It is a tragic measure of how far the world has changed – and the infinite capacity of modern man for taking offence – that there are no two subjects that can get you more swiftly into political trouble than motherhood and apple pie.

The last time I tentatively suggested that there was something to be said in favour of apple pie, I caused a frenzy of hatred in the healthy-eating lobby. It reached such a pitch that journalists were actually pelting me with pies, and demanding a retraction, and an apology, and a formal denunciation of the role of apple pie in causing obesity.

As for motherhood – the fertility of the human race – we are getting to the point where you simply can’t discuss it, and we are thereby refusing to say anything sensible about the biggest single challenge facing the Earth; and no, whatever it may now be conventional to say, that single biggest challenge is not global warming. That is a secondary challenge. The primary challenge facing our species is the reproduction of our species itself.

Depending on how fast you read, the population of the planet is growing with every word that skitters beneath your eyeball. There are more than 211,000 people being added every day, and a population the size of Germany every year.

As someone who has now been travelling around the world for decades, I see this change, and I feel it. You can smell it in the traffic jams of the Middle East. You can see it as you fly over Africa at night, and you see mile after mile of fires burning red in the dark, as the scrub is removed to make way for human beings.

You can see it in the satellite pictures of nocturnal Europe, with the whole place lit up like a fairground. You can see it in the crazy dentition of the Shanghai skyline, where new skyscrapers are going up round the clock.

You can see it as you fly over Mexico City, a vast checkerboard of smog-bound, low-rise dwellings stretching from one horizon to the other; and when you look down on what we are doing to the planet, you have a horrifying vision of habitations multiplying and replicating like bacilli in a Petri dish.

The world’s population is now 6.7 billion, roughly double what it was when I was born. If I live to be in my mid-eighties, then it will have trebled in my lifetime.

The UN last year revised its forecasts upwards, predicting that there will be 9.2 billion people by 2050, and I simply cannot understand why no one discusses this impending calamity, and why no world statesmen have the guts to treat the issue with the seriousness it deserves.

How the hell can we witter on about tackling global warming, and reducing consumption, when we are continuing to add so relentlessly to the number of consumers? The answer is politics, and political cowardice.

There was a time, in the 1960s and 1970s, when people such as my father, Stanley, were becoming interested in demography, and the UN would hold giant conferences on the subject, and it was perfectly respectable to talk about saving the planet by reducing the growth in the number of human beings.

But over the years, the argument changed, and certain words became taboo, and certain concepts became forbidden, and we have reached the stage where the very discussion of overall human fertility – global motherhood – has become more or less banned.

We seem to have given up on population control, and all sorts of explanations are offered for the surrender. Some say Indira Gandhi gave it all a bad name, by her demented plan to sterilise Indian men with the lure of a transistor radio.

Some attribute our complacency to the Green Revolution, which seemed to prove Malthus wrong. It became the received wisdom that the world’s population could rise to umpteen billions, as mankind learnt to make several ears of corn grow where one had grown before.

And then, in recent years, the idea of global population control has been more or less stifled by a pincer movement from the Right and the Left. American Right-wingers disapprove of anything that sounds like birth control, and so George W. Bush withholds the tiny contribution America makes to the UN Fund for Population Activities, regardless of the impact on the health of women in developing countries.

As for the Left, they dislike suggestions of population control because they seem to smack of colonialism and imperialism and telling the Third World what to do; and so we have reached the absurd position in which humanity bleats about the destruction of the environment, and yet there is not a peep in any communiqué from any summit of the EU, G8 or UN about the population growth that is causing that destruction.

The debate is surely now unavoidable. Look at food prices, driven ever higher by population growth in India and China. Look at the insatiable Chinese desire for meat, which has pushed the cost of feed so high that Vladimir Putin has been obliged to institute price controls in the doomed fashion of Diocletian or Edward Heath.

Even in Britain, chicken farmers are finding that the cost of chickenfeed is no longer exactly chickenfeed, and, though the food crisis may once again be solved by the wit of man, the damage to the environment may be irreversible.

It is time we had a grown-up discussion about the optimum quantity of human beings in this country and on this planet. Do we want the south-east of Britain, already the most densely populated major country in Europe, to resemble a giant suburbia?

This is not, repeat not, an argument about immigration per se, since in a sense it does not matter where people come from, and with their skill and their industry, immigrants add hugely to the economy.

This is a straightforward question of population, and the eventual size of the human race.

All the evidence shows that we can help reduce population growth, and world poverty, by promoting literacy and female emancipation and access to birth control. Isn’t it time politicians stopped being so timid, and started talking about the real number one issue?