A green myth is on the march. It wants to blame the world’s overbreeding poor people for the planet’s peril. It stinks. And on World Population Day, I encourage fellow environmentalists not to be seduced.

Some greens think all efforts to save the world are doomed unless we “do something” about continuing population growth. But this is nonsense. Worse, it is dangerous nonsense.

For a start, the population bomb that I remember being scared by 40 years ago as a schoolkid is being defused fast. Back then, most women round the world had five or six children. Today’s women have just half as many as their mothers — an average of 2.6. Not just in the rich world, but almost everywhere.

This is getting close to the long-term replacement level, which, allowing for girls who don’t make it to adulthood, is around 2.3. Women are cutting their family sizes not because governments tell them to, but for their own good and the good of their families — and if it helps the planet too, then so much the better.

This is a stunning change in just one generation. Why don’t we hear more about it? Because it doesn’t fit the doomsday agenda.

Half the world now has fewer than the “replacement level” of children. That includes Europe, North America, and the Caribbean, most of the Far East from Japan to Thailand, and much of the Middle East from Algeria to Iran.

Yes, Iran. Women in Tehran today have fewer children than their sisters in New York — and a quarter as many as their mothers had. The mullahs may not like it, but those guys don’t count for much in the bedroom.

And China. There, the communist government decides how many children couples can have. The one-child policy is brutal and repulsive. But the odd thing is that it may not make much difference any more. Chinese women round the world have gone the same way without compulsion. When Britain finally handed Hong Kong back to China in 1997, it had the lowest fertility in the world — below one child per woman. Britain wasn’t running a covert one-child policy. That was as many children as the women in Hong Kong wanted.

What is going on? Family-planning experts used to say that women only started having fewer children when they got educated or escaped poverty — like us. But tell that to the women of Bangladesh.

Recently I met Aisha, Miriam, and Akhi — three women from three families working in a backstreet sweatshop in the capital Dhaka. Together, they had 22 brothers and sisters. But they told me they planned to have only six children between them. That was the global reproductive revolution summed up in one shack. Bangladesh is one of the world’s poorest nations. Its girls are among the least educated in the world, and mostly marry in their mid-teens. Yet they have on average just three children now.

India is even lower at 2.8. In Brazil, hotbed of Catholicism, most women have two children. And nothing the priests say can stop millions of them getting sterilized. The local joke is that they prefer being sterilized to other methods of contraception because you only have to confess once. It may not be a joke.

Women are having smaller families because, for the first time in history, they can. Because we have largely eradicated the diseases that used to mean most children died before growing up. Mothers no longer need to have five or six children to ensure the next generation, so they don’t.

There are holdouts, of course. In parts of rural Africa, women still have five or more children. But even here they are being rational — they need the kids to mind the animals and work in the fields.

But most of the world now lives in cities. And in cities, children are an economic burden. You have to get them educated before they can get a job. And by then they are ready to leave home.

The big story is that rich or poor, socialist or capitalist, Muslim or Catholic, secular or devout, with tough government birth-control policies or none, most countries tell the same story: Small families are the new norm.

That doesn’t mean women don’t still need help to achieve their ambitions of small families. They need governments or charities to distribute modern contraception. But this is now about rights for women, not “population control.”

It is also true that population growth has not ceased yet. We have 6.8 billion people today, and may end up with another 2 billion before the population bomb is finally defused. But this is mainly because of a time lag while the huge numbers of young women born during the baby boom years of the 20th century remain fertile.

With half the world already at below-replacement birthrates, and with those rates still falling fast, the world’s population will probably be shrinking within a generation.

This is good news for the environment, for sure. But don’t put out the flags. Another myth put out by the population doom-mongers is that it’s all those extra people that are wrecking the planet. But that’s no longer the case.

Rising consumption today is a far bigger threat to the environment than a rising head count. And most of that extra consumption is still happening in rich countries that have long since given up growing their populations.

Virtually all of the remaining population growth is in the poor world, and the poor half of the planet is only responsible for 7 percent of carbon emissions.

The carbon emissions of one American today are equivalent to those of around four Chinese, 20 Indians, 40 Nigerians, or 250 Ethiopians. How dare rich-world greens blame the poor world for the planet’s perils?

Some greens need to take a long, hard look at themselves. They should remember where some of their ideas came from.

The granddaddy of demographic doomsters was Bob Malthus, an English clergyman who got famous by warning 200 years ago about population growth. He believed that the world’s population would keep increasing till it was cut down by disease or famine. Back in the ferment of the Industrial Revolution, he was a favorite of the evil mill owners and a scourge on anyone with a social conscience.

Malthus hated Victorian charities because he said they were keeping poor people alive to breed. Better that they die, he said. He believed the workhouses, where the destitute ended up, were too lenient, and he successfully campaigned for a get-tough law known at the time as Malthus’s Law.

The novelist Charles Dickens, a social reformer, attacked Malthus in several of his books. When Oliver Twist asked for more gruel in the workhouse, that was a satire on Malthus’s Law. In A Christmas Carol, Ebenezer Scrooge was a caricature of Malthus. In Hard Times, Thomas Gradgrind, the unfeeling headmaster of Coketown, had a son called Malthus.

I think Karl Marx, another contemporary, was spot on when he called Malthusian ideas “a libel on the human race.” And we are seeing the truth of that today as, round the world, women are voluntarily cutting their family sizes. No compulsion needed.

The population bomb is being defused right now — by the world’s poor women. Sadly, the consumption bomb is still primed and ever more dangerous. Now that would be a proper target for environmentalists.

Editor’s note: Read a rebuttal to Pearce’s post by Robert Walker of the Population Institute.

Read more about population and the option of going childfree: