Jonathon Porritt: Over-population: the global crisis that dare not speak its name

This coming Monday, 31 October, is the day the human species crashes through the seven billion barrier, give or take a generous margin of error. It is just 12 years since we went through the six billion barrier, and the UN now tells us that growth could continue throughout the century before stabilising between nine and 11 billion.

From a perspective of the future of the Earth, these are just about the most important numbers we have to deal with. But once they've had their media moment in the sun, as the seven billionth human being arrives, they will slide back again into obscurity, ignored by politicians, religious leaders and most of the world's NGOs, charities and pressure groups.

"Either we reduce our numbers voluntarily, or Nature will do it for us brutally." So said the great Maurice Strong, secretary general of the Rio Earth Summit in 1992. And I've no doubt he'll be saying so again in June next year, even more forcefully, when we're invited to "celebrate" the progress made over the past 20 years at the landmark Rio +20 Conference.

As it happens, there will be some good things to celebrate on the population front. The rate of population growth has fallen, and many countries have achieved impressive reductions in total fertility. The basic rights of women to manage their own fertility have been formally recognised, from the Cairo Conference in 1994 through to the Millennium Development Goals.

But that's about it. Our population is still growing by about 80 million every year. There are still many countries (particularly in Africa, the Middle East and Asia) where total fertility rates remain stubbornly high.

And there are still 215 million women who do not have access to contraception, an abiding injustice which contributes directly to the 53 million unwanted pregnancies every year and the deaths of 70,000 women every year from illegal abortions and complications in pregnancy.

Worse yet, there has been little progress on the political front. A unique coalition of religious fundamentalists, bigoted and craven politicians, and "politically correct" NGOs in the West, has contrived to keep population off anybody's serious "to do" list.

I find this more baffling by the day. The empirical reality of ecological stress – a rapidly worsening reality – is indisputable. Damage to soil, fresh water, forests, biodiversity and fisheries affects both the rich world and the poor world, and cannot any longer be blamed on "over-consumption in the West". On fresh water, for instance, alarm bells are already ringing all around the world, with the UN warning that 65 per cent of human kind will be living in water-stressed and water-scarce countries by 2025.

Of course we can get much smarter in our use of water (and there's a tonne of good stuff already happening on that score), but if you're intent on "better managing our collective water footprint" (to quote some standard NGO blurb), you've got to think about the number of feet as well as the footprints.

And there are political sensitivities involved. The continuing drought in the Horn of Africa is a humanitarian disaster, demanding (and getting) the highest level of compassion from both governments and individuals. The causes are complex. But to ignore the contribution that overpopulation makes is deeply dishonest. The total fertility rate for the women of Ethiopia, Uganda, Somalia and Kenya is between 4.6 and 6.5 children per woman of childbearing age. The combined population of these four countries was 40 million in 1960; now it's 167 million. The population of Ethiopia was 23 million in 1960; now it's 83 million.

So how many times will we have to get out the same old begging-bowls to ensure temporary relief, while continuing to ignore the root causes? Back in June, Baroness Tonge, chair of the All Party Parliamentary Group on Population Development and Reproductive Health, put it as trenchantly as she could. "Disaster relief is an essential response to humanitarian crises. However, prevention of future disaster is also important. Unless we address population growth and reproductive health, the children we save now will be bringing their families to the same feeding centres in 20 years."

Without that kind of investment prioritisation, the problems will get worse every year. Twenty million people were displaced by flooding in Pakistan in 2010 – in a disaster that many link directly to accelerating climate change – and the media coverage was intense and global. This year, a mere 11 million people were displaced in Pakistan by flooding, to which the prevailing media response was "been there, done that". Pakistan has many intractable problems, but to ignore overpopulation as one of them is, yet again, deeply dishonest.

But ignore it we do. Witness the latest blockbuster from the mighty Foresight programme from the department of Sir John Beddington, the Government's chief scientist. Published last week, Migration and Global Environmental Change is an astonishing piece of work. But even in the policy framework section, it just cannot bring itself to say that by far the best bang for every aid or development buck would be to invest in family planning and reproductive healthcare.

It was exactly the same with last year's report from Foresight on food security, which prattled on eloquently about "promoting the agency of women in ways that will accelerate hunger reduction", but had not a word to say about those women's rights to manage their own fertility, including the timing, number and spacing of children.

The simple truth is that continuing population growth is a multiplier of every one of today's converging sustainability pressures, including climate change. We roughly know how many billions of tonnes of CO2 and other greenhouse gases we can afford to put into the atmosphere and still manage to avoid runaway climate change. Effectively managing that "quantum" (assessed by the Potsdam Institute at about 900 billion tonnes) depends both on the number of people and emissions per person – back to the footprint and feet. And as per capita emissions come down (gradually) in the West and rise (rapidly) in developing and emerging economies, the obsessive focus of environmental NGOs of over-consumption in the West looks more and more ill-judged.

Population is a "both/and" story (rich world and poor world), not "either/or". Every country needs a population strategy, including the US and the UK, which are the only OECD countries still to have growing populations. But advocating such an approach gets the fluffy progressives in the green movement even more incensed. Arguing, for instance, that for a couple to decide to have no more than two children represents a much bigger commitment to sustainable living than flaunting your Prius or lagging your loft, induces apoplexy.

Jonathon Porritt is former chairman of the government's Sustainable Development Commission, a former director of Friends of the Earth and the founder-director of the think tank Forum for the Future