Cast your mind back to the 'Day of 7 Billion'. And now take a look at the population figure at the top of this page. In the three weeks since we marked the birth of fragile earth's seven billionth child, there are now 12 million more people on this planet. And that number is increasing by over 200,0000 per day.

Faced with the multiple whammy of topsoil loss, salination of fertile croplands, increased desertification, and the awakening monster of runaway climate change, every year our room for manoeuvre grows more cramped, our belief in a scientific salvation that bit more strained, and our insouciance in the face of our gravest ever challenge ever more eerie.

Following the sceptic-led outcry about the IPCC's projections of wholesale Himalayan-glacier meltdown by 2030, an unprecedentedly thorough scrutiny of the science now demonstrates that most of the glaciers will be very likely gone by 2040 to 2050, rather than all the glaciers very likely gone by 2035. One and a half billion people rely totally on this glacier melt water. Don't you feel reassured?

World human fertility is in decline, no question: the average woman now gives birth to 2.59 children, down from 2.8 in 2002 and 5.0 in 1965; free contraception, birth control, and women's education have undoubtedly had a profound effect, but the greatest check on numbers and boost to the length and quality of life still comes from industrialisation. Countries like Brazil an Iran have hugely lowered birth rates, but at the cost of billions of tons of CO2 emissions.

Yet with a growing carbon constraint surely this is no longer an option for the whole planet. If those millions and the billions already here are ever going to rise from statistics to full lives we simply do not have the option of letting Brazil-level industrialisation bring fertility levels down. The race is now on to humanely and fairly limit our population without the business-as-usual expectation that a couple of trillion tons of carbon from worldwide industrialisation, or 'development' as it's sometimes called, will sort it all out.

This is all excruciatingly difficult to say, and, most of all, it's not a blame game. Many of the world's poor currently release minute amounts of CO2 compared to us guzzlers. But our whole economic model as it now operates strives every sinew to turn minute consumers and emitters into citizens of Beverley Hills. This surely, has been the experience of China of the last twenty years. And can the world afford five new Chinas?

We need a new acceptance that industrialisation cannot be the answer to our present population increase, until a stabilisation has come first. There just isn't the sheer, raw stuff to go around, or the atmosphere, the rivers, lakes, or even seas enough to dump its waste products in.

We need our best minds and hearts working on supporting those in the developing world who'd like to stop at a few kids but feel they can't because there is no social security: we need a complete overhaul of the iniquities of world trade that keeps so much of the poorer world in harness; we need to set it as a good that China's State Economic Planning Agency's one-child policy has avoided the emission of 330 billion tons of CO2 since 1979: and we in the West need to change utterly too.

And most of all, now, more than ever we need to talk about population. In a clear, compassionate and civilized way we need to talk ourselves out of indifference and intolerance and into positive action and support. And first and most pressingly we need the aid and green NGOs right at the forefront of this radical detoxification of the population taboo.

Because - let's be frank about human nature - the longer this silence on population continues, the slimmer the chances of a complete turnaround of how we all live become, and the harder the hearts in the developed world will grow.

And that would be a tragedy beyond belief.