This is a column I don’t want to write. Its subject is ugly; it makes me instinctively recoil. I have chastised people who bring it up at environmentalist meetings. The people who talk about it obsessively have often been callous about human life, and consistently proved wrong throughout history. And yet… there is a grain of insight in what they say.

The subject is overpopulation. Is our planet over-stuffed with human beings? Are we breeding to excess? These questions are increasingly poking into public debate, and from odd directions. Phillip Mountbatten – husband of the British monarch Elizabeth Windsor – said in a documentary screened this week: “The food prices are going up, and everyone thinks it’s to do with not enough food, but it’s really [that there are] too many people. It’s a little embarrassing for everybody, nobody knows how to handle it.” He is not alone. A strange range of people have voiced the same sentiments over the past few months, from the Dalai Lama to Hu Jintao, from Conservative mayor Boris Johnson to Democratic Governor Bill Richardson.

They start by listing the sums, which are indeed startling. Every year, world population grows by 75 million people a year – equivalent to another Britain and Ireland whooshing fully-populated from the oceans. At the turn of the eighteenth century, there were 600 million people on earth. At the turn of the this century, there were 6.6 billion. By the time I am in my sixties, there will be more than 9 billion – at which point there will be more people alive simultaneously than in the first eighteen centuries after Christ combined.

The overpopulation lobby say this will inevitably leave more and more people chasing after a diminishing amount of resources on an ecologically ravaged planet. At their most pessimistic, they say human beings will, in the long sweep of planetary history, look like a big-brained version of a locust cloud. They eat everything in sight and multiply fifty-fold – until they have consumed everything, when they turn in desperation on each other, munch off their siblings’ heads, and then fall out of the sky dead.

They say with a frown that this global swarming is driving global warming. How can you be prepared to cut back on your car emissions and your plane emissions but not on your baby emissions? Can you really celebrate the pitter-patter of tiny carbon-footprints?

Yet this subject seems to leech out all the dark toxins of environmentalism – a movement I believe is the most urgent and important in the world. There has always been an element of green thinking that viewed human beings as a parasitic infestation, wrecking the pristine Eden of planet earth. The green philosopher John Gray calls our species “homo rapiens”; the founder of Earth First!, Dave Foreman, called us “Humanpox” and wrote: "The Aids epidemic, rather than being a scourge, is a welcome development in the inevitable reduction of human population ... If [it] didn't exist, radical environmentalists would have to invent [it]."

If environmentalism sounds – or is – misanthropic, we will lose the argument. Most human beings (including me) will never think the world would be better off without us. The overpopulation problem always seems to be somebody else’s: nobody thinks they are the surplus human being who should not have been born. These strident arguments hand a huge gift to the anti-greens, who always said we were anti-human beneath the surface.

It also looks like displacement. The places where population is growing fastest – sub-Saharan Africa, rural China, and Bangladesh – have virtually no carbon emissions, and pitiful food consumption rates. The gap is so huge that to be responsible for as many gas emissions as one British person, a Cambodian woman would need to have 262 children. Can we really sit in our nice homes, with a fridge-full of food we will mostly chuck away and an SUV in the drive, and complain that she is the problem?

Once this gut-reaction has kicked in, I then think of the horrible history of overpopulation predictions. In 200AD, Tertullian called for a “pruning” of human numbers through “pestilence and war” because “we are burdensome to the world, the resources are scarcely adequate for us.” Most famously, the eighteenth century demographer Thomas Malthus said mass starvation was inevitable because population increases geometrically while food productions grows arithmetically. He said the poor laws were pointless, and the poor should be left to starve, starting with “illegitimate” children because they were “of no value to society.”

He did not anticipate the coming of the Industrial Revolution, which transformed agricultural production and distribution. Similarly, the neo-Malthusians who emerged in the 1960s and 1970s – like Paul Ehrich with his book ‘The Population Bomb’ – didn’t see the Green Revolution that was galloping around the corner of history. So it is tempting to say now that the overpopulation argument will smack its head into the brick wall of some new technological development. After all, t’s not quite true to say there is a diminishing amount of resources, because the genius of human beings is to find new ways to use what is there. Two centuries ago, nobody could have conceived that the sun’s rays or the waves in the ocean were a resource to be used - but solar and tidal power make it so.

And yet, and yet…. why do my own arguments leave me echoing with doubt? A dark voice in my head says: you would accept that, to pluck an absurd number, 100 billion people would be too many. You don’t think human genius is infinitely expansive; there is a limit to what it can solve. So isn’t the question just where you draw the line? If 100 billion is too much, why not 9 billion?

Hmm. You should always take on the best arguments of your opponents, not the worst. There are good people – a world away from the British royals or the human-hating fringes – who are sincerely concerned about population levels: people like Professor Chris Rapley of the British Antarctic Survey, Professor John Guillebaud, or the writer Bill McKibben. They argue that although the swelling billions are not now emitting large amounts of greenhouse gases, they will see that we are doing it and will (totally understandably) want to join in the carbon-bonfire.

The environmentalist writer James Howard Kunsler has an interesting take on this. He says that Malthus was proved wrong by the discovery of fossil fuels, which enabled us to use up millennia of stored energy in one frenetic burst. But now those fossil fuels are running out – and we can’t use what’s left behind without cooking the planet. Unless we put in place a massive transition to renewable energy sources, he believes we are sagging back towards a more Malthusian reality.

But if this is a problem, is there a solution that isn’t abhorrent? Some people seem to reach instinctively for authoritarian answers. The government of China has bragged that its “greatest contribution” to the fight against global warming has been its policy of punishing, imprisoning or sterilising women who have more than one child. Some environmentalists – a small minority – eye this idea jealously.

There is a far better way – and it is something we should be pursuing anyway. It is called feminism. Where women have control over their own bodies – through contraception, abortion and general independence – they choose not to be perpetually pregnant. The UN Fund For Population Activities has calculated that 350 million women in the poorest countries to have didn’t want their last child, but didn’t have the means to prevent it. I’ve met these women – from the slums of South America to the warzones of Africa to the shanty-towns of the Indian subcontinent. We should be helping them by building a global anti-Vatican, distributing the pill and the words of Mary Wollstonecraft.

There is a fat menu of other policies that will bring down population numbers, simply as a consequence of doing something that’s already desirable. One – which might seem counter-intuitive at first – is to make sure more babies live. As the economist Professor Jeffrey Sachs explains: “You want poor households to cut their fertility and to have fewer children – assure them the fewer children they have will survive, they won’t be carried off by a mosquito bite.” It has been proven repeatedly: improving infant mortality makes birth-rates fall. Another population-reducing policy is to introduce pensions for the poor world. If people know they will be secure in their old age, they don’t feel the need to have so many children to look after them. One of the reasons why Europe has a declining birth-rate is because we have achieved all these goals.

So after studying the evidence, I am left in a position I didn’t expect. Yes, the argument about overpopulation is distasteful, it is often discussed inappropriately, and it is far from being a panacea-solution – but it can’t be dismissed entirely. It will be easier for six billion people to cope on a heaving, boiling planet than for nine or ten billion – and we will only get there by freeing women to make their own reproductive choices. To achieve this green goal, it’s necessary need to mix some oestrogen into the environmentalist palette.