chose Rokia. In another study, people were asked to give $300,000 to fight cancer. One group was told the money would save the life of a single child, another that it would save eight children. Perversely, people gave almost twice as much to save just one child rather than eight.

The authors have followed the lessons of these psychological studies They do have statistics in the book, many of which are harrowing. The equivalent of five jumbo jets' worth of women die in labour each day. Every 10 seconds a girl somewhere in the world is pinned down, her legs pulled apart and a part of her genitals cut off, mainly without anaesthetic.

But because the studies warn of the perils, they keep numbers in their place, and instead focus on individual stories. Such as that of Mukhtar Mai, who grew up in a peasant village in southern Punjab and was gang-raped as a child by members of a higher-status local clan. After that she was expected to commit suicide – that's what women who have been gang-raped do. Instead, she went to the police, and with the $8,300 she received in compensation set up a village school.

Or Sunitha Krishnan, who stands just four-and-a-half-feet tall but who has become a legendary fighter in the war against sex trafficking. She set up an organisation in Hyderabad called Prajwala that has helped some 1,500 women escape from prostitution and into new careers such as carpentry or welding.

As well as focusing on the personal, the authors relentlessly accentuate the positive; constantly firing out examples where terrible wrongs have been overcome, proving that seemingly immutable problems can be shifted. The most powerful case for WuDunn is also the most personal: her grandmother's feet were bound in the rural village where her family originates, yet today feet binding in China is unknown. "I'm so conscious of how lucky I am there was a movement in China and abroad that put a stop to a practice that was centuries old." The authors also remind us that genital mutilation was practised regularly in England until the 1860s. It too was eradicated.

"The research stresses the importance of the positive," says Kristof. "People want to be part of something that is successful, and that's one reason why, even though a lot of the stories we profile plumb some really desperate moments, we also try and show it is possible to make a difference. We don't want to sound manipulative – but we do want readers to care about these issues. We followed research in terms of writing in a way that would engage people, and Half the Sky was a kind of experiment in trying to use these approaches to reach a broader audience. From that regard, I think it worked remarkably."

More than 300,000 copies have been sold in the US, a four-hour public broadcasting TV documentary is in the works, and a videogame version of the book will be launched in an attempt to reach a younger audience. More importantly, the authors' call to arms has been heard, with people across the US who first engaged with Half the Sky through reading groups transforming their networks into mini-aid organisations, raising money and making contact with projects abroad. Some 580 book clubs, with 6,500 members, signed up to a Half the Sky project organised by the global aid agency Mercy Corps that raised $20,000. Beyond the money, book club members held speaking engagements, wrote to elected officials, placed opinion pieces in the local paper, and generally caused a stir that in turn inspired others to get involved.

Kristof and WuDunn hand me a letter. "I have a strong passion for helping women," writes its female author. "After reading your book, I decided to take my passion and turn it into my life. I travelled to Uganda, where I saw first-hand how difficult life is for those living in extreme poverty."

And here's one from a woman in Ohio: "After reading your book I felt I had to do something, so I enlisted the members of my water aerobics class at the Cleveland Racquet Club. We now have $640 to foster girls' education." And another from Saratoga Springs: "This year, for the holidays, I gave 25 copies of your book Half the Sky. The result has been the enthusiastic desire of a group of us to establish a multi-generational women's giving circle."

It's the kind of feedback any author or publisher would die for. "I think it's truly taking off," Kristof says.

As for his slaves, it hasn't been easy for them. Momm slid back into prostitution to feed the methamphetamine addiction she had already acquired when Kristof "bought" her. But she eventually became free for a second time following a government crackdown on brothels, and married one of her customers. In 2008, Kristof went to Cambodia to see her again. She called herself a housewife, and said she had left her old life behind for ever.

Neth had her troubles too. She was diagnosed with HIV and thought her life was over, but found medical treatment and also eventually married and had a son. When Kristof last saw her, she told him she planned to set up a hairdressing salon. She said she would name it after him.

Half the Sky: How to Change the World by Nicholas D. Kristof and Sheryl WuDunn is published by Virago, price £12.99. To order a copy for £10.99 with free UK p&p go to guardian.co.uk/bookshop or call 0330 333 6846

• This article was amended on 19 August 2010. The original referred to the slave trade at its peek in the 1780s. This has been corrected.

