80 million more births than deaths (equalling the current population of Egypt or Germany) are added each year to the 7000 million that will be here on earth this coming October 31st. A city for 1.5 million is built, somewhere, every week - inevitably paving over land, destroying habitats and increasing energy use.

Authoritative (UN, WWF) reports on the planet's health have regularly found that water, land, plants, animals and fish stocks are all in "inexorable decline". The more we are, combined with our ever rising world-average environmental footprints, means the less of our finite little planet's dwindling, but still rich legacy we each have to live on. It's not rocket science. Indeed the Chief Scientist and the last President of the Royal Society have both referred to the approaching "perfect storm" of population growth, climate change, and peak oil, leading inexorably to more food, water and energy insecurity.

Ironically, that future chaos stems from today's success: poor women are often blamed for having more children. Not so. Globally, the number of infant deaths per 1,000 births fell from 126 in 1960 to 57 in 2001. A good thing in itself, but the unintended consequence is too many survivors for sustainability, given a finite planet - of which 70 percent is salt-water ocean and half the rest is desert, mountain or icecap (or fast-disappearing biodiversity-rich rain forest).

Yet: in the voluminous outputs from UN Summits - on Climate change and on the ongoing 6th great extinction of world biodiversity (the first ever through unremitting growth of one species destroying habitats for most others) - there is rightly much about environmental footprints, but a deafening silence about population: ie the possibility of fewer feet.

Never since refuted, Ehrlich and Holdren's 1972 equation defined 3 factors or drivers of human environmental impact:

P - Number of persons
A - Affluence, resource consumption and pollution per person - and

T - The 'green-ness' of technologies - on average, per person;

Only three factors, so why is the P-factor "the elephant in the room that no-one talks about". I think this comes from the excessive zeal in the past of some who - not often, but definitely too often, and with a major if disproportionate effect on the thinking of many good people about this subject - have run indefensible programmes of "population control" (two words that should NEVER be put together). These trampled on people's reproductive rights. Past examples include the on-the-ground coercive Indira and Sanjay Gandhi programme in India in the mid-1970s and aspects of the implementation of the controversial Chinese One-child Policy. So come the notions that any quantitative concern about human numbers must, intrinsically, be coercive; or else exclusive, of the many other crucial measures for social justice and for 'making poverty history'. Yet neither of these need to be nor should they be true.

Making a taboo of population is bizarre when the win-win solution to factor P (voluntary, accessible family planning within a rights-based framework) is actually so life-saving, a humanitarian measure. Life-saving? yes, the lives of women (WHO says the outrageous avoidable mortality of 1000 mothers every 24 hours could reduce by 35%, since so many women are dying through a pregnancy they did not want and would have avoided with realistic access to voluntary family planning); and of children, since child survival is always increased by well-spaced births.

The good news is that the average family size of the world has halved since 1950 when it was over 5 and now (2011) stands at about 2.5 (where around 2.1 would be replacement level). The bad news is that the 58 highest fertility countries have family sizes from 7.2 (Niger) to 3.3 (Philippines): and these countries are projected to triple their numbers by 2100, to 4200 million. That's all the humans there were on earth in 1977" (UN data). In most countries there is also population momentum - created by the "bulge" of young people born already in the high fertility years. In Niger and Uganda, one in two of the population is currently a child, under age 15, and this along with current rates of childbearing would result in their present populations tripling sooner - in 40 years!

I welcome the Cambridge Festival of Ideas, launching without inhibitions or any more name-calling, a long overdue reflective, adult conversation on this subject of voluntary interventions that will reduce/reverse rather than forever accommodating to population growth - futilely on a finite planet. We know what works. Brazil, Costa Rica, Cuba, Iran, Korea, Mexico, Sri Lanka, Taiwan, Thailand and SOUTH India (even) all reduced their to around 2, as quickly as China, but without coercion. Let's deal with the taboo and set about removing the barriers to women: so that education and voluntary family planning services are fully accessible to all and provided always wisely and compassionately, respecting and protecting human rights.

"There is no major problem facing our planet that would not be easier to solve with fewer people, or harder - and ultimately impossible - with ever more." (Sir David Attenborough, 2010, speaking as Patron of Population Matters).

"We have not inherited the world from our grandparents, we have borrowed it from our grandchildren" (Kashmiri saying)