The "population bomb" is on the environmental agenda once more.
Green guru Jonathon Porritt recently lambasted the politically correct for ignoring the demographic elephant in the living room - "exponential population growth". Is he right?
When Paul Ehrlich wrote The Population Bomb in 1968, our rising numbers were seen as the primary threat to the planet's future. Only strict birth control could prevent doomsday. Some demographers said we should require a licence to have children, as you would for a dog.
After scandals about forced vasectomies in India, and China's draconian one-child policy, such views became too hot for progressive circles. And Ehrlich's prediction that there would be hundreds of millions of deaths from famine in the 1980s was put on hold when scientists developed new high-yielding varieties of staple crops like rice, wheat and corn - the green revolution.
But now the "p-word" is blamed for climate change, and rising oil and food prices. This is a false analysis for two reasons. The first is that there is no exponential population growth any more. The population "bomb" is fast being defused. Poor women across the developing world are choosing to have dramatically fewer babies. They are doing it for the good of themselves and their families. If this helps the planet too, then so much the better.
Half a century ago, the average woman across the world had between five and six kids. Now she has 2.6. This is approaching the replacement level, which is around 2.3.
Most of southern and eastern Europe now registers fewer than 1.4 babies per woman. Japan, South Korea, China, Taiwan, Singapore, Thailand and Vietnam also have fertility rates well below par.
There are holdouts where women have four children or more - in much of Africa, and some fundamentalist Muslim countries. But not Iran, where today's women have 1.7 children, a quarter as many as their mothers in the early days of the Islamic revolution. Rich or poor, socialist or capitalist, Muslim or Catholic, secular or devout, with government birth control policies or none, most countries tell the same story of fast-declining family sizes.
The world's population is still growing for now. It has reached 6.8 billion, and is rising by 70 million a year. But this is because of the huge numbers of young women born during the 20th century baby boom. They are still fertile. So while they may only have two children each, that is still a lot of babies.
Once they cease to be fertile, the population bomb will be finally defused. In fact, if fertility rates continue to decline, the world's population could start to fall within a few decades - for the first time since the Black Death of the 14th century. That's good news for the environment, but its effect will only be marginal. It certainly won't solve problems like climate change, disappearing forests, wrecked soils and dried-up rivers.
Because the second reason there is no elephant in Porritt's living room is this: the problem is not over-breeders in the poor world, but over-consumers in the rich world.
Take CO2 emissions, the main cause of climate change. Stephen Pacala, director of the Princeton Environmental Institute, calculates that the world's richest half billion people - that's about 7 percent of the global population - are responsible for 50 percent of the world's emissions. Meanwhile the poorest 50 percent are responsible for just 7 percent. So even if the poor world doubled its numbers (which it won't) the effect on global CO2 emissions would be small.
Or take our overall "ecological footprint", the number of hectares required to provide each of us with food, clothing and other resources. The average American takes 9.5 hectares, while Australians require 7.8, Britons 5.3, Germans 4.2 and Chinese 2.0. In India and Africa, where the majority of future world population growth will take place, the average person requires 1.0 hectares or less.
Of course, fast-rising populations can create serious local environmental crises. But, viewed at the global scale, it is overconsumption that matters. Economists predict the world's economy will grow by 400 percent by 2050. If it does, then only a tenth of that growth will be due to rising human numbers.
Yes, some of those extra poor people will become richer. But it is the height of hubris to downgrade the culpability of our own environmental footprint because future generations of poor people might one day be as rich and destructive as us. In any case, they take their cue on how to become rich from us. The truth is we have to find a path to environmentally friendly living.
And every time we talk about too many babies in Africa or India, we are denying that simple fact.
Fred Pearce is the author of Peoplequake, published by Eden Project Books, £12.99.