There's something about David Attenborough that instantly transports me to a happy place. I think it's the affable headmaster tones and the promise of escape from everyday life into a beautifully shot natural world where exotic animals roam free.

The paradox being, of course, that they don't. And it's not just their futures man has jeopardised, it's his own. It was a bold call in the week of the Copenhagen Summit to side-step the obvious environmental debate and shine the spotlight instead on the elephant in the corner: the global population explosion.

But that's what Attenborough did in the fascinating Horizon: How Many People Can Live on Planet Earth? (9pm, BBC2, 9 December). Although he clearly had an agenda he supports the sinister-sounding but well-meaning (I hope) Optimum Population Trust he threw welcome light on this crucial, yet overlooked element of the climate change equation.

If he looked as though he were presenting Jackanory from his comfortable living room, what he had to say made for anything but comfortable listening. In the past 50 or so years, the number of human beings has soared from 2.5 billion to 6.8 billion and it's expected to hit nine billion by 2050.

"We are living in an era in which the biggest threat to human beings and the earth might well be ourselves," he said ominously. The problem is not that we're having too many kids but too many of us are having them, and the rising population is already placing huge pressure on water supplies. No water means no food, yet we need to double the amount of food to feed our growing population. And developed countries are making things worse by consuming more than their fair share.

The upshot is that we have to curb population growth, though Attenborough didn't suggest anything as draconian as compulsory sterilisation. Instead he highlighted the importance of educating women in Kerala, which has one of the highest literacy rates in the world, women tend to marry later and have fewer kids and the need to consume less.

Overall, it was a pretty persuasive argument, even though it definitely did not take me to a happy place.

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