Save the planet? Somehow it seems so last year. Cancún – a climate change summit of modest achievement – rates 81 sparse lines of coverage in the Sunday Times, while Chris Huhne's apparent decision not to move in with his mistress rates 118. The BBC, having overspent on Chile's miners, duly hacked back on coverage of Mexico's major meeting. I didn't see one global warming placard in Parliament Square the other day. Protesting youth has other things on its mind.

You can explain the fading of interest – and fear – in many ways, of course. Too much snow in November. Too many lectures from the pope. Too much concentration on the here and now of pinched pocketbook politics. Too many XYZ factors. But David Cameron, ostensibly the greenest PM of them all, hasn't exactly been planting rainforests since he moved into Downing Street: and Ed Miliband, who knows the issues inside out, has simply left them out of his initial equation. Maybe Cancún, in its general agreement at least, is good rather than bad news: but it isn't big news for Britain 2011. And the worst news of all is how little the ecological agenda impinges on real debate.

Take the great child benefit row, and look at what wasn't said around its edges. In a rational world – of the kind passionately championed by Jonathon Porritt and his Forum for the Future – no system would pay you more government money to have more children. Child benefit is the absolute logical opposite of what's needed. Do we want a UK population spurting to 77 million before we're halfway through this century? Do we want Britain to add an extra 1,000 a day? It isn't immigration that's principally fuelling such figures any longer: it's "natural change" (aka known as births against deaths). Yet the furore that greets any shrinkage in benefit range or cash signally fails to register population impact. It's as though the issue doesn't exist.

But it does, of course. It exists in the antechambers of Cancún, where delegates (yet again) base their climate change forecasts on estimates of world population that are frail beyond imagining, because underpinned by no policies that address them. Look at Pakistan, up to 171 million now, and still growing fast; look at Bangladesh, with 164 million mouths to feed somehow, or Nigeria with 156 million. Look at India, at nearly 1.2 billion, hard on China's heels.

And look at China itself, consider those 1.3 billion Chinese. Will the world in 2051 have grown from 6.8 billion to the 9-10 billion range of current cautious expectation? Even those figures cast a giant shadow over food supplies, sustainability and the rest. "A perfect storm" of crises by 2030, according to HMG's chief scientific adviser. Yet nothing is done, nothing ensues.

China's "one child" policy – which may have stopped 250-400 million births, on official calculations – is not a polite subject for discussion anywhere in the west. Indeed, it's often lumped into Beijing's long list of human rights abuses. David and Sam, Ed and Justine, have their "happy events". Some year soon, perhaps, William and Kate will join in. But set all that alongside some LSE research last year for the Optimum Population Trust. It costs £5 on family planning to abate a tonne of CO 2 – against £15 for wind power and £31 for solar power. In short, too many happy events equal global misery. It's the harsh truth where Cancún communiques fall silent.

And the difficulty in even writing in such terms is clear once you start. Think Jonathan Swift and his A Modest Proposal (roasting plump Irish babies for the gentry). Would Dave, Nick or Ed ever dare to sign pledges on fewer babies? Electoral suicide. See the red faces as Howard Flight (and his crass characterisation of child benefits as a "breeding" incentive for the lower classes) was ushered into oblivion. Would any politician – in the steps of Swift – dare to go further and say that an NHS policy targeted on longer life, and thus on a swelling generation of very old dependents, didn't make huge economic sense either. We need more young to look after the old: but if we don't need 77 million, we also need less of everything.

See? There are some areas where democracy can't tread, some subjects too vexed for manifesto treatment. So we're left with very modest proposals indeed; with Cancún, small headlines and small reasons to be cheerful. Last year saving the planet was a challenge that couldn't be shirked. This year you can just reach for the remote.