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I will never forget the moment, back in the early 1960's, when Sir Peter Scott, founder of the World Wildlife Fund (WWF), came to visit me one evening in my laboratory at the University of Cambridge, where I was studying reproduction in African elephants (why do they have intra-abdominal testes?).

He said "You know, when we first set up WWF, our objective was to save endangered species from extinction. But we have failed completely; we haven't managed to save a single one. If only we had put all that money into condoms, we might have done some good."

I was flabbergasted! But of course he was right, because it is the habitat destruction caused by the ever-increasing human population that is the ultimate threat to all our wildlife. That remark was a turning-point in my career. I vowed to devote the rest of my life to trying to find new ways of controlling human population growth.

Fifty years later, I must admit that I too have failed. Australia has the highest rate of mammalian extinctions of any country in the world, thanks to human habitat destruction in our fragile environment. The Asian elephant, the Hindu God Ganesha, is now an endangered species, with only about 30,000 left in the wild, thanks to deforestation.

When I was born in 1930, the world's population was only two billion. Today, it has just passed 6.8 billion, and is expected to reach 9.1 billion by 2050. As a microcosm of this global scenario, Australia's current population of 22 million will reach 35 million by 2050 if our politicians have their way.

Every economist believes that growth must be good, whereas every ecologist knows that there must be limits to growth. (Why do elephants breed so slowly? They use breastfeeding, Mother Nature's contraceptive, to keep their births spaced at least 4 years apart).

So what are the factors currently limiting our population growth? We have abandoned prolonged breastfeeding in exchange for Infant Formula, thereby increasing the potential birth rate to one a year (it used to be four years, like elephants), and we have greatly prolonged life expectancy through the conquest of disease. Where will it end?

We have been suddenly brought up short by the realisation that the ultimate constraint to excessive human population growth is the sky. Our rape of the natural environment through deforestation, burning of non-renewable fossil fuels, the methane emissions from the farts and belches of our domestic ruminants, and the unquenchable thirst of our modern lifestyles has resulted in global warming. Left uncontrolled, we could self-destruct. So what is the solution?

It is so simple; we have already invented a pill that will prevent global warming - the oral contraceptive pill. A recent study by Thomas Wire of the London School of Economics has shown that each $7 spent on meeting the unmet demand for Family Planning between 2010 and 2050 would reduce CO2 emissions by over a tonne, which is four times more cost-effective than any conventional solution.

If women the world over were given easy access to the contraceptive pill, every birth could become a wanted birth, and human population growth would almost come to a halt. But getting that simple message across to our self-seeking politicians and economists will probably take another lifetime, and so the Asian elephant will become extinct after all.

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